McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Chapter 5 Data Link Layer

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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. After reading this chapter, the reader should be able to: O BJECTIVES Understand the addressing mechanism used in the data link layer and how network layer addresses are mapped to data layer addresses. Understand how error control is handled at the data link layer.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 DUTIES OF THE DATA LINK LAYER DUTIES OF THE DATA LINK LAYER 5.1

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-1 Data link layer in the Internet model

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-2 Data-link layer duties

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 HOP-TO-HOPDELIVERYHOP-TO-HOPDELIVERY 5.2

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-3 Hop-to-hop delivery

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 PACKETIZINGPACKETIZING 5.3

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 ADDRESSINGADDRESSING 5.4

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Addresses in Local Area Networks The 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, changing the network card requires reconfiguration of the computer.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-4 ARP operation

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 ERROR CONTROL 5.5

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Data can be corrupted during transmission. For reliable communication, errors must be prevented, or detected and corrected. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 In a single-bit error, only 1 bit in the data unit has changed. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-5 Single-bit error

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A burst error means that two or more bits in the data unit have changed. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-6 Burst error of length five

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-7 Redundancy

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-8 Detection methods

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 In vertical redundancy check (VRC), a parity bit is added to every data unit so that the total number of 1s becomes even. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-9 Even parity VRC concept

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-10 LRC

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 In 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:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-11 CRC

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-12 Stop and wait ARQ

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Procedure for Stop-And-Wait ARQ The sending device keeps a copy of the last frame transmitted until it receives an acknowledgment for that frame Both data frames and ACK frames are numbered 0 and 1 alternately. A data 0 frame is acknowledged by 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.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-13 Sliding-window ARQ

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Technical Focus: Procedure for Sliding-Window ARQ The sending device keeps copies of all transmitted frames until they have been acknowledged. In addition to ACK frames, a receiver can return a NAK frame if the data have been received damaged. The NAK frame tells the sender to retransmit a damaged frame. Like stop-and-wait ARQ, the sending device in sliding-window ARQ is equipped with a timer to enable it to handle lost acknowledgments.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 FLOW CONTROL 5.6

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Flow control refers to a set of procedures used to restrict the amount of data the sender can send before waiting for acknowledgment. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 MEDIUM ACCESS CONTROL CONTROL 5.1

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-14 Medium access methods

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-15 Select

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-16 Poll

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 The poll/select method is mostly used in time-sharing systems when a central computer is used to control other computers. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Figure 5-17 Token passing network

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 Token passing is used mostly by local area networks (LANs). We discuss LANs in Chapter 9. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 CSMA/CD is used in the Ethernet LAN discussed in Chapter 9. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 CSMA/CA is used in the wireless LANs discussed in Chapter 10. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 DATA LINK PROTOCOLS PROTOCOLS 5.8

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 A data link protocol is a set of specifications used to implement the data link layer. Note:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 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:

McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 All bit-oriented protocols are related to high-level data link control (HDLC), a bit-oriented protocol. Note: