SUBJECT: COMPUTER NETWORK (PGDCA 2 / ADHNT 2 / BCA 4) Page 73 of 73

SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 1 of 76

Note: This question-bank contains three sections. Section-A contains 1 mark Multiple Choice Objective type questions. Section-B contains 5 marks subjective questions. Section-A contains 10 marks subjective questions. Section – A 1 Marks Questions [QUESTIONS 1 TO 104] [PAGE 1 TO 14]

Q1. Which of the following condition is used to transmit two packets over a medium at the same time? A. Contention B. Collision C. Synchronous D. Asynchronous E. None of the above

Q2. Frames from one LAN can be transmitted to another LAN via the device A. Router B. Bridge C. Repeater D. Modem

Q3. You have a class A network address 10.0.0.0 with 40 subnets, but are required to add 60 new subnets very soon. You would like to still allow for the largest possible number of host ids per subnet. Which subnet mask should you assign? A. 255.240.0.0 B. 255.248.0.0 C. 255.252.0.0 D. 255.254.0.0 E. 255.255.255.255

Q4. What is the default subnet mask for a class C network? A. 127.0.0.1 B. 255.0.0.0 C. 255.255.0.0 D. 255.255.255.0 E. None of the above

Q5. Which of the following is used for modulation and demodulation? A. Modem B. Protocols C. Gateway D. Multiplexer E. None of the above

Q6. Which of the following is not a disadvantage of wireless LAN? A. Slower data transmission B. Higher error rate C. Interference of transmissions from different computers D. All of the above

Q7. The Internet Control Message Protocol (ICMP) A. Allows gateways to send error a control messages to other gateways or hosts B. Provides communication between the Internet Protocol Software on one machine and the Internet Protocol Software on another C. Only reports error conditions to the original source, the source must relate errors to individual application programs and take action to correct the problem D. All of the above E. None of the above Q8. Your company has a LAN in its downtown office and has now set up a LAN in the manufacturing plant in the suburbs. To enable everyone to share data and resources between the two lans, what type of device(s) are needed to connect them? Choose the most correct answer. A. Modem B. Cable C. Hub D. Router

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100 SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 2 of 76

Q9. The term 'duplex' refers to the ability of the data receiving stations to echo back a confirming message to the sender. In full duplex data transmission, both the sender and the receiver A. Cannot talk at once B. Can receive and send data simultaneously C. Can send or receive data one at a time D. Can do one way data transmission only E. None of the above

Q10. Contention is A. One or more conductors that serve as a common connection for a related group of devices B. A continuous frequency capable of being modulated or impressed with a second signal C. The condition when two or more stations attempt to use the same channel at the same time D. A collection of interconnected functional units that provides a data communications service among stations attached to the network E. None of the above

Q11. Which of the following TCP/IP protocol is used for transferring electronic mail messages from one machine to another? A. FTP B. SNMP C. SMTP D. RPC E. None of the above

Q12. Which of the following device is used to connect two systems, especially if the systems use different protocols? A. Hub B. Bridge C. Gateway D. Repeater E. None of the above

Q13. A distributed network configuration in which all data/information pass through a central computer is A. Bus network B. Star network C. Ring network D. Point-to-point network E. None of the above

Q14. Which of the following TCP/IP protocol allows an application program on one machine to send a datagram to an application program on another machine? A. UDP B. VMTP C. X.25 D. SMTP E. None of the above

Q15. A remote batch-processing operation in which data is solely input to a central computer would require a: A. Telegraph line B. Simplex lines C. Mixedband channel D. All the above E. None of the above

Q16. ICMP (Internet Control Message Protocol) is A. A TCP/IP protocol used to dynamically bind a high level IP Address to a low-level physical hardware address B. A TCP/IP high level protocol for transferring files from one machine to another C. A protocol used to monitor computers D. A protocol that handles error and control messages E. None of the above

Q17. The slowest transmission speeds are those of A. Twisted-pair wire B. Coaxial cable C. Fiber-optic cable D. Microwaves

Q18. What can greatly reduce TCP/IP configuration problems? A. WINS Server B. WINS Proxy C. DHCP Server D. PDC E. None of the above

Q19. Which of the following statements is correct for the use of packet switching? A. The subdivision of information into individually addressed packets in conjunction with alternative routing arrangement enabled the transmission path to be altered in the event of congestion or individual link failure B. The employment of additional intelligence within the network enabled more sophisticated error control and link control procedures to be applied C. By employing wide bandwidth circuits for the trunk networks substantial economies through extensive sharing of capacity could be achieved. D. All of the above E. None of the above

Q20. The interactive transmission of data within a time sharing system may be best suited to A. Simplex lines B. Half-duplex lines C. Full duplex lines D. Biflex-lines

Q21. Which of the following statement is incorrect? A. The difference between synchronous and asynchronous transmission is the clocking derived from the data in synchronous transmission. B. Half duplex line is a communication line in which data can move in two directions, but not at the same time. C. Teleprocessing combines telecommunications and DP techniques in online activities D. Batch processing is the prefered processing mode for telecommunication operation.

Q22. Which of the following is not a transmission medium? A. Telephone lines B. Coaxial cables C. Modem D. Microwave systems

Q23. Which of the following statement is incorrect? A. Multiplexers are designed to accept data from several I/O devices and transmit a unified stream of data on one communication line B. HDLC is a standard synchronous communication protocol. C. RTS/CTS is the way the DTE indicates that it is ready to transmit data and the way the DCW indicates that it is ready to accept data D. RTS/CTS is the way the terminal indicates ringing

Q24. Which of the following is an advantage to using fiber optics data transmission? A. Resistance to data theft B. Fast data transmission rate C. Low noise level D. All of above

Q25. In OSI network architecture, the dialogue control and token management are responsibility of A. Session layer B. Network layer C. Transport layer D. Data link layer E. None of above

Q26. In OSI network architecture, the routing is performed by A. Network layer B. Data link layer C. Transport layer D. Session layer E. None of above

Q27. Which of the following performs modulation and demodulation? A. Fiber optics B. Satellite C. Coaxial cable D. Modem E. None of the above

Q28. The process of converting analog signals into digital signals so they can be processed by a receiving computer is referred to as: A. Modulation B. Demodulation C. Synchronizing D. Digitising

Q29. How many OSI layers are covered in the X.25 standard? A. Two B. Three C. Seven D. Six E. None of above

Q30. Layer one of the OSI model is A. Physical layer B. Link layer C. Transport layer D. Network layer E. None of above Q31. The x.25 standard specifies a A. Technique for start-stop data B. Technique for dial access C. DTE/DCE interface D. Data bit rate E. None of above

Q32. Which of the following communication modes support two-way traffic but in only one direction at a time? A. Simplex B. Half duplex C. Three-quarters duplex D. All of the above E. None of the above

Q33. Which of the following might be used by a company to satisfy its growing communications needs? A. Front end processor B. Multiplexer C. Controller D. Concentrator E. All of the above

Q34. What is the number of separate protocol layers at the serial interface gateway specified by the X.25 standard? A. 4 B. 2 C. 6 D. 3 E. None of the above

Q35. A form of modulation In which the amplitude of a carrier wave is varied in accordance with some characteristic of the modulating signal, is known as A. Aloha B. Angle modulation

C. Amplitude modulation D. modem E. None of the above

Q36. CSMA (Carrier Sense Multiple Access) is A. a method of determining which device has access to the transmission medium at any time B. a method access control technique for multiple-access transmission media. C. a very common bit-oriented data link protocol issued by ISO. D. network access standard for connecting stations to a circuit-switched network E. None of the above

Q37. Which of the following summation operation is performed on the bits to check an error-detecting code? A. Codec B. Coder-decoder C. Checksum D. Attenuation E. None of the above

Q38. The standard suit of protocols used by the Internet, intranets, extranets, and some other networks. A. TCP/IP B. Protocol C. Open Systems D. Internetwork processor Q39. Networks that follow the 802.5 standard appear to be in a star topology but are actually operating in what type of topology? A. Linear bus B. Modified star C. Modified ring D. Ring E. Hybrid hub

Q40. A communications device that combines transmissions from several 1/ O devices into one line is a: A. concentrator B. modifier C. multiplexer D. full-duplex line E. None of the above

Q41. The main difference between synchronous and asynchronous transmission is A. the clocking is derived from the data in synchronous transmission B. the clocking is mixed with the data in asynchronous transmission C. the pulse height is different. D. the bandwidth required is different E. None of the above

Q42. ARP (Address Resolution Protocol) is A. a TCP/IP protocol used to dynamically bind a high level IP Address to a low-level physical hardware address B. a TCP/IP high level protocol for transferring files from one machine to another C. a protocol used to monitor computers D. a protocol that handles error and control messages E. None of the above

Q43. Which of the following uses network address translation? A. Routers B. Network adapter drivers C. Hubs D. Windows 95

Q44. The X.25 standard specifies a

A. technique for start-stop data B. technique for dial access C. DTE/DCE interface D. data bit rate E. None of the above

Q45. The most important part of a multiple DHCP configurations is to make sure you don't have which of the following in the different scopes? Select the best answer. A. Duplicate addresses B. Duplicate pools C. Duplicate subnets D. Duplicate default gateways E. None of the above

Q46. The 802.5 standard implements a way for preventing collisions on the network. How are collisions prevented when using this standard? A. CSMA/CD B. Token passing C. Collision detection D. Time sharing E. Switched repeaters Q47. A communication network which is used by large organizations over regional, national or global area is called A. LAN B. WAN C. MAN D. Intranet E. None of the above

Q48. Information systems with common hardware, software, and network standards that provide easy access for end users and their networked computer systems. Select the best fit for answer: A. TCP/IP B. Protocol C. Open Systems D. Internetwork processor

Q49. Which of the following TCP/IP protocol is used for file transfer with minimal capability and minimal overhead? A. RARP B. FTP C. TFTP D. TELNET E. None of the above

Q50. Sending a file from your personal computer's primary memory or disk to another computer is called A. uploading B. downloading C. logging on D. hang on

Q51. What frequency range is used for microwave communications, satellite and radar? A. Low Frequency: 30 kHz to 300 kHz B. Medium Frequency: 300 kHz to 3 MHz C. Super High Frequency: 3000 MHz to 30000 MHz D. Extremely High Frequency: 30,000 MHz E. None of the above

Q52. Which of the following statements is incorrect? A. The difference between synchronous and asynchronous transmission is the clocking derived from the data in synchronous transmission B. Half-duplex line is a communication line in which data can move in two directions, but not at the same time. C. Teleprocessing combines telecommunications and DP techniques in online activities. D. Batch processing is the preferred processing mode for telecommunication operations E. None of the above

Q53. If the ASCII character H is sent and the character I is received, what type of error is this? A. single-bit B. multiple-bit C. burst D. recoverable

Q54. UDP is: A. Not a part of the TCP/IP suite B. Connection oriented and unreliable C. Connection orientated and reliable D. Connectionless and unreliable

Q55. The interactive transmission of data within a time sharing system may be best suited to A. simplex lines B. half-duplex lines C. full-duplex lines D. biflex-line E. None of the above

Q56. What OSI layer handles logical address to logical name resolution? A. Transport B. Physical C. Presentation D. Data Link

Q57. The MAC (Media Access Control) address of the network card is used in both Ethernet and To- ken-Ring networks and is essential for communication. What does MAC provide? A. A logical address that identifies the workstation B. A physical address that is randomly assigned each time the computer is started C. A physical address that is assigned by the manufacturer D. The logical domain address for the workstation E. An alias for the computer name

Q58. Which of the following is an example of a client-server model? A. TELNET B. FTP C. DNS D. All of the above E. None of the above

Q59. Alice is setting up a small network in her home so that she can study for her MCSE exams. She doesn't have a lot of money to spend on hardware, so she wants to use a network topology that requires the least amount of hardware possible. Which topology should she select? A. Star B. Right C. Token-Ring D. Ethernet E. Bus

Q60. The CCITT Recommendation X.25 specifies three layers of communications: A. application, presentation and session B. Session, transport and network C. physical datalink and network D. datalink, network and transport

Q61. Which of the following technique is used for allocating capacity on a satellite channel using fixed-assignment FDM? A. Amplitude modulation B. Frequency-division multiple access C. Frequency modulation D. Frequency-shift keying E. None of the above

Q62. Which of the following allows a simple email service and is responsible for moving messages from one mail server to another? A. IMAP B. DHCP

C. SMTP D. FTP E. POP3

Q63. What does 192 translate to in binary? A. 11000000 B. 00111110 C. 00001111 D. 00000011 E. None of the above

Q64. Many large organizations with their offices in different countries of the world connect their computers through telecommunication satellites and telephone lines. Such a communication network is called A. LAN B. WAN C. ECONET D. EITHERNET E. None of the above

Q65. A network which is used for sharing data, software and hardware among several users owning microcomputers is called A. WAN B. MAN C. LAN D. VAN E. None of the above

Q66. What frequency range is used for TV transmission and low power microwave applications? A. Very Low Frequency : 3 kHz to 30 kHz B. Medium Frequency : 300 kHz to 3 MHz C. Ultra High Frequency : 300 MHz to 3000 MHz D. Super High Frequency : 3000 MHz to 30000 MHz E. None of the above

Q67. The data-link layer, in reference to the OSI model, specifies A. data link procedures that provide for the exchange of data via frames that can be sent and received B. the interface between the X.25 network and packet mode device C. the virtual circuit interface to packet-switched service D. All of the above E. None of the above

Q68. What function does a serial interface perform in data communication? A. Converts serial data into audio signals B. Converts analog signals into digital signals C. Converts parallel data into a stream of bits D. Decodes incoming signals into computer data E. None of the above

Q69. You need to determine whether IP information has been assigned to your Windows NT. Which utility should you use? A. NBTSTAT B. NETSTAT C. IPCONFIG D. WINIPCFG E. PING

Q70. You have been contracted to install a windows NT network in an office that is located in a strip mall. The office is located next to the power plant of the building, so a UPS (uninterruptible power sup - ply) has already been installed. What type of cable should you use for the network cabling? Choose the best answer.

A. TI B. UTP C. Fiber-optic D. PSTN E. STP

Q71. Communication circuits that transmit data in both directions but not at the same time are operating in A. a simplex mode B. a half-duplex mode C. a full-duplex mode D. an asynchronous mode E. None of the above

Q72. In a synchronous modem, the digital-to-analog converter transmits signal to the A. equalizer B. modulator C. demodulator D. terminal E. None of the above

Q73. If the client receives an address from a DHCP server, what is the only information available in the Network Properties dialog box? A. The IP address B. The subnet address C. That the client is receiving its address from DHCP D. The default gateway E. None of the above

Q74. Which of the following TCP/IP protocol is used for transferring files from one machine to another? A. RARP B. ARP C. TCP D. FTP E. None of the above

Q75. Which of the following is an example of bounded medium? A. Coaxial cable B. Wave guide C. Fiber optic cable D. A and C E. All of the above

Q76. Which transmission mode is used for data communication along telephone lines? A. Parallel B. Serial C. Synchronous D. Asynchronous E. None of the above

Q77. Which of the following TCP/IP internet protocol a diskless machine uses to obtain its IP address from a server? A. RDP B. RARP C. RIP D. X.25 E. None of the above

Q78. In OSI network architecture, the dialogue control and token management are responsibilities of A. session layer B. network layer C. transport layer D. data link layer E. None of the above

Q79. Which of the following transmission systems provides the highest data rate to an individual device A. Computer bus B. Telephone lines C. Voice band modem D. leased lines E. None of the above

Q80. The IEEE 802 project of the 1980s involved further defining the lower two layers of the OSI model. A number of standards were agreed upon during that time. Which of the following is the standard for Ethernet? A. 802.2 B. 802.3 C. 802.4 D. 802.5 E. 802.6

Q81. Which of the following device is used with an X.25 network to provide service to asynchronous terminals A. repeater B. bridges C. gateway D. Packet assembler/disassemble E. None of the above

Q82. A medium access control technique for multiple access transmission media is A. Aloha B. Amplitude C. Angle modulation D. Attenuation E. None of the above

Q83. Which of the following TCP/IP protocol is used for remote terminal connection service? A. TELNET B. FTP C. RARP D. UDP E. None of the above

Q84. The frequency range : 300 kHz to 3 MHz is used for A. AM radio transmission B. FM radio transmission C. TV transmission D. microwave communications, satellite and radar E. None of the above

Q85. With an IP address of 201.142.23.12, what is your default subnet mask? A. 0.0.0.0 B. 255.0.0.0 C. 255.255.0.0 D. 255.255.255.0 E. None of the above

Q86. A smart modem can dial, hang up and answer incoming calls automatically. Can you tell who provides the appropriate instruction to the modem for this purpose? A. Communications software B. Error detecting protocols C. Link access procedure (LAP) D. Telecommunications E. None of the above

Q87. IEEE project 802 divides the data link layer into an upper _____ sublayer and a lower _____ sublayer. A. HDLC, PDU B. PDU, HDLC C. MAC, LLC D. LLC, MAC

E. None of the above

Q88. A devices that links two homogeneous packet-broadcast local networks, is A. gateway B. repeater C. bridge D. hub E. None of the above

Q89. A computer that handles resource sharing and network management in a local area network. Se- lect the best fit for answer: A. Network Server B. Virtual Private Network C. Network operating system D. OSI

Q90. Repeaters are often used on an 802.3 network to help strengthen the signals being transmitted. As with the length of segments and the number of segments, a limit exists as to how many repeaters can be used between any two nodes. What is the maximum number of repeaters that can be used? A. Five B. Two C. Four D. Six E. Three

Q91. Layer one of the OSI model is A. physical layer B. link layer C. transport layer D. network layer E. None of the above

Q92. What is the term used to describe addresses available on a DHCP server? A. Pools B. Scopes C. Ranges D. Notes E. None of the above

Q93. What is the default subnet mask for a class B network? A. 127.0.01 B. 255.0.0.0 C. 255.255.0.0 D. 255.255.255.0 E. None of the above

Q94. What is the standard protocol for network management features? A. SNA B. FTP C. SNMP D. SMS

Q95. Brad is in charge of a small network and wants to make it simple but secure. The users want to have full control over their data and still be able to share data with the rest of the office. The networking knowledge of the office staff is excellent. Which network(s) would be the best to set up? A. Peer-to-peer B. Master domain C. Server-based D. Ethernet E. Share-level Q96. Modulation is the process of A. sending a file from one computer to another computer B. converting digital signals to analog signals C. converting analog signals to digital signals D. echoing every character that is received E. None of the above

Q97. Which multiplexing technique shifts each signal to a different carrier frequency?

