The DCF Physical carrier sense/Virtual carrier sense/ Contention window back-off allow different stations to synchronize usage of the air medium.
However, the one drawback of the DCF mechanism was that it was a fair mechanism for all Application level user priorities. It treated all user application priorities at the same priority. This caused packet loss in highly sensitive packet transmissions such as voice or video. A mechanism was needed that would treat different application priorities with a different yardstick and allow higher priority packets more access to the Air medium.
By this methodology – Voice was given highest priority, followed by video, Best Effort and Background in respective order. Each of the Traffic classes was to be placed in their own queue and the contention requirement for each traffic class was also altered.
To obtain the transmission medium for data transmission, the Voice Traffic class had to wait for a lesser time, followed by Video, Best effort and Background traffic classes. This set of rules of un-fairness to provide better transmit opportunities for higher traffic classes was termed as Wi-Fi Multi-media (WMM) as the Wi-Fi Organization initially came up with this Methodology and ratified the WMM standard.
The 802.11 standard’s body also started working on the same problem via Task-Group- E. However, The Wi-Fi organizations WMM standard was ratified first. The 802.11 standard’s body amalgamated the changes of the WMM standard and incorporated other enhancements into one standard – termed 802.11e which was ratified at a later date.
The standard defined two mechanisms.
- Enhanced Distributed Co-ordination Function – (EDCF)
- Hybrid Co-ordination Function – (HCF)
Currently The QoS access mechanism normally employed in WLAN is EDCF. The HCF mechanism was not widely implemented due to the difficulty in implementing the same.
The 802.11e standard is now combined into the 802.11-2012 standard. You can refer the WMM specifications and usage at the below link.
In the next article – we will try and understand the different application packet user priorities for which EDCF mechanism was developed. Later we shall look at the EDCF mechanism itself.