WMM Access Categories

Each Access Category has its own transmit queue. Traffic is assigned to the appropriate Access Category based on type of service (ToS) information that is provided by either the application or the firewall. SonicWall security appliances assign ToS either through access rules or VLAN tagging.

The following table shows how the WMM Access Categories map to 802.1D user priorities.

Wi-Fi Multimedia Access Categories
Priority	User Priority (Same as 802.1D user priority)	802.1D designation	WMM Access Category (AC)	WMM AC Designation (informative)
Lowest Highest	1	BK	AC_BK	Background
	2	–	AC_BK	Background
	0	BE	AC_BE	Best Effort
	3	EE	AC_BE	Best Effort
	4	CL	AC_VI	Video
	5	VI	AC_VI	Video
	6	VO	AC_VO	Voice
	7	NC	AC_VO	Voice

WMM prioritizes traffic through a process known as Enhanced distributed channel access (EDCA). EDCA is a contention-based mechanism for governing access to the transmission channel among the four WMM Access Categories. EDCA requires uses a listen-then-talk method where clients must wait for a random “backoff” period of time to observe if any other devices are transmitting before they transmit. The backoff times are randomized to reduce the likelihood of collisions and to give all devices a fair chance. WMM prioritizes traffic by defining a different range of “backoff” periods for each Access Category. The WMM backoff periods are defined by two parameters:

•

Arbitration Inter-Frame Space (AIFS) – The time interval between the wireless channel becomes idle and when the AC can begin negotiating access to the channel.

•

Contention Window (CW) – The range of possible values for the random backoff periods. A range of time that specifies the random backoff period. The CW is defined by a minimum and maximum value:

•

Minimum contention window size (CWMin) – The initial upper limit of the length of the CW. The AC will wait for a random time between 0 and CWMin before attempting to transmit. Higher priority AC with higher priority is assigned a shorter CWMin.

•

Maximum contention window size (CWMax) – The upper limit of the CW. If a collision occurs, the AC doubles the size of the CW, up to the CWMax, and attempts to transmit again. The CWMax must be larger than the CWMin.

Higher priority ACs are generally given lower values for AIFS, CWMin, CWMax.

NOTE: The unit of measure for AIFS, CWMin, and CWMax is multiples of the slot time for the 802.11 standard that is being used. For 802.11b, one slot is 20 microseconds. For 802.11a and 802.11g, one slot is 9 microseconds. The slot time for the 802.11n and the 802.11ac is 9 microseconds.

Separate WMM parameters are configured for Access Points (SonicPoints) and for wireless clients (such as smart phones or laptops). The following tables show the default WMM parameters for SonicPoints and wireless clients.

Default WMM Parameters for SonicPoints
WMM Access Category (AC)	WMM AC Designation (informative)	CWMin	CWMax	AIFS
AC_BE(0)	Best Effort	4	6	3
AC_BK(1)	Background	4	10	7
AC_VI(2)	Video	3	4	1
AC_VO(3)	Voice	2	3	1

Default WMM Parameters for Wireless Clients
WMM Access Category (AC)	WMM AC Designation (informative)	CWMin	CWMax	AIFS
AC_BE(0)	Best Effort	4	10	3
AC_BK(1)	Background	4	10	7
AC_VI(2)	Video	3	4	2
AC_VO(3)	Voice	2	3	2