The ISM Band and Channel Allocation

The working band assigned to IEEE 802.11b corresponds to an industrial,
scientific, and medical (ISM) band centered on 2.4 GHz. This band is regulated in a different way in Europe, Japan, and the United States. Even at
a European level, France has a particular assignation of this band that motivates
its separate inclusion in Table 4.5, which shows the set of channels
defined for operation with an infrastructure of the IEEE 802.11b type working
in DS spread spectrum mode.
In Table 4.5, the frequencies indicated correspond to the central frequency
of each channel. Given that the bandwidth of each channel is 22
MHz, these channels overlap in part of the band assigned to each one. Figure
4.27 shows this situation for the U.S. case.
Most equipment available in the market permits the choice of one
channel or another through a configuration menu provided by the manufacturers
for this purpose. Thus, Figure 4.28 shows the configuration menu of
an Avaya access point that permits this selection.
Due to the overlap of the part of the frequency band that can be produced
among the different channels when more than one WLAN infrastructure
is operating in neighboring environments, it is necessary to maintain a
minimum separation among the channels to be used. The limit case is produced
when an AP has the capacity to place two PCMCIA cards with the aim of having, for example, two WLAN infrastructures working independently.
In this case, Figure 4.29, obtained from the specifications provided by
Avaya, shows the combinations among channels that enable the cohabitation
of the wireless segments.