As mentioned earlier, WLANs operate in the unlicensed spectrum. 802.11b and
802.11g operate in the 2.4-GHz band. 802.11a operates in the 5-GHz band. Within
the 2.4-GHz and 5-GHz bands, the frequencies are not licensed. However, these
bands have a limited size, which is set by regulation. This means that the
shared media is prone to collisions and therefore needs a method of dealing
with this possibility.
The technique currently used is called carrier
sense multiple access, with collision avoidance (CSMA/CA). It is similar in
many respects to CSMA/CD in Ethernet. The CSMA/CA protocol is designed to
reduce the collision probability between multiple devices accessing a medium,
at the point when collisions are most likely to occur. Just after the medium
becomes idle, following a busy medium is when the highest probability of a
collision exists. This is because multiple devices could have been waiting for
the medium to become available again. This is when a random backoff procedure
is used to resolve medium contention conflicts.
The access CSMA/CA
method uses both a physical and a virtual carrier-sense mechanism. The physical
carrier-sense mechanism works just as it does for CSMA/CD. The virtual
carrier-sense mechanism is achieved by distributing reservation information
announcing the impending use of the medium. The exchange of RTS and CTS frames
prior to the actual data frame is one way to distribute this medium reservation
information. TThe RTS and CTS frames contain a duration field that defines the
period of time that the medium is needed for transmittal of the actual data
frame, the returning ACK frame, and all interframe spaces (IFSs). All devices
within the reception range of either the origination, which transmits the RTS,
or the destination, which transmits the CTS, will learn of the medium
reservation. The RTS/CTS exchange also performs a type of fast-collision
inference and a transmission path check. Figure
illustrates the
RTS/CTS exchange. This access method is referred to as the distributed
coordination function (DCF).
The IEEE 802.11 MAC can also incorporate an
optional access method, called the point coordination function (PCF), which
creates contention-free access to the medium, using a type of polling, whereby
the AP is the polling master.