UNIT-III DATALINK LAYER: ERROR DETECTION AND CORRECTION

Error-Correcting Codes

Error control in computer networks is typically achieved by error detecting using cyclic redundancy check (CRC), which is the checksum inside a frame, combined with retransmission

For this purpose, systematic cyclic or polynomial generated codes are often used, which are special linear block codesSome basic concepts in error control coding are first reviewedAn n-bit frame, which consists of m-bit data and r check bits, is called a codeword. AllCode words form the codebook

Hamming distance between two code words is number of bits in which two code words differ

For example, 10001001 and 10110001 have a Hamming distance of 3, where is bit-wise exclusive OR (modulo 2)

The minimum Hamming distance d min of a codebook is the smallest Hamming distance between any pair of code words in the codebook

A codebook with dmin can detect up to dmin − 1 errors and correct up to (dmin − 1)/2 errors in each codeword.

Use of a Hamming code to correct burst errors

Hamming codes can only correct single errors. However, there is a trick that can be used to permit Hamming codes to correct burst errors. A sequence of k consecutive codewords is arranged as a matrix, one codeword per row. Normally, the data would be transmitted one codeword at a time, from left to right. To correct burst errors, the data should be transmitted one column at a time, starting with the leftmost column. When all k bits have been sent, the second column is sent, and so on.

2.6.2 Error-Detecting Codes

Error detection is most commonly realized using a suitable hash function (or checksum algorithm). A hash function adds a fixed-length tag to a message, which enables receivers to verify the delivered message by recomputing the tag and comparing it with the one provided.

There exists a vast variety of different hash function designs. However, some are of particularly widespread use because of either their simplicity or their suitability for detecting certain kinds of errors

A repetition code is a coding scheme that repeats the bits across a channel to achieve error-free communication. Given a stream of data to be transmitted, the data is divided into blocks of bits. Each block is transmitted some predetermined number of times. For example, to send the bit pattern "1011", the four-bit block can be repeated three times, thus producing "1011 1011 1011". However, if this twelve-bit pattern was received as "1010 1011 1011" – where the first block is unlike the other two – it can be determined that an error has occurred.

A parity bit is a bit that is added to a group of source bits to ensure that the number of set bits (i.e., bits with value 1) in the outcome is even or odd. It is a very simple scheme that can be used to detect single or any other odd number (i.e., three, five, etc.) of errors in the output. An even number of flipped bits will make the parity bit appear correct even though the data is erroneous.

A checksum of a message is a modular arithmetic sum of message code words of a fixed word length (e.g., byte values). The sum may be negated by means of a ones'- complement operation prior to transmission to detect errors resulting in all-zero messages.

A cyclic redundancy check (CRC) is a single-burst-error-detecting cyclic code and non-secure hash function designed to detect accidental changes to digital data in computer networks. It is not suitable for detecting maliciously introduced errors. It is characterized by specification of a so-called generator polynomial, which is used as the divisor in a polynomial long division over a finite field, taking the input data as the dividend, and where the remainder becomes the result.

ELEMENTARY DATA LINK PROTOCOLS

1. Physical, data link, and network layer are independent processes that communicate by passing messages - Each layer can operate simultaneously with the other. 2. A reliable, connection-oriented service between machines is desired - Essentially, the network layers of two machines can communicate on the assumption that whatever sent is received correctly (i.e. same sequence, no duplicates, no other errors). 3. The sender always has an infinite supply of data to send - When the data link layer of the sender asks for data, the network layer always has some available. 4. Data link assumes that the entire packet from network layer is data - When the data link layer receives a network layer packet it only need add header information for communication with the receiving machine's data link layer.

Communication between A and B machines physically occurs vertically through the network, data link, and physical layers but logically through the virtual connections between peer layers. Transmitting the message "Hello" from A to B network layer is illustrated in the following diagram, note the header information added to the message at each layer for communication with its peer.

An Unrestricted Simplex Protocol - Protocol 1

Assumptions

1. Simplex - Communication is one direction only. 2. Reliable channel - The communication channel never introduces errors (i.e. no duplicate, no missing, or damaged frames). 3. Receiver never gets behind, it can process incoming data infinitely fast (no need for flow control).

Important points of note are:

1. Independent processes - Both sender and receiver are independent processes whether running on different or the same machine. 2. Infinite execution - Both processes are assumed to start at the same time (in reality, receiver B must start before sender A to avoid missing frames sent by A), and run forever due the while(true) execution loop in each. 3. Frame - The data link layer passes frames between the physical layer. A frame contains a network layer packet. 4. Packet - The data link layer passes packets between the network layer. A packet is network layer data.

The frame sent uses only an info field containing the network packet:

A Simplex Stop-and-Wait Protocol - Protocol 2

The key assumptions are:

1. Simplex - Communication is one direction only. 2. Reliable channel - The communication channel never introduces errors (i.e. no duplicate, no missing, or damaged frames). 3. The most unrealistic assumption that is dropped:  Receiver never gets behind, it can process incoming data infinitely fast.

