SMC Networks SMC2552W-G2-17 manual WMM Access Categories Access Category, Description 802.1D Tags

6 System Configuration

Table 6-1. WMM Access Categories

Access Category	WMM Designation	Description	802.1D Tags
AC_VO (AC3)	Voice	Highest priority, minimum delay. Time-sensitive	7, 6
		data such as VoIP (Voice over IP) calls.
AC_VI (AC2)	Video	High priority, minimum delay. Time-sensitive data	5, 4
		such as streaming video.
AC_BE (AC0)	Best Effort	Normal priority, medium delay and throughput.	0, 3
		Data only affected by long delays. Data from
		applications or devices that lack QoS capabilities.
AC_BK (AC1)	Background	Lowest priority. Data with no delay or throughput	2, 1
		requirements, such as bulk data transfers.

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.

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