System information

 Wi-Fi Multimedia
Working Principles of Basic Functions
WMM Operating
 WMM is an extension to the legacy CSMA/CA-based DCF mechanism
 Each time when AP transmits a packet, it wait for a specific period of time,
defined by AIFS (Arbitration Interframe Space ) and listen for contention.

After the waiting time of AIFS AP starts to count down a random backoff

After the waiting time of AIFS
,
AP starts to count down a random backoff
wait time from 0 to the value defined by the Minimum Contention Window
(cwMin).
 If the random backoff time ends and a collision is detected, the AP select a
andom time again and et b t do bles the time ange The et and
r
andom time again and
r
et
ry,
b
u
t do
u
bles the time
r
ange
.
The
r
et
ry
and
doubling behavior continues (if collisions remain there) until reach the value
specified in the Maximum Contention Window (cwMax) or packet has been
sent/discarded.
0 – cwMin
0 – 2x cwMin
= Random Backoff
Wait Time
Initial transmission
1st retransmission
2nd retransmission
0
–
4x
cwMin
AIFS
AIFS
AIFS
= AIFS
95
0 – cwMax
2nd retransmission
0
4x
cwMin
AIFS
AIFSXth retransmission