The main problem then is to prevent the sender from overwhelming the receiver, this is commonly known as flow control. A simple solution is to require that the sender never transmits a new frame until the receiver has confirmed it has processed the previous frame. Data is still only sent in one direction but an empty frame is transmitted by the receiver as confirmation requiring the sender to wait before sending another data frame.

sender frame uses only the info field containing the network packet:

receiver frame sent is empty:

Improvements:

1. Flow control by requiring sender to wait for a confirmation from receiver.

Problems:

1. Deadlock when either transmit or confirmation frame lost. No way to stop waiting on either sender2 or receiver2. 2. Delay while sender2 waits for confirmation, under-utilizing the channel.

A Simplex Protocol for a Noisy Channel - Protocol 3

The assumption is:

 Simplex - Data communication is one direction only.

The assumption dropped:

 Reliable channel - The communication channel never introduces errors (i.e. no duplicate, no missing, or damaged frames). Deadlock was possible in Protocol 2 over an unreliable channel where lost frames can occur. Protocol 3 does not deadlock in the face of errors but does still force the sender to wait until a confirmation arrives. There are two essential implications:

1. After a fixed wait the sender will timeout and retransmit the data. This avoids deadlock when frames are lost. 2. Each frame sent is numbered 0 or 1 and a new data frame is not sent until the previous frame is confirmed. This prevents lost or out of sequence data frames.

 Sender frame - The seq and info field are used :

 Receiver frame - The ack field is used :

Improvements:

1. No deadlock due to the use of a timer on the sender. If the sender does not receive a confirmation within the allocated time, the same frame is resent. 2. No duplicates due to the use of sequence numbers (0 and 1). If the sender receives a confirmation to a frame sequence number different than the most recently transmitted, the same frame is resent. Using a binary sequence number allows only one outstanding frame at a time.

Problems:

1. With only one outstanding frame, the utilization of the channel is not optimal (below 100%). 2. Timeout should be longer than the round trip time (the time a frame travels to the receiver and a confirmation returns). Otherwise, time wasted sending duplicate messages unnecessarily.

2.8 SLIDING WINDOW PROTOCOLS

The main problem addressed by sliding window protocols is the poor channel utilization due to the need of the sender to await confirmation to the one outstanding frame before another can be transmitted. The solution is to have several frames outstanding at a time rather than only one, the optimum number dependent upon the round trip time for the frame to travel from the sender and its acknowledgement to return. For example, if the full round trip time was 10 seconds and only a single frame was outstanding at a time, the wait would be 10 seconds for the acknowledgement to fully arrive, utilization would be only 10% for a frame of one second duration. With multiple outstanding frames and acknowledgments, for one second long frames the channel could hold 5 one second long frames from the sender and 5 acknowledgments coming back to completely fill the channel, utilization would be 100%. Utilization Examples - Consider sending frames under the stop-and-wait regime of Protocol 2.

1. Suppose all frames are 1 second long and channel time is 10 seconds. Emitting a frame is 1 second, transmit time 10 seconds before the last bit reaches the receiver, 1 second to emit an acknowledgement, and 10 seconds for the last acknowledgement bit to reach the sender. The total time for sending a 1 second frame and receiving the acknowledgement would be 22 seconds, the utilization would be 1 second/22 seconds or about 4.5%. 2. Suppose that a long frame were sent and an equal size acknowledgement, one that filled the channel completely or 10 seconds. Then one frame takes 10 seconds to emit, 10 seconds before the last bit is received, 10 seconds to emit acknowledgement, and 10 seconds before the last acknowledgement bit reaches the sender. The total time between sending frames is 40 seconds, the utilization would be 10 seconds/40 seconds or 25%. In general, sending longer frames improves utilization. 3. Suppose that a really long frame were sent, a 5 second long frame on a channel with propagation time of 1 second. The first sender bit would arrive at the receiver after 1 second. The last bit of the sent frame would arrive after 6 seconds. If the receiver instantly transmitted a one bit acknowledgement it would arrive at the sender 7 seconds after it had sent the first bit. To send 5 seconds of data requires 7 seconds. Utilization would be 5/7 or about 71%.

Sender Window

 As in Protocol 3, a timer determines when to retransmit unacknowledged frames.  The sender must be able to buffer all unacknowledged frames transmitted before a timeout occurs, the number of frames buffered is termed the sender window size. Ideally, the timeout is slightly longer than the round trip time to prevent resending a frame that is about to be acknowledged. Some sliding window implementations (e.g. TCP/IP) modify the window size dynamically making it smaller when timeouts occur so fewer frames are outstanding.

Receiver Window

 The receiver must ensure that its network layer is passed data in the same sequence passed by the sender's network layer.  The sender and receiver window do not need to be the same size but for optimal performance, the number of receiver buffers should be at least one larger than the sender window. Optimal performance under errors occurs when only the lost frame is resent.

A 1-bit Sliding Window Protocol - Protocol 4 Assumptions:

 Still assume that network layer always has data available to send.

The protocol is symmetric so both ends are sender and receiver. Both ends must have data to send before acknowledging data received. Obviously this must be ignored for the first frame numbered 0 on each end when both must send a bogus confirmation.

