Data Link Layer

Chapter 4

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Outline4 Data Link Layer

4.1 Media Access Control4.2 Error Control4.3 Data Link Protocols

2Copyright 2010 John Wiley & Sons, Inc

Network Layers

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Computer 1 Computer 2

Data Link Layer - Introduction

Responsible for moving messages from one device to another

Controls the way messages are sent on media

Organizes physical layer bit streams into coherent messages for the network layer

Major functions of a data link layer protocol Media Access Control

Error Control

Message Delineation

Data Link Layer

Physical Layer

Network Layer

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Recap – Media Access Control and Error Detection

Media Access Control• Control• Contention

Error Control• Source of errors

Error Detection• Parity Checks • CRC-16 & CRC-32

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4.2.3 Error Correction

Once detected, the error must be corrected

Error correction techniques

Retransmission (or, backward error correction)

Forward Error Correction

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4.2.3.1 Automatic Repeat reQuest (ARQ)

Process of requesting a data transmission be resent

Main ARQ protocols

Stop and Wait ARQ (A half duplex technique)

Continuous ARQ (A full duplex technique)

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4.2.3.1 Stop and Wait ARQ

Sender Receiver

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4.2.3.1 Stop and Wait ARQ – Timeouts

Sender Receiver

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4.2.3.1 Stop and Wait ARQ – Timeouts

Sender Receiver

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4.2.3.1 Continuous ARQ

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4.2.3.1 Flow Control with ARQ

Ensuring that sender is not transmitting too quickly for the receiver

Stop-and-wait ARQ

Continuous ARQ

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Flow Control Example

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4.3 Data Link Protocols

Classification Asynchronous transmission Synchronous transmission

Differ by Message delineation Frame length Frame field structure

frame k frame k+1frame k-1

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4.3.1 Asynchronous File Transfer

Used on: Point-to-point asynchronous circuits Typically over phone lines via modem Computer to computer for transfer of data files

Characteristics of file transfer protocols Designed to transmit error-free data

Popular File transfer Protocols Xmodem, Zmodem, and Kermit

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4.3.1 Asynchronous Transmission

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Remember the Physical Layer…Bi-Polar Transmission

4.3.2 Synchronous Transmission

• Data sent in a large block

• Includes addressing information

• Includes a series of synchronization (SYN) characters Used to help the receiver recognize incoming data

• Synchronous transmission protocols categories Bit-oriented protocols: SDLC, HDLC Byte-count protocols: Ethernet Byte-oriented protocols: PPP

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4.3.2.1 Ethernet (IEEE 802.3ac & II – Ethernet 2)

Most widely used LAN protocol, developed jointly by Digital, Intel, and Xerox, now an IEEE standard

Uses contention based media access control

Byte-count data link layer protocol

No transparency problem uses a field containing the number of bytes (not flags) to

delineate frames

Error correction: optional

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4.3.2.1 Ethernet II Frame

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4.3.2.2 Point-to-Point Protocol (PPP)

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Protocol Size Error Detection Retransmission Media Access

Asynchronous Xmission 1 Parity Continuous ARQ Full Duplex

         

File Transfer Protocols        

XMODEM 132 8-bit Checksum Stop-and-wait ARQ Controlled Access

XMODEM-CRC 132 8-bit CRC Stop-and-wait ARQ Controlled Access

XMODEM-1K 1028 8-bit CRC Stop-and-wait ARQ Controlled Access

ZMODEM * 32-bit CRC Continuous ARQ Controlled Access

KERMIT * 24-bit CRC Continuous ARQ Controlled Access

         

Synchronous Protocols      

SDLC * 16-bit CRC Continuous ARQ Controlled Access

HDLC * 16-bit CRC Continuous ARQ Controlled Access

Token Ring * 32-bit CRC Stop-and wait ARQ Controlled Access

Ethernet * 32-bit CRC Stop-and wait ARQ Contention

SLIP * None None Full Duplex

PPP * 16-bit CRC Continuous ARQ Full Duplex

* Varies depending on message length.

4.3 Data Link Protocol Summary

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4.3 Transmission Efficiency

An objective of the network: Move as many bits as possible with minimum errors

higher efficiency and lower cost

Factors affecting network efficiency: Characteristics of circuit (error rate, speed)

Speed of equipment, Error control techniques

Protocol used• Information bits (carrying user information)• Overhead bits ( used for error checking, frame

delimiting, etc.)

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4.3 Transmission Efficiency

Total number of info bits to be transmittedTotal number of bits transmitted

=

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4.3 Optimum Packet Size

Trade-off between packet size and throughput

(more costly in terms of circuit capacity to retransmit if there is an error)(less likely to contain errors)

Acceptable range

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Transmission Efficiency of Protocols

Async Transmission:

Ethernet II Transmission

However, large packets likely to have more errors and are more likely to require retransmission wasted capacity

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Throughput

A more accurate definition of efficiency

Total number of information bits received per second; takes into account: Overhead bits (as in transmission efficiency) Need to retransmit packets containing errors

Complex to calculate; depends on: Transmission efficiency Error rate Number of retransmission

Transmission Rate of Information Bits (TRIB) Used as a measurement of throughput

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TRIB =

K (M – C) (1 – P)

(M / R) + T

Info bits per character

Average number of non-info

characters per block

Probability that a block will

require retransmission

Time between blocks (in seconds) (propagation time +

turnaround time) (a.k.a., reclocking

time)

Packet length in

characters

Data xmission

rate in char per second

Number of info bits accepted

Total time required to get the bits

TRIB =

Ex:K=7 bits/characterM = 400 char/blockR= 4.8 Kb/sC = 10 char/blockP = 1%T = 25 ms

7(400-10)(1-0.01)

(400/600)+0.025)

= 3.908 Kb/s

TRIB =

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Implications for Management

Provide a few, widely used data link layer protocols for all networks Minimize costly customization Minimize costly translation among many protocols Less training, simpler network management Bigger pool of available experts Less expensive, off-the-shelf equipment

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Next Week:

Start Chapter 5 – Network and Transport Layers

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