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Improvements
in ALOHA:
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1-persistent
CSMA: listen for idle channel before
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broadcasting.
Collisions can still occur:
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due
to propagation delay
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two
stations may wait for a third to finish and then start
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broadcasting
simultaneously
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nonpersistent
CSMA: listen to the channel, if not
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busy,
start broadcasting. Otherwise, wait for a
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random
interval and try again. Utilization approaches
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1
under heavy load
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p-persistent
CSMA: listen for an idle channel. Then
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with
probability <p broadcast frame (otherwise, delay
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and
try again)
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Yet
another improvement: abort broadcast as soon as
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collision
is detected (called CSMA/CD)
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frames
must be long enough so that all transmitting
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stations
can detect a collision: the farther stations can
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be
apart, the larger the minimum packet length
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Ethernet:
maximum network length is 2.5 km,
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minimum
frame length is 512 bytes
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