 Sender frame - The seq, ack and info field are used :

 Receiver frame - The seq, ack and info field are used :

Improvements:

1. Data is bi-directional. 2. Sender and receiver are symmetrical.

Problems:

1. With only one outstanding frame, the utilization of the channel is not optimal (below 100%). 2. Timeout should be slightly longer than the round trip time (the time a frame travels to the receiver and a confirmation returns). Otherwise, time wasted sending duplicate messages unnecessarily. 3. As the above tables indicate, the two processes continue to alternate sending duplicates.

Go Back N Example: The following diagram illustrates a go back n Sliding Window Protocol when the receiver window is size 1 and a sender window of 4. The sender has a timeout interval of 3 so must at least buffer three unacknowledged frames and the one frame in error. The arrows point to the time interval when a frame is received. When errors occur, the receiver discards all frames received until the frame with the sequence number following the last correctly received frame number arrives. The sender must go back to the oldest unacknowledged frame and start resending all subsequent frames. If the sender's timeout interval is 3 frames long allowing 4 frames to be unacknowledged, all 4 frames will necessarily need to be resent in the face of an error.

1. Each frame has a timer. 2. Sender A sends frame 0, 1, 2, and 3 starting a timer for each after frame completely sent. 3. Sender A receives an acknowledgement of frame 0 before the 3 frames long timeout occurs. Timeout would occur at the end of sending frame 3 if frame 0 not acknowledged. 4. Frame 1 is not received so no acknowledgement is returned. A timeout occurs for frame 1 on the sender after frame 4 is completely sent. 5. Receiver B discards frames 2-4 as they arrive because frame 1 must be delivered to the network layer first and the receiver window holds only 1 frame. 6. Sender A resends unacknowledged frames 1-4 as the timeout occurs for each. 7. Frame 5 is sent after an acknowledgement of frame 2 is received since the timeout interval is 3 frames long and 3-5 is within the interval. 8. GoBackN

Selective Repeat Example: The following diagram illustrates a selective repeat Sliding Window Protocol when the receiver window size is large, able to buffer at least 4 frames in the case where the sender timeout interval is 3 frames (4 frame buffer required for the 3 frames sent during timeout interval + 1 frame being transmitted). The arrows point to the time interval when a frame is received.

1. A sends frame 0, 1, 2, and 3 starting a timer for each after being completely sent. 2. A receives an acknowledgement of frame 0 before the 3 frame long timeout occurs so sends frame 4. 3. Frame 1 is not received so frame 0 acknowledgement (i.e. last correctly received frame) is returned with data from B and a sender timeout occurs for frame 1 on A. 4. B buffers frame 2-4 because frame 1 must be delivered to the network layer first. 5. A resends unacknowledged frame 1. 6. B receives frame 1 and acknowledges frames 1-4 by acknowledging frame 4. 7. Selective Repeat with 1 error and 4 buffers

Even though frame 1 timed out and must be retransmitted, the text algorithm optimistically assumes that frames 2, 3, etc. will be acknowledged later. This is only possible where ample buffers are available on the sender.

1. A sends frame 0, 1, 2, and 3 starting a timer for each after being completely sent. 2. A receives an acknowledgement of frame 0 before the 3 frame long timeout occurs so sends frame 4. 3. Frame 1 is not received so frame 0 acknowledgement (i.e. last correctly received frame) is returned with data from B and a sender timeout occurs for frame 1 on A. 4. B buffers frame 2-4 because frame 1 must be delivered to the network layer first. 5. A resends unacknowledged frame 1 followed by frame 5 and resets the timers on 1-4. 6. B receives frame 1 and acknowledges frames 1-4 when acknowledging frame 4. Delivers frames 1-4 to network layer. 7. Ack 4 arrives at A before frame 1 timeout. 8. B receives, acknowledges and delivers to network layer frame 5, 6, 7, ...

Selective Repeat with 1 error and more than 4 buffers on Sender and Reciever

Selective repeat with 2 errors and more than 4 buffers

Improvements:

Piggyback data - Data is always sent along with an acknowledgement, possible only because of assumption that network layer always has data available.

Bidirectional - No longer simplex data.

Symmetric - The same algorithm runs on both processes. Problem is how to start both at same instant without sending duplicates or acknowledging frame not yet received.

Problems:

Blocks - Blocks receive network layer when send network layer has no data. It is important to note that go back n and selective repeat have the same utilization characteristics when no errors, the sender's window size determines the utilization when errors. If the sender has sufficient buffers to hold all outstanding frames, utilization can be 100%. They differ in utilization when errors occur and frames must be resent.

Sender Window Size Calculation - The sender window defines the frames sent but not acknowledged (i.e. frames buffered before timeout occurs). How large a window size is required at a minimum to buffer outstanding frames on the sender? The relevant parameters usually known are number of bits per frame, transmission rate, propagation rate, and distance. Consider the following:  Frame size - 1000 bits  Transmission rate - 1 Mbps  Distance - 1000 km. one way  Propagation rate - 20 msec/km.

MEDIUM ACCESS CONTROL SUB LAYER

The Media Access Control (MAC) data communication protocol sub-layer, also known as the , is a sublayer of the Data Link Layer specified in the seven-layer OSI model (layer 2). It provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multi-point network, typically a local area network (LAN) or metropolitan area network (MAN). The hardware that implements the MAC is referred to as a Medium Access Controller.