A. FDM B. synchronous TDM C. asynchronous TDM D. All of the above

Q98. Which of the following protocol is connection-oriented? A. IPX B. UDP C. NetBEUI D. TCP

Q99. Which layer of international standard organization's OSI model is responsible for creating and recognizing frame boundaries? A. physical layer B. data link layer C. transport layer D. network layer E. None of the above Q100. Different computers are connected to a LAN by a cable and a/an A. modem B. interface card C. special wires D. telephone lines E. None of the above Q101. When a host knows its physical address but not its IP address, it can use _____. A. RARP B. ARP C. IGMP D. ICMP E. None of the above Q102. After you have verified that TCP/IP is installed correctly, what is the next step in verifying the TCP/IP configuration? A. Ping the broadcast address B. Ping the Microsoft Web site C. Ping a distant router. D. Ping the address of the local host E. None of the above Q103. In OSI model, which of the following layer transforms information from machine format into that understandable by user A. application B. presentation C. session D. physical E. None of the above Q104. Which of the following is a wrong example of network layer: A. Internet Protocol (IP) ARPANET B. X. 25 Packet Level Protocol (PLP) - ISO C. Source routing and Domain naming - Usenet D. X.25 level 2 - ISO E. None of the above

1. B 9. B 17. A 25. A 33. E 41. A 49. C

2. B 10. C 18. C 26. A 34. D 42. A 50. A

3. D 11. C 19. D 27. D 35. A 43. B 51. D

4. D 12. C 20. B 28. D 36. B 44. C 52. D

5. A 13. B 21. D 29. B 37. C 45. A 53. A

6. D 14. A 22. C 30. A 38. A 46. B 54. D

7. D 15. B 23. D 31. C 39. D 47. B 55. B

8. D 16. D 24. D 32. B 40. C 48. C 56. A

57. C 64. B 71. B 78. A 85. D 92. B 99. B

58. D 65. C 72. A 79. A 86. A 93. C 100. B

59. D 66. C 73. C 80. B 87. D 94. C 101. A

60. C 67. A 74. D 81. D 88. C 95. A 102. D

61. B 68. C 75. D 82. A 89. A 96. B 103. B

62. C 69. C 76. B 83. A 90. C 97. A 104. D

63. A 70. E 77. B 84. A 91. A 98. D

Section – B 5 Marks Questions [QUESTIONS 1 TO 78] [PAGE 15 TO 55]

Q1. Explain multiplexer. Explain TDM (Time Division Multiplexing). Ans. Multiplexer :The technique to divide a single physical channel into two or more logical channels so that each logical channel could be used to send different signals is known as multiplexing and the device used for this is Multi- plexer. Multiplexer is a device that enables several low-speed devices to share the same high-speed line. Multiplexer takes input from more than one terminal, combines the signal and lead the combined signal over a single channel, which is used to transfer the information to remote host. Further after receiving a combined signal it may divide the signal, at destination, to various logical channels and send each separate signal to the desired terminal. The following diagram helps to understand:-

Time-division multiplexing (TDM) is a type of digital multiplexing in which two or more channels are derived from a given frequency spectrum, i.e. bit stream, by interleaving pulses representing bits from different channels. Time Division Multiplexing (TDM) is the means by which multiple digital signals (or analogue signals carrying digital data) can be carried on a single transmission path by interleaving portions of each signal in time. Interleaving can be done at bits or blocks of bytes. It enables digitally encoded speech signals to be transmitted and switched optimally within a circuit-switched network. Q2. Discuss the methods for flow control. Ans. Flow Control: - In computer networking, flow control is the process of managing the rate of data transmission between two nodes. It may also be defined as the control of the rate at which data are transmitted from a terminal so that the data can be received by another terminal. It may occur between data terminal equipment (DTE) and a switching centre, via data terminating equipment (DCE), or between two DTEs. By managing a compatible data transfer rate between sending and receiving ends, it prevents network congestion. Data Terminal Equipment:- DTE includes any unit that functions either as a source of or as a destination for binary digital data. It can be a terminal or Printer etc. Data Circuit-Terminating Equipment:- DCE includes any functional unit that transmits or receives data in the form of an analog or digital signal through a network. It can be a MODEM.

There are two methods that have been developed to control the flow of data across communication links. Stop-and-Wait:- In a Stop-and-Wait method of flow control, the sender waits for an acknowledgement after every frame it sends. Only when an acknowledgement has been received is the next frame sent. This process of alternately sending and waiting repeats until the sender transmits an end of the transmission frame.

Stop & Wait Sliding Window:- In this method of flow control several data frames (as specified by the sliding window) can be sent over the network for transmission before needing an acknowledgement. Frames can be sent one right after another, that the link capacity can be used efficiently. After the successful receiving of the sent frames receiver sends the ACK to the receiver. The window slides to the next set of frames according to the ACK received. It will note move for the frames for which no ACK is received and transmit again

Q3. What is congestion? What are the causes for congestion? How can congestion be controlled? Ex- plain various congestion prevention policies? Ans. Congestion management:- Network congestion is the situation in which an increase in data transmission results in a proportionately smaller increase, or even a reduction, in throughput. Congestion results from applications sending more data than a network devices can accommodate, thus causing the buffers on such devices to fill up and possibly overflow. A buffer is a portion of a device’s memory that is set aside as a temporary holding place for data is being sent to or received from another device. This can result in delayed or lost packets, thus causing applications to retransmit the data, thereby adding more traffic and further increase the congestion. To control the congestion we can follows these concept:- Traffic Shaping:- Networks that experience traffic that is bursty tend to have congestion problems. Bursty traffic is defined as idle transmission moments intermixed with sudden increase in traffic for short periods of time. One way to help overcome congestion under these conditions is to force the packets to be transmitted at the managed rate. This methodology is called traffic shaping. One way to implement a regulated rate of traffic is with the use of leaky bucket algorithm. Leaky Bucket algorithm for Flow Control The congestion in a network can be simulated by using a leaky bucked. If a bucket has a small hole at the bottom, The water leaks from the bucket at the constant rate as long as the water is in the bucket. The rate at water leaks does not depends upon the rate at which water enters into the bucket. The input can vary but the output remains constant. It is obvious then if more water enters the bucket then is leaked, the time will come when the bucket over - flows. The same occurs in a network when sending device sends the data at higher rate than the processing rate of the receiving device. Load Shedding:- Load shedding is the final efforts by the routers wvhen other congestion control methods are not solving the congestion problem. Load shedding simply means that the routers will dump packets they cannot process. However, routers can be selective in which packets they discard instead of just dropping ones at random. Jitter Control:- For some applications it is important that the information arrive at the destination at a constant rate. The actual time it takes to transfer the information is not as important as the rate of information delivery. For example, with video and audio data, it is important that the information arrive at a constant rate; otherwise, the information will “jump” and cause jitters. To accomplish this when a packet arrives at a router in the path, the router checks to see if the packet is ahead or behind its delivery time. If it is behind, the router attempts to pass it on as quickly as

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100 SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 16 of 76 possible. If the packet is ahead of its schedule, the router will hold it back a little as the packet gets back on its timetable.

Q4. Differentiate between Internet and intranet.

Ans. INTERNET:- A global network connecting millions of computers. More than 100 countries are linked into exchanges of data, news and opinions. Unlike online services, which are centrally controlled, the Internet is decentralized by design. Each Internet computer, called a host, is independent. Its operators can choose which Internet services to use and which local services to make available to the global Internet community. Remarkably, this anarchy by design works exceedingly well. There are a variety of ways to access the Internet. Most online services, such as America Online, offer access to some Internet services. It is also possible to gain access through a commercial In- ternet Service Provider (ISP).

Internet Intranet. An intranet is a local area network that is in a building or origination. We can also say tha it is a LAN. The same concepts and technologies of the Internet such as clients and servers running on the Internet protocol suite are used to build an intranet. HTTP and other internet protocols are commonly used as well, especially FTP and email. An organization that has many computers usually owns and operates a private network, called an intranet, which connects all the computers within the organization. To provide Internet service, the organization connects its intranet to the Internet. Unlike public access networks, intranets are restricted to provide security. Only authorized computers at the organization can connect to the intranet, and the organization restricts communication between the intranet and the global Internet. The restrictions allow computers inside the organization to exchange information but keep the information confidential and protected from outsiders.

Intranet 1

Q5. How Internet works?

Ans. All information is transmitted across the Internet in small units of data called packets. Software on the sending computer divides a large document into many packets for transmission; software on the receiving computer re- groups incoming packets into the original document. Similar to a postcard, each packet has two parts: a packet header specifying the computer to which the packet should be delivered, and a packet payload containing the data being sent. The header also specifies how the data in the packet should be combined with the data in other packets by recording which piece of a document is contained in the packet. Hardware devices that connect networks in the Internet are called IP routers because they follow the IP protocol when forwarding packets. A router examines the header in each packet that arrives to determine the packet’s desti - nation. The router either delivers the packet to the destination computer across a local network or forwards the packet to another router that is closer to the final destination. Thus, a packet travels from router to router as it passes through the Internet. Protocol software also ensures that data arrives complete and intact. If any packets are missing or damaged, protocol software on the receiving computer requests that the source should resend them. Only when the data has arrived correctly the protocol software makes it available to the receiving application program, and therefore to the user. Q6. Explain the need of computer networks. Explain the networking elements. Ans. Computer network is a group of computers that have been connected together in some fashion so that these two things can be shared: 1 Information 2 Resources: - The design and implementation of a network requires the three components to be present: 3 Something that needs to be shared 4 A physical pathway between network “nodes” 5 Rules to control communication Today, network is a need of every office, organization, company, institute etc. Computer networks are used for electronic mail, teleconferences etc. While sitting in front of a computer, you can collect information about any place, institute etc. This can be done with the help of a program called Netscape Navigator. You can talk to your friend through a special software sitting thousands of miles away from you and see his face on the screen. Some of the Networking elements/Components are; Network Examples of alternatives available component Media Twisted pair wire, coaxial cable, fiber optics, Microwave, satellites, cellular radio, point to point trans receivers. Processors Modems, multiplexers, concentrators, routers, bridges, gateways, Front-end processors, client and server computers. Software Communication software, Network operating system Netware, Slip or PPP, POP, SMTP, MSN Internet Explorer or Netscape Navigator, middleware. Channels Analog/digital, switched/non-switched, circuit/message/packet switching, simplex/duplex, synchronous/asynchronous, speed and baud rate. Topology Point-to-point, multidrop, star, ring, mesh, Ethernet. Architecture OSI, IEEE, ISDN, PSTN Q7. Explain networking services. Ans. Network is established so that a number of expensive hardware and relevant data can be shared among different computers. This technology is used to take advantage of the reduced price and increased performance in the workplace. For this, a network supports the following services. 1) File services: - In computer terminology processed information or unprocessed raw data is stored in the form of files. These files are arranged in hierarchical order in folders so that they can be traced easily. File services provide a mean to access these files in a network. Files are stored in a hard disk and other computers access them. A computer in the network with proper permissions can access, read, modify, create, delete or copy these files from a remote computer. 2) Print Services: - It is not possible for every organization to purchase a separate printer for each computer installed in the organization. If the computers are networked even though it is not advisable to move the printer from one computer to another where it is required. Print Services allow a printer to be installed on one computer and other computers can share the same printer through network. There is no need to transfer printer from one computer to another.

3) Application Services: - These services discard the need of installing applications in separate computers. This may also be used to share a database among various computers. Database queries, updating, and maintenance can be done at a single computer depending on the instructions given by other computers. This helps in synchro- nized data storage and helps to store multiple copies of same database in different computers. 4) Messaging Services: - Messaging services enable users to use several programs to transfer information in various forms (text, graphics, voice and video) across the network to other users. Electronic mail is the most common example of how message services can be used.

Q8. What do you mean by network categories? Or Differentiate between peer-to-peer and client/server network. Ans. Networks can be broadly categorized as following: 1. Client/server Networks: - These network use a dedicated computer serving as a server and providing all the necessary network services such as printing, application and database sharing etc. All the other computers are known as clients. These ask for various resources and the server in turn serve them with all the resources available to it and the permission granted to that particular client computer. Client/server network can cover a much larger geographical area, from one building to areas located distantly.In a client/server environment, all tasks are divided between a back end (server), which stores and distributes data, and a front end (client), which requests specific data from the server. The most common client/server application is a database management system using SQL. In a client/server environment, the database query is sent from the client processed on the server. Only the results are sent across the network back to the client.

Characteristics (I) Area covered is quite large (ii) It may go for one building LAN to an international WAN (iii) A client/server network can have specialized servers to meet specific Services. (iv) Security and performance are important issues. (v) Reduced network traffic 2. Peer-to-Peer Networks: - In this type of network there is no need for a dedicated server and each computer uses the network to share resources among its independent peers. These are usually small in size. They use simple cabling system and users perform most administrative duties themselves. Users may stop the accessibility of certain files or folders by making it as ‘not-shared’. The technology used is simple. Login and password system may be employed if the user wishes to take part in the network otherwise he is free to use his computer totally on his own. Each node can be either a client or a server, or both. These networks are used for file sharing, printer sharing, e-mail, tight budgets, and easy installations. They usually contain 2 to 10 users. Users simply share disk space and resources, such as printers and faxes etc. Any user can use any resource once he logs into one peer on the network. Sharing of the resources can be controlled by a password also. The user cannot specify which users on the network can access the resource. The largest disadvantage of peer-to-peer network is that each shared resource must have its own password. To protect the resources the number of passwords on the network may become huge (depends on the number of users and the resources to be shared).

Characteristics (i) Simple cabling scheme (usually twisted pair cable with a small hub) (ii) Small geographical area covered (single floor, or a part of a floor, small office) (iii) The degree of performance depends on the number of users and sharing of resources (v) Growth of network is anticipated to be fairly limited. (iv) This is cost effective network. Q9. Explain in detail the advantages of computer networks. Ans. Various advantages of a network include: 1) Sharing files:-The network enables computers to share information with one another. The one way is to transmit the file or data from your computer to the other computer directly. The other way is to send data to an intermediate place, from where the other person can pick it up later. 2) Sharing hardware: - It permits various departments and people to share hardware. For example, a high-speed laser printer may be an unaffordable device for a small office but a company can easily afford it because many users and departments can share it. Similarly, several users can share a hard disk in the main computer. A hard disk must be set up as a shared resource in order to share files with other users. 3) Communication: - The network permits communication between various organizations. The information sharing is of great significance for the organization. 4) Electronic transfer of text: - irculars, notices and memos etc. Each user has a mail-box located in the memory of his computer. The messages to be sent are stored in the appropriate mail-box and are retrieved as and when needed by the user. 5) Coast saving:- With the help oft time saving & hardw3rare sgharing we can save our coast. 6) Speed: - Networks provide a very rapid method for sharing and transferring files. Without a network, copying them to floppy disks, then carrying or sending the disks from one computer to another shares file. 7) Faster Communication: - Using E-mail and other means of communication in the network the communication can be made in timely manner.

Q10. Explain the disadvantages of computer networks. Ans. Networks are helpful in many ways to share resources and saving time and money to buy extra hardware and resources but this technology has some drawbacks. The main drawbacks could be summarized as follows: 1) Data Security:- When all the files are shared among the users there is no security to important data stored in one computer. This data can be altered or used un-authoritatively by other users. This problem can be solved by is - suing proper permissions to various users according to their nature of work. Any desired file or folder may be made inaccessible to other users of the network or it can be made available to them through proper password validation check. 2) System Crash: - Since all the important resources and data are shared, failure of one computer in the network may cause huge loss to the organization. This may also be taken care of by proper disaster recovery measures and taking backups at regular intervals. Various topologies help in arranging computers in a network in such a fashion that the unavailability of one may not hinder the overall working of the

network. 3) Privacy: - If the network is poorly implemented, improper communication may take place. Other users can see private or important mails and this may result in loss of privacy.

Q11. Explain the roles of protocols. Or What is the importance of networking protocols in a network? Ans. Roles of Protocols: - Protocols or procedures are defined to help the computers to communicate through a network in a more efficient manner. The major parts are: 1. Data Sequencing: - A huge stream of data sent at one go might cause an error and the error occurred in this case can be difficult to find out. The error may be loss of data or corruption of data due to attenuation (attenuation means the tendency of a signal to weaken as it travels through a transmission medium). To avoid this, huge data is divided into packets, known as data sequencing and these packets and are sent across the network. These packets are further divided into frames. Frames are formed with proper structure containing the destination address, actual data and error detection codes. 2. Data Routing: - Data has to be sent from one computer to another and there may be many paths available to reach the desired host or terminal. Protocols define proper routing algorithm so that data is sent to desired location at minimum time and without any loss. 3. Flow Control:- Different computers and devices are involved in network, obviously their speed may also vary. Communication protocol takes care of it and pads the timing difference if any. 4. Error Control:- In data transfer, any error may occur due to attenuation, faulty media or by corrupt control algorithm. Communication protocol also defines the error detecting and recovering algorithms in case of data loss. The most common method of data recovery is to retransmit the block. 5. Precedence and order of transmission: - In network the nature and precedence of the work and stations should be defined. In case of a clash, the work or station with highest priority should be processed first. Communication protocol provide mean to solve such situations. 6. Connection establishment: - Whenever one station wants to send an information to another station through a network, it must be seen that other station is ready to be connected or not. Sender should make sure that the receiver is listening and it is sending the information to the correct authorized station only. All this is covered by protocols. 7. Data Security: - Data sent through a network is passed through several stations and devices. Communica- tion protocol sets the security standards that must be followed in order to carry out safe and secure data transmission. 8. Log Information: - Data communication software can normally log different activities like what time the network was activated, how many nodes are connected at which time, how many bytes of data is transferred to and fro through the network and other security measures.

Q12. What is OSI Reference model? Ans. In 1983, the International Standards Organization (ISO) developed a model, which would allow the sending, and receiving of data between two computers. It works on a layer approach, where each layer is responsible for performing certain functions. When we think of how to send data from one computer to another, there are many different things involved. There are network adapters, voltages and signals on the cable, how the data is packaged, error control in case something goes wrong, and many other concerns. By dividing these into separate layers, it makes the task of writ - ing software to perform this much easier. In the Open Systems Interconnect model, which allows dissimilar computers to transfer data between themselves, there are SEVEN distinct layers.

Q13. Explain the functions of OSI layers Ans. Functions of various layers described in OSI model are as follows:- 1) Physical Layer: - Controls the transmission of the actual data onto the network cable. It defines the electrical signals, encoding of the data and the connector types used. Repeaters are an example of devices that work at the Physical Layer. 2) The Data-Link Layer: - This layer takes the data frames or messages from the Network Layer and provides for their actual transmission. At the receiving computer, this layer receives the incoming data and sends it to the network layer for handling. It also provides error-free delivery of data between the two computers by using the physical layer. It does this by packaging the data from the Network Layer into a frame which includes error detection information. 3) Network Layer: - This is responsible for addressing messages and data so they are sent to the correct destination, and for translating logical addresses and names (like a machine name Lab1_1) into physical addresses. This layer is also responsible for finding a path through the network to the destination computer. 4) Transport Layer: - Ensures that data is delivered error free, in sequence and with no loss, duplications or corruption. This layer also repackages data by assembling long messages into lots of smaller messages for sending, and repackaging the smaller messages into the original larger message at the receiving end. 5) Session Layer: - Allows two applications to establish, use and disconnect a connection between them called a session. Provides for name recognition and additional functions like security which are needed to allow applications to communicate over the network. 6) Presentation Layer: - This layer provides independence from differences in data representation by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats data to be sent across a network. 7) Application Layer: - This layer supports application and end-user processes. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist en- tirely in the application level.

Q14. Differentiate between broadband and base band. Ans. Broadband:- A transmission method in which the network’s range of transmission frequencies is divided into separate channels and each channel is used to send a different signal. Broadband transmission is often used to send signals of different kind simultaneously, such as video and voice and data. Base band:- A transmission method in which a network uses its entire transmission frequency range to send a single communication or signal The construction and shielding of the coaxial cable give base band. coaxial cables are a good combination of high bandwidth and excellent noise immunity. The bandwidth possible depends on the cable length. Coaxial cables used to be widely used within the telephone system but have now largely been replaced by fiber optics on long-haul routes. Whereas broadband cables can be used up to 300 MHz and can run for nearly 100 km due to the analog signaling, which is much less critical than digital signaling.

One key difference between base band and broadband is that broadband systems typically cover a large area and therefore need analog amplifiers to strengthen the signal periodically. These amplifiers can only transmit signals in one direction, so a computer outputting a packet will not be able to reach computers from it if an amplifier lies between them.

Q15. Define cable media. Ans. In cable media, cables are used to transfer data. Some of the common used cable media are: Twisted Pair Cable: This is one of the most common transmission methods. These are used in local telephone communication and short distance digital data transmission up to about 1 km. Data transmission speed of up to 9600 bits per second (baud) can be obtained for a distance of about 100 meters. For longer distances local telephone lines are used but that reduces the transmission speed to about 1200 bits per second. Insulated copper wires are twisted on each other to reduce interference by adjacent wires. But the major disadvantages are noise interface and low transmission rate.

Coaxial Cable: It is used for long distance service by telephone companies and for both baseband and broadband transmission in local area network. These cables are used for high-speed data transfer over a long distance and are immune to electrical noise. Baseband coaxial cable transmits a single signal at a time at a very high speed, while a broadband coaxial cable can transmit many simultaneous signals using different frequencies. A broadband coaxial cable can transmit only analog signals, so it must be used in conjunction with a modem. It is not easy to use it in a network. Coaxial cables are groups of specially wrapped insulated wire lines.

Fiber -Optic cables: Fiber-optic cables are made of glass or transparent plastic. They are as thick as human hair (thickness is of the order of few microns). The light enters at one end of the light-pipe. After multiple reflections at various points inside the pipe, the light comes out without any significant loss in energy. On this cable data is transmitted as pulses of light, which passes through the cables. These cables are secured, and transmit large volumes of data over long distances.

Q16. Why is co-axial cable superior to twisted pair cable? Ans. Coaxial cables are used for long distance service by telephone companies and for both baseband and broadband transmission in local area network. These cables are used for high-speed data transfer over a long distance and are immune to electrical noise. Baseband coaxial cable transmits a single signal at a time at a very high speed, while a broadband coaxial cable can transmit many simultaneous signals using different frequencies. These are used in local telephone communication and short distance digital data transmission up to about 1 km. It is mainly used for local area networks. A broadband coaxial cable can transmit only analog signals, so it must be used in conjunction with a modem. Insulated copper wires are twisted on each other to reduce interference by adjacent wires. It is not easy to use it in a network. Coaxial cables are groups of specially wrapped insulated wire lines. Twisted pair cable suffers from noise and low transmission rate. But the major disadvantages are noise interface and low transmission rate.

