Thomas Kunz
Systems and Computer Engineering
170
802.11 MAC: CSMA/CA
–station
ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)
–if the
medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on
service type)
–if the
medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off
time (collision avoidance, multiple of
slot-time)
–if another
station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)
slot time
direct access if
medium is free ³ DIFS
medium is free ³ DIFS
t
medium busy
DIFS
DIFS
next frame
contention window
(randomized back-offmechanism)