User's Manual

Chapter 6

| Wireless Settings

Wi-Fi Multimedia (WMM)

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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.

Table 2: WMM Access Categories

Access