HP ProCurve Switch / Wireless - Using 802.11e and WMM on the ProCurve Wireless Edge Services Module

Using 802.11e and WMM on the ProCurve Wireless Edge Services Module

HP ProCurve Networking

1. Introduction

IEEE 802.11e is an enhancement to the 802.11 standard that adds Quality of Service for wireless LAN applications

such as Voice over Wireless LAN (VoWLAN) or IP Streaming Multimedia. This important standard has been added to

the published 802.11-2007 standard.

WMM, which stands for WiFi MultiMedia, is also known as WME (Wireless Multimedia Extensions). It is a WiFi

Alliance interoperability certification based on the 802.11e standard. WMM/WME provides basic QoS features to

802.11 networks, prioritizing traffic according to four queues, without guaranteed throughput.

2. Details of the WMM protocol

On a wired network, stations use CSMA-CD (Carrier Sense Multiple Access with Collision Avoidance) to improve

communications performance. On an 802.11 network, a similar mechanism, known as the distributed coordination

function (DCF), is used. When a wireless station wants to transmit, it selects a random backoff time and listens for

contention. After a default 3-millisecond interframe space (IFS), the station starts the countdown of its backoff timer;

when the backoff timer reaches zero, the station transmits. Using this mechanism, all stations are equal in the

competition for the wireless media, which can cause delay-sensitive applications such as VoWLAN to exhibit poor

Quality of Service characteristics.

WMM, on the other hand, defines four access category (AC) queues:

• Background

• Best effort

• Video

• Voice

For each queue, four parameters are defined:

• AIFSN, Arbitration IFS Number, the default time before countdown. This is similar to the IFS in DCF.

• CW minimum (CWmin), the minimum contention window.

• CW maximum (CWmax), the maximum contention window.

• Transmit Ops (TXOP), the opportunity to transmit. This is the time during which a station that has won the

control of the shared medium can retain it.

For example, the AC queue for voice has smaller values for contention, so that WiFi phones can gain the control of

the radio more easily.

3. Implementing WMM on a ProCurve WESM

WiFi Multimedia is supported on the ProCurve Wireless Edge Services Module (WESM). When you enable WMM:

• Radios can place traffic into the four queues according to 802.1p priority. (You can also specify DSCP

priority.)

• WMM-enabled stations can use QoS marking to queue traffic as well; otherwise, traffic goes to the Best Effort

queue by default.