Wireless QoS - Part 2

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Part 2 - IEEE 802.11e Principles

 802.11e and WMM QoS Overview

 The initial 802.11 Standard did not give priority to different types of Traffic. This really doesn't works well in the real world. We would like to have more priority to traffics like Voice and Video over other type of Data, but this wasn't addressed initially. With 802.11e Amendment, it addressed this issue by giving more priority creating 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:

 Establishment of 4 priority queues for traffic (Access Categories), implemented on a per-station basis.
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
Random Backoff timers defining Contention Window minimum (CWmin) and maximum (CWmax) values for each of the 4 priority queues
Queuing Structure

 4 priority queues are established to prioritize data frames within each station. These 4 priority queues align with the 4 access categories mentioned previously. Each frame that a station wishes to transmit is classified and placed into one of the appropriate queues.

The inter-frame spacing and random backoff timers are calculated independently and decremented in parallel for each queue. If an internal collision occurs, whereby frames from two or more queues are ready to transmit at the same time, the higher priority queue is granted access to transmit and the other queues act as if a physical collision occurred during transmission, increment their retry counter, and increase their contention window values according to binary exponential backoff. In this manner, one physical station emulates four logical stations, one for each traffic queue. The following figure illustrates these priority queues.

Next in the Series:
Wireless QoS - Part 3

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