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Chapter 5. Data Link Layer. O BJECTIVES. 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.

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Chapter 5

Data Link

Layer


OBJECTIVES

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 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.

Understand the functions of the data link layer.

After reading this chapter, the reader should be able to:


5.1

DUTIES OF THE

DATA LINK LAYER


Figure 5-1

Data link layer in the Internet model


Figure 5-2

Data-link layer duties


5.2

HOP-TO-HOP

DELIVERY


Figure 5-3

Hop-to-hop delivery


5.3

PACKETIZING


5.4

ADDRESSING


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.


Figure 5-4

ARP operation


5.5

ERROR CONTROL


Note:

Data 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.


Figure 5-5

Single-bit error


Note:

A burst error means that two or more bits in the data unit have changed.


Figure 5-6

Burst error of length five


Note:

Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.


Figure 5-7

Redundancy


Figure 5-8

Detection methods


Note:

In vertical redundancy check (VRC), a parity bit is added to every data unit so that the total number of 1s becomes even.


Figure 5-9

Even parity VRC concept



Note:

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.



Figure 5-12

Stop and wait ARQ


The sending device keeps a copy of the last frame transmitteduntil 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.

Technical Focus:Procedure for Stop-And-Wait ARQ


Figure 5-13 transmitted

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.

Technical Focus:Procedure for Sliding-Window ARQ


5.6 until

FLOW CONTROL


Note: until

Flow control refers to a set of procedures used to restrict the amount of data the sender can send before waiting for acknowledgment.


5.1 until

MEDIUM ACCESS

CONTROL


Figure 5-14 until

Medium access methods


Figure 5-15 until

Select


Figure 5-16 until

Poll


Note: until

The poll/select method is mostly used in time-sharing systems when a central computer is used to control other computers.


Figure 5-17 until

Token passing network


Note: until

Token passing is used mostly by local area networks (LANs). We discuss LANs in Chapter 9.


Note: until

CSMA/CD is used in the Ethernet LAN discussed in Chapter 9.


Note: until

CSMA/CA is used in the wireless LANs discussed in Chapter 10.


5.8 until

DATA LINK

PROTOCOLS


Note: until

A data link protocol is a set of specifications used to implement the data link layer.


Note: until

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: until

All bit-oriented protocols are relatedto high-level data link control(HDLC), a bit-oriented protocol.


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