Introduction
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LAN technologies are designed with constraints of speed, distance and costs
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Typical LAN technology can span, at most, a few hundred meters
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How can a network be extended to cover longer distances; e.g., the Bucknell
campus?
LAN design for distance
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Many LANs use shared medium - Ethernet, token ring
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Length of medium affects fair, shared access to medium
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CSMA/CD - delay between frames, minimum frame length
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Token passing - circulation time for token
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Length of medium affects strength of electrical signals and noise immunity
LAN extensions
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Several techniques extend diameter of LAN medium
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Most techniques use additional hardware
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LAN signals relayed between LAN segments
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Resulting mixed technology stays within original engineering constraints
while spanning greater distance
Fiber optic extensions
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Can extend connection to a computer using fiber optic cable
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Insert fiber modems and fiber optic cable into AUI cable
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Fiber modems:
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Convert AUI signals to digital signal
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Transmit digital signals via fiber optic cable to other modem
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Most often used to connect two LANs - typically through a bridge - different
buildings
Repeaters
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May want to extend LAN medium
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Ethernet - timing constraints allow longer medium
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Signal strength constraints limit length
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Repeater - bidirectional, analog amplifier that retransmits analog signals
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One repeater can effectively double the length of an LAN segment
Ethernet repeaters
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Simply copy signals between segments
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Do not understand frame formats
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Do not have hardware addresses
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Any Ethernet segment is limited to 500 meters
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Repeater can double to 1,000 meters
Limits on repeaters
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Can't extend Ethernet with repeaters indefinitely
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CSMA/CD requires low delay; if medium is too long, CSMA/CD won't work
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Ethernet standard includes limit of 4 repeaters between any two Ethernet
stations
Repeater architecture
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With four repeaters, can extend Ethernet through a building
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FOIRL - Fiber Optic Intra-Repeater Link - can be used to connect bridges
Characteristics of repeaters
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Very easy to use - just plug in
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Repeaters simply re-transmit analog signals
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Collisions affect entire network
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Transient problems - noise - propagates throughout network