The purpose of this blog post is to hopefully provide a better understanding of wireless QoS without doing deep.
Wireless QoS simply put it is a method of prioritising certain types of frames, so that it spends less time waiting to transmit.
When talking about Wireless QoS most people are referring to either Wi-Fi Multimedia (WMM) or 802.11e.
WMM is a certification created by the Wi-Fi alliance to assist with the need for frame prioritisation while the 802.11e amendment was being signed off. The 802.11e and WMM are somewhat similar in their structure.
Wireless QoS was defined in 802.11e amendment that is part of the 802.11-2016 standard, it was created to address particular requirements around latency and jitter for voice, video and audio traffic that is sent over the wireless medium, as the original 802.11 standard did not account for that type of traffic.
Two channel access methods are defined in the 802.11e amendment:
- Enhanced distributed channel access(EDCA) and
- Hybrid coordination function controlled channel access (HCCA)
Of the two, EDCA was adopted,
EDCA is used by both WMM and 802.11e capable clients. EDCA is a channel access method that allows certain types of traffic to be assigned to 4 queues called access category (AC).
The 4 AC are listed highest to lowest
- Best effort
The AC are mapped to user priorities of which there is 8, within each AC there are two UP
- UP 7 & 6 = Voice
- UP 5 & 4 = Video
- UP 3 & 0 = Best Effort
- UP 2 & 1 = Background
The AC that a frame is placed in will determine how often it gets access to the wireless medium. I.e. a frame in the video queue will get more transmit opportunities than a frame in the Background queue.
Part of the 802.11 arbitration process is a wireless client must check that the medium is available before it transmit, this involves performing the following:
- Carrier sense checks for further information on this refer to this post
- Inter-frame space (IFS)
- A set period of time a STA cannot transmit a frame
- For an 802.11e frame it is called arbitration inter-frame space (AIFS), basically the higher the AC the frame is placed into the short the AIFS timer will be
- Random back off time
- A random range of values called the Contention window (CW)
- Each AC has minimum CW and maximum CW value. The higher the AC the lower the CW min and max value will be.
For a QoS frame, the IFS and the Random back off timer are extremely important, as it these timers that determine the period of time the client is waiting before it can send its frame. Less time waiting to transmit equals more transmit opportunities.
What has not be discussed is how this maps up to the wired side with layer 2 (COS) and layer 3 (DSCP) QoS markings, this will be covered in another post.
Westcott, David A.. CWAP Certified Wireless Analysis Professional Official Study Guide. Wiley. Kindle Edition.
Coleman, David D.. CWNA Certified Wireless Network Administrator Study Guide (p. 267). Wiley. Kindle Edition.