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Data Link Layer Chapter 5: Data Link Layer Abdullah Konak School of Information Sciences and Technology Penn State Berks Learning Objectives Understand the functions of the data link layer. Understand the concept of the hop-to-hop delivery compared to host-to-host delivery and application-to-application delivery. Understand the concept of access method and define different access methods used in LANs and WANs . Understand how error control is handled at the data link layer. Understand the addressing mechanism used in the data link layer and how network layer addresses are mapped to data layer addresses. A. Konak IST 220/Ch5: Data link layer 2 1 Data Link Layer Hop-to-Hop Delivery TheThe Data Data link link layer layer providesprovides hop-to-hop hop-to-hop Server deliverydelivery in in a a closed closed network. Application: network. Web Server Client Application: Web browser Data Link Layer A. Konak IST 220/Ch5: Data link layer 3 Hop-to-hop delivery Hop-to-hop delivery means Hop-to-hop delivery means delivery of packets from the delivery of packets from the Hop-to-hop delivery means host's network interface card one router interface to the delivery of packets from the (NIC) to the router's interface other router's interface. one router interface to the host's NIC. Host Hop-to-hop delivery means delivery of packets from one A. Konakhost's network interface card IST 220/Ch5: Data link layer 4 (NIC) to the other’s NIC 2 Functions of the Data Link Layer Addressing Packetizing Flow control Error Detection and Control Medium Access Control A. Konak IST 220/Ch5: Data link layer 5 Addressing In hop-to-hop delivery, physical addresses of the devices are used to identify devices. The format of a physical address depends on the network type. For example, Ethernet LAN technology use 48-bit (6- byte) addresses. Each Ethernet network device has a Network Interface Card (NIC) with its own unique hardware address that is normally written in hexadecimal notation An Ethernet Physical Address in Hexadecimal notation Ethernet Broadcast Address 06-01-02-01-2C-4B FF-FF-FF-FF-FF-FF A. Konak IST 220/Ch5: Data link layer 6 3 Address Resolution In the data link layer (hop-to-hop delivery), physical addresses are used to identify devices, but the IP header carries information only about IP address (network layer address). The Address Resolution Protocol (ARP) is responsible for finding the physical address of a given IP address in a LAN. A. Konak IST 220/Ch5: Data link layer 7 ARP Operation IP=192.168.0.2 IP=192.168.0.3 IP=192.168.0.4 IP=192.168.0.5 MAC=12-23-EF-78-50-00 MAC=12-23-EF-78-20-30 MAC=12-23-EF-50-21-05 MAC=12-23-EF-53-31-54 A B C D ARP Request Destination IP=192.168.0.5 Source IP=192.168.0.2 Destination MAC=FF-FF-FF-FF-FF-FF Source MAC=12-23-EF-78-50-00 LookingLooking for for the the physical physical addressaddress of of a a node node with with IP IP addressaddress of of 192.168.0.5 192.168.0.5 A B C D ARP Response Destination IP=192.168.0.2 TheThe node node physical physical address address is is 12-23-EF-53-31-54 Source IP=192.168.05 A. Konak12-23-EF-53-31-54 Destination IST 220/Ch5: MAC=12-23-EF-78-50-00 Data link layer 8 Source MAC=12-23-EF-53-31-54 4 Paketizing (Framing) Transport Layer Data Link Data Link Header Header Trailer IP Header (TCP or UDP) Preamble Field, Source & Destination Cyclical Source & Destination port Data Source & Destination IP Addresses, Protocol, Redundancy Numbers , Sequence Physical Addresses, Time to live ..... Check Number Length of the data, number, Acknowledgment (used for error protocol..... number, window size, detection)..... checksum....... Segment Packet Packet or Datagram Frame A. Konak IST 220/Ch5: Data link layer 9 Error Control Data can be corrupted during transmission. For reliable communication, errors must be prevented, or detected and corrected. Source of Errors & How to prevent them: White noise Impulse noise Crosstalk Echo Jitter Attenuation Distortion A. Konak IST 220/Ch5: Data link layer 10 5 Error Types In a single-bit error, only 1 bit in the data unit has changed. A burst error means that two or more bits in the data unit have changed A. Konak IST 220/Ch5: Data link layer 11 Error Detection Methods Error Detection Methods Using Redundancy Check Vertical Redundancy Longitudinal Redundancy Cyclic Redundancy Check (VRC) Check (LRC) Check (CRC) Data Bits Redundant Bits A. Konak