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Chapter 4 Data Link Layer
S305 – Network Infrastructure
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Announcements and Outline
Recap 3 Physical Layer 3.1 Circuits 3.2 Media 3.3 Digital Transmission (Digital Data) 3.4 Analog Transmission (Digital Data) 3.5 Digital Transmission (Analog Data) Outline 4 Data Link Layer 4.1 Media Access Control 4.2 Error Control 4.3 Data Link Protocols Copyright 2010 John Wiley & Sons, Inc 2
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Network Layers Computer 1 Computer 2
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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 Network Layer Data Link Layer Physical Layer 4
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Data Link Layer – Ethernet Address
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4.1 Media Access Control (MAC)
Controlling when and what computer transmit Why used: When to use: Two possible approaches Controlled access Contention based access 6
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Controlled Access Controlling access to shared resources Commonly used by mainframes (or its front end processor) Also used by some LAN protocols 7
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Contention Collisions ? Common Use: Problematic? 8
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4.1.3 MAC - Relative Performance
Depends on network conditions Cross-over point 9
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4.2 Error Control Handling of network errors caused by problems in transmission Network errors Human errors Categories of Network Errors Corrupted Lost data 10
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4.2 Error Control (Cont.) Error Rate Burst Error 11
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4.2.1 Sources of Errors Line noise and distortion – major cause
More likely on electrical media Undesirable electrical signal Introduced by equipment and natural disturbances Degrades performance of a circuit Manifestation Extra bits “Flipped” bits Missing bits 12
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Sources of Errors White Noise Impulse Noise 13
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Sources of Errors Cross Talk Echo 14
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4.2.1 Sources of Errors Source Examples
What do you think is a major cause of impulse noise? 15
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Sources of Errors and Prevention
Source of Error What causes it How to prevent it Line Outages Faulty equipment, Storms, Accidents (circuit fails) White Noise Movement of electrons (thermal energy) Increase signal strength (increase SNR) Impulse Noise Sudden increases in electricity (e.g., lightning, power surges) Shield or move the wires Cross-talk Multiplexer guard bands are too small or wires too close together Increase the guard bands, or move or shield the wires Echo Poor connections (causing signal to be reflected back to the source) Fix the connections, or tune equipment Attenuation Gradual decrease in signal over distance (weakening of a signal) Use repeaters or amplifiers Intermodulation Noise Signals from several circuits combine Move or shield the wires Jitter Analog signals change (small changes in amp., freq., and phase) Tune equipment Harmonic Distortion Amplifier changes phase (does not correctly amplify its input signal) More important mostly on analog 16
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4.2.2 Major Functions of Error Control
Error prevention Error detection Parity checks Cyclic Redundancy Check (CRC) Error correction 17
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Error Detection Sender calculates an Error Detection Value (EDV) and transmits it along with data Receiver recalculates EDV and checks it against the received EDV Mathematical calculations Mathematical calculations DATA EDV DATA EDV DATA ? = EDV EDV 18
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4.2.2 Error Detection Techniques
Parity checks Cyclic Redundancy Check (CRC) 19
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4.2.2.1 Parity Checking One of the oldest and simplest
A single bit added to each character Even parity: Odd parity: Receiving end recalculates parity bit If one bit has been transmitted in error the received parity bit will differ from the recalculated one 20
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4.2.2.1 Examples of Using Parity
Sending the letter A ( ) using odd parity bit sender receiver 21
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4.2.2.2 Cyclic Redundancy Check (CRC)
Treats entire message as a number, divides by a preset number and uses the remainder as the check code in the message Preset number is chosen by sender and receiver to ensure that the remainder is no longer than 16 or 32 bits depending on detection mode. Detection modes: CRC-16 CRC-32 22
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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 23
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