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|
3.6 | |
Multiple Access and Bandwidth | |
| 3.6.2 | |
WLAN DSSS and CSMA/CA |
| 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
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.
|
illustrates
the RTS/CTS exchange. This access method is referred to as the
distributed coordination function (DCF). 
