December 2002
Intermediate to advanced
192 pages
5h 30m
English
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Structure of 802.11 MAC
Regardless of the underlying PHY used, the MAC is the same for all currently deployed 802.11 wireless technologies. The 802.11 MAC provides several functions: access to the wireless medium, joining and leaving a network, and security services.
Access to the wireless medium is controlled by a contention-based protocol called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). This is a similar method to the one used in wired Ethernet. Like wired Ethernet, 802.11 devices are contending for the same physical transmission medium. If two or more devices are transmitting at the same time, their signals collide and it is impossible for the receiving station to discern the one signal from the other. CSMA/CA provides a way for 802.11 devices to probe the medium to see if it is in use and then lock the medium while they transmit.
Since 802.11 wireless networks use a shared medium, the more devices that are trying to access it, the lower the effective throughput will be. This is similar to standard wired Ethernet. When an 802.11 device is transmitting, no other device in the network may transmit data. If there are multiple devices trying to send large amounts of data, there will be heavy contention for the airwaves. This congestion gets worse as more machines are added or more data is being transmitted.
BSS and IBSS
The core unit of an 802.11 network is called a Basic Service Set (BSS). A BSS consists of a central access point (AP) and client stations. ...
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