Q17. Compare features of bridges and switches. Ans. Bridges: Bridges are used to decrease network traffic by dividing a network into segments, which increases the maximum possible size of your network. Each segment that a bridge connects is considered to be from a different network. Bridges connect network segments. Bridges had only two ports, one for each network. However, unlike hubs, bridges actually inspect the data that passes through them and make decisions about whether to send it to the other network or not. Switches: As networks grew even larger and the amount of data transmitted by each computer increased, seg- menting networks become even more important. Two-port bridges were no longer sufficient. Although they have much more functionality now, switches began as multiport bridges and are considered layer 2 devices. Most switch - es have 12 or 24 ports, but many are modular and can have several hundred ports. Another distinction is that switches can handle several conversations at the same time. Switches also employ buffers. These buffers are memory that can be used to store frames until the frame can be transmitted.

Q18. Explain the concept of routers. On which layer do switches and routers work. Ans. Routers are devices that connect two or more de similar networks. Routers include the physical interfaces to the various networks in the inter-network. These interfaces can be Token Ring, Ethernet, TI, Frame Relay, Asyn- chronous Transfer Mode (ATM), or any other technology. Routers act as the boundary between broadcast domains. Similar to the way bridges and switches read and act upon the layer 2 headers, routers read and make decisions based on the layer 3 headers, such as the TCP/IP or IPX headers. Therefore, we say that routers are layer 3 devices. A router’s job is to inspect each packet sent to it and determine if it belongs to the local IP or IPX network or to a remote network. If the destination of the packet is a remote network, and the router knows how to reach that network, the router forwards the packets; otherwise the packet is discarded. Routers are also used almost exclu- sively to connect remote networks via WAN links, but this is unrelated to the actual function of routing. Routers often use sophisticated algorithms and routing protocols to communicate with other routers to discover the best way to reach remote networks.

Q19. Explain various LAN topologies. Or What are the different network topologies? Explain. Ans. Various available LAN topologies include: Topologies define the arrangement of computers taking part in the network. These should be properly planned and implemented because a good selection of topology can result in proper and adequate use of resources while a mismanaged arrangement of computers can lead to inefficient use of resources, loss of data and time. Various available topologies include:-

BUS TOPOLOGY: - Bus topology comprises of stations that are connected to a single communication line. This single communication line is referred to as a bus. Information frames originating from a station are propagated away from the station in both directions on the bus. Each station on the bus interrogates the information frame destination address field for its own address. If the destination field does not match the stations address, the station discards the information frame back on to the bus. If the destination address matches the station address, it accepts the information frame and processes the frame.

Advantages: 1) Simple installation 2) Failure of a station affects that station only. 3) No concept of dependability on a central node, like in the case of star topology. Disadvantages: 1) Time to transfer information may be large because each node tests the information packet floating in the network that whether it is for him. 2) Failure of cables may result in serious implication on the network. Cable failure can be extremely difficult to locate on large bus networks. STAR TOPOLOGY: - tar topology comprises of a number of stations connected directly to a central station or controller. Communications on the connecting links between the stations and the central station of star topography can be bi-directional and are point-to-point. A station on this type of network passes an information frame to the central controller, which then forwards the information the destination station. The central controller manages and controls all communications between stations on the network. Diagrammatic representation of the star topology is given below.

Advantages: 1) If a computer crashes, it has no effect on the entire network. 2) It involves minimal number of cables. 3) Introduction or removal or node from the network has no optimal effect over other computers in the network. Disadvantages: 1) Whole system is dependent on a central node, so it should be powerful and for that matter it may be costly. 2) If the central node, fails whole of the network may come down.

RING TOPOLOGY: - Ring topology is like a circuit as shown in the diagram below. Local area networks that have each station attached to an adjacent station using point-to-point links form a physical ring. Each station attached and active to the ring regenerates the information frame, then, re-transmits the information frame on the ring. The ring itself is logically circular and the flow of information is unidirectional.

Advantages: 1) Since the transmission of frames is unidirectional simple, software algorithms could be used. 2) There is no dependency on a single node. Disadvantages: 1) Failure of one node stops the working of whole network. 2) A control mechanism is required so as to determine who should start up the ring. 3) Since transmission is unidirectional, the time taken to reach the information at desired location may be long. TREE TOPOLOGY: - Tree topology is considered as a corollary of the bus topology, the difference being the existence of branches with multiple nodes is possible. The transmission medium is a branching cable with no closed loop.

Q20. What is traffic shaping. What are the various methodologies used in traffic shaping. Ans. Traffic Shaping:- Networks that experience traffic that is bursty tend to have congestion problems. Bursty traffic is defined as idle transmission moments intermixed with sudden increase in traffic for short periods of time. One way to help overcome congestion under these conditions is to force the packets to be transmitted at the managed rate. This methodology is called traffic shaping. One way to implement a regulated rate of traffic is with the use of leaky bucket algorithm.

A second approach to regulating traffic is with the use of token bucket algorithm. This is similar to the leaky bucket except that as the rate of incoming packets rises, the output of the system speeds up slightly. The bucket in this scheme holds tokens and in order for a packet to leave the bucket, it must have a token. If the network is idle, the

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100 SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 26 of 76 number of tokens in the bucket grows so if burst packets arrive, there are more tokens available for the packets to use to leave the bucket. Once all the tokens are used up, the packets must wait until tokens are regenerated before they can leave the bucket. Thus, we have controlled the rate of traffic but the rate can vary somewhat, depending on the rate at which the packets arrive.

Q21. What is error correction? What are the methods used to control errors? Ans. Most computer networking protocols use error correction schemes to implement requests to the sender to retransmit the missing information. To ensure that the information arrives, acknowledgement techniques are used so that the sender has some type of feedback from the receiver that the information did or did not arrive. Acknowl- edgements are usually special control frames that contain either positive or negative acknowledgements about the incoming information. To ensure that the retransmitted packets of the same information are not all processed, se - quence numbers are assigned to outgoing packets so that the receiver can tell which are the originals and which are the retransmissions. The methods used for error control include the combinations of the following mechanisms: a)Error detection: packets arriving at the receiver that do not pass the error checking routine are discarded. b)Positive acknowledgement: When the receiver determines that a packet has passed the error-checking routine, a positive acknowledgement is sent back to the sender. c)Negative acknowledgement: when the receiver determines that a packet has failed the error-checking routine, a negative acknowledgement is sent back to the sender. d)Retransmission after timeout: If the sender has not received an acknowledgement within a certain time frame from a packet it has sent, the sender retransmits the packet. As a group, these methods are known as automatic repeat requests (ARQ). There are two implementations of ARQ found in network communication protocols. The first is called stop-and –wait ARQ. In this scheme, the sender transmits a packet, waits for the acknowledgement and then sends the next packet. In the second procedure, go-back-n ARQ, the sender transmits a series of packets that contain sequence numbers. The receiver analyzes the transmitted packets and if any failure occurs in the error-checking routine, the sender retransmits the damaged packet and the packets following the damaged packet.

Q22. Explain types of networks. Ans. Based largely on a network’s geographic size, we can divide networks into two categories: Local Area Networks (LANs): - It is a network of computers largely located in vicinity such as a floor of a building or a building itself. They developed from point-to-point connections, where a single wire joined the two systems.

LAN These typically have the following characteristics: § covers small geographical area §Cables with twisted-pair wire, coaxial or fiber-optic cable are used. Metropolitan Area Network: - A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to entire cities. Its geographic scope falls between a WAN and LAN. MAN provide Internet con - nectivity for LANs in a metropolitan region, and connect them to wider area networks like the Internet

MAN Wide Area Networks (WANs): - When the organization is spread over a wide area then it becomes more difficult and costly to link them through cables. For this situation, the various units of the organization are linked using Wide area networks. Connecting links are made over telephone lines leased from the various telephone companies. In some cases, WANs are created with satellite links, radio or microwave transceivers. The Internet has become the largest and least expensive WAN in the world. WANs suffer from extremely limited bandwidth. The fastest commercially feasible wide area data links are many times slower than the slower local area links. Generally, WAN links are used only for inter-process communications to route short messages, such as e-mail or HTML (World Wide Web) traffic.

These cover wider area. These networks help to share information among units. A WAN is also known as a long haul network. These typically have the following characteristics: § Covers large geographical area including different states and even countries. § Linking medium used vary from telephone line and satellites.

Q23. Differentiate between LAN and WAN. Ans. LAN WAN 1. Restricted to a limited geographical area Area covered could be unlimited. it could be of a few kilometers. statewide, nation wide worldwide. 2. The cost of operation is negligible, once The cost of installation and operations rise in installed and be operated by and large by proportion to the medium for transmission used users themselves. and it requires technical manpower to implement the system. 3. Medium of transmission used could be Mediums used involve telephone lines and twisted or coaxial cables or fiber-optic ca- satellites using microwave or radio waves. bles. 4. Computers are usually physically con- There is no physical link between computers. nected to each other. 5. Data transmission speed is high. It is in Data transmission speed is in the order of 1800 the range of 0.1 to 100 mega bits per sec- to 9600 bits per second. ond.

6. Because of closer vicinity of computers Data loss could occur because of wider area to loss of data is much lower and lost data can be covered by the information to reach the desti- be easily recovered or transmitted again im- nation. Loss can also occur because sometimes mediately. the medium may change from one to another like from telephone line to satellites and reverse back from satellites to telephone line. We can used switch or HUB to create a We can used Router arrange WAN. LAN.. Q24. Explain LAN access methods. Or Differentiate between CSMA/CD and Token Passing. Ans. used techniques are as given below: - If all the devices at the same time begin to access the network, there would be some sort of disorder. Therefore there are many methods of managing access to network:-

(i) Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) (ii) Token passing Carries-Sense Multiple Access with Collision Detection (CSMA/CD): - The problem occurs only when two devices sense channel at the same time and transmit data when found idle. In this situation the data packets may col - lide and results in loss of packets. During the transmission, a device continues listening to detect collision. If a collision is detected, it stops the transmission immediately and waits for a random period of time before goes back to step transmission again. Token passing: -As name signifies, it uses a token or series of bits to allow a node to transmit. The device after capturing token can transmit data into the network. When that particular node completes sending its data, the node passes the token along to the next node in the network. Protocol specification signifies how long a device may keep the token.

Q25. Explain wireless communication media. Ans. Radio Transmission: - Radio waves are used for transmission purpose. These waves are electromagnetic in nature and are not disturbed by external electric and magnetic fields. These waves find use normally in voice communication. These are more flexible, less expensive, and allow portable workstations. These waves suffer from security and low rate of data transfer. Microwave Transmission: This is another transmission media which is becoming rapidly popular as it does not require any material like cables etc. for transmission of data and is less expensive. Microwaves are high frequency electromagnetic in nature and are not disturbed by external electric and magnetic fields. These are similar in nature to the radio waves. These waves are used to transmit data without the use of cables. These waves are transmitted by antennas placed on local peaks, such as the tops of buildings or mountains. Transmission rate is very high. Data transmission speed is of the order of about 16 giga bits per second. Their range of transmission is limited to about 25 – 30 km. Microwave transmission is widely used for broadband communication and telephone service. To link a number of locations of an organization we need microwave links today.

Satellite Transmission: The material obstacles such as topographical features or tall buildings, mountains create the main problem with microwave communication. Therefore, microwaves must be relayed (received, amplified and transmitted by another antenna) for long-range transmission. Due to this reason, several repeater stations are need-

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100 SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 29 of 76 ed for compensating the loss of intensity. It increases the cost of transmission. Both microwaves signals and telephone signals can be relayed to an earth station for transmission to a communication satellite. The earth station consists of satellite dish which functions as an antenna and communication equipment to transmit and receive data from satellites passing overhead.

High frequency transmission is weather sensitive. Dozens of satellites are in the orbit to handle domestic and international data, voice and video signals. INSAT-1B is a geo-synchronous satellite and is accessible from any place in India. Infrared Waves: Infrared waves exist between the visible spectrum i.e. the rainbow and microwave. Infrared technology is typically used in very local setting, such as in remote control on television VCDs and DVDs and the infrared ports on most laptop computers.

Q26. Explain frame relay. Ans. Frame relay is a service for people who want an absolute connection-oriented way to move bits over A to B at reasonable speed and low cost. Frame relay can be best thought of as a virtual leased line. It is based on a concept called a virtual circuit. A virtual circuit is a bi-directional path through the network which is defined in software. The main benefit of frame relay is that many virtual circuits can be used across a single physical connection. For ex - ample, a company’s headquarter needs to communicate with three remote offices. Instead of leasing three point-to- point circuits to connect each of these three offices, a single circuit connects each to the frame relay network and three virtual circuits are used.

Q27. What is ISDN? Ans. ISDN (Integrated Services Digital Network): - ISDN is a network architecture in which digital technology is used to convey information from multiple networks to the end user. This service is available with different data rates from 64 Kbps to 2 Mbps. There are two basic types of ISDN service , Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI consists of two 64 Kbps B channels and one 16 Kbps D channel for a total of 144 Kbps. This basic sevice is intended to met the needs of most individual users. PRI is intended for users with greater capacity requirements. Typically the channel structure is 23 B channels plus one 16 Kbps D channel for a total of 1536 Kbps. ISDN uses multiplexing mechanosm to merge different channels. Some standardized channel types are shown be - low:- A- 4KHz analog telephone channel. B- 64 Kbps digital channel for voice and data. C- 16 Kbps digital channel. D- 16 Kbps or 64 Kbps digital channel for out-of-band signaling. E- 64 Kbps digital channel for internal ISDN signaling.

Q28. Discuss Ethernet technologies. Ans. Ethernet is by far the most popular LAN technology. Its popularity has more to do with the cost per port to manufacture than with any practical advantages. In fact, there are faster, more secure, technologies that are capa - ble of communicating over much greater distances, but nothing is cheaper than Ethernet.

Ethernet frame Types: - Because of its long history, there are many versions of Ethernet frames and many specifications for Physical layer implementations. There are four common frame types. All four frame formats are very simple and lightweight. They provide the following features: 1 A destination address field 2 A source address field 3 A mechanism for identifying the contents of the payload 4 A payload field, which carries the data 5 A checksum Frame type Cisco Novell Notes Version II ARPA Ethernet II Often called DIX IEEE 802.3 LLC Ethernet 802.2 Include the 802.3 LLC header IEEE 802.3 SNAP SNAP Ethernet SNAP Used for compatibility Novell’s format NOVELL Ethernet 802.3 Proprietary Version II Ethernet The frame format for version II Ethernet is as follows: 6 bytes 6 bytes 2 bytes variable 4 bytes Destination address (uni- Source address Ethertype Payload FCS cast, broadcast, or multicast The destination and source address fields carry the six-byte MAC address of the recipient and sender, respectively. This is fairly straightforward, but there are three types of destination addresses: 1 Unicast, which identifies a single node on the network 2 Broadcast, which is sent to every node on the network 3 Multicast, which is sent to a group of nodes on the network

IEEE 802.2 and 802.3 Ethernet 6 bytes 6 bytes 2 bytes 3 byte Variable 4 bytes Destination ad- Source ad- Length LLC Payload FCS dress dress header The 3 byte LLC header 1 byte 1 byte 1 byte Destination service access point (DSAP) Source service access Control point (SSAP) SNAP Frame format 3 bytes 2 bytes Vendor code Local code Q29. Explain types of WAN. Ans. The essential purpose of any Wide Area Network is to link separate locations in order to move data around. A WAN allows these locations to access shared computer resources and provides the essential infrastructure for developing widespread distributed computer systems. We will now discuss the different types of WAN, which are commonly used. Public Networks: - Public Networks are those networks, which are installed, and run by the telecommunication authorities and are made available to any organization or individual who subscribe to it. Examples include Public Switched Telephone Networks (PSTN), Public Switched Data Networks (PSDN), Value Added Services (VANs/VADs) and the Integrated Services Digital Networks (ISDN). Public Switched Telephone Network (PSTN): - The features of the PSTN are its low speed, the analog nature of transmission, a restricted bandwidth and its widespread availability. As PSTN is designed for telephones, modems are required when it is used for data communication. It is seldom advisable to use PSTN as the sole communications medium for building a network system. Costs are high, as data connections last for a considerable time. Also, the links set up are unreliable and can terminate with - out warning. PSTN connections are usually easy to obtain at short notice, and are widely available and cover almost

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100 SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 31 of 76 every location where people live and work. PSTN is most useful for occasional user or as backup to private circuits. It is also for facsimile (FAX) machines. Public Switched Data Networks (PSDN): The term PSDN covers a number of technologies, although currently it is limited to Public Packet Switched Networks available to the public. The main feature of all PSDNs is their high level of reliability and the high quality of the connections provided. They can support both low and high speeds at appropriate costs. Like the PSTN, a PSDN is very useful for a private network for backup and occasional access purposes. It can also be used to link computer systems and networks of one organization to several other organizations. PSDN is very popular for connecting public and private mail systems to implement electronic mail services with other companies. Value Added Services (VANs/VADs): In Value Added Services, the provider of such services must process, store and manipulate the data that is carried on the network, that is, add value to it. The technique can be used in specific type of business in which it is advantageous to be able to share information with other companies on the same line. Integrated Services Digital Network (ISDN): The ISDN is a networking concept providing for the integration of voice, video and data services using digital transmission media and combining both circuit and packet switching techniques. The motivating force behind ISDN is that telephone networks around the world have been making a transition towards utilizing digital transmission facilities for many years. Private Networks:The basic technique used in all forms of private WAN is to use private (or more usually leased) circuits to link the locations to be served by the network. Between these fixed points the owner of the network has complete freedom to use the circuits in any way they want.

Q30. Explain frame format of PPP Protocol. Ans. PPP consists of a high-level data control, link control protocol and a set of protocols called network control protocols (NCPs). High-level data link control is used to encapsulate datagrams over serial links. Link control proto - col establishes, configures, and tests the data-link connection. Network control protocol is used to establish and configure one or more network-layer protocols. Point-to-point protocol operates between data terminal equipment (DTE) and data communications equipment (DCE) interfaces. The link between these devices must be duplex, and it may operate in either synchronous or asynchronous mode. A PPP frame is shown below:- 1 Byte 1 Byte 1 Byte 2 Byte Variable 2 or 4 Bytes Flag Address Control Protocol Data FCS The flag simply marks the beginning of a frame. It is always 01111110 binary. The address field is always 11111111, which is a broadband address because PPP doesn’t define station addresses. The control field is always 00000011, which indicates a connectionless link service similar to LLCI. The data field, of course, contains the data gram, which is theoretically a max of 1500 bytes, but can be changed in some circumstances. The frame check sequence(FCS) is a 16-or 32-bit calculation used to detect errors in the frame. It just works like FCS field in nearly every other data link protocol.

Q31. What do you mean by FDMA and TDMA? Ans. FDMA:- Frequency Division Multiplexing:- In FDM, multiple channels are combined onto a single aggregate signal for transmission. The channels are separated in the aggregate by their frequency. There are always some unused frequency spaces between channels, known as guard bands. These guard bands reduce the effects of overlapping between the adjacent channels and therefore reduce a condition more commonly referred to as cross- walk. Time-division multiplexing:- Time-division multiplexing (TDM) is a type of digital multiplexing in which two or more signals seems to be transferred simultaneously as sub-channels in one communication channel, but physically are taking turns on the channel. The time domain is divided into several regular timeslots of fixed length, one for each sub-channel. TDM can be implemented in two ways: Synchronous TDM or Asynchronous TDM. Synchronous TDM:-Here synchronous means that the multiplexer allocates exactly the same time slot to each device at all times. Whether or not to a device has anything to transmit. Time slot A, for example, is assigned to device A alone and cannot be used by any other device. Each time its allocated time slot comes up, a device has the op - portunity to send a portion of its data. If a device is unable to transmit or does not have data to send, its time slot re - mains empty which is the wastage of bandwidth and time.

Asynchronous TDM:-Asynchronous TDM or Statistical TDM is designed to avoid this type of waste. As with the term synchronous, the term asynchronous means something different in multiplexing than it means in other areas of data communication. Here it means flexible or not fixed. In STDM time is allocated to lines only when it is required. This is achieved with the use of intelligent devices.