The MAC sub-layer acts as an interface between the Logical Link Control (LLC) sublayer and the network's physical layer. The MAC layer emulates a full-duplex logical communication channel in a multi-point network. This channel may provide unicast, multicast or broadcast communication service.

MULTIPLE ACCESS PROTOCOLS

The channel access control mechanisms provided by the MAC layer are also known as a multiple access protocol. This makes it possible for several stations connected to the same physical medium to share it. Examples of shared physical media are bus networks, ring networks, hub networks, wireless networks and half-duplex point-to- point links. The multiple access protocol may detect or avoid data packet collisions if a packet mode contention based is used, or reserve resources to establish a logical channel if a circuit switched or channelization based channel access method is used. The channel access control mechanism relies on a physical layer multiplex scheme.

The most widespread multiple access protocol is the contention based CSMA/CD protocol used in Ethernet networks. This mechanism is only utilized within a network , for example an Ethernet bus network or a hub network. An Ethernet network may be divided into several collision domains, interconnected by bridges and switches.

A multiple access protocol is not required in a switched full-duplex network, such as today's switched Ethernet networks, but is often available in the equipment for compatibility reasons. Examples of common packet mode multiple access protocols for wired multi-drop networks are:

 CSMA/CD (used in Ethernet and IEEE 802.3)  Token bus (IEEE 802.4)  (IEEE 802.5)  Token passing (used in FDDI)

Examples of common multiple access protocols that may be used in packet radio wireless networks are:

 CSMA/CA (used in IEEE 802.11/WiFi WLANs)  Slotted ALOHA  Dynamic TDMA  Reservation ALOHA (R-ALOHA)  Mobile Slotted ALOHA (MS-ALOHA)  CDMA  OFDMA

ETHERNET

Definition: Ethernet is a physical and data link layer technology for local area networks (LANs). Ethernet was invented by engineer Robert Metcalfe.

When first widely deployed in the 1980s, Ethernet supported a maximum theoretical data rate of 10 megabits per second (Mbps). Later, so-called "Fast Ethernet" standards increased this maximum data rate to 100 Mbps. Today, Gigabit Ethernet technology further extends peak performance up to 1000 Mbps.

Higher level network protocols like Internet Protocol (IP) use Ethernet as their transmission medium. Data travels over Ethernet inside protocol units called frames.

The run length of individual Ethernet cables is limited to roughly 100 meters, but Ethernet networks can be easily extended to link entire schools or office buildings using network bridge devices. thernet is the most widely-installed local area network ( LAN) technology. Specified in a standard, IEEE 802.3, Ethernet was originally developed by Xerox from an earlier specification called Alohanet (for the Palo Alto Research Center Aloha network) and then developed further by Xerox, DEC, and Intel. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. Ethernet is also used in wireless

LANs. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD ) protocol. Fast Ethernet or 100BASE-T provides transmission speeds up to 100 megabits per second and is typically used for LAN backbone systems, supporting workstations with 10BASE-T cards. Gigabit Ethernet provides an even higher level of backbone support at 1000 megabits per second (1 gigabit or 1 billion bits per second). 10-Gigabit Ethernet provides up to 10 billion bits per second.

Ethernet was named by Robert Metcalfe, one of its developers, for the passive substance called "luminiferous (light-transmitting) ether" that was once thought to pervade the universe, carrying light throughout. Ethernet was so- named to describe the way that cabling, also a passive medium, could similarly carry data everywhere throughout the network.

The early development of Ethernet was done by Xerox research. The name "Ethernet" was a registered trademark of Xerox Corporation. The technology was refined and a second generation called Ethernet II was widely used. Ethernet from this period is often called DIX after its corporate sponsors Digital, Intel, and Xerox. As the holder of the trademark, Xerox established and published the standards.

Obviously, no technology could become an international standard for all sorts of equipment if the rules were controlled by a single US corporation. The IEEE was assigned the task of developing formal international standards for all Local Area Network technology. It formed the "802" committee to look at Ethernet, Token Ring, Fiber Optic, and other LAN technology. The objective of the project was not just to standardize each LAN individually, but also to establish rules that would be global to all types of LANs so that data could easily move from Ethernet to Token Ring or Fiber Optics.

This larger view created conflicts with the existing practice under the old Xerox DIX system. The IEEE was careful to separate the new and old rules. It recognized that there would be a period when old DIX messages and new IEEE 802 messages would have to coexist on the same LAN. It published a set of standards of which the most important are:

 802.3 - Hardware standards for Ethernet cards and cables  802.5 - Hardware standards for Token Ring cards and cables  802.2 - The new message format for data on any LAN

The 802.3 standard further refined the electrical connection to the Ethernet. It was immediately adopted by all the hardware vendors. Today all cards and other devices conform to this standard.

