3Com WL-575, 3CRWEASYA73 manual WMM Backoff Times

Radio Interface

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 33

High Priority

Low Priority

WMM Backoff Times

		Time
CWMin		CWMax
AIFS	Random Backoff
Minimum Wait Time	Random Wait Time
	CWMin	CWMax
AIFS		Random Backoff
Minimum Wait Time		Random 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.

To configure WMM, select the Radio Settings page, and scroll down to the WMM configuration settings.

5-47