. Q32. Explain the centralized and DECENTRALISED computer system. Or Differentiate between centralized and DECENTRALISED computer system. Ans. Centralized System: In this system, all computers are big mainframe computers and located at the head office. Regional offices wishing to make the use of central computers do so on a time-sharing, multi-programming basis. The cost of such computers is few millions. They had limited use and were installed by big institutions and re - search organizations. Benefits of Centralized Systems (i) It is easier to carry out operations with such a system. Organizational standards, system testing and other procedures may be implemented and enforced easily with such a system. (ii) A larger computers center that utilizes the latest data base management system and advanced and sophisticated management information system may be more appealing to the computer people and professionals. (iii)Economies of scale the use of larger and powerful computers involves less processing cost, if located at a central place. Duplication in record storage and program preparation may be avoided to some extent. Decentralised Computer systems: Computer users felt the need of a new computer system, which could overcome the limitations of centralized system. They wanted low cost systems installed at local sites and a better facility for processing data for day-to-day work and to have direct control over their own equipment as well as data formats. They also wanted some choice of hardware, which could exactly suit their particular requirements. But it did not eliminate all the problems. Such a system is called decentralized system. Benefits of decentralized computer systems (i) Management information system used at a central place may not be equally suitable and useful to other users at different locations. The needs may be different. Thus, users at different locations may be more ready to use suitable software packages for their own purpose and create customized programs for streamlining whole work. Though the hardware will be slower than the centralized system, it does not have to be allocated to the needs of several users groups. Priorities are to be given to the jobs of every user because delay of information causes delay in implementation. Thus, prompt processing of a job by a small machine may provide users with faster turnaround time. (ii) Less downtime risk if there is a breakdown in one user group, it does not affect the operations in other user group. (iii) Greater interest and motivation as the needs of various user groups are different therefore users should have control of their own computers and programs. It increases the accuracy of input data and the better utilization of equipments to meet particular needs of a group.

Q33. Explain DISTRIBUTED computer systems with their merits and demerits. Ans. In this system, the usability of computers is increased many fold. The reducing trend in prices of small computers has made it possible to integrate the systems into daily business activities at various locations at which these activities take place. The usability of computers is more because they are brought closer to end-user. The advance- ment in communication systems and computer systems have increased the feasibility of placing data processing power throughout all of the departments or locations of the business firm. Messages, data, program, information and other resources are transmitted between the cooperating processors and terminals. Users sitting far away from each other can share many hardware and important software resources, which definitely reduces the time for action and the cost involved.

ADVANTAGES OF DDP SYSTEM 1 In this system, computers or terminals are placed at local or regional sites, thereby providing access power to the mainframe computer resources. 2 Each computer can be used to process data like a decentralized system. 3 A computer at one location can also transfer data and processed jobs to and from computers at other locations. 4 DDP systems may be used for single organization and also for many organizations. This network may use star

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100 SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 33 of 76 or ring arrangement. 5 Users can share mass storage devices, plotters, data-base and complete library of application programs. 6 The telecommunication costs are reduced. DISADVANTAGES OF DDP SYSTEM 1 Complete control on the user programs and data is not possible because they are transmitted over the network channels. It is easy to tap a network line. A proper security control is needed. 2 The smooth functioning of the network is not achieved fully yet due to lack of adequate communication stan - dards. 3 As the computers are dispersed at various locations, therefore it is not easy to have a control from a central point. It makes the system more complex. . Q34. Differentiate between CSMA and CSMA/CD protocols. Ans. Carrier-Sense Multiple Access/Collision Avoidance (CSMA/CA):- In this case when a device wants to transmit , it first sense (listen to) the medium to insure whether another transmission is in progress or not. If the channel is in use, it must wait else it may transmit data. The problem occurs only when two devices sense channel at the same time and transmit data when found idle. The data packets collides and results in loss of packets. Carries-Sense Multiple Access with Collision Detection (CSMA/CD): - The problem occurs only when two devices sense channel at the same time and transmit data when found idle. In this situation the data packets may col - lide and results in loss of packets. During the transmission, a device continues listening to detect collision. If a collision is detected, it stops the transmission immediately and waits for a random period of time before goes back to step transmission again.

Q35. Explain the concept of handoff. What are the two ways, handoff can be done. Ans. At any instant, each mobile telephone is logically in one specific cell and under the control of that cell’s base station. When a mobile telephone physically leaves a cell, its base station notices the telephone’s signal fading away and asks all the surrounding base stations that how much power they are getting from it. The base station then transfers ownership to the cell getting the strongest signal, that is, the cell where the telephone is now located. The telephone is then informed of its new boss, and if call is in progress, it will be asked to switch to a new channel. This process is called handoff. It takes 300 msec. Handoff can be done in two ways: 1. Soft handoff 2. Hard handoff. 1. In a soft handoff, the telephone is acquired by new base station before the previous one signs off. In this way, there is no loss of continuity. The downside here is that the telephone needs to be able to tune to two frequencies at the same time. 2. In a hard handoff, the old base station drops the telephone before the new one ac- quires it. If the new one is unable to acquire it, the call is disconnected. Q36. Define ALOHA, Slotted ALOHA and Pure ALOHA. Ans. ALOHA:- It was developed by University of Hawaii in 1970 under the able expertise of Norman Abramson and Franklin Kuo. Pure ALOHA:- .Pure ALOHA used same frequency for each node and therefore require contention management. On an ALOHA multiple-access network, when a device needs to transmit data, it begins transmission without performing any checks. The sender waits for an acknowledgment, and if it does not receive one after a period of time, the sender assumes the packet was lost. Then sender waits for a random period of time before re-transmitting the packet. The peak possible good put of an ALOHA implementation is only 18 Percent. Slotted ALOHA:- Slotted ALOHA:- As one of the problems with pure ALOHA is that as soon as a device transmits a packet, it can collide with another packet already on the media. To avoid this In slotted ALOHA , the time base for the environment is divided into time slots of the same duration. The node transmit the data only its time slot comes. This protocol does improve the overall channel utilization, by reducing the probability of collisions by a half. environment is divided into time slots of the same duration. The node transmit the data only its time slot comes. This protocol does improve the overall channel utilization, by reducing the probability of collisions by a half.

Q37. What are the four-sonet layers? Explain their functions.

Ans. SONET :- SONET is a high-speed fibre-optic data transmission system. It can move data at rates faster than 1 Gbps. It is a connectionless, cell based service that can provide many-to many connections between a vari - ety of sites. SONET defines how fibre-optic technologies can deliver data, video, and voice over a network at speeds over 1 Gbps. SONET is a set of physical standards for high speed WAN’s, the same way Ethernet is a set of physical standards for LAN’s. The following are standard SONET transmission rates:- OC Level Line Rate OC-1 51.8Mbps OC-3 155.5Mbps OC-9 466.5Mbps OC-12 622.0Mbps OC-18 933.1Mbps OC-24 1.24Gbps OC-36 1.86Gbps OC-48 2.48GbpS

Q38. What is the concept behind CDMA technology? Ans. The CDMA technology is the solution to the multiple access problems by combining the time and domain technologies. There are two possible combinations and the first one is frequency-hopping CDMA or FH/CDMA. In this technique, the transmitting device begins sending information on one frequency and then after a short period of time, it changes to another frequency. Thus the transmitting device jumps from frequency to frequency throughout the duration of the conversation. The receiving device must use the same frequency pattern to be able to properly understand the conversation. In this scheme, the devices are using both time slots and frequency technologies. The time slot is the amount of time used on a specific frequency and the frequency element is the usage of different fre - quencies. The second one is the Direct sequence CDMA or DS/CDMA, which is more complicated than FH/CDMA. In this technique a code word is assigned to represent a binary 1 and each device uses a different code word. For example, one device may use 00101101 to represent a binary 1, while another device uses 10110110 to indicate a binary 1. each of these code words is transmitted for the same time duration and across different frequencies. Be- cause each of the recipient uses a different code word dictionary, the recipients message can be extracted from of all the bits spread across the different frequencies.

Q39. Explain briefly the different Ethernet versions. Ans. The three different versions of Ethernet are commonly called 10 Mbps Ethernet, 100 Mbps fast Ethernet and 1000 Mbps Gigabit Ethernet. Although they are all ethernet, they are actually quite different at the physical layer because the use of different encoding schemes. There are several physical specifications with 10-Mbps Ethernet which are: Ethernet Verisons:- • 10 Base 2 (Thin net) • 10 Base 5 (Thick net) • 10 Base T (Twisted Pair Ethernet) • 100 Base T (Fast Ethernet) • Gigabit Ethernet a) 10 Base 2: Commonly called thin Ethernet , 10Base 2 operates at 10 Mbps and has a maximum distance of 200m. this specification uses a thin coaxial cable and is physically a bus topology. b) 10Base 5: It is called thick Ethernet and uses similar thicker coaxial cable. This cable is less suitable for connecting PCs because the cable is much more rigid. c) 10Base T: In this case the ‘T’ stands for twisted pair cable. It deviates from the other two versions as it is physically a star topology. One of the major advantages of this specification is that a cable can be plugged and un - plugged from the hub without disturbing the other devices. d) 10Base F: In this case the ‘F’ stands for fiber-optic.10 Base F also uses star topology

Much like 10mbps, fast Ethernet has several specifications for different physical media but it also has a few optional modes of operation. The first is 100Base T which is very similar to 10Base T. Similarly there are other versions such as100Base-TX and 100Base-T4. The Gigabit Ethernet has also some specifications.One of the most common specifications is IEEE 802.3z which includes 1000Base-CX,1000Base-LX and 1000Base-SX.

Q40. Explain the binary exponential back off algorithm for CSMA. Ans. Binary exponential backoff systems use a collision detection scheme in addition to the carrier sensing techniques. In the backoff algorithm, if the transmitting device realizes that a collision has occurred, it stops for some time and remains idle for a random period of time. Two chooses this random time from a time interval calculated by multiplying the maximum propapagation delay. After this random time has elapsed, the device retransmits the data that was originally lost in the collision. If the transmitting station detects another collision from this retransmission, it again backs off for a random time. However, on this second collision detection, the time interval is four-times the maximum propagation delay. The device retransmits again after the second idle time period has passed. Each time the station detects a collision after retransmit ting data, the time interval length is doubled. The effect of doubling the time interval gives rise to an exponential backoff timer.

Q41. Explain the 1-persistent, no persistent and p-persistent CSMA techniques. Ans. A device that is ready to transmit can sense traffic on the transmission media Via two mechanisms. The first is called persistent. The station that wants to transmit waits until the media is idle and then transmits. This type is known as 1-persistent. The second approach known as nonpersistent has the device set a timer, wait for a period of time and check again if the media is free. If the media is free, the station will immedi - ately transmit the data. A problem with the persistent approach is that if two stations hear the same idle time at the same moment , they will both transmit and a collision will occur. The probability of collisions increases as the num - ber of devices that want to transmit increases. In p-persistent CSMA, the device that wants to transmit does not immediately send data when it detects that the media is idle. Instead it uses a random scheme that gives t a fifty-fifty chance to either send the data immediately or to wait and then repeat the process. The reasoning behind this concept is that if two stations detect the same idle moment, the chance of a collision is reduced.

Q42. Explain Busy tone multiple access(BTMA) and multiple access collision avoidance. Ans. BTMA and MACA are the two approaches that are used in environments that suffer from the hidden terminal and exposed terminal situations. In the hidden terminal situation, computer A can hear computer B and comput - er C. but computer B cannot hear computer C and computer C cannot hear computer B. Collisions will occur if computer B and computer C transmit at the same time because they cannot hear each other. The exposed terminal situation is the opposite problem where a station that wants to transmit hears too much and thinks that the media is in use. There are two approaches to avoid these problems: BTMA and MACA. BTMA divides the available frequency band into the data message channel and a busy tone channel. When a device is receiving information, it places data, or a tone on the busy tone channel. Other devices that want to send data to the receiving station hear the busy tone on its channel and know not to send the data. Now , with BTMA computer B knows computer C is transmitting because computer B can sense the busy tone on computer A’s busy tone channel. The second approach, MACA helps to overcome a problem with BTMA. Since BTMA requires the frequency to be subdivide, there is the chance of cross talk or interference be - tween the data and busy tone channels. If this occurs, then a station may detect a busy tone when there is not any data, or a device may detect data but without an associated busy tone so it can determine who is using the media. In MACA, the busy tone concept is used, except that the busy tone is sent as a special message so that all the stations know that a receiver is busy and the media is in use.

Q43. What does the term MODEM stand for? Explain the asymmetry of 56K modems. Ans. A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio.

Asymmetry of 56K Modems: - 56Kbps modems are asymmetric at speeds above 33.6Kbps. Asymmetric means the upstream speed (the data you send) is different than the downstream speed (the data you receive). For V.90’s, your downstream (receive) maximum speed is 53.3Kbps, and your upstream (send) maximum speed is 33.6Kbps. For V.92’s, your downstream max is 53.3Kbps, and upstream is 48Kbps.

Q44. What is DQDB? What is the advantage of implementing DQDB in a ring configuration? Ans. DQDB: a Distributed-Queue Dual-Bus Network (DQDB) is a distributed multi-access network that: (a) Supports integrated communications using a dual bus and distributed queuing (b) Provides access to local or metropolitan area networks (c) Supports connectionless data transfer, connection-oriented data transfer, and isochronous communications, such as voice communications. IEEE 802.6 is an example of a network providing DQDB access methods.

Q45. List two design issues of network layer. Ans. The network layer has been designed with the following goals: 1. The services provided should be independent of the underlying technology. Users of the service need not be aware of the physical implementation of the network - for all they know, they're messages could be transported via carrier pigeon! This design goal has great importance when we consider the great variety of networks in operation. In the area of Public networks, networks in underdeveloped countries are nowhere near the technological prowess of those in the countries like the US or Ireland. The design of the layer must not disable us from connecting to networks of different technologies. 2. The transport layer (that is the host computer) should be shielded from the number, type and different topologies of the subnets he uses. That is, all the transport layer want is a communication link; it need not know how that link is made. 3. Finally, there is a need for some uniform addressing scheme for network addresses.

Q46. Explain Circuit switching? Ans. Circuit switching creates a direct physical connection between two devices such as phones or computers. For example, in fig. Instead of point-to-point connection between the three computers on the left (A,B and C) to the four computers on the right (D,E,F,G), requiring 12 links. We can use four switches to reduce the number and the total length of the link. In fig. computer, A is connected through switches I,II,III to computer D. By moving the levers of the switches, any computer on the left can be connected to any computer on the right. A circuit switch is a device with n inputs and m outputs that creates a temporary connection between an input link and output link. The number of inputs does not have to match the number of outputs.

Q47. Differentiate between packet switching and message switching. Ans PACKET SWITCHING:- In a packet switched network, data are transmitted in discrete units of potentially variable length blocks called packets. The max. length of the packet is established by the network. Longer transmissions are broken up into multiple packets. Each packet contains not only data but also a header with control information (Such as source and destination addresses etc.). The packets are sent over the network node to node. At each node, the packet is stored briefly then routed according to the information in its header. In packets switching the packets are routed through different paths and it is possible that they are received at the destination in different sequence as they sent. Packets are re-arranged at the destination with the help of packet number.

MESSAGE SWITCHING:- Message switching is best known by the descriptive term store and forward. In this mechanism, a node receives a message, stores it until the appropriate route is free, then sends it along. Store and forward is considered a switching technique because there is no direct link between the sender and receiver of a transmission. A message is delivered to the node along one path then rerouted along another to its destination. Note that in message switching, the messages are stored and relayed form secondary storage while in packet switching the packet are stored and forwarded from primary storage.

Q48. What are the primary elements of a switch. Explain briefly. Ans. The internal physical structure of a switch is composed of four primary elements: 1. Input buffers:- Switches include buffers to store packets arriving at their input connections. Some switches have buffers for all the inputs and others have small buffers that are only used to hold data when contention for the switch fabric is under way. 2. Port mapper:- This element is found only in packet switches and is not used on circuit switches. The packet switch contains a table that associates, or maps, each input to an output 3. Switch fabric:- The switch fabric is the hardware and software elements of the switch that move data from a switch input to a switch output. 4. Output buffers: these buffers hold data that has been processed through the switch fabric. These buffers may be small or large and may contain a scheduler that handles media access for the out - put connections.

Q49. Explain Flooding? Ans. Flooding:- It is a simple routing algorithm in which every incoming packet is sent through every outgoing link. There are so many advantages in our network:- Advantages:- 1. If a packet can be delivered, it will (probably multiple times). 2. Since flooding naturally utilizes every path through the network, it will also use the shortest path.

3. This algorithm is very simple to implement. Disadvantages :- 1. Flooding can be costly in terms of wasted bandwidth and, as in the case of a Ping flood or a Denial of service attack, it can be harmful to the reliability of a computer network. 2. Duplicate packets may circulate forever, unless certain precautions are taken: 3. Use a hop count or a time to live count and include it with each packet. This value should take into account the number of nodes that a packet may have to pass through on the way to its destination. 4. Have each node keep track of every packet seen and only forward each packet once.

Q50. Explain ATM switching or cell switching. Ans. Asynchronous Transfer Mode (ATM) first began to appear in the 1980s and early 1990s. This protocol provides several features, such as discovering the route across the ATM network and allocating resources at the switches used by a connection. Used fixed-length packets has some advantages. For example, the amount of Asynchronous Transfer Mode is International Telecommunication Union – Telecommunication Standardization sector (ITU-T) for cell relay wherein information for multiple services such as voice, video, or data, is conveyed in small, fixed-size cells. Cells are re- assembled at the destination point. ATM networks are connection oriented. It can provide medium to high bandwidth and a virtual dedicated link between ends for the delivery of the voice, data and video. Each cell is broken into two main sections, header (5 bytes) and payload (48 bytes). The header contains information that allows the cell to be forwarded to its destination. The payload is the portion that caries the actual information- Whether it be voice, data, or video.

Q51. Discuss the spanning tree algorithm. Ans. Infinite loops occur in a network containing multiple bridges as a result of one of the following conditions: The bridges are forwarding all frames to all the segments. The arrangement of bridges is such that redundancy or cycling of bridges results. When a bridge is powered on, it sends out special types of packets called bridge protocol data units (BPDUs) so the bridge can communicate with the other bridges on the network. All the bridges then execute the distributed spanning tree (DST) algorithm. When a bridge in a spanning tree configuration, then bridges inform all the other bridges of its vital statistics by sending out the information in BPDUs. The vital statistics consist of the following information; 1 The ID of the bridge. 2 The ID of the bridge that the bridges sending out the BPDUs is the root bridge. 3 The distance between the root bridge and the bridge sending out the BPDUs. The vendor may set the bridge ID value, or on some bridges the administrator can set it. From the collection of ID numbers, the bridges go through a decision process and determine which of the bridges has the lowest ID number. The one bridge in the collection of bridges that has the lowest ID number becomes the root bridge for the network. Once the root is determined, the remaining bridges calculate the shortest path to the root bridge. Within each group of redundant bridges, one of the bridges becomes the designated bridge and will forward frames between the two attached segments. The remaining bridges in the group will not forward frames. The map of the routes from the root bridge to the designated bridges is called the spanning tree. If a bridge fails, the entire process of determining the root, designated bridges, and the paths is recalculated. One drawback of the spanning tree configuration is that some bridges are idle and do not participate in frame for - warding. For example, during bursts of traffic, the single forwarding bridge between two segments may become congested, and the other redundant bridge(s) cannot be used to help in alleviating the traffic load.

Q52. Explain the concept of virtual LANs with suitable example. Ans. When you add more bridges to a LAN, the size of the network can become quite large and the traffic load increases. This growth and size become an issue when you are dealing with broadcast packets. Bridges forward any broadcast packets they receive to all of their ports except the port they received the broadcast from. In a large environment it may not be necessary for all devices to broadcast to all of the systems on the network. Instead there may be grouping of machines that need to see each other’s broadcasts but no broadcasts from outside the group. One way to divide the LAN into logical groups is with VLAN. Virtual LANs make it possible for you to assign network segments to the same or different VLANs. The effect is like having individual separate LANs within the LAN. To dis-

tinguish each VLAN from another, a unique ID is assigned to each VLAN. Any device within a VLAN can communicate with any other devices in the same VLAN but not outside its VLAN.

Q53. What is hierarchical naming? Ans. The type of addressing scheme that includes information about the location of a system is called hierarchical. A hierarchical naming system also makes it simpler and more efficient to route or move the packets from network to network. Once the packets are on the appropriate network, each device can examine the remaining portion of the address to determine if the packets are for that device. This structure makes it easier for devices to properly read the address information and keeps the packet header a reasonable size. Another important consideration for a hierarchical naming structure that is used by different networks is the authority to catalog and control the different names. This type of structure prevents duplication of names and confusion on the network. The naming authority can also provide logical organization of names if it decides to implement categorization, such as type of users or companies.