However, the 802.2 standard would require a change to the network architecture of all existing Ethernet users. Apple had to change its Ethertalk, and did so when converting from Phase 1 to Phase 2 Appletalk. DEC had to change its DECNET. Novell added 802 as an option to its IPX, but it supports both DIX and 802 message formats at the same time. The TCP/IP protocol used by the Internet refused to change. Internet standards are managed by the IETF group, and they decided to stick with the old DIX message format indefinitely. This produced a deadlock between two standards organizations that has not been resolved.

IBM waited until the 802 committee released its standards, then rigorously implemented the 802 rules for everything except TCP/IP where the IETF rules take precedence. This means that NETBEUI (the format for NETBIOS on the LAN) and SNA obey the 802 conventions.

So "Ethernet" suffers from too many standards. The old DIX rules for message format persist for some uses (Internet, DECNET, some Novell). The new 802 rules apply to other traffic (SNA, NETBEUI). The most pressing problem is to make sure that Novell clients and servers are configured to use the same frame format.

Access and Collisions

Ethernet uses a protocol called CSMACD. This stands for "Carrier Sense, Multiple Access, Collision Detect". The "Multiple Access" part means that every station is connected to a single copper wire (or a set of wires that are connected together to form a single data path). The "Carrier Sense" part says that before transmitting data, a station checks the wire to see if any other station is already sending something. If the LAN appears to be idle, then the station can begin to send data.

An Ethernet station sends data at a rate of 10 megabits per second. That bit allows 100 nanoseconds per bit. Light and electricity travel about one foot in a nanosecond. Therefore, after the electric signal for the first bit has traveled about 100 feet down the wire, the station has begun to send the second bit. However, an Ethernet cable can run for hundreds of feet. If two stations are located, say, 250 feet apart on the same cable, and both begin transmitting at the same time, then they will be in the middle of the third bit before the signal from each reaches the other station.

This explains the need for the "Collision Detect" part. Two stations can begin to send data at the same time, and their signals will "collide" nanoseconds later. When such a collision occurs, the two stations stop transmitting, "back off", and try again later after a randomly chosen delay period.

While an Ethernet can be built using one common signal wire, such an arrangement is not flexible enough to wire most buildings. Unlike an ordinary telephone circuit, Ethernet wire cannot be just spliced together, connecting one copper wire to another. Ethernet requires a repeater. A repeater is a simple station that is connected to two wires. Any data that it receives on one wire it repeats bit-for-bit on the other wire. When collisions occur, it repeats the collision as well.

In common practice, repeaters are used to convert the Ethernet signal from one type of wire to another. In particular, when the connection to the desktop uses ordinary telephone wire, the hub back in the telephone closet contains a repeater for every phone circuit. Any data coming down any phone line is copied onto the main Ethernet coax cable, and any data from the main cable is duplicated and transmitted down every phone line. The repeaters in the hub electrically isolate each phone circuit, which is necessary if a 10 megabit signal is going to be carried 300 feet on ordinary wire.

Every set of rules is best understood by characterizing its worst case. The worst case for Ethernet starts when a PC at the extreme end of one wire begins sending data. The electric signal passes down the wire through repeaters, and just before it gets to the last station at the other end of the LAN, that station (hearing nothing and thinking that the LAN is idle) begins to transmit its own data. A collision occurs. The second station recognizes this immediately, but the first station will not detect it until the collision signal retraces the first path all the way back through the LAN to its starting point.

Any system based on collision detect must control the time required for the worst round trip through the LAN. As the term "Ethernet" is commonly defined, this round trip is limited to 50 microseconds (millionths of a second). At a signaling speed of 10 million bits per second, this is enough time to transmit 500 bits. At 8 bits per byte, this is slightly less than 64 bytes.

To make sure that the collision is recognized, Ethernet requires that a station must continue transmitting until the 50 microsecond period has ended. If the station has less than 64 bytes of data to send, then it must pad the data by adding zeros at the end.

In simpler days, when Ethernet was dominated by heavy duty coax cable, it was possible to translate the 50 millisecond limit and other electrical restrictions into rules about cable length, number of stations, and number of repeaters. However, by adding new media (such as Fiber Optic cable) and smarter electronics, it becomes difficult to state physical distance limits with precision. However those limits work out, they are ultimately reflections of the constraint on the worst case round trip.

It would be possible to define some other Ethernet-like collision system with a 40 microsecond or 60 microsecond period. Changing the period, the speed, and the minimum message size simply require a new standard and some alternate equipment. AT& T, for example, once promoted a system called "Starlan" that transmitted data a 1 megabit per second over older phone wire. Many such systems are possible, but the term "Ethernet" is generally reserved for a system that transmits 10 megabits per second with a round trip delay of 50 microseconds.

To extend the LAN farther than the 50 microsecond limit will permit, one needs a bridge or router. These terms are often confused:

 A repeater receives and then immediately retransmits each bit. It has no memory and does not depend on any particular protocol. It duplicates everything, including the collisions.  A bridge receives the entire message into memory. If the message was damaged by a collision or noise, then it is discarded. If the bridge knows that the message was being sent between two stations on the same cable, then it discards it. Otherwise, the message is queued up and will be retransmitted on another Ethernet cable. The bridge has no address. Its actions are transparent to the client and server workstations.  A router acts as an agent to receive and forward messages. The router has an address and is known to the client or server machines. Typically, machines directly send messages to each other when they are on the same cable, and they send the router messages addressed to another zone, department, or subnetwork. Routing is a function specific to each protocol. For IPX, the Novell server can act as a router. For SNA, an APPN Network Node does the routing. TCP/IP can be routed by dedicated devices, UNIX workstations, or OS/2 servers.

