Wireless networking expert Jim Geier discusses the IEEE 802.11 standard for wireless LANs.
Jim Geier is the author of Wireless LANs: Implementing Interoperable Networks (1999, Macillan Technical Publishing).
In June 1997, the IEEE finalized the initial IEEE 802.11 standard for wireless LANs. The 802.11 standard specifies a single Medium Access Control (MAC) layer and several physical layers. The MAC layer implements functions that enable the sharing of the common air medium. The physical layers provide for the actual transmission of data using differing methods.
The distributed coordination function (DCF) is a mandatory form of medium access for 802.11-compliant wireless LANs; therefore, all vendors implement the DCF in their 802.11-compliant products. The DCF implements a carrier-sense protocol and supports only asynchronous communications. The DCF is suitable for sending information such as bar codes and data files, but it is not very efficient for sending broadband, time-critical information such as video.
The 802.11 standard provides for an optional point coordination function (PCF) medium access mechanism that is implemented in the access points and (in addition to the mandatory DCF) that provides delivery of time-bounded data via synchronous communications using station-polling mechanisms. This mechanism is capable of transporting video effectively as compared to the DCF. Very few 802.11-compliant products on the market today offer the PCF as an option. The process of enhancing an access point to include PCF operation generally requires only a firmware upgrade.
The 802.11 standard provides the optional wired equivalent privacy (WEP), which offers frame transmission privacy by generating secret shared encryption keys for source and destination stations. Many of the wireless LAN vendors now offer WEP as an option to their product offerings. WEP encrypts only the payload of MAC layer frames, not the frame headers.