Q54. Define various classes of IP addresses. Ans. IPV4:- The numbering schema use for IPV4 is a 32-bit number that incorporates a two part hierarchical structure. The IP number contains a network portion that is shared by all devices on the same network segment, and a unique number for each interface. Packets that are sent out on an IP network include an IP header that contains many items, including the source and destination devices IPV4 header details displayed from a packet capture program. It is divided into five classes. A, B, C, D and E. This separation of the address space into classes is re - ferred to as classful IP addressing Class Start address Finish address A 0.0.0.0 127.255.255.255 B 128.0.0.0 191.255.255.255 C 192.0.0.0 223.255.255.255 D 224.0.0.0 239.255.255.255 E 240.0.0.0 255.255.255.255 N A CLASS MAXIMUM HOST CAN BE 1,67,77,214. IN B CLASS MAXIMUM HOST CAN BE 65534. IN C CLASS MAXIMUM HOST CAN BE 254.

Q55. Explain the concept of RPC. Ans. Remote Procedure Call:- RPC is a network programming model for point-to-point communication within or between software applications. In RPC, the sender makes a request in the form of a procedure, function, or method call. RPC translates these calls into requests sent over the network to the intended destination. The RPC recipient then processes the request based on the procedure name and argument list, sending a response to the sender when complete. RPC applica - tions generally implement software modules called "proxies" and "stubs" that broker the remote calls and make them appear to the programmer the same as local procedure calls (LPC). RPC calling applications usually operate synchronously, waiting for the remote procedure to return a result. RPC incorporates timeout logic to handle network failures or other situations where RPCs do not return.

RPC has been a common programming technique in the Unix world since the 1990s. The Open Systems Foun - dation (OSF) Distributed Computing Environment (DCE) and Sun Microsystems Open Network Computing (ONC) li- braries both were widely deployed. More recent examples of RPC technologies include Microsoft DCOM, Java RMI, and XML-RPC and SOAP.

Q56. Explain ARP (Address Resolution Protocol). What is the limitation of ARP. How is it re- solved? Ans. ARP (Address Resolution Protocol): - The address resolution protocol associates an IP address with physical address. ARP is used to find the physical address of the node when its internet address is known. Any time a host, or a router, needs to find the physical address of another host on the network, it formats an ARP query packet that includes the IP address and broadcasts it over the network. Every host on the network receives and processes the ARP packet, but only the intended recipient recognizes its IP address and sends back its physical address.

Q57. What is a cross point in crossbar switch. What is limiting factor of this Switch? How does a multistage switch elevate the problem? Ans. Space-division switching mechanisms take each input packet on a different path through the switch. The path a specific packet will take depends on the destination of the packet. There are a couple of ways in which space-divi - sion switching can work. The first, called a crossbar is the simplest method of providing switching for multiplexed data. The input and output lines are arranged as a grid perpendicular to each other. The intersection of an input and output line is called a cross point and if the cross point is active, then data can travel from input line over to the out - put line. If the cross point is not active, data on the input line cannot cross over to the output line. If the switch has eight input and eight output lines, then the crossbar switch is referred to as an 8*8 switch. If the input line coming to a switch is not multiplexed, then the input lines will always switch to the same output lines. If the incoming links are multiplexed, the individual elements will have different destinations. In this case, a controller is required to produce a schedule that specifies which cross points are active at each time unit of the multiplexed data. As the number of input lines increase, it takes longer for the controller to communicate to all the cross points to turn them on and off. This problem led to the development of multistage crossbar switches. A multistage crossbar provides for multiple paths between groups of inputs and outputs. The switch divides the input lines into groups, which are themselves switched internally by a crossbar.

Q58. Explain the Link State Routing? Ans. The idea behind link state routing is simple and can be stated as five parts. Each router must do the following: 1 Discover its neighbors and learn their network addresses. 2 Measure the delay or cost to each of its neighbors. 3 Construct a packet telling all it has just learned. 4 Send this packet to all other routers. 5 Compute the shortest path to every other router.

Learning about the neighbors: When the router is booted, its first task is to learn who its neighbors are. It accom- plishes this goal by sending a special HELLO packet on each point-to-point line. The router on the other end is expected to send back a reply telling who it is. These names must be globally unique. Measuring Line Cost: The link state routing algorithm requires each router to know the delay to each of its neighbors. The most direct way to determine this delay is to send over the line a special ECHO packet that the other side is required to send back immediately. By measuring the round-trip time and dividing it by two, the sending router can get a reasonable estimate of the delay. For even better results, the test can be conducted several times, and the average used. Building Link State Packets: Once the information needed for exchange has been collected, the next step is for each router to build a packet containing all the data. The packet starts with the identity of the sender, followed by sequence number and a list of neighbors. For each neighbor, the delay to that neighbor is given. Distributing the link state packets: As the packets are distributed and installed, the routers getting the first ones will change their routes. Consequently, the different routers may be using different versions of the topology, which can lead to inconsistencies, loops, unreachable machines, and other problems. To keep the flood in check, each packet contains a sequence number that is incremented for each new packet sent. Routers keep track of all the pairs they see. When a new link state packet comes in, it is checked against the list of packets already seen. If it is new, it is forwarded on all lines except the one it arrived on. If it is a duplicate, it is discarded. Computing new routes: Once a router has accumulated a full set of link state packets, it can construct the entire subnet graph because every link is represented. Every link is represented twice, once for each direction. Two values can be averaged or used separately. Now Dijkstra’s algorithm can be run locally to construct the shortest path to all possible destinations.

Q59. What are application gateways? Ans. The firewall can check the packet header and determine the IP protocol or higher OSI level of service con - tained in the packet. This level of filtering along with the layer 3 filtering can provide a much better firewall. For example, on an e-mail gateway server, the firewall could be configured to reject all packets coming into the e-mail gateway except those needed by the e-mail gateway software. Thus, any web traffic, FTP traffic or anything that is not permitted in e-mail traffic could not enter the private network through the firewall. In another example, protocol filtering can also apply to outgoing traffic. For example, in a public library, the administration wants to make sure that the three internet computers available for public can access the internet but not other services. The firewall could be configured to check the addresses of the Internet machines. If the HTTP protocol is not in - cluded in the packet, the packet is rejected at the firewall. Thus, users attempting to use the library’s internet machines to access a game server would be denied access because the game packets would not be allowed to leave the private network. Firewalls that provide upper OSI filtering are sometimes called application-layer gateways or proxies.

Q60. What is blocking. Differentiate between internal and external blocking. Ans. In networks, there are two types of switches that are distinguished by the connection types. Line switches connect individual telephone lines and must connect a specific input to a specific output Transit switches connect multiplexed links and must be able to connect an input to any one of the outputs that is going in the same direction. On both of these types of switches, a call may be blocked if there is no path between the input and the output. There are two types of blocking that can occur: internal and output. When internal blocking occurs, there is no path to an available output. Packets building up at the buffer waiting for the same output will prevent other packets from getting a chance at their output port. In an output blocking situation, two inputs are in competition for the same output port and one does not get access to the output.

Q61. Compare hierarchical and multicast routing. Ans. Hierarchical routing: - It is a method of dividing the network that allows routing protocols to scale very well. An example of this is the relationship between interior(internal) and exterior(external) routing protocols. Another example is the area structure of OSPF. In both these examples, one method in particular is used to gain the bulk of the efficiency. This method is called summarization or routing aggregation.

Multicast routing is special because of its need to reach at many different clients with a single packet sent to a group address. Instead of sending a unicast packet to a single destination along a single path, multicast must send

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100 SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 42 of 76 a packet to an arbitrary number of destinations, which necessarily means an arbitrary number of paths. To accomplish this, a number of special routing protocols can be used. These include: Distance Vector Multicast Routing Protocol (DVMRP) Multicast Open Shortest Path First (MOSPF) Protocol Independent Multicast (PIM) Multicast routing uses one of two distribution methods: Source trees In the source tree distribution model, a tree diagram can be created to describe the path between each multicast source and destination. The source and destination are typically represented with the following nota- tions: (S, G) where, S=Source IP address G=Group IP address Shared tree : Unlike source tree, where every (S,G) pair has its own unique tree where the source is always the root of the tree, in the shared tree distribution model, there is only one tree shared among all sources. The notation for this is (*, G) because the source does not longer matters.

Q62. Discuss the important features of DNS. Ans. Domain Name System it is basically used for change name to IP & IP to name resolution. The assignment of friendly names to systems and services help humans to interact with the network. When the different devices communicate with each other, IP addresses and physical (MAC) addresses are used to transfer the information among the senders and receivers. It is much easier for people to remember a string of words or characters rather than a string of numbers. Therefore, in order to provide convenience to the user but allow the systems to communicate, there needs to be a mechanism to translate the friendly names to the IP protocol suite that provides this service. No single DNS server maintains a complete list of the registered domain names found on the Internet. The Domain name System is hierarchical. A DNS name for a device or service consists of alphanumeric characters separated by dots. Each item between the dots specifies the name of a group or organization of systems. The left-most item in a DNS name is the name of the device or service, and the right-most item is the name of the top of the hierarchy and is called the most significant segment of the name. The names are not case sensitive and each segment can be up to 63 characters.

Q63. Discuss FTP and TFTP. Ans. FTP is one of the oldest IP protocols. It is the most popular protocol used on the Internet to transfer files be - tween different systems. In fact, when you go to a web site and download a file, the system may use FTP to transfer the file In the early days of the Internet, before the introduction of graphical element, approximately one-third of all Internet traffic was FTP-related. FTP also includes the ability to handle interactive and batch file transfers. To pro - vide these features and functions, FTP operates as a client-server model. The system that accepts files runs an FTP service and the machine that sends and receive the files runs an FTP client piece so the device receiving files can also send files using FTP. TFTP: The IP protocol suite includes another file transfer protocol called Trivial File Transfer Protocol (TFTP). One of the big differences between FTP and TFTP is the underlying IP protocol. FTP uses TCP, and TFTP used UDP. TFTP does not allow for user authentication, the contents of remote directories cannot be displayed, and only file transfer is supported which permits global access. Q64. How do you determine the net id and host bits of a given IP address? Ans. The designers of the IPV4 addressing scheme gave the following specifications for the different classes.. Class Number of network bits No. of host bits A 8 24 B 16 16 C 24 8 Taking into account the number of bits for the network and host portions of the IP address, we can also say that the first byte of class A address is the network number, the first two bytes of class B are the network number and the first 3 bytes of the class C are network number. If we take into account the bit pattern in the first byte and the number of bits for the network address, we can list the network address ranges for the first byte in each class of IP addresses as follows: Class Beginning address Ending address

A 0 127 B 128 191 C 192 223

Q65. Explain the concept of SNMP. Or Explain the important features of SNMP. Ans.Simple network Management Protocol (SNMP) is a protocol that can be used by software and hardware that are designed to monitor various network and system components. In addition to noting failure or problems, system and network administrators may want to monitor activities over a period of time to look for trend and/or changes in the environment. These type of items, and many more can be captured or trapped through the use of the SNMP protocol. The basic structure includes a software piece called a manager, which collects all the information and acts on the data. The other components are agents, which run on the different network devices and send information to or accept directives from the manager. The SNMP protocol, which runs on the top of UDP, specifies the syntax and format of the information passed between the agents and manager components. For each possible request and reply, SNMP specifies the exact meaning of these messages. Information passed between the managers and agents is encoded using the abstract syntax notation (ASN.10 standard. The messages functions as fetch-store operations where store is used to pass information, such as values, to a device and fetch is used to retrieve information or val - ues from devices. The SNMP manager could then retrieve and accumulate the information so an administrator could determine when the events occurred and how often. In order to collect information from the proper devices and to keep track of the information from the different devices, each object has to have a unique name. In addition, both the manger and agent pieces must agree on the same names and on the definitions of the different store and fetch operations. The objects can then be accessed by SNMP through the MIB.

Q66. Explain the types of message services. Ans. There are four main types of message services: 1) Electronic mail: With e-mail, you can easily send a message to another user on the network or other networks, including the Internet. 2) Workgroup applications: Workgroup applications process common sets of tasks among multiple users. There are two types of workgroup applications: workflow management applications and linked-object documents. Manage- ment applications coordinate documents, forms and notices. Scheduling programs are examples of workflow applications. Linked-object documents provide compound documents with links to separate files that are displayed inside the compound document but can be edited and updated separately. An example of a linked-object document would be an HTML document with links to objects such as graphic images, ActiveX components, and ODBC compatible databases. 3) Object-oriented applications: Object-oriented applications are programs that can accomplish complex tasks by combining smaller applications, called objects, into large tasks. Message services facilitate communication between these objects by creating agents that pass the data between objects. 4) Directory services: Directory services is network application that follow easy lookup of usernames and services. A typical directory service is organized into a hierarchical tree made up of many OUs (organization unit). An organizational unit is a container that holds other objects that contain users, printers, and other services.

Q67. What are the main types of specialized servers? Or Differentiate between various types of specialized servers. Ans. There are three main types of specialized servers 1. File Servers 2. Print Servers 3. Application Servers 1. FILE SERVERS: - These servers offer services that allow network users to share files. File services are the network applications that store, retrieve, and move data. When using network file services, users can exchange, read, write and manage shared files and the data contained in them. These servers are designed specifically to support the file services for a network. Because of constant file access, file servers place great demands on their hard drives. These servers need to contain several fast hard drives.

2. PRINT SERVERS: - An important feature of networking is the ability to share printers. Print servers manage and control printing on a network, allowing multiple and simultaneous access to printing facilities. The network operating system achieves this by using print queues, which are special storage area where print jobs are stored and then sent to the printer in an organized fashion.

The following are the characteristics of print servers. I. Allow users to share printers II. Allow you to place printers where they are convenient, not just near individual computers III. Achieve better workstation performance by using high-speed network data transfer, print queues, and print spooling IV. Allow users to share network fax services APPLICATION SERVERS: application server is a program that handles all application operations between users and an organization's backend business applications or databases. An application server is typically used for complex transaction-based applications. To support high-end needs, an application server has to have built-in redundancy, monitor for high-availability, high-performance distributed application services and support for complex database access.

Q.67 What are Advantages and disadvantages of UDP? Ans: UDP stands for user datagram protocol. It is a generalized protocol and is not restricted to a connection based communication model. There are no guarantees with UDP. a packet may not be delivered, or delivered twice, or delivered out of order as it is an unacknowledged service. TCP is really working in the same environment; you get roughly the same services from IP and UDP. However, TCP makes up for it fairly well, and in a standardized manner. UDP has no implementation is the duty of user programs. Routers are quite careless with UDP. They never retransmit it if it collides, and it seems to be the first thing dropped when a router is short on memory. UDP suffers from worse packet loss than TCP. Complex data processing tasks to be centralized on a high performance computer for better performance, control and management of the application and its data are accomplished by these servers .A key to understanding client/server applications is to remember that processing is taking place on at least two computers cooperatively; the client computer and the server computer. Application servfers can be dedicated servers set up specifically for the purpose of the providing application services, or they can serve multiple functions. A single server, for example, can provide file, print, communication, and database services. Q68. Explain various security policy issues.

Ans. Authorization: Implementing a security policy means that somebody is in charge. Security levels must be set and modified as needed. Most security polices involve multiple individuals with different levels of authorization so no one person has the keys to entire kingdom. Accountability: Most security policies included the ability to audit events and activities. When auditing is implemented, there needs to be an individual or group of individuals accountable for the audited information. Data availability: Users need to know they will have access to the information when they need it. Data integrity: Measures must be in place to make sure that data is not modified during delivery between communication systems. Data confidentiality: Users who request confidential information must be comfortable that the information is inaccessible to unauthorized individuals. Privacy: In some systems it is important that the identity of a user remain anonymous. In some properly secured and treated with respect so an individual’s private information is kept confidential.

Q69. Discuss the IP security issue with example. Ans. IP security (IPSec) is an IP protocol suite standard that was developed to address some of the security limi - tations of Ipv4. its design provides three main features and functions: Modularity: The security administrators can choose an encryption algorithm and security protocols. Security services: There are a number of security services the administrators can choose from. These include access control, message integrity, authentication, and replay protection so packets cannot be resent from an unauthorized party and privacy. Level of detail of security services application: Administrators can choose to place a restriction on all packets meeting a specific criterion or on a subset of those packets based on other factors. IPV4 packet format is: NextHdr PayloadLength Reserved SPI SeqNum Authentication Data The principal role of the authentication header is to ensure that the integrity and origin of the data are authentic. It also prevents the replay of packets so an unauthorized user cannot resend the packet in an attempt to confuse systems or to repeat particular activities.

Q70. Explain the methods to implement network security. Ans. FIREWAL:- The firewall examines, filters, and reports on all information passing through the network to ensure its appropriateness. These functions help prevent saturation of input capabilities that otherwise might deny usage to legitimate users, and they ensure that information received from an outside source is expected and does not contain computer viruses. It also provides security to an organizations data. Data encryption/decryption:- Data encryption/decryption and signature techniques are the most effective means of ensuring that the data is understood only by the intended recipients Another common technique that is used to protect access to information is the use of accounts, passwords, and access control. IP SECURTIES :- IPSec provides a set of security algorithms plus a general framework that allows a pair of communicating entities to use whichever algorithms to provide security appropriate for the communication. APPLICATION OF IP SECURTY:-  Secure branch office connectivity over the Internet  Secure remote access over the Internet  Establsihing extranet and intranet connectivity with partners  Enhancing electronic commerce security WEB SECURITY:- There are a lot of dangers in putting up a Web page. From invasions of privacy to actual hackers cracking your security. Learn how to secure your Web server and site as well as hacks and patches to keep yourself safe.  Secure Sockets Layer – SSL:- Setting up a secure Web site requires that you use SSL or the secure sockets layer to encrypt the data that comes across. These links will help you secure your pages and set up SSL certificates on your Web sites  Basic Authentication:- A simple user ID and password-based authentication scheme, and provides the

following: o To identify which user is accessing the server o To limit users to accessing specific pages (identified as Universal Resource Locators, URLs EMAIL SECURITY:- _email is one of the most widely used and regarded network services _currently message contents are not secure –may be inspected either in transit –or by suitably privileged users on destination system EMAIL SECURITY ENHANCEMENTS:- •Confidentiality –Protection from disclosure •Authentication –Of sender of message •Message integrity –Protection from modification •Non-repudiation of origin –Protection from denial by sender

Q71. Discuss e-mail security. Ans. When you send an e-mail message to an individual at a different location, the message passes through several routers and network equipment to get from you to the recipient. At any location in the pathway your message could be picked up by somebody and your contents read. The unauthorized individual picking up your message might put it back on the transmission media in a modified form and may respond to you in such a manner that you think that the response is correct and comes from the actual recipient. To provide an environment that makes e-mail more difficult to understand when it is captured by a hacker, individuals and products use encryption technique to encrypt the e-mail messages. Two systems that are in use are: 1) Pretty Good Privacy (PGP): PGP can be used to encrypt any type of data, not just e-mail. This system provides the following functions and features: a) Privacy b) Authentication c) Digital signatures d) Compression 2) Privacy Enhanced e-mail (PEM): This concept is similar to PGP but the technique is different. A PEM message is first converted to a canonical form so that all the messages have the same format for white spaces, carriage re- turns and line feeds

Q72. Discuss any one technique of error detection and error recovery. Ans:- Error detection and error correction: - Most computer networking protocols use error correction schemes to implement requests to the sender to retransmit the missing information. To ensure that the information arrives, acknowledgement techniques are used so that the sender has some type of feedback from the receiver that the information did or did not arrive. Acknowledgements are usually special control frames that contain either positive or negative acknowledgements about the incoming information. To ensure that the retransmitted packets of the same information are not all processed, sequence numbers are assigned to outgoing packets so that the receiver can tell which are the originals and which are the retransmissions. The methods used for error control include the combinations of the following mechanisms: a) Error detection: packets arriving at the receiver that do not pass the error checking routine are discarded. b) Positive acknowledgement: When the receiver determines that a packet has passed the error-checking routine, a positive acknowledgement is sent back to the sender. c) Negative acknowledgement: when the receiver determines that a packet has failed the error-checking routine, a negative acknowledgement is sent back to the sender. d) Retransmission after timeout: If the sender has not received an acknowledgement within a certain time frame from a packet it has sent, the sender retransmits the packet.

