Uninformed: Informative Information for the Uninformed

Vol 5» 2006.Sep

802.11 Basics

Standard 802.11-1999 specifies Medium Access Control (MAC) and Physical (PHY) layer protocols. There are two types of MAC protocols described, Point Coordination Function (PCF) and Distributed Coordinated Function (DCF). It is possible to alternate between them. When the PCF is operating, the medium is in a contention-free period since the point coordinator, an access point, controls all access to the medium. When end stations compete for the medium, including the access point, they use the DCF MAC protocol. This period is called a contention period.

The standard specifies three different frame types: control, management, and data. Control frames are used for medium reservation and acknowledgements, and have a real-time processing requirement. Medium reservation control frames are not confined to a single network; they are intended to be processed by all stations on a given channel even though they may belong to different wireless networks, or Basic Service Sets (BSS). These frames carry a duration field that is essentially an announcement of a station's intention to use the medium for a period of time. Stations operating on the same channel should observe the announcement regardless of the BSS to which they belong. Otherwise they risk interference with their own transmissions. In this way, multiple Basic Service Sets can coexist on the same channel.

MAC management in 802.11 includes authentication and association with an access point. It also includes provisions for locating networks via probe requests and beacon packets. Management frames handle all of these tasks.

Finally, data frames are used to transmit data.