Voice over Wireless LAN Solution Guide v1.0 December 2005
______________________________________________________________________________________________________
Page 27
radio power setting. Another option is to set the minimum data rate before client-driven power
changes occur to be 2 Mbps or less. This ensures that the handsets will roam (if possible) before
power increases occur. In some cases it is better to turn off the auto-tune power feature
altogether for the 802.11b radios.
2.2.3.2 WLAN Security S witch 2300 connectivity
As mentioned previously, the ports on all WSS 2300 models can be grouped as an MLT so that
you have active-active link redundancy between the WSS and other network devices. For high
resiliency, this can be used in coordination with the SMLT capability on the ERS 8600.
2.2.3.3 N+1 WLAN Security Switch 2300 redundancy
The WSS 2300 has N+1 redundancy capabilities, but because of the varying sizes of models,
figuring out the number of WSSs can be a little more involved than with other N+1 redundancy
schemes. If the network is homogeneous with respect to WSS models, the calculation is simple.
For example, after you determine the number of WSS 2380s needed to support the desired
number of APs, add an additional WSS 2380 to the network.
But if there is a mix of WSS models, the N+1 calculation becomes more complex. The first step is
to figure out the number of APs and number and types of WSSs needed. If this includes WSS
2380s, do not forget to include license information (that is, one 2380 might have 40 licenses and
another 80, according to the network design). After you determine this information, you attain N+1
redundancy by identifying the largest WSS in the design and duplicating it. For N+2 redundancy,
identify the largest WSS followed by the second largest, and duplicate both of them in the
network design. For example, if the network has one WSS 2380 with 120 licenses, one WSS
2380 with 40 licenses, and 10 WSS 2360s, N+2 redundancy is achieved by adding one more
WSS 2380 with 120 licenses and one more with 40 licenses. Alternatively, adding two WSS
2380s with 80 licenses each accomplishes the same thing.
AP failover preferences are configurable on a per-AP basis. This means that instead of the AP
simply finding a surviving WSS and attaching to it, you can configure a backup WSS for the AP to
go to in case its preferred WSS fails. Several primary (high bias) and several backup (low bias)
WSSs can be assigned per AP. In the case of multiple WSSs at the same bias level, the WSS
with the most available AP slots is preferred. When configuring explicit fallback WSSs, keep in
mind that each backup AP configuration counts against the configured AP limit of a WSS. This
limit is not the same as the active AP limit of a WSS, but is roughly 2.5 times higher for each
WSS model. For example, a WSS 2380 can support up to 120 active APs, but can have up to
300 AP configurations stored. This means a WSS 2380 can control any of 300 different APs (if
configured this way) as long as no more than 120 of them are active APs at any given time. The
WSS 2360 can have up to 30 configured APs, with only 12 active at a time, and the WSS 2350
can have up to eight configured APs with only three active at a time.
As a general best practice, after you determine the number of WSSs, divide the number of APs
evenly, proportionally speaking, among the WSSs. Then configure the backup preferences as
desired. Another option is to use the auto-DAP feature to let APs find any available surviving
WSS rather than explicitly configuring backup low bias connections to specific WSSs. To do this,
you configure a generic auto-DAP on the WSS 2300 as though it is a real DAP. When a DAP
boots up, it will attempt to find its primary WSS 2300 (that is, its high bias WSS). If that WSS is
not available, it is then directed to the least loaded WSS with an auto-DAP profile configured. The
generic auto-DAP configuration is applied to that AP, and the AP becomes operational.
If a WSS 2300 fails, there is a time lag between that event and the detection of the event by the
AP 2330. During this interval, the traffic streams of any associated devices are interrupted
because they are associated to an AP that has no data path to the network. The AP has not yet
detected that the WSS 2300 is down, so it will not disassociate devices or force them to roam. In