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Ch 8 LAN Technologies and Network Topology

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ScopeScope

Describes the concepts underlying local network technologies

Describes basic network topologyExamines examples of popular local network

technologies

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Classification TerminologyClassification Terminology

Network technologies classified into three broad categories Local Area Network (LAN) Metropolitan Area Network (MAN) Wide Area Network (WAN)

LAN and WAN most widely deployed

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Scientific Justification for LANs

Scientific Justification for LANs

A computer is more likely to communicate with computers that are nearby than with computers that are distant

A computer is more likely to communicate with the same set of computers repeatedly

Known as the locality principles

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LANsLANsMany LAN technologies exist

Designed for sharing (needs medium access control, MAC)

IEEE 802.3, 802.4, 802.5, 802.11Key features of a LAN

High throughputRelatively low costLimited to short distanceOften rely on shared media rather than direct

connections (or said point-to-point connections)

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Network TopologiesNetwork TopologiesSpecifies general “shape” of a network

Star Ring Bus

Each topology has advantages and disadvantages

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Star TopologyStar Topology

Central point of network known as hubEach computer has separate connection to

hub

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Ring TopologyRing Topology

To be connected in a closed loopConnections go directly from one computer

to anotherNo central facility

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Bus TopologyBus Topology

Shared medium forms main interconnectBroadcasting orientedOnly one computer sends a signal at any time

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Example Bus Network: Ethernet

Example Bus Network: Ethernet

Most popular LANsIEEE standard 802.3Several generations

Same frame format Different data rates (10/100/1000 Mbps) Different wiring schemes (e.g., 10Base2,

10BaseT)

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Manchester EncodingManchester Encoding

Hardware can detect a change in voltage easily than a fixed value Use rising and falling edges to encode data 1 and 0 One slot for a bit Voltage change occur exactly half-way through a slot

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Manchester EncodingManchester EncodingA preamble is used to have the receiver know when

each time slot begins The preamble consists of 64 alternating 1’s and 0’s

sent before the frame

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Sharing on an EthernetSharing on an Ethernet

Signal propagates across entire cable (terminator located at both ends)

All stations receive transmission (only the dest. can accept the frame)

Only one station transmits at any timeCSMA/CD media access scheme

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CSMA/CD ParadigmCSMA/CD Paradigm

Multiple Access (MA)Multiple computers attach to shared mediaEach uses same access algorithm

Carrier Sense (CS)Wait until medium idleBegin to transmit frame

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Two simultaneous transmissionsInterfere with one anotherCalled collision

CSMA plus Collision Detection (CD)Listen to medium during transmissionDetect whether another station’s signal interferesBack off from interference and try again

CSMA/CD ParadigmCSMA/CD Paradigm

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Backoff After CollisionBackoff After Collision

When collision occursWait random time t, 0 ≤ t ≤ d (tome slot)Use CSMA and try again

Double range for each successive collisionCalled exponential backoff

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Wireless LANWireless LAN IEEE 802.11

switch

AP

LAN

DHCP

server

Cat 5

Cat 5

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Wireless LANWireless LANWireless ADSL

line

ADDSL router

Cat 5

straight

line

ADDSL router

Private

IP

4-in-1 頻寬分享器

(NAT, DHCP and Hub)

Cat 5

CO

CO

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Wireless LANWireless LANWireless ADSL

line

ADDSL router

Cat 5

AP

Cat 5

4-in-1 頻寬分享器

(NAT, DHCP and Hub)

CO

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Limited range (hidden terminal problem)Not all stations receive all transmissionsCannot use CSMA/CDE.g., STA2 can detect the collision

CSMA/CACSMA/CA

STA1 STA2 STA3

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CSMA/CACSMA/CAPurpose: inform all stations in range of X or Y

before transmissionKnown as Collision Avoidance (CA)

STA1STA2 STA3

RTS

CTS

Area cleared by RTS (Request To Send)

Area cleared by CTS (Clear To Send)

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Source Destination

ACK

Data

CTS

RTS

CSMA/CACSMA/CA4-way MAC frame exchange protocol

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Token Passing Ring Transmission

Token Passing Ring Transmission

Station waits for token before sendingSignal travels around entire ringSender receives its own transmission

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Token PassingToken PassingToken

Special, reserved message Small bit pattern differs from normal data frames

Station Waits for the token to arrive Transmits one packet around ring Transmits token around ring

When no station has data to send Token circulates continuously

Guarantees fair access

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Strengths of Token Ring Approach

Strengths of Token Ring Approach

Easy detection of broken ring interference (by the sender) hardware failures (passing mode)

No collision

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Weaknesses of Token Ring Approach

Weaknesses of Token Ring Approach

Broken wire disables entire ringPoint-to-point wiring

Awkward in office environment Difficult to add / move stations

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Token Passing Ring Technologies

Token Passing Ring Technologies

LocalTalk Operated at 10 Mbps (CSMA/CA)

IBM Token Ring Originally operated at 4 Mbps Later version operated at 16 Mbps

FDDI (Fiber Distributed Data Interconnect ) Operated at 100 Mbps

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FDDI Failure RecoveryFDDI Failure Recovery

Uses two fiber ringsAutomatic failure recovery

Dual-attachedCounter rotating (data travels in the reverse

direction across the second ring)Self healing (the process of reconfiguring to

avoid failure)

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Illustration of FDDIFailure Recovery

Illustration of FDDIFailure Recovery

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FDDI TerminologyFDDI Terminology FDDI

Uses optical fibers High reliability Immune to interference

CDDI FDDI over copper Same frame format Same data rate Less noise immunity

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FDDI Hub TechnologyFDDI Hub Technology

Part of FDDI standardStations attach to hubSame frame format and data rate as FDDICalled star-shaped ringAdvantages

WiringReliability

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The End

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Example Star Network: ATM

Example Star Network: ATM

Asynchronous Transfer Mode (ATM)Designed by telephone companiesIntended to accommodate

VoiceVideoData

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Building block known as ATM switchEach station connects to switch (star topology)Switches can be interconnectedOnly propagate data to the communicating pair

Example Star Network: ATM

Example Star Network: ATM

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Details of ATM ConnectionDetails of ATM Connection

Full-duplex connectionsTwo fibers requiredOperates at 155 Mbps or faster

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ATM CharacteristicsATM Characteristics

High data rates (e.g. 155 Mbps)Fixed size packets

Called cellsImportant for voice

Cell size is 53 octets48 octets of data5 octets of header