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| Wireless Settings
C
8
HAPTER
QoS
WMM Operation — WMM uses traffic priority based on the four ACs; Voice,
Video, Best Effort, and Background. The higher the AC priority, the higher
the probability that data is transmitted.
When the access point forwards traffic, WMM adds data packets to four
independent transmit queues, one for each AC, depending on the 802.1D
priority tag of the packet. Data packets without a priority tag are always
added to the Best Effort AC queue. From the four queues, an internal
"virtual" collision resolution mechanism first selects data with the highest
priority to be granted a transmit opportunity. Then the same collision
resolution mechanism is used externally to determine which device has
access to the wireless medium.
For each AC queue, the collision resolution mechanism is dependent on two
timing parameters:
AIFSN (Arbitration Inter-Frame Space Number), a number used to
◆
calculate the minimum time between data frames
CW (Contention Window), a number used to calculate a random backoff
◆
time
After a collision detection, a backoff wait time is calculated. The total wait
time is the sum of a minimum wait time (Arbitration Inter-Frame Space, or
AIFS) determined from the AIFSN, and a random backoff time calculated
from a value selected from zero to the CW. The CW value varies within a
configurable range. It starts at CWMin and doubles after every collision up
to a maximum value, CWMax. After a successful transmission, the CW
value is reset to its CWMin value.
Figure 51: WMM Backoff Wait Times
CWMin
High Priority
AIFS
Minimum Wait Time
Low Priority
AIFS
Minimum Wait Time
For high-priority traffic, the AIFSN and CW values are smaller. The smaller
values equate to less backoff and wait time, and therefore more transmit
opportunities.
– 100 –
CWMax
Random Backoff
Random Wait Time
CWMin
Random Backoff
Random Wait Time
Time
CWMax
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