Wireless LAN QoS - Introduction

Wireless LANs operate in the same mannor as wired LANS at OSI layer 3 and above.

Where wired LANs implement CSMA/CD, wireless LANs (802.11) are not able to detect collisions, so implement collision avoidance - CSMA/CA. Collision Avoidance uses RF carrier sense, random back-off, inter-frame spacing.

802.1e is an approved standard for implementing QoS on wireless networks. Prior to 802.1e being standardised, Wifi Multimedia WMM created a QoS policy based on 4 queues, to prioritise traffic.

WLAN data from a client is sent between the LWAP and controller using LWAPP (Light Weight Access Point Protocol). A LWAP running LWAPP layer 2 will need to be in the same broadcast domain and IP subnet as the controller, however a LWAP in layer 3 mode doesn't have to be in the same IP subnet or broadcast domain. In layer 2 mode the LWAPP protocol is an Ethernet frame, but in layer 3 mode it is a UDP packet.

The following shows the process of QoS continuity is as follows:

1. WLAN controller frame packet marked with 802.q (CoS)
2. WLAN controller encapsulates frame with LWAPP, copies inner DSCP fields to outer LWAPP DSCP field, then maps the DSCP field to the outer CoS field using a mapping table.
3. LWAP forwards the IP packet to the client, it uses the 802.11e layer 2 QoS marking