As a group, these methods are known as automatic repeat requests (ARQ). There are two implementations of ARQ found in network communication protocols. The first is called stop-and –wait ARQ. In this scheme, the sender transmits a packet, waits for the acknowledgement and then sends the next packet. In the second procedure, go- back-n ARQ, the sender transmits a series of packets that contain sequence numbers. The receiver analyzes the transmitted packets and if any failure occurs in the error-checking routine, the sender retransmits the damaged packet and the packets following the damaged packet.

Q73. Explain in detail sliding window protocol. Ans: Sliding Window Protocol :- It is a bi-directional data transmission protocol in the data link layer (OSI model). It is used to keep a record of the packet sequences sent and their respective acknowledgements received by both the users.In transmit flow control, sliding window is a variable-duration window that allows a sender to transmit a specified number of data units before an acknowledgment is received or before a specified event occurs.An exam - ple of a sliding window in packet transmission is one in which, after the sender fails to receive an acknowledgment for the first transmitted packet, the sender "slides" the window, i.e. resets the window, and sends a second packet. This process is repeated for the specified number of times before the sender interrupts transmission. Sliding window is sometimes (loosely) called acknowledgment delay period.First, the sender creates a sequence number for each frame as it is transmitted. Throughout the communication, it maintains the send window size, the last acknowledg - ment received, and the last frame sent. To ensure that the window does not overflow, the sender ensures that the window size is greater than the sequence number of last frame sent minus the sequence number last acknowledg - ment received.

Q74. What are the options and trade-offs involved in designing a LAN? Ans:- Ethernet is now the most common data link layer protocol and IP as a network layer protocol, many different options have been used, and some continue to be popular in niche areas. Smaller LANs generally consist of a one or more switches linked to each other - often with one connected to a router, cable modem, or DSL modem for Inter- net access.Larger LANs are characterized by their use of redundant links with switches using the spanning tree protocol to prevent loops, their ability to manage differing traffic types via quality of service (QoS), and to segregate traffic via VLANs. Larger LANS also contain a wide variety of network devices such as switches, firewalls, routers, load balancers, sensors and so on.LANs may have connections with other LANs via leased lines, leased services, or by 'tunneling' across the Internet using VPN technologies. Depending on how the connections are made, se - cured, and the distance involved they become a Metropolitan Area Network (MAN), a Wide Area Network (WAN), or a part of the internet.

Q75. Write short note on wireless networks. Ans:- Wireless Networks:- Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometres for deep-space radio communications. It encompasses various types of fixed, mobile, and portable two-way radios, cellular telephones, personal digital assistants (PDAs), and wireless networking. Other examples of wireless technology include GPS units, Garage door openers or garage doors, wireless computer mice, keyboards and Headset (audio), headphones, radio receivers, satellite television, broadcast television and cordless telephones.

Section – C 10 Marks Questions [QUESTIONS 1 TO 20] [PAGE 56 TO 89] Q1. What is switching? Explain the role of switching in network. Explain its all types? Ans. Switching: - Every time you access the internet or other network outside your location. Your messages are sent through a way of transmition media and connection devices. The way of moving information between different networks and network segment is called switching. The internal physical structure of a switch is composed of four primary elements: Input buffers: switches include buffers to store packets arriving at their input connections. Some switches have buffers for all the inputs and others have small buffers that are only used to hold data when contention for the switch fabric is under way. Port mapper: this element is found only in packet switches and is not used on circuit switches. The packet switch contains a table that associates, or maps, each input to an output Switch fabric: the switch fabric is the hardware and software elements of the switch that move data from a switch input to a switch output. Output buffers: these buffers hold data that has been processed through the switch fabric. These buffers may be small or large and may contain a scheduler that handles media access for the output connections. Types of switching:- Circuit Switching (TDS, SDS, TSS) Packet Switching (Port Mapper, Blocking, ATM) Massage Switching (Crossbar, Broadcast) 1) Circuit Switching: - the most common usage of circuit switching is the telephone system. When you initiate a telephone call, the entire path from your telephone to the recipient’s telephone is determined when the call is made. It is up to the switching equipment between the two telephones to determine the physical communication path. Once this path is determined, the same paths used for the entire duration of the conversation. When another call is made to the same recipient at another time, the same physical path may or may not be used. One the end-to-end path has been established, data can begin traveling across the network. Once the connection is made, congestion will not occur on the link since only the sender and receiver are using the channel. In circuit switching, packets do not contain metadata information such as destination and source addresses. The source and destination address are determined from the signals on the transmission media. Space division switching: - This kind of switch was specially developed for an analog environment. Space-division switching mechanisms take each input packet on a different path through the switch. The path a specific packet will take depends on the destination of the packet. Time division switching: - this is used for digital data transmition due to multiplexing. 2) Packet switching:- in packet switching when a packet arrives at a packet switch, it is stored and then transmitted later. This means that the arrival time of a packet is not a factor in determining the destination of the packet, since the switch does not contain the predefined delivery paths based on a timing scheme. In this scheme, a port mapper component is used to determine the output port of the packets based on their address information. Packet switches are known as store and forward switches because the frames are stored in the switch before they are transmitted on the other side. ATM Switching:- asynchronous transfer mode (ATM) first began to appear in the 1980s and early 1990s. This protocol provides several features, such as discovering the route across the ATM network and allocating resources at the switches used by a connection. The primary purpose of these features is to provide quality of service options. When an ATM connection is established, the signaling message includes the address of the destination device. There are different formats for the signaling embedded address. The address is not the same as the mac address. The term cell is used on an ATM network instead of the term packet because the size of the packets is fixed in length. Most other packet-switching networks support variable-length packets, and the term packet is typically used to refer to networks with different length packets. Used fixed-length packets have some advantages. For example, the amount of overhead used by other sys - tems to accommodate variable length packets is not needed. In addition, the fixed length packet simplifies an ATM hardware switch. Fixed packet lengths also enable the switches to perform tasks in parallel because each of the systems takes the same time to do a task. This approach permits scalability of switch designs. The fixed-length cells

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100 SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 49 of 76 also make possible more consistent management of buffers, or queues at the switches. ATM queues can still become filled when network traffic increases, but it is easier to handle buffer issue when packets are the same size Switching Fabric:- switched fabric, switching fabric, or just fabric, is a network topology where network nodes connect with each other via one or more network switches (particularly via crossbar switches, hence the name). The term is popular in telecommunication, fibre channel storage area networks and other high-speed networks, including infiniband. The term is in contrast to a broadcast medium, such as early forms of ethernet. Switched fabrics can offer better total throughput than broadcast networks, because traffic is spread across multiple physical links. Types:- Crossbar Switching:- Space-division switching mechanisms take each input packet on a different path through the switch. The path a specific packet will take depends on the destination of the packet. There are a couple of ways in which space-division switching can work. The first, called a crossbar is the simplest method of providing switching for multiplexed data. The input and output lines are arranged as a grid perpendicular to each other. The intersection of an input and output line is called a cross point and if the cross point is active, and then data can travel from input line over to the output line. If the cross point is not active, data on the input line cannot cross over to the output line. If the switch has eight input and eight output lines, then the crossbar switch is referred to as an 8*8 switch. If the in - put line coming to a switch is not multiplexed, then the input lines will always switch to the same output lines. If the incoming links are multiplexed, the individual elements will have different destinations. In this case, a controller is required to produce a schedule that specifies which cross points are active at each time unit of the multiplexed data. As the number of input lines increase, it takes longer for the controller to communicate to all the cross points to turn them on and off. This problem led to the development of multistage crossbar switches. A multistage crossbar provides for multiple paths between groups of inputs and outputs. The switch divides the input lines into groups, which are themselves switched internally by a crossbar. Crossbar switching 1 Broadcast switching:- In the case of broadcast packets (broadcast radiation) over a switching loop, the situation may develop into a broadcast storm. 3) Message switching Message switching is best known by the descriptive term store and forward. In this mechanism, a node receives a message, stores it until the appropriate route is free, then sends it along. Store and forward is considered a switching technique because there is no direct link between the sender and receiver of a transmission. A message is delivered to the node along one path then rerouted along another to its destination note that in message switching, the messages are stored and relayed form secondary storage, while in packet switching the packet are stored and forwarded from primary storage.

Q2. What is Computer network and explain there advantages and disadvantage in complete detail? Ans. In information technology, networking is the construction, design, and use of a network, including the physical cabling (hub, bridge, switch, router, etc.), the selection and use of telecommunication protocol and computer software for using and managing the network, and the establishment of operation policies and procedures related to the network.

1) INTRODUCTION ABOUT COMPUTER NETWORK:- COMPUTER NETWORK:- A computer network consist of two or more than two computers that can share data, hardware devices like printer CDROM, etc. it can be possible through cables, telephone, Wi-Fi, radio waves, satellites etc.

ADVANTAGES, DISADVANTAGES, NEEDS

The network components may be located at many remote locations or within the same office of an organization. All the computers in the network are hooked together with cables. To make the system work, a special network adaptor is needed inside your computer, which connects your computer to the other computers and to the mainframe. There are major benefits of network, but networking needs enough skill and wisdom of the professionals involved in laying out the network and managing it. Every computer connected to the network is said to be on net - work. The technical term for such a computer is 'node'. When a computer is switched on, and is able to access the network, the computer is said to be on-line. If com - puter is not able to access the network, then it is said to be off-line. There could be several reasons for the comput - er to be off-line. The connecting cable may be defective or it may be unplugged etc. The inter-connection of the computers is done through a communication link also known as physical layer of the network. This link maintains the transparency of the network to the users sitting at faraway places. For creating communication link some interfacing software are used which are known as protocols. These protocols enable any user to access another computer, database, application program in another location. Computers are interconnected by telephone lines, microwave transmitters and receivers and other high-speed communication links. Networking has been developed on all levels, from local to international, in diverse sectors of society. For retrieval of information from database at a central place and those used by financial institutions and oth - er commercial houses the network of computer is must. Rapid action can be taken if information is disseminated at the right time to the right person. For example, modem is used for sharing the information through a telephone line. This device converts the digital signals into analog signals to be transmitted over the telephone line and vice-versa. All the computers in a network follow a set of procedures. LAN (local area network) is a communication network consisting of many computers placed in a small region not exceeding one kilometer. Advantages of Networks Sharing files: - The network enables computers to share information with one another. Depending upon the way you set your network, you can do it in one of the following ways. The one way is to transmit the file or data from your computer to the other computer directly. The other way is to send data to an intermediate place, from where the oth - er person can pick it up later. There is another way to permanently store the file at an intermediate place, where both of you (sender and the receiver) can get the file wherever and whenever you want. Some files in a office may be used as common files. For example, in a revenue collection system, all the files are kept in a central computer in the main office and are constantly updated to reflect the latest financial data. The network gives each office an access to the files and the programs in the central computer. Sharing hardware device: - It permits various departments and people to share hardware. A cost effective hardware is always the choice of everybody. For example, a high-speed laser printer may be an unaffordable device for a small office but a company can easily afford it because it can be shared by many users and departments. Each department of a company can justify the use of laser printer. Similarly, a hard disk in the main computer can be shared by several users. A hard disk must be set up as a shared resource in order to share files with other users. Communication: - The network permits communication between various organizations, which need to cooperate with each other in performing certain task rapidly. The information sharing is of great significance for the organization. Decentralization of various data processing functions: - All the functions cannot be performed at one central place in an organization. Some data processing and analysis functions are performed by data processing departments in a decentralized manner. Then the network enables the complete files to be transferred to the mainframe computer for final analysis and action thereupon. Electronic transfer of text: - By using electronic mail system, it is easy to distribute copies of reports, circulars, notices and memos etc. Each user has a mail-box located in the memory of his computer. The messages to be sent are stored in the appropriate mail-box and are retrieved as and when needed by the user. If there is a message, it is displayed on the screen of the monitor. A printout can be taken of the message if required. Response can be sent through the same system. Cost saving: - Cost saving is the main advantage of the computer networks. With the help of hardware utilization, time saving we can save our coast.

DISADVANTAGES OF A NETWORK : - Networks are no doubt helpful in many ways to share resources and saving time and money to buy extra hardware and resources but this technology has some drawbacks. The main drawbacks could be summarized as follows: Data Security: - When all the files are shared among the users there is threat to important data stored in one computer. This data can be altered or used un-authoritatively by other users. This can be taken care of by issuing proper permissions to various users according to their nature of work. Any desired file or folder may be made inaccessible to other users of the network or it can be made available to them through proper password validation check. System Crash: - No program is guaranteed to work at 100% secure and safe condition. Each program has some percentage of bugs in it, no one is perfect. This may result in crash of the network software or sometimes the hardware may fail. Since all the important resources and data are shared, failure of one computer in the network may cause huge loss to the organization. This may also be taken care of by following proper disaster recovery measures and taking backups at regular intervals. Various topologies (discussed later) help in arranging computers in a network in such a fashion that the unavailability of one may not hinder the overall working of the network. Privacy: - If the network is poorly implemented, improper communication may take place. Private or important mails can be seen by other users and this may result in loss of privacy or corrupt information system.

Q3. HISTORY OF COMPUTER NETWORK Ans: Earlier In Time near about 1960 mainframe computers were standalone devices. There were hardly any electronic communications (communications via the magnetic tapes) between computers of different models and from different manufacturers. In 1960s, US Defense Department’s Advance Research Projects Agency (ARPA) commissioned a project to linked computers together. The idea for ARPANET was formed in 1967. In 1969, ARPANET linked 4 nodes (computer) together. In 1973, Vint Cerf and Bob Kahn outlined Transmission Control Protocol (TCP), connecting computers from different operating systems. In 1977, TCP was split into 2 protocols (TCP/IP) with TCP responsible for reliable transfer of data; while Internet Protocol (IP) handles the format and routing of datagram (independent packet). TCP became more popular when UC Berkeley modified the UNIX operating system to include TCP/IP in 1981. Various versions of TCP/IP have been used by the Internet. Today, computer networks are the core of modern communication. All modern aspects of the Public Switched Telephone Network (PSTN) are computer-controlled, and telephony increasingly runs over the Internet Protocol, although not necessarily the public Internet. The scope of communication has increased significantly in the past decade, and this boom in communications would not have been possible without the progressively advancing computer network. Computer networks and the technologies needed to connect and communicate through and between them, continue to drive computer hardware, software, and peripherals industries. This expansion is mirrored by growth in the numbers and types of users of networks from the researcher to the home user. Need of Networks : -Computer network has imp. Advantage of time and space both. Computers can process data so quickly that it is justified to say that computer conquers time. Space constraints are removed by the computer networks. Today, network is a need of every office, organization, company, institute etc. Computer networks are used for electronic mail teleconferences etc. While sitting in front of a computer, you can collect information about any place, institute etc. This can be done with the help of a program called Netscape Navigator. You can talk to your friend through a special software sitting thousands of miles away from you and see his face on the screen. The amazing new world of computer connectivity, sometimes called cyber space has reduced the cost of sharing information, resources and has led the way to acquire knowledge in a much easier and quicker way.

Q4. What is network media? Explain the guided and unguided media? Ans. Transmission Media: - There are a number of components which are used to build networks. An understanding of these is essential in order to support networks:- 1) NIC (Network Interface Card):- The network interface card (NIC) provides the physical connection between the network and the computer workstation. Most NICs are internal, with the card fitting into an expansion slot inside the computer. Network interface cards are a major factor in determining the speed and performance of a network. It is a good idea to use the fastest network card. Transmission Media: - The physical channel used for transmission in the network is called transmission media. The media may be guided or un-guided.

Guided Media: - Guided Transmission Media uses a "cabling" system that guides the data signals along a specific path. The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media. Un-guided Media: - Unguided Transmission Media consists of a means for the data signals to travel but nothing to guide (by Air) them along a specific path. The data signals are not bound to a cabling media and as such are often called Unbound Media. Guided media Twisted Pair Cable: - This is one of the most common transmission methods. These are used in local telephone communication and short distance digital data transmission up to about 1 km. Data transmission speed of up to 9600 bits per second (baud) can be obtained for a distance of about 100 metres. For longer distances local telephone lines are used but that reduces the transmission speed to about 1200 bits per second. Insulated copper wires are twisted on each other to reduce interference by adjacent wires. Out of all the communication channels for LAN, it is the easiest one to lay the network. This cable suffers from noise and low transmission rate. The major disadvantages are noise interface and low transmission rate.

Characteristics (i) Inexpensive (ii)Easy to install (iii)Easy to tap (iv)Low noise level (v)Seed up to 9600 bits per second (vi)Up to 1000 devices can be accommodated (vii)Adequate for network span up to 1 km Coaxial Cable: - It is used for long distance service by telephone companies and for both baseband and broadband transmission in local area network. These cables are used for high-speed data transfer over a long distance and are immune to electrical noise. Baseband coaxial cable transmits a single signal at a time at a very high speed, while a broadband coaxial cable can transmit many simultaneous signals using different frequencies. A baseband cable transmits digital data at a speed of about 2 million bits per second but must be amplified every 250 metres or so. It is mainly used for local area networks. A broadband coaxial cable can transmit only analog signals, so it must be used in conjunction with a modem. It is not easy to use it in a network. Coaxial cables are groups of specially wrapped insulated wire lines. A coaxial cable also uses two wires, but one is a tube woven from very fine strands of metal; the second one passes through the center along the length of the first.

A PVC insulation is used surrounding the central copper wire. A thicker PVC material is used for shielding the metal sleeve as shown in diagram. The signal is transmitted by the inner copper wire. Characteristics (i) Offer much higher band width (ii) Capable of transmitting digital signals at very high rates of 10 megabits per second. (iii) Extensively used in long distance telephone lines and as cables in closed TV circuits. (iv)Well-packaged large cable can handle more than 16000 telephone lines simultaneously. (v) Immune to noise (vi) Less distortion or loss of signal (vii)Mre expensive than twisted-pair wiring (viii)Moredifficult to tap than twisted-pair wiring (ix) idely available FIBER-OPTIC CABLES: - Fiber-optic cables are made of glass or transparent plastic. They are as thick as human hair (thickness is of the order of few microns). The light enters at one end of the light-pipe. After Fiber -Optic cables Multiple reflections at various points inside the pipe, the light comes out without any significant loss in energy.On this cable data is transmitted as pulses of light, which passes through the cables. These cables are secured, and transmit large volumes of data over long distances.

Optical fiber systems consist of a transmitter, the glass fiber filaments along which data travel as high-speed pulses of light, and a receiver. Optical fibers carry a huge amount of data. Transmission is not disrupted by electromagnetic interference and data are highly secure. Being optical in nature, fibers do not radiate electronic signals. Simultaneous transmission of about 50,000 messages by means of advanced optical techniques based upon laser technology has been tested by USA scientists. Semiconductor lasers transmit information in the form of light along optical fibers at the speed of light (186,000 miles per second) with no significant loss of intensity over long distances. Fiber optic cables increase the capacity of a like-sized coaxial cables by at least a factor of 10. This makes them valuable for use in variety of applications, specially in the communication networks. It is likely that they may prove to be an alternative to satellite communication. At the inlet (source) of the fiber, electrical signals are converted into light signals. At the outlet (sink), the light

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100 SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 54 of 76 signals are converted back into electrical signals by photoelectric diodes. These signals after amplification are sent to the receiver.

(Transmission through optical fiber) Characteristics of optical fiber (i) Transmission of signal is at a very high-speed. Transmission speed is about 1 giga bits per second at low error rate (1 in 109 bits). (ii) Asingle fiber in glass fiber cable has a thickness of about 2 microns (2 x 10-6 m). They occupy very less sace in coparison to additional bulky copper wires. (iii) The use of optical fibers is very cost-effective. The conduits used to house copper wires are of fixed size ad very epensive whereas the optical fibers are much smaller in size. (iv) Optical fibers may be used to transmit either analog or digital signals. In analog signal transmission, the light intensity changes continuously whereas in digital signal transmission, the light source is turned on and off. (v) Optical transmission is not affected by electromagnetic interference. Noise level is low and distortion is rduced to ery small amount. (vi) Optical fibers are most useful for point-to-point one-way communication link. This is due to the reason that fiber optical cables cannot be tapped at various points. (vii) Unauthorized tempering of information is very difficult.

Q5. Explain the following:-  Connectors  Router  Repeater  Bridge  Gateway Ans: Connectors are used to connect the network cable with NIC and other connecting devices (Hub, Router, Bridge Etc.). The type of connector depends on the cable type used. Following are the types of connectors:- a). RJ-45 The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry.

