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Chapter 11

Extending Networks

(Repeaters, Bridges, Switches)

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MotivationMotivation

RecallEach LAN technology has a distance limitationExample: CSMA/CD cannot work across arbitrary

distanceHowever

Users desire arbitrary distance connectionsExample: two computers across a corporate

campus are part of one workgroup

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Extension TechniquesExtension Techniques

Must not violate design assumptionsOften part of original designExample technique

Use connection with lower delay than copper

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Illustration Of ExtensionFor One Computer

Illustration Of ExtensionFor One Computer

Optical fiberHas low delayHas high bandwidthCan pass signals within specified bounds

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RepeaterRepeater

Hardware deviceConnects two LAN segmentsCopies signal from one segment to the otherConnection can be extended with Fiber Optic

Intra-Repeater Link

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Repeater (continued)Repeater (continued)

Amplifies signals from one segment and sends to the other

Operates in two directions simultaneouslyPropagates noise and collisions

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Repeaters and the OriginalEthernet Wiring Scheme

Repeaters and the OriginalEthernet Wiring Scheme

Designed for officeOnly two repeaters between any pair of stations

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HubHub

PhysicallySmall electronic deviceHas connections from several computers (e.g., 4 or 20)

LogicallyOperates on signalsPropagates each incoming signal to all connectionsSimilar to connecting segments with repeatersDoes not understand packetsExtremely low cost

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Connection MultiplexingConnection Multiplexing

ConceptMultiple stations share one network connection

MotivationCost Convenience of wiringHardware device required

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Illustration of Connection Multiplexing

Illustration of Connection Multiplexing

Multiplexing device attached to networkStations attach to devicePredates hubs

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Modern Equivalent OfConnection MultiplexingModern Equivalent Of

Connection MultiplexingHubs used nowConnections on a hub

One for each attached computerOne for another hub

Multiple hubsCan be interconnected in a daisy chainOperate as one giant hubCalled stacking

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BridgeBridge

Hardware deviceConnects two LAN segmentsForwards framesDoes not forward noise or collisionsLearns addresses and filtersAllows independent transmission

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Bridge AlgorithmBridge Algorithm

Listen in promiscuous modeWatch source address in incoming framesMake list of computers on each segmentOnly forward if necessaryAlways forward broadcast / multicast

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Illustration of a BridgeIllustration of a Bridge

Bridge uses source address to learn location of computersLearning is completely automated

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Extending a BridgeExtending a Bridge

Typically optical fiberCan span buildings

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Satellite BridgingSatellite Bridging

Can span arbitrary distance

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Apparent ProblemApparent Problem

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

cycleConsider a broadcast frame

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Spanning Tree AlgorithmSpanning Tree Algorithm

Allows cyclesUsed by all bridges to

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

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SwitchSwitch

Electronic devicePhysically similar to a hubLogically similar to a bridge

Operates on packetsUnderstands addressesOnly forwards when necessary

Permits separate pairs of computers to communicate at the same time

Higher cost than hub

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Conceptual Switch Function

Conceptual Switch Function

Conceptual operationOne LAN segment per hostBridge interconnects each pair of segments

NOT an actual implementation

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SummarySummary

LANsHave distance limitationsCan be extended

Fiber can be used between computer and LANRepeater

Connects two LAN segmentsRepeats and amplifies all signalsForwards noise and collisions

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Summary (continued)Summary (continued)

BridgeConnects two LAN segmentsUnderstands framesUses addressesDoes not forward noise or collisionsAllows simultaneous transmission on the

segments

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Summary (continued)Summary (continued)

HubCentral facility in star-shaped networkOperates like a repeater

SwitchCentral facility in star-shaped networkOperates like a set of bridged segments