IST 220/Ch5: Data link layer 12 6 Vertical Redundancy Check (Parity Check) In vertical redundancy check (VRC), a parity bit is added to every data unit so that the total number of 1s becomes even. Data unit Even Parity Generator (Sender) Even Parity Checker (Receiver) 1 Sum of bits is Sum of bits is an even Data unit Parity bit No an even number? number? Yes No Yes Bits have not changed 0 Bits have changed. A. KonakRetransmit IST 220/Ch5: Data link layer 13 Longitudinal Redundancy Check (LRC) A. Konak IST 220/Ch5: Data link layer 14 7 Cyclic Redundancy Check (CRC) A. Konak IST 220/Ch5: Data link layer 15 Error Correction Error Correction Using Retransmission If the sender has not receive positive acknowledgment in due time, the frame is retransmitted. Stop-and-Wait Automatic Repeat Request (ARQ) Sliding-Window ARQ Forward Error Correction The receiver detects and fix errors without any retransmission. Usually requires complex algorithms and a lot redundancy. Therefore, it is used when retransmission requires long delays such as in satellite transmission. A. Konak IST 220/Ch5: Data link layer 16 8 Flow Control and Error Correction Using Retransmission & Positive Acknowledgment Stop and Wait Automatic Repeat Request Sliding-Window ARQ (ARQ) A. Konak IST 220/Ch5: Data link layer 17 Medium Access Methods MediumMedium AccessAccess MethodsMethods ControlledControlled RandomRandom (CSMA/CD) (CSMA/CD) (CSMA/CA)(CSMA/CA) Poll/SelectPoll/Select CarrierCarrier Sense Sense CarrierCarrier Sense Sense Token/RingToken/Ring Multiple Access (Mainframe(Mainframe Multiple Access MultipleMultiple Access Access (Token/Ring(Token/Ring DummyDummy Terminal Terminal withwith Collision Collision withwith Collision Collision LAN)LAN) Detection Communication)Communication) Detection AvoidanceAvoidance (Ethernet LAN) (Ethernet LAN) (Wireless(Wireless LAN) LAN) A. Konak IST 220/Ch5: Data link layer 18 9 Poll (Select/Poll) Poll procedure is used to receive data from secondary computers. Secondary Secondary Secondary A B C Mainframe (Primary) 1) A ! Do you need data Poll transmission? 2) No. Thank you! NAK 3) B! Do you need data Poll transmission? 4) Not this time. NAK 5) C! Do you need data Poll transmission? 6) Yes. Here is my data. Data A. Konak7) Ok. I got it. ACK IST 220/Ch5: Data link layer 19 Select (Select/Poll) Select procedure is used to send data to secondary computers. Secondary Secondary Secondary A B C Mainframe (Primary) 1) B! I have got something for you? Are you available to Select accept data. 2) Yes. I am. ACK 3) Here you go. Data 4) I got it. Thank you very much. ACK A. Konak IST 220/Ch5: Data link layer 20 10 Token Ring B B B Token D D D D 1 A 2 A A 3 D D C B C C B (2) Comp. A detects and seizes (1) Comp. A whishes to begin (3) Comp. B wants to initialize a the token. Then, it begins to transition, but it can't since it does transmitting. However, it can't transmit. since it does not have a token. not have the token. Comp D receives the frame and copies it. B B (4) When Comp. D A detects its D (5) Now, Comp returning frame, A 4 D C seizes the 5 it releases the token, and it can A token. begin to B transmit. B C A. Konak IST 220/Ch5: Data link layer C 21 CSMA/CD Algorithm Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a simple protocol used in Ethernet LANs to control the usage of the common medium by the stations on the same network. Listen the Start medium YESIs the NO Continue to Wait a random Transmit medium listen the amount of time immediately idle? medium YES NO Collision detected? Continue to listen Immediately medium for cease Transmission transmitting the next transmitting is successful. frame A. Konak IST 220/Ch5: Data link layer 22 11 CSMA/CD Operation A B C D At time t0, station A has a frame addressed D to transmit. It listens the bus medium, and it begins transmitting the frame since the bus is empty. A B C D At t1, stations B and C are ready to transmit. B senses a transmission and so defers. C, however, begins transmitting since it is still unaware of A's transmission. A B C D When A's transmission reaches C, at t3, C detects the collision and cease transmission. However, A is still unaware of the collision and continues transmitting. A B C D At t4, C's transmission reaches A, and finally, A detects the collision and ceases A. Konak IST 220/Ch5: Data link layertransmission as soon as possible. 23 12.
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