Wired LAN and Backbone Requirements
NOTICE:
The 802.1p priority tag is not likely to be preserved if VLANs are not
configured throughout the wired network. If the packets travel across different
subnets, the router configuration needs to cope with preserving the 802.1p priori-
ty tag.
NOTICE:
Any device that assigns QoS information to a data frame must be con-
nected to a port in the LAN switch, which is defined as a trunk port. A trunk port in
a switch accepts a frame as legal when it is extended with a VLAN tag.
Normally an access port in a switch does not accept such a frame because the frame is not a
standard Ethernet frame.
NOTICE:
The priority tag can be changed by any intermediate device by an ad-
ministrator creating rules in the device.
Downlink : Access Point to Handset
As stated in the section about WMM, if QoS is configured properly, voice packets will gain high
priority and thereby minimize latency and packet inter-arrival jitter.
But how does an AP know which packets to prioritize? Two basic methods are defined:
• WMM default (Layer 2 to Layer 2 mapping)
The classification is done by translating the Layer 2 802.1p priority tag into one of four Ac-
cess categories and vice versa. This requires that the 802.1p priority tag is preserved in the
wired network all the way to the APs Ethernet interface. In most cases, this requires the use
of VLAN. A VLAN header includes the 802.1p priority tag.
• IP DSCP mapping (Layer 3 to Layer 2 mapping)
All IP packets contain a field used for prioritization. This value is called Differentiated Services
Code Point (DSCP). In the AP, a rule can be created that maps packets with a specific DSCP
value to the access category voice and thereby gain priority by using WMM channel access.
If no classification is done, the downlink packets (from the AP to the handset) will contend for
transmission time on the same conditions as all other data traffic. The impact will be bad speech
at random occasions when other clients might create load on the system by some heavy file
transfer, and so on.
A31003-M2000-P103-01-76A9, 29/04/2020
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System Planning, Planning Guide
