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13. Quality of Service (QoS)
Introduction
The Excel QoS model is based on the Wireless Multimedia Extensions (WME). This is also known as
Wi-Fi Multimedia (WMM) and is a Wi-Fi Alliance inter-operability certification, derived from the IEEE
802.11e standard. It provides basic Quality of service (QoS) features to IEEE 802.11 networks. WMM
prioritizes traffic according to four Access Categories (AC) - voice, video, best effort, and
background. However, it does not provide guaranteed throughput.
QoS Implementation in Access
The software driver basically classifies the PDU's based on VLAN priority (if present) and IP
Precedence bits. The first classification is with VLAN priority bits. If the incoming traffic has a VLAN
ID present the classification is done as shown in the figure and table below.
VLAN Priority
Class of Service
0
Best Effort
1
Background
2
Background
3
Best Effort
4
Video
5
Video
6
Voice
7
Voice
After the VLAN classification, the IP packets are classified according to Precedence bits (3 bits) of the
IP TOS field. These precedence bits map to Diff Serve Code Points (DSCP) bits (DS5-DS3). Bits 0
and 1 of the TOS field are reserved and are always 0.