BROAD BAND WIRELESS

According to the 802.16-2004 standard, broadband means "having instantaneous bandwidths greater than 1 MHz and supporting data rates greater than about 1.5 Mbit/s."[1] Wireless Broadband networks feature speeds roughly equivalent to wired broadband networks, such as that of ADSL or a cable modem. Wireless Broadband networks can also be Symmetrical, meaning bandwidth traveling in both directions (download/upload), which is most commonly associated with 'Fixed Wireless' networks. A Fixed link is a stationary terrestrial wireless connection (similar to satellite, but with far superior speeds), rather than mobile, which allows for greater speeds.

Technology and speeds A typical WISP Customer Premises Equipment (CPE) installed on a residence.

Few Wireless Internet Service Providers (WISPs) provide download speeds of over 100 Mbit/s; most broadband wireless access services are estimated to have a range of 50 km (30 miles) from a tower.[2] Technologies used include LMDS and MMDS, as well as heavy use of the ISM bands and one particular access technology is being standardized by IEEE 802.16, also known as WiMAX. WiMAX is highly popular in Europe but has not met full acceptance in the United States because cost of deployment does not meet return on investment figures. In 2005 the Federal Communications Commission adopted a Report and Order that revised the FCC’s rules to open the 3650 MHz band for terrestrial wireless broadband operations.[3] On November 14, 2007 the Commission released Public Notice DA 07-4605 in which the Wireless Telecommunications Bureau announced the start date for licensing and registration process for the 3650–3700 MHz band.[4] Recently the FCC adopted the TV White Space Rules (TVWS) and allowed some of the better none line of sight frequency (700 MHz) into the FCC Part-15 Rules (http://www.wispa.org/?p=3146). The Wireless Internet Service Providers Association, (http://www.wispa.org) a National association of WISPs, petition the FCC and won. Because of this we will see new products on the market that will be far superior to the Wi-Fi standard, allowing for better coverage in rural areas.

Initially, WISPs were only found in rural areas not covered by cable or DSL.[5] These early WISPs would employ a high-capacity T-carrier, such as a T1 or DS3 connection, and then broadcast the signal from a high elevation, such as at the top of a water tower. To receive this type of Internet connection, consumers mount a small dish to the roof of their home or office and point it to the transmitter. Line of sight is usually necessary for WISPs operating in the 2.4 and 5 GHz bands with 900 MHz offering better NLOS (non-line-of-sight) performance.

Mobile wireless broadband

Also called Mobile Broadband, wireless broadband technologies include new services from companies such as Verizon, Sprint, and AT&T Mobility, which allow a more mobile version of this broadband access. Consumers can purchase a PC card, laptop card, or USB equipment to connect their PC or laptop to the Internet via cell phone towers. This type of connection would be stable in almost any area that could also receive a strong cell phone connection. These connections can cost more for portable convenience as well as having speed limitations in all but urban environments.[citation needed]

On June 2, 2010, after months of discussion, AT&T became the first wireless Internet provider to announce plans to charge according to usage. As the only iPhone service in the United States, AT&T experienced the problem of excess Internet use more than other providers. About 3 percent of AT&T smart phone customers account for 40 percent of the technology's use. 98 percent of the company's customers use less than 2 gigabytes (4000 page views, 10,000 emails or 200 minutes of streaming video), the limit under the $25 monthly plan, and 65 percent use less than 200 megabytes, the limit for the $15 plan. For each gigabyte in excess of the limit, customers would be charged $10 a month starting June 7, 2010, though existing customers would not be required to change from the $30 a month unlimited service plan. The new plan would become a requirement for those upgrading to the new iPhone technology later in the summer.[6]

BLUETOOTH

In 1994, the L. M. Ericsson company became interested in connecting its mobile phones to other devices (e.g., PDAs) without cables. Together with four other companies (IBM, Intel, Nokia, and Toshiba), it formed a SIG (Special Interest Group, i.e., consortium) to develop a wireless standard for interconnecting computing and communication devices and accessories using short-range, low- power, inexpensive wireless radios. The project was named Bluetooth, after Harald Blaatand (Bluetooth) II (940-981), a Viking king who unified (i.e., conquered) Denmark and Norway, also withoutcables. By all this, in July 1999 the Bluetooth SIG issued a 1500-page specification of V1.0. Shortly thereafter, the IEEE standards group looking at wireless personal area networks, 802.15, adopted the Bluetooth document as a basis and began hacking on it. The IEEE 802.15 committee is standardizing only the physical and data link layers: the rest of the protocol stack falls outside its charter. IEEE approved the first PAN standard, 802.15.1, in 2002, the Bluetooth SIG is still active busy with improvements. Although the Bluetooth SIG and IEEE versions are not identical, it is hoped that they will soon converge to a single standard. Bluetooth Architecture The basic unit of a Bluetooth system is a piconet, which consists of a master node and up to seven active slave nodes within a distance of 10 meters. Multiple piconets can exist in the same (large) room and can even be connected via a bridge node, as shown in Fig. 4-35. An interconnected collection of piconets is called a scatternet.

