Information Technology Reference
and overlap. Besides, specifying the centre frequency of each channel, 802.11
also specifies a spectral mask defining the permitted distribution of power
across each channel. The mask requires that the signal can be attenuated by at
least 30 dB from its peak energy at ±11 MHz from the centre frequency, the
sense in which channels are effectively 22 MHz wide. One consequence is
that stations can only use every fourth or fifth channel without overlap .
IEEE 802.11 - amendments b and g - uses the 2.4 GHz ISM band.
Because of this choice of frequency band, 802.11b and g equipment may
occasionally suffer interference from microwave ovens, cordless telephones
and Bluetooth devices. 802.11b/g uses the DSSS signalling and orthogonal
frequency-division multiplexing (OFDM) methods, respectively.
OFDM is a scheme utilized as a digital multi-carrier modulation method.
A large number of closely spaced orthogonal sub-carriers are used to carry
data. The data is divided into several parallel data streams or channels, one for
each sub-carrier. Each sub-carrier is modulated with a conventional modulation
scheme such as quadrature amplitude modulation (QAM) or phase shift keying
(PSK) at a low symbol rate, maintaining total data rates similar to conventional
single-carrier modulation schemes in the same bandwidth.
The primary advantage of OFDM over single-carrier schemes is its ability
to cope with severe channel conditions - for example, narrowband inter-
ference and frequency-selective fading due to multipath - without complex
equalization filters. Channel equalization is simplified, because OFDM may
be viewed as using many slowly modulated narrowband signals rather than
one rapidly modulated wideband signal. The low symbol rate makes use of
a guard interval between symbols affordable, making it possible to handle
time-spreading and eliminate inter-symbol interferences. This mechanism
also facilitates the design of single-frequency networks, where several
adjacent transmitters send the same signal simultaneously at the same
frequency, as the signals from multiple distant transmitters may be combined
constructively, rather than interfering as would typically occur in a traditional
802.11a uses the 5 GHz band, which offers at least 19 non-overlapping
channels rather than the 4-5 offered in the 2.4 GHz ISM frequency band, and
data transfer rates up to 108 Mbps.
Infrared Data Association (IrDA)  was designed to allow for simple
short distance communications (<1 m). Devices involved in such communi-
cation need to be in sight, and this is one of the main IrDA disadvantages.
Besides, the short range, the angle between the infrared emitter and the