Quality of Service is a complex and often misunderstood topic, more so when it comes to wireless networks, as wireless and radio technologies are often misunderstood in general. Combining the two (that is QoS & 802.11 Wi-Fi) gives us a good place to start with the first post on this site. As this could be a lengthy write-up we will break it up into a number of posts, let’s begin with a bit of history and understanding what goes on when packets are transmitted over the air.
When the original IEEE 802.11 wireless standard was ratified all the way back in 1997 no provision was made for any form of traffic differentiation. Packets were transmitted in the order they arrived as there was no mechanism to queue certain high-priority traffic ahead of lower priority traffic. In contrast, outside of the wireless world, work on the various QoS standards was already underway since the mid-1990s, culminating in RFC2474 (Definition of the Differentiated Services Field in the IPv4 and IPv6 Headers) published in 1998 which laid the ground work for tagging the IP payload itself with a specific marking, which could then be used to define Per Hop Behaviour (PHB) as the packet traversed the various network hops along its path. Since this post is specifically around wireless technologies we will not be delving too far into the numerous historical QoS standards or RFCs, suffice to say that we had to wait for 2005 and the 802.11e amendment which specifically addressed traffic queuing over the air. Later 802.11e along with several other amendments were rolled up into what is known as the IEEE 802.11-2007 standard.
So, what happened in the decade or so between 1997 (the original 802.11) and 802.11e? Well, from the point of view of wireless QoS, not a great deal. Wireless networks were generally a best effort service. Which bring us neatly to the point of 802.11e and what it brought us. Of course, to better understand this we need to get into some details of what happens when a wireless station (STA) needs to transmit.
The next instalment of the wireless QoS series covers channel access.