Bridge slave Figure 4-35. Two piconets can be connected to form a scatternet. The seven active slave nodes in a piconet, there can be up to 255 parked nodes in the net. These are devices that the master has switched to a low power state to reduce the drain on their batteries. In parked state, a device cannot do anything except respond to an activation or beacon signal from the master. There are also two intermediate power states, hold and sniff, but these will not concern us here. At its heart, a piconet is a centralized TDM system, with the master controlling the clock and determining which device gets to communicate which time slot. All communication is between the master and a slave, direct slave-slave communication is not possible. Bluetooth Applications Most network protocols just provide channels between communicating entities and let applications designers figure out what they want to use them for example, 802.11 does not specify whether users should use their notebook computers for reading e-mail, surfing the Web, or something else. In contrast, the Bluetooth V1.1 specification names 13 specific applications to be supported and provides different protocol stacks for each one. The 13 applications, which are called profiles, are listed in below

Name Description Generic access Procedure for link management. Service discovery Protocol for discovering offered services. Serial port Replacement for a serial port cable. Generic object Defines client-server relationship for object movement. exchange LAN access Protocol between a mobile computer and a fixed LAN. Dial-up networking Allows a notebook computer to call via a mobile phones. Fax Allows a mobile fax machine to talk to a mobile phone. Cordless telephony Connects a handset and its local base station. Intercom Digital walkie-talkie. Headset Allows hand-free voice communication. Object push Provides a way to exchange simple objects. File transfer Provides a more general file transfer facility Synchronization Permits a PDA to synchronize with another computer.

Repeaters, Hubs, Bridges, Switches, Routers, and Gateways We have looked at a variety of ways to get frames and packets from one cable segment to another. We have mentioned repeaters, bridges, switches, hubs, routers, and gateways. To start with, these devices operate in different layers, as illustrated in (Refer class notes Fig). The layer matters because different devices use different pieces of information to decide how to switch. In a typical scenario, the user generates some data to be sent to a remote machine. Those data are passed to the transport layer, which then adds a header, for example, a TCP header, and passes the resulting unit down to the network layer. The network layer adds its own header to form a network layer packet, for example, an IP packet. (Refer class notes fig.)We see the IP packet shaded in gray. Then the packet goes to the data link layer, which adds its own header and checksum (CRC) and gives the resulting frame to the physical layer for transmission, for example, over a LAN. Now let us look at the switching devices and see how they relate to the packets and frames. At the bottom in the physical layer, we find the repeaters. These are analog devices that are connected to two cable segments. A signal appearing on one of them is amplified and put out on the other. Repeaters do not understand frames, packets or headers. They understand volts. Classic Ethernet, for example, was designed to allow four repeaters, in order to extend the maximum cable length from 500 meters to 2500 meters. Next we come to the hubs. A hub has a number of input lines that it joins electrically. Frames arriving on any of the lines are sent out on all the others. If two frames arrive at the same time, they will collide, just as on a coaxial cable. The entire hub forms a single collision domain. All the lines coming into a hub must operate at the same speed. Hubs differ from repeaters in that they do not (usually) amplify the incoming signals and are designed to hold multiple line cards each with multiple inputs, but the differences are slight. Like repeaters, hubs do not examine the 802 addresses or use them in any way. A hub is shown in Fig. 4-47(a).

Figure 4-47. (a) A hub. (b) A bridge. (c) A switch.

Now let us move up to the data link layer where we find bridges and switches. We just studied bridges at some length. A bridge connects two or more LANs, as shown in Fig. 4-47(b). When a frame arrives, software in the bridge extracts the destination address from the frame header and looks it up in a table to see where to send the frame. For Ethernet, this address is the 48-bit destination address shown in Fig. 4-17. Like a hub, a modem bridge has line cards, usually for four input lines of a certain type. A line card for Ethernet cannot handle, say token ring frames, because it does not know where to find the destination address the frame header. A bridge may have line cards for different network types and different speeds. With a bridge, each line is its own collision domain, contrast to ahub. Switches are similar to bridges in that both route on frame addresses. Many people uses the terms interchangeably. The main difference is that a switch is most often used to connect individual computers, as shown in Fig. 4-47(c). As a consequence, when host A in Fig. 4-47(b) wants to send a frame to host B bridge gets the frame but just discards it. In contrast, in Fig. 4-47(c), the switch must actively forward the frame from A to B because there is no other way for frame to get there. Since each switch port usually goes to a single computer switches must have space for many more line cards than do bridges intended connect only LANs. Each line card provides buffer space -for frames arriving on its ports. Since each port is its own collision domain, switches never lose frames to collisions. If frames come in faster than they can be retransmitted the switch may run out of buffer space and have to start discarding frames.

