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| – |
downlink/forward
channel: no contention, only one
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sender:
the MDBS. All frames are broadcasted, each
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M-ES
picks out the ones destined for it or for everyone
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uplink/reverse
channel: contention is a problem
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n |
access
to channel follows a DSMA/CD protocol:
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uses
time slots of 60 bit times (see structure of forward channel)
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“digital
sense”: watch forward channel to determine whether
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reverse
channel is busy or idle (busy/idle flags every 60 bits)
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if
busy, skip a random number of slots and try again. If still busy,
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wait
for longer period (statistically twice as long) and retry
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if
idle, start transmitting
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“collision
detection”: decode flag in forward channel indicates
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with
delay whether there was a collision
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keep
sending until collision is detected or until maximum
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number
of slots is set or until MDBS tells
M-ES to shut down
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