Each
QoS data frame also carries its user priority value in the MAC frame header. An 802.11e
station shall implement four channel access functions, where a channel access function is
an enhanced variant of the DCF, as shown in Figure 4.9. Each frame arriving at the MAC
with a user priority is mapped into an access category (AC) as shown in Table 4.2, where
one of the four channel access functions is used for each AC. Note the relative priority of
UP 0 is placed between 2 and 3. This relative priority is obtained from the IEEE 802.1d
bridge specification [4].
Figure 4.9: IEEE 802.11e EDCA channel access.
Busy
Medium
SIFS
AIFS[AC]
Backoff Window
SlotTime
Defer Access Select Slot and decrement backoff as
long as medium stays idle
AIFS[AC]
Contention Window
from [0,CW[AC]]
Immediate access when
medium is idle >= AIFS[AC]
Next Frame
PIFS
Basically, a channel access function uses AIFS12[AC], CWmin[AC], and CWmax[AC]
(instead of DIFS, CWmin, and CWmax of the DCF respectively) when contending to transmit
a frame belonging to access category AC. AIFS[AC] is determined by
AIFS[AC] = SIFS + AIFSN[AC]???SlotTime, (52)
12 AIFS: Arbitration Interframe Space, referring to IEEE 802.
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