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	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
Highest	7	NC	AC_VO	Voice

WMM prioritizes traffic through a process known as Enhanced distributed channel access (EDCA). It 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 waits 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.

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.

Separate WMM parameters are configured for the access points and for the station (the SonicWall security appliance). The following tables show the default WMM parameters for the access points and SonicWall security appliances.

Default WMM Parameters for Access Points

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 SonicWall Security Appliances

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