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1) WMM Access Categories
At present, WMM defines traffic into 4 access categories.
AC_VO: Voice (highest priority)
AC_VI: Video (high priority)
AC_BE: Best effort (medium priority)
AC_BK: Background (low priority)
802.11 uses DCF (Distributed Coordination Function) scheme of the CSMA/CA
(Carrier Sense Multiple Access / Collision Avoidance) protocol to reduce the
chances of packets collision while one more devices access the wireless
media at the same time. A client wishing to transmit has to first listen to the
channel for a predetermined amount of time so as to check for any activity on
the channel. If the channel is sensed "idle" then the client is permitted to
transmit. If the channel is sensed as "busy" the station has to defer its
transmission. The random interval provides a fair transmission chance for all
the devices.
When each priority queue waits for sending packets, it has to wait a fixed time
AIFSN and a random time CW. They define time values by multiple time slots.
For 802.11b, its time slot is 20ms. The time slot of 802.11a and 802.11g is 9 ms.
CW insures the random delay time of DCF, so that the packets collision among
the devices with the same access category can be avoided. If collision occurs,
CW is doubled until exceeds its maximum value. After every successful
transmission, CW returns to the minimum value.
The priority queue that succeeds in the competition of sending packets, it will
acquire Txop time to send packets. If the txop value is 0, it is limited to be a
MSDC (MAC Service Data Unit).
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