Thomas Kunz
Systems and Computer Engineering
172
IEEE 802.11 DCF Protocol
nSense media before transmission
nIf media is free, transmit if media stays idle for a
fixed amount of time (DCF Interframe Space,
DIFS)
nDefer:
–wait until
end of current transmission, plus DIFS
–apply
random backoff procedure: pick number between 0 and 7, check whether medium is idle during each backoff slot
–if media is
busy, suspend backoff process at beginning of current slot
–after media
was idle for selected number of slots, transmit immediately
–if this
transmission results in collision, backoff again, doubling the backoff
nUpon receipt of packet:
–receiver
waits short interval (Short Interframe Space, SIFS)
–transmits
acknowledgement frame (ACK) back to sender
nIf sender receives no ACK within ACKTimeout interval,
assume collision