ABSTRACT
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A wireless local area network (WLAN ) is used for providing location-
independent network access by using radio waves rather than a cable infrastructure.
In WLAN architecture, the region to be covered is subdivided into cells and the net-
work communication follows the IEEE 802.11 protocol. Each cell is controlled by
an access point (AP ) which coordinates all communications that take place in that
cell. The network is formed by several APs connected through a backbone network.
The backbone is called the distribution system or DS and is typically an Ethernet.
The stations (STAs) access the network through these APs. The main benefits of
WLAN lie in increased mobility of the nodes and cost-effective setup. In general,
WLAN supports different data rates. For example, IEEE 802.11b WLAN supports
1, 2, 5.5 and 11Mbps data rates, respectively, for the ISM 2.4GHz frequency band
[19]. It uses the direct sequence spread spectrum (DSSS) technique for 1 Mbps
and 2 Mbps data rates. In DSSS, each information bit is combined via an XOR
operation with a pseudo-random numerical (PN ) sequence resulting in a high speed
digital stream which modulates a carrier frequency using differential binary phase
shift keying (DBPSK) and differential quadrature phase shift keying (DQPSK)
for 1 Mbps and 2 Mbps operations, respectively. However, to provide the higher
data rates of 5.5 and 11 Mbps, it uses the extension of DSSS to a high rate tech-
nique known as the direct sequence spread spectrum (RH/DSSS) in combination
with 4-bit and 8-bit complementary code keying (CCK), respectively [19]. It also
uses dynamic rate shifting which allows the data rates to be automatically adjusted
to changing radio channel conditions. For each rate, there is an optimal range for the
successful operation. When WLAN devices move beyond the optimal range for a
higher rate operation, they will transmit at the next lower rate. Likewise, if the device
moves back within the optimal range of a higher rate transmission, the connection
will automatically speed up again. The optimal range for a rate depends on the ra-
dio channel condition which, in turn, depends primarily on the Euclidean distance,
though other factors are also present. Most IEEE 802.11 vendors provide the optimal
range for each data rate [22, 23] in accordance with their supplied devices. The basic
access method for WLAN is the distributed coordination function (DCF ) which
uses carrier sense multiple access with collision avoidance (CSMA/CA) . It re-
quires each station to sense the channel before transmitting. If the channel is found
to be idle, the station transmits. If the channel is busy, a station defers its transmis-
sion until the channel becomes idle and then starts transmitting. If a collision occurs,
the colliding stations wait for a random time using a binary exponential backoff al-
gorithm and then try again. In the infrastructure mode, authentication is established
between an AP and STA before they start communicating.
