1

Ch 11 Extending LANs

Fiber modems, Repeaters, Bridges, and Switches

2

Distance LimitationEach LAN technology has a distance limitation

LAN hardware is engineered to emit a fixed amount of power

LAN use shared comm. medium for saving costFair access mechanism, such as CSMA/CD, takes

time proportional to the size of networkFrequent collision caused by mass population

3

Extension TechniquesUse connection with lower delay than copper

(fiber)Repeaters or HubsBridges or switches

4

Fiber Optical Extensions

Optical fiberHas lower delay, higher bandwidthWithout changing the original LANTo connect computers with a remote LAN

(several Km)

5

Typically optical fiberCan span buildingsBridge has local traffic be local

Fiber Optical Extensions

6

Repeaters

Connects two LAN segmentsCopies signal from one segment and

amplifies it to the otherDo not understand the frame format (work at

Layer 1)Propagates noise and collisions

7

RepeatersOperates in two directions simultaneouslyNetwork will not operate correctly if more

than 4 repeaters separate any pair of stations

8

Repeaters and the OriginalEthernet Wiring Scheme

Designed for officeOnly two repeaters between any pair of stations

R1

Segments on floor 3

R2

Segments on floor 2

R3

Segments on floor 1

Vertical

Segment

9

HubsPhysically

Multi-port repeaterHas connections from several computers

LogicallyOperates on signalsPropagates each incoming signal to all

connectionsDoes not understand packets (work at Layer 1)

Hub

10

Multiple hubsCan be interconnected in a daisy chainOperate as one giant hub, called stackingForms 1-BC domain and 1-collision domains

Hubs

Hub Hub

Hub

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BridgesHardware device that connects two LAN

segmentsForwards frames if necessary (work at Layer 2)

Does not forward noise or collisionsIsolate problemsForms 1-BC domain and 2-collision domains

Bridge

Segment

BC domain

collision domain

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Frame Filtering

13

Frame FilteringListen in promiscuous modeUses source MAC address to learn location

of computersMake list of computers on each segmentLearning is completely automated

Watch source address in incoming framesOnly forward if necessary (in the steady state)

Always forward broadcast / multicast

14

Planning a Bridged Network

Two segments can be used simultaneousAllows independent transmissionsImprove the performance of an existing LAN

by dividing the LAN into 2 segments

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Bridging Across Long Distance

Satellite connection can span arbitrary distanceEach site has bridge HW to filter local traffic (low BW)Supports buffering and flow control

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A Cycle of Bridges

Complex bridge connections may not be apparentAdding one more bridge inadvertently introduces a

cycle

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A Cycle of Bridges

Consider a broadcast frame issued by station on segment a

Computer on all segments receive a infinite number of copies

Segment a

Segment c

Segment b

Segment dB2 B3B1

B4

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Spanning Tree AlgorithmUsed by all bridges to

Discover one anotherBreak cycle(s)Known as Distributed Spanning Tree (DST)

Segment a

Segment c

Segment b

Segment dB2 B3B1

B4

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SwitchingElectronic device with multiple portsOnly forwards packets when necessary (work

at Layer 2)Permits separate pairs of computers to

communicate at the same timeHigher cost than hub

HubCategory 3/5

UTP

Switch

Share 10 Mbps

10 Mbps

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SwitchingMaximal throughput = RN/2

R: data rateN: total number of port

Hub Hub

SwitchCollision BC domain

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Exercise11.6, 11.7