Wireless QoS Part 2 - IEEE 802.11e Principles

Read the Entire Wi-Fi Quality of Service 5-Part Series:
  1. Part 1 - Background Information
  2. Part 2 - IEEE 802.11e Principles
  3. Part 3 - User Priorities, Access Categories and Queues
  4. Part 4 - Arbitration Interframe Spacing
  5. Part 5 - Contention Window and Final Thoughts
Part 2 - IEEE 802.11e Principles
Now, let's dig into the guiding principles behind wireless QoS design and the 802.11e amendment. This is part 2 of this series on wireless QoS, building on the background information from the first post in this series, which described the original Distributed Coordination Function (DCF) 802.11 medium contention protocol.

802.11e and WMM QoS Overview

The amended IEEE 802.11 wireless protocol supports a priority scheme that can provide up to 8 priority classes for traffic. This work was done as part of the IEEE 802.11e working group and was added as an amendment to the standard. Certifications of compliance with portions of this amendment are done by the WiFi Alliance through the WiFi Multimedia (WMM) certification process.

802.11e defines 8 user priorities for Quality of Service (QoS) of wireless traffic. These 8 user priorities are grouped into 4 Access Categories (AC) defined as Voice, Video, Best Effort, and Background. Each access category contains 2 different user priorities.

The design of this priority scheme is based on three major changes to the operation of the original 802.11 Distributed Coordination Function (DCF), which is now called Enhanced Distributed Coordination Access (EDCA). The three major changes are:

  1. Establishment of 4 priority queues for traffic (Access Categories), implemented on a per-station basis

  2. Arbitrated Inter-Frame Spacing (AIFS) values for each of the 4 priority queues to replace the single Distributed Coordination Inter-Frame Spacing (DIFS) value previously used for all data and management frames

  3. Random Backoff timers defining Contention Window minimum (CWmin) and maximum (CWmax) values for each of the 4 priority queues

Further information on WMM certification can be found the Wi-Fi Alliance website. In addition, the IEEE 802.11e amendment and current 802.11-2007 standard can be found on the IEEE website.

In future posts, I'll detail each of these design changes and how they're implemented. We'll also see why wireless QoS provides a statistical advantage for higher priority traffic, but cannot go so far as to perform or guarantee strict priority queuing due to the use of a shared medium and distributed contention among multiple stations. This is a fundamental difference for those familiar with QoS on wired switches, which have the ability to implement strict priority-queues over a contention-free full-duplex link.