WMM priority classes and COS (Layer 2) mapping

The 802.11e standard defines four WMM access categories (AC's) for handling the data traffic in compliance with the QoS requirements.

AC_BK (background)
AC_BE (best effort)
AC_VI (video)
AC_VO (voice)

802.11e also specifies a mapping between the LAN's Layer 2 (802.1d) Class of Service and the WLAN's WMM access categories.

Mapping table based on 802.11e

Priority	Layer 2 COS	WMM Access Category
Lowest	1	AC_BK (background)
	2	AC_BK (background)
	0	AC_BE (best effort)
	3	AC_BE (best effort)
	4	AC_VI (video)
	5	AC_VI (video)
	6	AC_VO (voice)
Highest	7	AC_VO (voice)

The access points (bintec W2003ac-ext™, bintec WI1003n™) have implemented the mapping in compliance with the 802.11e standard. The access points' WLAN driver does the mapping between the LAN's Layer 2 priority and the WLAN's WMM class, in both directions.

In the many IP networks (no VLAN or VLAN without Layer 2 priority), the QoS requirements for the data transmission are signalled using Layer 3 priority (TOS/DSCP). Therefore the WLAN access points need to support Layer 3 <- -> Layer 2 mapping.

Layer 3 / Layer 2 / WMM Mapping

DSCP Field Hex/Bin/Dec	Layer 2 Prio	Traffic Type	Acronym	WMM Access Category
0x38 / 111000 / 56	7	Network Control	NC	AC_VO
0x30 / 110000 / 48	6	Voice	VO	AC_VO
0x28 / 101000 / 40	5	Video	VI	AC_VI
0x20 / 100000 / 32	4	Controlled Load	CL	AC_VI
0x18 / 011000 / 24	3	Excellent Effort	EE	AC_BE
0x10 / 010000 / 16	2	Spare	--	AC_BK
0x08 / 001000 / 8	1	Background	BK	AC_BK
0x00 / 000000 / 0	0	Best Effort	BE	AC_BE

The mapping above only includes the top three bits in the TOS/DSCP field, so it is relatively fuzzy and leads to problems with VoWLAN devices.

But most VoWLAN devices use as Class of Service Expedited Forwarding (EF) with DSCP value (0x2E / 101110 / 46) in compliance with RFC 4594. The mapping table above would map this class in the WMM Class 'AC_VI'. But this is incorrect, because the VoWLAN telephone uses "WMM AC 'AC_VO" for the opposite direction.

To avoid this problem, bintec access points map data tagged with "EF" in the following way:

DSCP Field Hex/Bin/Dec	Layer 2 Prio	Traffic Type	Acronym	WMM Access Category
0x2E / 101110 / 46	6	Voice	VO	AC_VO

Note: When doing the installation, the user only needs to be aware that the VoWLAN telephone and the IP PABX use as Class of Service Expedited Forwarding (EF) with DSCP value (0x2E / 101110 / 46). In the case of the bintec be.IP plus™ this value is preset.