Modern switches start forwarding frames as soon as the destination header field has come in, but before the rest of the frame has arrived (provided the output line is available, of course). These switches do not use store-and-forward switching. Sometimes they are referred to as cut-through switches. Usually, cut-through is handled entirely in hardware, whereas bridges traditionally contained an actual CPU that did store-and-forward switching in software. But since all modem bridges and switches contain special integrated circuits for switching, the difference between a switch and bridge is more a marketing issue than a technical one. So far we have seen repeaters and hubs, which are quite similar, as well as bridges and switches, which are also very similar to each other. Now we move up to routers, which are different from all of the above. When a packet comes into a router, the frame header and trailer are stripped off and the packet located in the frame’s payload field (shaded in Fig refer class notes) is passed to the routing software. This software uses the packet header to choose an output line. For an IP packet, the packet header will contain a 32-bit (IPv4) or 128-bit (IPv6) address, but not a 48- bit 802 address. The routing software does not see the frame addresses and does not even know whether the packet came in on a LAN or a point-to-point line. Up another layer we find transport gateways. These connect two computers that use different connection-oriented transport protocols. For example, suppose a computer using the connection-oriented TCP/IP protocol needs to talk to a computer using the connection-oriented ATM transport protocol. The transport gateway can copy the packets from one connection to the other, reformatting them as need be. Finally, application gateways understand the format and contents of the data and translate messages from one format to another. An e-mail gateway could translate Internet messages into SMS messages for mobile phones.

UNIT – III Section – A 1) Computer data are always sent in block of ______A. Bits B. Bytes C. Characters D. Codes Ans: A Bits 2) ______Codes are referred to as forward error correction. A. Hamming B. Error correction C. Error Detection D. Debugging Ans: B Error correction 3) The no. of bit positions in which two code words differ is called the ______distance. A. Hamming B. Error correction C. Error Detection D. Debugging Ans: A Hamming 4) Hamming codes can only correct ______errors. A. All Type of B. Single C. Multiple D. None Ans: B Single 5) ______code is also known as Cycle Redundancy Check. A. Hamming B. Error correction C. Error Detection D. Polynomial Ans: D Polynomial 6) Protocols deal with only ______errors. A. Computer B. Crashing C. Communication D. Rebooting Ans: C Communication 7) ______is an enumerated type and can take on the values true and false. A. Character B. Boolean C. Number D. String Ans: B Boolean 8) ______is the unit of information exchanged between the network layer and data link layer on the same machine. A. Packets B. Frames C. Messages D. All of these Ans: A Packets 9) A frame is composed of ______fields. A. 2 B. 3 C. 4 D. 5 Ans: C 4 10) ______Field tells whether there are any data in the frame. A. Info B. Kind C. Seq D. Ack. Ans: B Kind 11) The sender in a protocol runs in the data link layer of the ______machine. A. Source B. Destination C. Both A & B D. None Ans: A Source 12) The receiver in a protocol runs in the data link layer of the ______machine. A. Source B. Destination C. Both A & B D. None Ans: B Destination 13) System in which multiple users share a common channel in a way that can lead to conflicts are known as ______system. A. Crashing B. Contention C. Condition D. Collision Ans: B Contention 14) When two frames try to occupy the channel at the same time there will be a ______. A. Crashing B. Contention C. Condition D. Collision Ans: D Collision 15) The problem of a station not being able to detect a potential competitor for the medium because the competitor is too far away is called ______station problem. A. Hidden B. Exposed C. Internal D. External Ans: A Hidden 16) Protocols in which stations listen for a transmission and act according are called______protocols. A. Carrier sense B. Collision free C. Bitmapped D. None. Ans: A Carrier sense 17) ______cabling called thick Ethernet. A. 10Base2 B. 10Base5 C. 10BaseT D. 10BaseF

Ans:B 10Base5 18) Bluetooth is a wireless technology for ______. A. Local Area Network B. Wide Area Network C. Metropolitan Area Network D. Personal Area Network. Ans: D Personal Area Network 19) Frame______allows a sender to transmit a concatenated sequence of multiple frames in a single transmission. A. Bursting B. Transfer C. Mailing D. Copying Ans: A Bursting 20) Bluetooth uses______A. Frequency hopping spread spectrum B. Orthogonal frequency division multiplexing C. Time division multiplexing D. None. Ans: A Frequency hopping spread spectrum

Section – B 1) Write a short note on Error detection & correction 2) Short note on Frames. 3) Explain about ALOHA 4) Analyze pure ALOHA & slotted ALOHA 5) Explain MACA & MACAW 6) Write about Ethernet cabling. 7) Explain about Switched Ethernet. 8) Discover the difference between 802.11 with 802.16 9) Write note on Bluetooth 10) Discuss about Wireless local loop.

Section – C 1) Explain about Error correcting codes in detail. 2) Explain about Error detecting codes in detail. 3) Discuss about the elementary data link protocol in detail. 4) Outline the Sliding window protocol 5) Discuss in detail about Multiple Access protocol. 6) Explain in detail about Wireless LAN protocol 7) Discuss in detail about the Ethernet. 8) Explain about Gigabit Ethernet in detail. 9) Analyze the Broadband wireless in detail. 10) Analyze the advantages & disadvantages of the Bluetooth.