BNC Connector: - The most common type of connector used with coaxial cables is the Bayonet-Neill-Concelman (BNC). Different types of adapters are available for BNC connectors, including a T-connector, barrel connector, and terminator. Connectors on the cable are the weakest points in any network. To help avoid problems with your network, always use the BNC connectors that crimp, rather than screw, onto the cable.

Bridges: - Bridges are used to decrease network traffic by dividing a network into segments, which increases the maximum possible size of your network. Each segment that a bridge connects is considered to be from a different network. Bridges connect network segments, and the use of a bridge increases the maximum possible size of the network. A bridge selectively determines the appropriate segment to which it should pass a signal. It does this by reading the address of all the signals it receives. The bridge reads the physical location of the source and destination computers from this address. Bridges should not be joined on a network to create loops of any kind. Connecting them would create a circular connection between all the bridges, which would cause bottlenecks from never-ending data transmissions. A network should use bridges in a hierarchical pattern to avoid this problem. Some bridges prevent this from happening with the use of a technique called STA (spanning tree algorithm). Bridges are incapable of connecting LAN segments with different architectures. For example, an Ethernet segment and a Token Ring segment normally cannot be connected with a bridge because each network type uses different physical addressing. However, a special bridge called a ‘translation bridge’ allows you to connect different network types.

Routers: - Routers are devices that connect two or more networks. Routers include the physical interfaces to the various networks in the inter-network. These interfaces can be Token Ring, Ethernet, TI, Frame Relay, Asynchronous Transfer Mode (ATM), or any number of other technologies, or any number of other technologies. Similar to bridges, routers can be used to segment a busy network to reduce network traffic by filtering network messages. A network address consists of two parts, where one part of the address identifies an entire network segment, and the other part identifies a node such as a computer residing on that network segment. Where bridges use routing tables containing individual computer’s addresses, routers store network locations. Routers match packet headers to a LAN segment and choose the best path for the packet, optimizing network performance. Routers are capable of determining the best path for data to follow, but bridges usually can’t. They can operate with different media access methods and network architectures, such as Ethernet and Token Ring. Routers pass packets to other routers. When packets are passed from router to router, the routers need to see the network addresses so they can send a packet across different network architectures. Although routers can distinguish between protocols, and therefore, can deal with different access methods and their protocols, neither routers nor bridges can link or translate two different protocols like IPX/SPX or TCP/IP. Routers can’t merely associate some IP address to a computer on some IPX number on another computer. Translation of protocols is performed by gateways. The following protocols are routable.  Decent  DDP (Apple Talk)  NWLink IPX  TCP/IP  OSI and XNS (SPX/IPX)

Gateways: - Gateways find use in connecting two dissimilar LANs. There is a subtle difference between gateways and routers. The two terms are used interchangeably. Gateways can be comprised of software, dedicated hardware, or a combination of both. Cable Interface Unit (Hub): - Hub is a device that connects multiple user stations, each via a dedicated cable. Hubs are used to create a physical star network while maintaining the logical bus or ring configuration of the LAN. There are many types of hubs. Passive hubs are simple combiners that group workstations into a single segment, whereas active hubs include a repeater function and are thus capable of supporting many more connections. Maximum speed of data traveling 10 mbps.

Repeaters: - Since a signal loses strength as it passes along a cable, it is often necessary to boost the signal with a device called a repeater. The repeater electrically amplifies the signal it receives and rebroadcasts it. Repeaters can be separate devices or they can be incorporated into a concentrator. They are used when the total length of your network cable exceeds the standards set for the type of cable being used.

Q6. OSI model Ans: OSI Reference Model: - The need for standards in data communication is obvious. But the protocols used must be universally recognizable. If each computer network administrator and implementer begins to design their own set of standards then it will be difficult to join these networks and to transfer data between two different networks. To avoid this situation International Standards Organization (ISO) established a sub-committee to develop an international standard on network architectures. The effort results in an Open System Interconnection (OSI) model. OSI standardized protocols made it easy to communicate between heterogeneous networks anywhere in the world. It is a model for how messages should be transmitted between any two points in a telecommunication network. The reference model defines a networking framework for implementing protocols in seven layers. Control is passed from one

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100 SUBJECT: ‘COMPUTER NETWORK’ (PGDCA – 2 / ADHNT 2 / BCA 4) Page 57 of 76 layer to the next, starting at the Application Layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy. One of the model’s soul purposes is to guide product implementers so that their products will consistently work with other products. Although OSI is not always strictly adhered to in terms of keeping related functions together in a well- defined layer, many if not most, products involved in telecommunication make an attempt to describe themselves in relation to the OSI model. The layered approach to OSI offers several advantages to system implementers by separating the job of networking into logical smaller pieces. This means that vendors can more easily solve network problems and difficulties. For example a product from one vendor that implements the Application Layer (Layer 7), will be much more likely to interoperate with another vendor's Presentation Layer (Layer 6) product when both vendors follow this standard model. Finally, the OSI layers afford extensibility and new protocols and other network services will generally be easier to add to a layered architecture than to a gigantic one.

In brief the OSI the seven layers are as below: The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers. The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, application, is closest to the end user. Both users and application-layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another layer in the OSI model. The lower layers of the OSI model handle data transport issues. The physical layer and data link layer are implemented in hardware and software. The other lower layers generally are implemented only in software. The lowest layer, the physical layer, is closest to the physical network medium (the network cabling, for example) and is responsible for actually placing information on the medium. Functions of layers Functions of various layers described in OSI model are as follows: Physical Layer: - The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define characteristics, such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. Physical-layer implementations can be categorized as either LAN or WAN specifications. In brief the functions of physical layer are: - transmission of bits over a channel - design issues deal largely with mechanical, electrical and procedural interfaces - volts representations of 1s and 0s.

Data-link Layer: - The data link layer provides reliable transit of data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer. Network topology consists of the data link layer specifications that often define how devices are to be physically connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time. The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC). The Logical Link Control (LLC) sublayer of the data link layer manages communications between devices over a single link of a network. LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services used by higher-layer protocols. IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share a single physical data link. The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one another at the data link layer. In brief the functions of data link layer are: - transformation of raw bits into a line free of undetected transmission errors to the network layer. - sending the input data up into data frames - transmitting the frames sequentally - processing of acknowledgement frames sent back by the receiver - retransmitting of the frame in case of the noise destruction - solving the problem of lost, damaged and duplicate frames - dealing with high speed transmitters and slow receivers - controlling access to shared channel. Network Layer: - The network layer provides routing and related functions that enable multiple data links to be combined into an internetwork. This is accomplished by the logical addressing (as opposed to the physical addressing) of devices. The network layer supports both connection-oriented and connectionless service from higher-layer protocols. Network- layer protocols typically are routing protocols, but other types of protocols are implemented at the network layer as well. Some common routing protocols include Border Gateway Protocol (BGP), an Internet interdomain routing protocol; Open Shortest Path First (OSPF), a link-state, interior gateway protocol developed for use in TCP/IP networks; and Routing Information Protocol (RIP), an Internet routing protocol that uses hop count as its metric. In brief the functions of network layer are: - routing the packets from source to destination - Controlling of congestion - to overcome the problems of differences in addressing schemes, types of networks etc. Transport Layer: - The transport layer implements reliable internetwork data transport services that are transparent to upper layers. Transport-layer functions typically include flow control, multiplexing, virtual circuit management, and error checking and recovery. Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process. Multiplexing enables data from several applications to be transmitted onto a single physical link. Virtual circuits are established, maintained, and terminated by the transport layer. Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves taking an action, such as requesting that data be retransmitted, to resolve any errors that occur. Some transport-layer implementations include Transmission Control Protocol, Name Binding Protocol, and OSI transport protocols. Transmission Control Protocol (TCP) is the protocol in the TCP/IP suite that provides reliable transmission of data. Name Binding Protocol (NBP) is the protocol that associates AppleTalk names with addresses. OSI transport protocols are a series of transport protocols in the OSI protocol suite. In brief the functions of transport layer are - accepts data from the session layer and passes this to the network layer

- creates a distinct network connection for each transport connection required by session layer - multiplexing several transport connections. - Keeping tracks that which message belong to which connection. Session Layer: - The session layer establishes, manages, and terminates communication sessions between presentation layer entities. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session-layer implementations include Zone Information Protocol (ZIP), the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP), the DECnet Phase IV session- layer protocol. In brief the functions of session layer are: - allows users to establish sessions between them - manages dialogue control. - Token management - allows a user to log into a remote timesharing system - allows a file transfer between two machines - Synchronization and insertion of check-points into the DataStream. Presentation layer:- The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system will be readable by the application layer of another system. Some examples of presentation-layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes. Common data representation formats, or the use of standard image, sound, and video formats, enable the interchange of application data between different types of computer systems. Conversion schemes are used to exchange information with systems by using different text and data representations, such as EBCDIC and ASCII. Standard data compression schemes enable data that is compressed at the source device to be properly decompressed at the destination. Standard data encryption schemes enable data encrypted at the source device to be properly deciphered at the destination. Presentation-layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding. Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF is a standard for compressing and coding graphic images. JPEG is another compression and coding standard for graphic images, and TIFF is a standard coding format for graphic images. In brief the functions of presentation layer are: - it is concerned with the syntax and semantics of the information transmitted - decides about the coding scheme of data i.e., ASCII will be used or Unicode will be used for coding. - encoding and decoding. Application Layer:- The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Two key types of application-layer implementations are TCP/IP applications and OSI applications. TCP/IP applications are protocols, such as Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP), that exist in the Internet Protocol suite. OSI applications are protocols, such as File Transfer, Access, and Management (FTAM), Virtual Terminal Protocol (VTP), and Common Management Information Protocol (CMIP), that exist in the OSI suite. In brief the functions of application layer are: - defining of an abstract network virtual terminal

- handling different terminal types - mapping of the functions of the network virtual terminal into the real terminal - file transfer between systems having different naming conventions, representations and other incompatibilities.

Q7. Explain the wide area network techonology? Ans. WIDE AREA NETWORK:- When the organization is spread over a wide area then it becomes more difficult and costly to link them through cables. For this situation, the various units of the organization are linked using Wide area networks. Connecting links are made over telephone lines leased from the various telephone companies. In some cases, WANs are created with satellite links, radio or microwave transceivers. The Internet has become the largest and least expensive WAN in the world. WANs suffer from extremely limited bandwidth. The fastest commercially feasible wide area data links are many times slower than the slower local area links. Generally, WAN links are used only for inter-process communications to route short messages, such as e-mail or HTML (World Wide Web) traffic.

TYPES OF WAN TECHNOLOGIES FRAME RELAY:- Frame relay is a service for people who want an absolute connection-oriented way to move bits over A to B at reasonable speed and low cost. Frame relay can be best thought of as a virtual leased line. It is based on a concept called a virtual circuit. A virtual circuit is a bi-directional path through the network which is defined in software. The main benefit of frame relay is that many virtual circuits can be used across a single physical connection. For example, a company’s headquarter needs to communicate with three remote offices. Instead of leasing three point-to-point circuits to connect each of these three offices, a single circuit connects each to the frame relay network and three virtual circuits are used. ISDN:- Integrated Services Digital Network is a set of standards for the transmission of digital data.ISDN hasten service levels: Basic rate Interface(BRI) and Primary Rate Interface(PRI). These services use two types of channels: ‘B’ channels and ‘D’ channels. The ‘B’ channels provide 64Kbps and are used to transport either voice or data.In a Basic Rate Interface there are two B channels and one D channel which is 16 Kbps. The D channel is used to carry control and signaling information.In the PRI there are 23 B channels at 64 Kbps each and one D channel. A BRI is used to carry one voice and one data circuit into a small office or home. BRIs are generally used in a dial-up connection whereas an ISDN modem connects only to the service provider. The PRI is usually a permanent connection. SONET:- Synchronous Optical NETwork (SONET) is set of standards defining the rates and formats for optical networks specified in ANSI TI.106, and ANSI TI.117. the synchronous digital hierarchy (SDH) is a similar standard defined by the ITU-T and used primarily in Europe. The frame format used by SONET is synchronous transport module (STM). This is said to be hierarchical because smaller signal levels can be multiplexed together to form larger ones. The SONET design had four major goals.:- First and foremost, SONET had to make it possible for different carriers to internetwork. Achieving this goal required defining a common signaling standard with respect to wavelength, timing, framing structure, and other issues. Second, some means were needed to unify the U.S, European, and Japanese digital systems, all of which were based on 64-kbps PCM channels, but all of them combined them in different ways. Third, SONET had to provide a way to multiplex multiple digital channels together. Fourth, SONET had to provide support for operations, administration, and maintenance (OAM). POINT-TO-POINT PROTOCOL (PPP): - is a data link protocol commonly used in establishing a direct connection between two networking nodes. It can provide connection authentication, transmission encryption privacy, and compression.

PPP is used over many types of physical networks including serial cable, phone line, trunk line, cellular telephone, specialized radio links, and fiber optic links such as SONET. Most Internet service providers (ISPs) use PPP for customer dial-up access to the Internet. Two encapsulated forms of PPP, Point-to-Point Protocol over Ethernet (PPPoE) and Point- to-Point Protocol over ATM (PPPoA), are used most commonly by Internet Service Providers (ISPs) to establish a Digital Subscriber Line (DSL) Internet service connection with customers

Q8. Explain HDLC LLC Ans; HDLC:- In data transmission and telecommunication bit stuffing is the insertion of non information bits into data. Bit stuffing is used for various purposes, such as for bringing bit streams that do not necessarily have the same or rationally related bit rates up to a common rate, or to fill buffers or frames. The location of the stuffing bits is communicated to the receiving end of the data link, Bit stuffing may be used to synchronize several channels before multiplexing or to rate- match two single channels to each other. LLC:- The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC). The Logical Link Control (LLC) sublayer of the data link layer manages communications between devices over a single link of a network. LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services used by higher-layer protocols. IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share a single physical data link.

Q9. Explain the concept of carrier sence multiple access? Ans: CSMA:- A discipline must be imposed on devices connected to the network to ensure a controlled access to the media. Access methods are the means or ways by which stations actually gain the use of the common channel to transmit messages. The right to transmit is an issue only in broadcast where workstations share a single channel. Two commonly used techniques are as given below: - (i) Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) (ii) Token passing CSMA/CD : - The CSMA/CD access method is primarily used with bus networks. The bus operates in a Multiple Access (MA) mode. A node is allowed to transmit on the bus, if it senses that the medium is free (carrier sense). Occasionally two or more nodes may simultaneously sense that the medium is free can begin the transmission. This creates a collision, as the contents of transmitted information frames will collide resulting in corruption of the information frame. The transmitting node detects the collision. The two (or more) nodes involved then wait for a further short random time interval before trying to retransmit a frame once again. CSMA/CA: - On some networks, the system cannot detect collisions because the power of the transmitting device is much stronger than the receiver’s power. In these situations we apply the CSMA/CA:- technique. This is a p-persistent technique with the addition of idle time management. When a device detects that the transmission media is idle, the device must wait for a specified time before it can compete for access to media. This specified waiting time is called interface spacing or IFS time.

Q10. Discuss any one technique of error detection and error recovery. Ans:- Error detection and error correction : - Most computer networking protocols use error correction schemes to implement requests to the sender to retransmit the missing information. To ensure that the information arrives, acknowledgement techniques are used so that the sender has some type of feedback from the receiver that the information did or did not arrive. Acknowledgements are usually special control frames that contain either positive or negative acknowledgements about the incoming information. To ensure that the retransmitted packets of the same information are not all processed, sequence numbers are assigned to outgoing packets so that the receiver can tell which are the originals and which are the retransmissions. The methods used for error control include the combinations of the following mechanisms: e)Error detection: packets arriving at the receiver that do not pass the error checking routine are discarded. f) Positive acknowledgement: When the receiver determines that a packet has passed the error-checking routine, a positive acknowledgement is sent back to the sender. g)Negative acknowledgement: when the receiver determines that a packet has failed the error-checking routine, a negative acknowledgement is sent back to the sender. h)Retransmission after timeout: If the sender has not received an acknowledgement within a certain time frame from a packet it has sent, the sender retransmits the packet.

As a group, these methods are known as automatic repeat requests (ARQ). There are two implementations of ARQ found in network communication protocols. The first is called stop-and –wait ARQ. In this scheme, the sender transmits a packet, waits for the acknowledgement and then sends the next packet. In the second procedure, go-back-n ARQ, the sender transmits a series of packets that contain sequence numbers. The receiver analyzes the transmitted packets and if any failure occurs in the error-checking routine, the sender retransmits the damaged packet and the packets following the damaged packet.

Q11. Define all LAN topologies Ans: Topologies define the arrangement of computers taking part in the network. These should be properly planned and implemented because a good selection of topology can result in proper and adequate use of resources while a mismanaged arrangement of computers can lead to inefficient use of resources, loss of data and time. Various available topologies include:- STAR TOPOLOGY: - Star topology comprises of a number of stations connected directly to a central station or controller. Communications on the connecting links between the stations and the central station of star topography can be bi- directional and are point-to-point. A station on this type of network passes an information frame to the central controller, which then forwards the information the destination station. The central controller manages and controls all communications between stations on the network. Diagrammatic representation of the star topology is given below.

Advantages: 1) If a computer crashes, it has no effect on the entire network. 2) It involves minimal number of cables. 3) Introduction or removal or node from the network has no optimal effect over other computers in the network. Disadvantages: 1) Whole system is dependent on a central node, so it should be powerful and for that matter it may be costly. 2) If the central node, fails whole of the network may come down. BUS TOPOLOGY: - Bus topology comprises of stations that are connected to a single communication line. This single communication line is referred to as a bus. Information frames originating from a station are propagated away from the station in both directions on the bus. Each station on the bus interrogates the information frame destination address field for its own address. If the destination field does not match the stations address, the station discards the information frame back on to the bus. If the destination address matches the station address, it accepts the information frame and processes the frame.

Advantages:

1) Simple installation 2) Failure of a station affects that station only. 3) No concept of dependability on a central node, like in the case of star topology. Disadvantages: 1) Time to transfer information may be large because each node tests the information packet floating in the network that whether it is for him. 2) Failure of cables may result in serious implication on the network. Cable failure can be extremely difficult to locate on large bus networks. RING TOPOLOGY: - Ring topology is akin to an omni bus circuit as shown in the diagram below. Local area networks that have each station attached to an adjacent station using point-to-point links form a physical ring. Each station attached and active to the ring regenerates the information frame, then, re-transmits the information frame on the ring. The ring itself is logically circular and the flow of information is unidirectional.

Advantages: 1) Since the transmission of frames is unidirectional simple, software algorithms could be used. 2) There is no dependency on a single node. Disadvantages: 1) Failure of one node stops the working of whole network. 2) A control mechanism is required so as to determine who should start up the ring. 3) Since transmission is unidirectional, the time taken to reach the information at desired location may be long. HYBRID NETWORKS: - Selection of topology depends on the following criteria: · Cost that can incur · Expandability · Availability of communication lines · Number of systems and area to be connected So, generally a combination of above discussed topology is used and such a network is known as Hybrid Network. Different topologies have their own benefits and drawbacks so hybrid network provides a means to take the benefit of these topologies collectively. A network making use of some of the above-mentioned topologies may look like as shown below:

TREE TOPOLOGY: - Tree topology is considered as a corollary of the bus topology, the difference being the existence of branches with multiple nodes is possible. The transmission medium is a branching cable with no closed loop.

Advantages: 1) There are no closed loops, hence failure of one branch does not affect whole system. 2) The branches may be extended without any delay or stopping the transmission of information from other branches. Disadvantages: 1) In contrast to bus topology, tree topology is bit complex and costly. 2) Only workstation can transmit information at a time.

Q12. Explain the concept of Data Link Layer protocol? Ans. Data-link layer has responsibility of Transferring datagram from one node to adjacent node over a link Understand principles behind data link layer services: Merror detection, correction Msharing a broadcast channel: multiple accesses Mlink layer addressing Mreliable data transfer, flow control Framing, link access: Encapsulate datagram into frame, adding header, and trailer

Channel access if shared medium “MAC” addresses used in frame headers to identify source, dest Different from IP address! Reliable delivery between adjacent nodes We learned how to do this already! Seldom used on low bit-error link (fiber, some twisted pair) Wireless links: high error rates

Q14. Why both link-level and end-end reliability? Ans: Flow control: Pacing between adjacent sending and receiving nodes

Error detection: Errors caused by signal attenuation, noise. Receiver detects presence of errors: Signals sender for retransmission or drops frame Error correction: Receiver identifies and corrects bit error(s) without resorting to retransmission Half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit, but not at same time In each and every host Link layer implemented in “adaptor” (aka network interface card NIC) Ethernet card, PCMCI card, 802.11 card Implements link, physical layer Attaches into host’s system buses Combination of hardware, software, firmware Adaptors Communicating

Datagram Datagram

Controller Controller

Sending host Datagram Receiving host Frame

Sending Side: Receiving Side 1. encapsulates datagram in frame 1. Looks for errors, rdt, flow control, etc 2. adds error checking bits, rdt, flow 2. Extracts datagram, passes to upper layer control, etc. at receiving side

Error Detection

EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable! • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction

Internet checksum

Sender: treat segment contents as sequence of 16-bit integers Checksum: addition (1’s complement sum) of segment contents Sender puts checksum value into UDP checksum field Receiver: compute checksum of received segment Check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless?

