In a LAN where it is essential to have communications, some customers will
require redundancy. With the direct sequence spread spectrum (DSSS) products of
a different vendor, both AP units would be set to the same frequency and data
rate, as pictured in Figure
. Since these
units timeshare the frequency, only one unit can be talking at a time. If that
one unit goes down for some reason, the remote clients will hand off to the
other active unit. While this does provide redundancy, it does not provide any
more throughput than a single AP provides.
With the Cisco DS systems, the
units are set to different channels. The remote clients will load balance, when
both units are active, as shown in Figure
. If one
unit goes down, remote clients will hand off to the remaining unit and continue
working. Load balancing can be configured based on the number of users, the bit
error rate, or signal strength.
Another option, when fault-tolerance and availability are critical, is a
hot-standby AP. In this case, there is no load balancing. For business-critical
deployments, a Cisco Aironet AP can be configured as a redundant hot standby to
another AP in the same coverage area. The hot-standby AP continually monitors
the primary AP on the same channel, and assumes its role in the rare case of a
failure of the primary AP. The standby will be ready to take over, if the
primary AP becomes unavailable. Note that both APs in Figure
use the same
channel, or Channel X.