Data Link Layer

Chapter 5Data LinkLayerUnderstand the functions of the data link layer.Understand the concept of the hop-to-hop delivery compared tohost-to-host delivery and application-to-application delivery.Understand the concept of access method and define differentaccess methods used in LANs and WANs .After reading this chapter, the reader should be able to:OBJECTIVESUnderstand the addressing mechanism used in the data link layer and how network layer addresses are mapped to datalayer addresses. Understand how error control is handled at the data link layer.DUTIES OF THEDATA LINK LAYER5.1Figure 5-1Data link layer in the Internet modelFigure 5-2Data-link layer dutiesHOP-TO-HOPDELIVERY5.2Figure 5-3Hop-to-hop deliveryPACKETIZING5.3ADDRESSING5.4Technical Focus: Addresses in Local Area NetworksThe physical address for most computers on local area networks is imprinted on the network card that is installed in the computer.If the user or network manager changes the network card(because of a failure, for example), the physical address of the computer is changed. In most cases, changingthe network card requires reconfiguration of the computer. Figure 5-4ARP operationERROR CONTROL5.5Data can be corrupted during transmission. For reliable communication, errors must be prevented, or detected and corrected.Note:In a single-bit error, only 1 bit in the data unit has changed.Note:Figure 5-5Single-bit errorA burst error means that two or more bits in the data unit have changed.Note:Figure 5-6 Burst error of length fiveError detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.Note:Figure 5-7RedundancyFigure 5-8Detection methodsIn vertical redundancy check (VRC), a parity bit is added to every data unit so that the total number of 1s becomes even.Note:Figure 5-9Even parity VRC conceptFigure 5-10LRCIn longitudinal redundancy check (LRC), a block of bits is divided into rows and a redundant row of bits is added to the whole block. Note:Figure 5-11CRCFigure 5-12Stop and wait ARQTechnical Focus: Procedure for Stop-And-Wait ARQThe sending device keeps a copy of the last frame transmitteduntil it receives an acknowledgment for that frameBoth data frames and ACK frames are numbered 0 and 1 alternately. A data 0 frame is acknowledgedby an ACK 1 frame.If an error is discovered in a data frame, a negative acknowledgment (NAK) frame is returned.If an expected acknowledgment is not received within an allotted time period, the sender assumes that the last data frame was lost in transit and sends it again.Figure 5-13Sliding-window ARQTechnical Focus: Procedure for Sliding-Window ARQThe sending device keeps copies of all transmitted frames until they have been acknowledged.In addition to ACK frames, a receiver can return a NAK frameif the data have been received damaged. The NAK frametells the sender to retransmit a damaged frame. Like stop-and-wait ARQ, the sending device in sliding-windowARQ is equipped with a timer to enable it to handle lost acknowledgments. FLOW CONTROL5.6Flow control refers to a set of procedures used to restrict the amount of data the sender can send before waiting for acknowledgment.Note:MEDIUM ACCESSCONTROL5.1Figure 5-14Medium access methodsFigure 5-15SelectFigure 5-16PollThe poll/select method is mostly used in time-sharing systems when a central computer is used to control other computers.Note:Figure 5-17Token passing networkToken passing is used mostly by local area networks (LANs). We discuss LANs in Chapter 9.Note:CSMA/CD is used in the Ethernet LAN discussed in Chapter 9.Note:CSMA/CA is used in the wireless LANs discussed in Chapter 10.Note:DATA LINK PROTOCOLS5.8A data link protocol is a set of specifications used to implement the data link layer. Note:In a character-oriented protocol, the frame is interpreted as a series of characters. In a bit-oriented protocol, the frame or packet is interpreted as a series of bits.Note:All bit-oriented protocols are relatedto high-level data link control(HDLC), a bit-oriented protocol.Note: