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Ch 11 Extending LANs
Fiber modems, Repeaters, Bridges, and Switches
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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
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Extension TechniquesUse connection with lower delay than copper
(fiber)Repeaters or HubsBridges or switches
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Fiber Optical Extensions
Optical fiberHas lower delay, higher bandwidthWithout changing the original LANTo connect computers with a remote LAN
(several Km)
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Typically optical fiberCan span buildingsBridge has local traffic be local
Fiber Optical Extensions
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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
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RepeatersOperates in two directions simultaneouslyNetwork will not operate correctly if more
than 4 repeaters separate any pair of stations
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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
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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
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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
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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
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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