Q15. DNS (domain name service) The internet primarily uses IP addresses for locating nodes. However, its humanly not possible for us to keep track of the many important nodes as numbers. Alphabetical names as we see would be more convenient to remember than the numbers as we are more familiar with words. Hence, in the chaotic organization of numbers (IP addresses) we would be much relieved if we can use familiar sounding names for nodes on the network. There is also another motivation for DNS. All the related information about a particular network (generally maintained by an organization, firm or university) should be available at one place. The organization should have complete control over what it includes in its network and how does it "organize" its network. Meanwhile, all this information should be available transparently to the outside world. Conceptually, the internet is divide into several hundred top level domains where each domain covers many hosts. Each domain is partitioned in subdomains which may be further partitioned into subsubdomains and so on... So the domain space is partitioned in a tree like structure as shown below. It should be noted that this tree hierarchy has nothing in com - mon with the IP address hierarchy or organization. The internet uses a hierarchical tree structure of Domain Name Servers for IP address resolution of a host name.

The top level domains are either generic or names of countries. eg of generic top level domains are .edu .mil .gov .org .net .com .int etc. For countries we have one entry for each country as defined in ISO3166. eg. .in (India) .uk (United Kingdom). The leaf nodes of this tree are target machines. Obviously we would have to ensure that the names in a row in a subdo- main are unique. The max length of any name between two dots can be 63 characters. The absolute address should not be more than 255 characters. Domain names are case insensitive. Also in a name only letters, digits and hyphen are allowed. For eg. www.iitk.ac.in is a domain name corresponding to a machine named www under the subsubdomain iitk.ac.in.

Q16. What is firewall And explain the benefits of the firewall in a network? Ans: Firewalls Introduction : - This lecture discusses about security mechanisms in the Internet namely Firewall . In brief, It's a configuration of routers and networks placed between an organization's internal internet and a connection to an external internet to provide security. In other words, Firewall is a mechanism to provide limited access to machines either from the outside world to internal internet or from internal world to outside world. By, providing these security mechanisms, we are increasing the processing time before one can access a machine. So, there is a trade-off between security and ease of use. A firewall partitions an internet into two regions, referred to informally as the inside and outside.

Firewall

Rest of Internet Intranet

Outside Inside

Security Lapses  Vulnerable Services NFS : A user should not be allowed to export certain files to the outside world and from the outside world also, someone should not be allowed to export our files.  Routing based attacks: Some kind of ICMP message should not be allowed to enter my network. e.g.. Source routing and change route ICMP's.  Controlled access to our systems: e.g. Mail server and web pages should be accessible from outside but our individual PC's should not be accessible from the outside world.  Authentication: Encryption can be used between hosts on different networks.

 Enhanced Privacy: Some applications should be blocked. e.g.. finger ...  PING & SYN attack: Since these messages are send very frequently, therefore you won't be able to do anything except reply to these messages. So, I should not allow these messages to enter my network. So. Whatever I provide for my security is called Firewall. It is a mechanism and not just a hardware or software. Firewall Mechanisms 1. Network Policy: Here, we take into consideration, what services are allowed for outside and inside users and the services which are allowed can have additional restrictions. e.g.. I might be allowed to download things from the net but not upload i.e.. some outside users cannot download the things from our net. Some exceptional cases might be there which have to be handled separately. And if some new application comes up then , we choose an appropriate network policy. 2. Authentication mechanism: An application can be designed which ask for a password for authentication. 3. Packet Filtering: Router have information about some particular packets which should not be allowed. 4. Application gateways: or proxy servers. Certain Problems with Firewall 1. Complacency: There are lots of attacks on the firewall from internal users and therefore, it's limitations should be under - stood. 2. Encapsulated packets: An encapsulated packet is an IP packet within another IP packet. If we ask the router to drop encapsulated packets then, it will drop the multicast packets also. 3. Throughput: So, in order to check which packets are allowed and which are not, we are doing some processing which can be an overhead and thus affects throughput. Authentication: We can use the following mechanisms:  One time passwords: passwords are used only once and then it changes. But only the user and the machine knows the changing passwords.  password aging : User are forced to change passwords after some time on regular intervals.  smart cards : swipe through the PC.  biometrics : eyes or finger prints are used. Packet Filtering : Terms associated:  Source IP address  Destination IP address  Source port #  Destination port #  protocol  interface

Q17. What is network security? Explain there types? Ans. Network Security: - Data on the network is analogous to possessions of a person. It has to be kept secure from others with malicious intent. This intent ranges from bringing down servers on the network to using people's private information like credit card numbers to sabotage of major organizations with a presence on a network. To secure data, one has to ensure that it makes sense only to those for whom it is meant. This is the case for data transactions where we want to prevent eavesdroppers from listening to and stealing data. Other aspects of security involve protecting user data on a computer by providing password restricted access to the data and maybe some resources so that only authorized people get to use these, and identifying miscreants and thwarting their attempts to cause damage to the network among other things. The various issues in Network security are as follows: 1. Authentication: We have to check that the person who has requested for something or has sent an e-mail is in - deed allowed to do so. In this process we will also look at how the person authenticates his identity to a remote machine. 2. Integrity: We have to check that the message which we have received is indeed the message which was sent. Here CRC will not be enough because somebody may deliberately change the data. Nobody along the route should be able to change the data. 3. Confidentiality: Nobody should be able to read the data on the way so we need Encryption

4. Non-repudiation: Once we sent a message, there should be no way that we can deny sending it and we have to accept that we had sent it. 5. Authorization: This refers to the kind of service which is allowed for a particular client. Even though a user is au- thenticated we may decide not to authorize him to use a particular service. For authentication, if two persons know a secret then we just need to prove that no third person could have generated the message. But for Non-repudiation we need to prove that even the sender could not have generated the message. So authentication is easier than Non-repudiation. To ensure all this, we take the help of cryptography. We can have two kinds of encryption: 1. Symmetric Key Encryption: There is a single key which is shared between the two users and the same key is used for encrypting and decrypting the message. 2. Public Key Encryption: There are two keys with each user : a public key and a private key. The public key of a user is known to all but the private key is not known to anyone except the owner of the key. If a user encrypts a message in his private key then it can be decrypted by anyone by using the sender's public key. To send a mes - sage securely, we encrypt the message in the public key of the receiver which can only be decrypted by the user with his private key. Symmetric key encryption is much faster and efficient in terms of performance. But it does not give us Non-repudiation. And there is a problem of how do the two sides agree on the key to be used assuming that the channel is insecure ( others may snoop on our packet ). In symmetric key exchange, we need some amount of public key encryption for authentication. However, in public key encryption, we can send the public key in plain text and so key exchange is trivial. But this does not authenticate anybody. So along with the public key, there needs to be a certificate. Hence we would need a public key infrastructure to distribute such certificates in the world. Key Exchange in Symmetric Key Schemes We will first look at the case where we can use public key encryption for this key exchange. . The sender first encrypts the message using the symmetric key. Then the sender encrypts the symmetric key first using it's private key and then using the receiver's public key. So we are doing the encryption twice. If we send the certificate also along with this then we have authentication also. So what we finally send looks like this :

Z : Certificatesender + Publicreciever ( Privatesender ( Ek ) ) + Ek ( M )

Here Ek stands for the symmetric key and Ek ( M ) for the message which has been encrypted in this symmetric key. However this still does not ensure integrity. The reason is that if there is some change in the middle element, then we will not get the correct key and hence the message which we decrypt will be junk. So we need something similar to CRC but slightly more complicated. This is because somebody might change the CRC and the message consistently. This function is called Digital Signature. Digital Signatures: - Suppose A has to send a message to B. A computes a hash function of the message and then sends this after encrypting it using its own private key. This constitutes the signature produced by A. B can now decrypt it, recompute the hash function of the message it has received and compare the two. Obviously, we would need the hash functions to be such that the probability of two messages hashing to the same value is extremely low. Also, it should be difficult to compute a message with the same hash function as another given message. Otherwise any intruder could replace the message with another that has the same hash value and leave the signatures intact leading to loss of integrity. So the message along with the digital signature looks like this :

Z + Privatesender ( Hash ( M ) )

Digital Certificates: - In addition to using the public key we would like to have a guarantee of talking to a known person. We assume that there is an entity who is entrusted by everyone and whose public key is known to everybody. This entity gives a certificate to the sender having the sender's name, some other information and the sender's public key. This whole information is encrypted in the private key of this trusted entity. A person can decrypt this message using the public key of the trusted authority. But how can we be sure that the public key of the authority is correct? In this respect Digital signatures are like I-Cards. Let us ask ourselves the question : How safe are we with I-Cards? Consider a situation where you go to the bank and need to prove your identity. I-Card is used as a proof of your identity. It contains your signature. How does the bank know you did not make the I-Card yourselves? It needs some proof of that and in the case of I-Cards they contain a counter signature by the director for the purpose. Now how does the bank know the signature I claim to be of the director indeed belongs to him? Probably the director will also have an I-Card with a counter signature of a higher authority. Thus we will get a chain of signing authorities. Thus in addition to signing we need to prove that the signatures are genuine and for that purpose we would probably use multiple I-Cards each carrying a higher level of signature-counter signature pair. So in order to distribute the public key of this authority we use certificates of higher authority and so on. Thus we get a tree structure where the each node needs the certificates of all nodes above it on the path to the root in order to be trusted. But at some level in the tree the public key needs to be known to everybody and should be trusted by everybody too.

Q18. Explain FTP AND TFTP services> Ans. FTP: - The file transfer facilities are usually provided by a mechanism known as the File Transfer Protocol (FTP). It is a simple featured file moving utility that allows users to transfer files from or to thousands of host computers on the internet. To transfer a file, the user invokes the host, FTP utility specifies file name, type and remote destination.

FTP offers many facilities:  Interactive Access : Most implementations provide an interactive interface that allows humans to easily interact with remote servers.  Format (representation) specification: FTP allows the client to specify the type and format of stored data.  Authentication Control: FTP requires client to authorize themselves by sending a login name and password to the server before requesting file transfers. FTP Process Model FTP allows concurrent accesses by nultiple clients. Clients use TCP to connect to the server. A master server awaits connections and creates a slave process to handleeach connection. Unlike most servers, the slave process does not perform all the necessary computation. Instead the slave accepts and handles the control connection from the client, but uses an additinal process to handle a separate data transfer connection. The control connection carries the command that tells the server which file to transfer.

Data transfer connections and the data transfer processes that use them can be created dynamically when needed, but the control connection persists throughout a session. Once the control connection disappears, the session is terminated and the software at both ends terminates all data transfer processes. In addition to passing user commands to the server, FTP uses the control connection to allow client and server processes to coordinate their use of dynamically assigned TCP protocol ports and the creation of data transfer processes that use those ports. Proxy commands - allows one to copy files from any machine to any other arbitrary machine i.e. the machine the files are being copied to need not be the client but any other machine. Sometimes some special processing can be done which is not part of the protocol. eg. if a request for copying a file is made by issuing command 'get file_A.gz' and the zipped file does not exist but the file file_A does , then the file is automatically zipped and sent. Consider what happens when the connection breaks during a FTP session. Two things may happen, certain FTP servers may again restart from the beginning and whatever portion of the file had been copied is overwritten. Other FTP servers may ask the client how much it has already read and it simply continues from that point.

TFTP: - TFTP stands for Trivial File Transfer Protocol. Many applications do not need the full functionality of FTP nor can they afford the complexity. TFTP provides an inexpensive mechanism that does not need complex interactions between the client and the server. TFTP restricts operations to simple file transfer and does not provide authentication. Diskless devices have TFTP encoded in read-only memory(ROM) and use it to obtain an initial memory image when the machine is powered on. The advantage of using TFTP is that it allows bootstrapping code to use the same underlying TCP/IP protocols. that the operating system uses once it begins execution. Thus it is possible for a computer to bootstrap from a server on another physical network. TFTP does not have a reliable stream transport service. It runs on top of UDP or any other unreliable packet delivery system using timeout and retransmission to ensure that data arrives. The sending side transmits a file in fixed size blocks and awaits acknowledgements for each block before sending the next. Rules for TFTP : - The first packet sent requests file transfer and establishes connection between server and client. Other specifications are file name and whether it is to be transferred to client or to the server. Blocks of the file are numbered starting from 1 and each data packet has a header that specifies the number of blocks it carries and each acknowledgement contains the number of the block being acknowledged. A block of less than 512 bytes signals end of file. There can be five types of TFTP packets . The initial packet must use operation codes 1 or 2 specifying either a read request or a write request and also the filename. Once the read request or write request has been made the server uses the IP address and UDP port number of the client to identify subsequent operations. Thus data or ack msgs do not contain filename. The final message type is used to report errors. TFTP supports symmetric retransmission. Each side has a timeout and retransmission. If the side sending data times out, then it retransmits the last data block. If the receiving side times out it retransmits the last acknowledgement. This ensures that transfer will not fail after a single packet loss. Problem caused by symmetric retransmission - Sorcerer's Apprentice Bug When an ack for a data packet is delayed but not lost then the sender retransmits the same data packet which the receiver acknowledges. Thus both the acks eventually arrives at the sender and the sender now transmits the next data packet once corresponding to each ack. Therefore retransmissions of all the subsequent packets are triggered. Basically the receiver will acknowledge both copies of this packet and send two acks which causes the sender in turn to send two copies of the next packet. The cycle continues with each packet being transmitted twice. TFTP supports multiple file types just like FTP i.e. binary and ascii data. TFTP may also be integrated with email . When the file type is of type mail then the FILENAME field is to be considered as the name of the mailbox and instead of writing the mail to a new file it should be appended to it. However this implementation is not commonly used.

Q19. Difference between switch and hub? Ans. The Difference between Hubs and Switches: - Ethernet hubs and switches are both used for networking workstations, file servers, printers, and many other components (each of which is referred to as a 'node' of the network). The main difference between hubs and switches is the way in which the nodes communicate with the network. There are advantages and disadvantages to both, and which one you use will depend on the size of your network, the amount of traffic that it handles, and its purpose. A Quick Tour of Ethernet Hubs: - Hubs form the heart of a network, with every separate node of the network connected to the hub through its ports. Anything from a file server to a workstation to a print server can be connected to the hub, making it accessible to every other node of the network. There are three main points to remember about hubs:  Many kinds of nodes can be connected to the hub with networking cable.  All hubs can be uplinked together, either with straight-through cable or cross-over cable, depending on whether or not the hub has an uplink port.  Performance will decrease as the number of users is increased. When choosing a hub, the main consideration should be performance. If the hub is 100Mbps and there are five users, each user will receive only 20Mbps of bandwidth. If the hub is intended for a small network with very little traffic, this should not be a problem. However, if the network is large or expected to expand, a switch is a better option in the end. When considering a hub, always remember that the network may expand in the future. Try to buy a hub that has enough ports to allow expansion without more hardware purchases. If expansion is not an immediate concern, remember that in the future you can add more hubs.

The standard hub is great for starting a small network, or providing a network to one department. There are two basic types: 10baseT, which will support a speed of 10Mbps; and 100baseTX, which supports 100Mbps. A standard 10baseT hub cannot connect to hardware that runs at 100Mbps unless a switch or hub with auto-sensing capabilities is used between them. If your network is small and will not require expansion in the near future, a standard hub is the perfect solution. Many hubs have one or more of the following extra features, which can increase ease of use, efficiency, and expandability:  Auto-sensing or dual-speed 10/100Mbps. Hubs with this feature can support hardware running at both speeds, increasing the length of time you can use your old 10baseT hardware.  Stackable hubs. Hubs with this feature are very expandable, operating as a single hub when stacked together. While standard hubs can only be uplinked through four hubs, a stacked hub is considered a single hub and there is no up linking required.  SNMP (Simple Network Management Protocol). Hubs that support SNMP allow configuration of the hub from anywhere on the network. This feature can be extremely useful in LANs that span two or more floors, or have more than 50 users. Final note on hubs: Always remember that hubs can only communicate in half duplex mode, which means that a computer on the network can only send data when it is not receiving. If the network requires full duplex mode, use a switch rather than a hub. Ethernet Switches: - Switches have many features that make them different than hubs. The most compelling reason to choose a switch rather than a hub is bandwidth. When a 100Mbps hub has five workstations, each receives 20Mbps of bandwidth. When a 10/100Mbps switch is used, each workstation receives 100Mbps of bandwidth, dramatically increasing the speed of the connection. Switches also run in full duplex mode, which allows data to be sent and received across the network at the same time. Switches effectively double the speed of the network when compared to hubs, which only sup - port half duplex mode. A 10/100Mbps switch can also support hardware running at either 10Mbps or 100Mbps, allowing the continued use of older technology and delaying replacement. Switches will increase the speed and efficiency of networks in any of the following situations:  Any network that calls for a 10/100 hub will benefit from a 10/100Mbps switch. The switch will increase available bandwidth, drastically increasing the speed that the network is running at.  Any network that requires enhanced performance for file servers, workstations, Web servers, etc. Any critical components should be connected directly to a 10/100Mbps switch.  Any network that uses high-speed applications including multimedia and video. Any workstation or file server using the intensive applications should be connected directly to a 10/100Mbps switch.  Any network that uses Fiber optic cabling should use a 10/100Mbps switch rather than a hub. A hub will not take full advantage of the speed possible with Fiber optic cabling.

Q20. What is Multiplexing techinique? Explain it in detail with there types? Ans. MULTIPLEXING: - Multiplexing technology is used on computer networks and especially over wide area network (WAN) exchanges. On computer networks, different carrier frequencies enable the use of multiple, simultaneous computer conversations over the same transmission media. When different carrier frequencies are used to carry different signals, they can use the same transmission media without interfering with each other.It use the concept of MUX & Demux(A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX).)

There are three types of multiplexing:-  Frequency division multiplexing  Wave division multiplexing.  Time division multiplexing.  Code division multiplexing.

Frequency Division Multiplexing:- In FDM, multiple channels are combined onto a single aggregate signal for transmission. The channels are separated in the aggregate by their frequency. There are always some unused frequency spaces between channels, known as guard bands. These guard bands reduce the effects of overlapping between the adjacent channels and therefore reduce a condition more commonly referred to as crosstalk.

Time-division multiplexing:- Time-division multiplexing (TDM) is a type of digital multiplexing in which two or more signals seems to be transferred simultaneously as sub-channels in one communication channel, but physically are taking turns on the channel. The time domain is divided into several regular timeslots of fixed length, one for each sub-channel.

TDM can be implemented in two ways: Synchronous TDM or Asynchronous TDM. Synchronous TDM:-Here synchronous means that the multiplexer allocates exactly the same time slot to each device at all times. Whether or not to a device has anything to transmit. Time slot A, for example, is assigned to device A alone and cannot be used by any other device. Each time its allocated time slot comes up, a device has the opportunity to send a portion of its data. If a device is unable to transmit or does not have data to send, its time slot remains empty which is the wastage of bandwidth and time. Asynchronous TDM:-Asynchronous TDM or Statistical TDM is designed to avoid this type of waste. As with the term synchronous, the term asynchronous means something different in multiplexing than it means in other areas of data communication. Here it means flexible or not fixed. In STDM time is allocated to lines only when it is required. This is achieved with the use of intelligent devices. Wavelength division multiplexing:- In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths light to carry different signals. This allows for a multiplication in capacity, in addition to making it possible to perform bidirectional communications over one strand of fiber.

Code Division Multiplexing:- Code Division Multiplexing (CDM) is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses. Code Division Multiplexing assigns each channel its own code to separate them from each other. These unique codes when decoded, restore the original desired signal. CDM is widely used in second-generation(2G) and third-generation (3G) wireless communication. This technology is used in ultra-high-frequency cellular telephone systems in the 800-MHz and 1.9-MHz bands.

Prepared By: - Vaishnoo Maa Computers, SCO 145, Chotti Baradari, Patiala. Ph. 0175-2205100, 2215100