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Sequence Spread Spectrum (DSSS) technology. IEEE 802.11b offers 5.5 and 11 Mbit/s
physical data rates. The multirate control of 802.11 MAC protocol allows a terminal to switch
from the 11 Mbit/s link speed down to 5.5 and even further down to 2 and 1 Mbit/s if the radio
channel quality for some reason cannot accommodate higher data rates.
The fundamental access method of 802.11 MAC is distributed coordination function (DCF)
known as the carrier sense multiple access with collision avoidance, or CSMA/CA. It is
implemented in all stations for use both within independent and infrastructure networks.
CSMA/CA is a "listen before you talk" algorithm – similar to the protocol used in fixed
Ethernet -- where the station wishing to transmit senses the transmission medium to
determine whether it is idle or busy. If the medium is not busy the transmission may proceed.
If the medium is busy the station defers until the end of the current transmission. After
deferral or prior to attempting to transmit again immediately after a successful transmission,
the station must select a random backoff interval before it starts the transmission. This
reduces the probability of a collision at a point where it would most likely occur. This moment
is just after the medium becomes free after transmission. Random backoff of different
stations ensures that not all the stations begin transmission immediately.
The transmission time is limited to ensure fair access to the channel for all the stations.
There has to be a gap of some minimum duration (Interframe Space) between two
transmissions from one station.
2.1.3 Architecture
The standard defines the logical architecture of an 802.11 network. The architectural basic
components are the station, wireless medium, access point (AP), distribution system and
portal. These components are used to form the 802.11 network. There are two fundamental
alternatives for the 802.11 architecture, namely independent (ad hoc) and infrastructure
networks. These are depicted in Figure 1 and Figure 2 respectively.
BSS 1
STA 1
STA 2
BSS 2
STA 3
STA 4
Figure 1. Independent networks BSS1 and BSS2.
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