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2
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
3
Extension TechniquesExtension Techniques
Must not violate design assumptionsOften part of original designExample technique
Use connection with lower delay than copper
4
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
14
Illustration of a BridgeIllustration of a Bridge
Bridge uses source address to learn location of computersLearning is completely automated
17
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
20
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
22
Summary (continued)Summary (continued)
BridgeConnects two LAN segmentsUnderstands framesUses addressesDoes not forward noise or collisionsAllows simultaneous transmission on the
segments