radio channel to which the AP is set. Periodically it surveys all 802.11 channels in
order to access whether a different AP would provide it with better performance
characteristics. If it determines that this is the case, it reassociates with the new
AP, tuning to the radio channel to which that AP is set. Reassociating usually
occurs because the wireless station has physically moved away from the original
AP, causing the signal to be weakened. In other cases, reassociating occurs due to
changes in radio characteristics in the building, or due to high network traffi c on
the original AP. In the latter case this function is known as load balancing, since
its primary function is to distribute the total WLAN load most effi ciently across
the available wireless infrastructure.
The process of dynamically associating and reassociating with APs allows
network managers to set up WLANs with very broad coverage by creating a series
of overlapping 802.11b cells throughout a building or across a campus. To be
successful, the IT manager ideally will employ channel reuse, taking care to set up
each access point on an 802.11 DSSS channel that does not overlap with a chan-
nel used by a neighboring AP (see Figure 21.17).
As noted above, while there are 14 partially overlapping channels specifi ed
in 802.11 DSSS, there are only 3 channels that do not overlap at all and these are
the best to use for multicell coverage (refer to Table 21.5). If two APs are in range
of one another and are set to the same or partially overlapping channels, they
may cause some interference for one another, thus lowering the total available
bandwidth in the area of overlap.
