Design Considerations; Wireless Mesh Constraints - Cisco aironet 1522 Design And Deployment Manual

SNR Smoothing
One of the challenges in WLAN routing is the ephemeral nature of RF. This must be considered when
analyzing an optimal path and deciding when a change in path is required. The SNR on a given RF link
can change substantially from moment to moment, and changing route paths based on these fluctuations
results in an unstable network, with severely degraded performance. To effectively capture the
underlying SNR but remove moment-to-moment fluctuations, a smoothing function is applied that
provides an adjusted SNR.
In evaluating potential neighbors against the current parent, the parent is given 20% of bonus-ease on
top of the parent's calculated ease, in order to reduce the ping-pong effect between parents. This implies
that a potential parent must be significantly better in order for a child to make a switch. Parent switching
is transparent to CAPWAP and other higher-layer functions.
Loop Prevention
To ensure that routing loops are not created, AWPP discards any route that contains its own MAC
address. That is, routing information apart from hop information contains the MAC address of each hop
to the RAP; therefore, a mesh access point can easily detect and discard routes that loop.

Design Considerations

Each outdoor wireless mesh deployment is unique, and each environment has its own challenges with
available locations, obstructions, and available network infrastructure. Design requirements driven by
expected users, traffic, and availability needs are also major design criteria. This section describes
important these design considerations and provides an example of a wireless mesh design.

Wireless Mesh Constraints

Here are a few system characteristics to consider when designing and building a wireless mesh network.
Some of these apply to the backhaul network design and others to the CAPWAP controller design:
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Cisco Aironet 1520, 1130, 1240 Series Wireless Mesh Access Points, Design and Deployment Guide, Release 6.0
OL-20213-01
Cisco recommends setting the backhaul rate to auto.
When the bridge data rate is set to auto, the mesh backhaul chooses the highest rate possible given
its link quality and the sustainability of that rate. Bridge data rate is set on each access point
individually. It is not a global setting.
Typically, 24 Mb/s is chosen as the optimal backhaul rate because it aligns with the maximum
–
coverage of the WLAN portion of the client WLAN of the MAP; that is, the distance between
MAPs using 24 Mb/s backhaul should allow for seamless WLAN client coverage between the
MAPs.
A lower bit rate might allow a greater distance between mesh access points, but there are likely
–
to be gaps in the WLAN client coverage, and the capacity of the backhaul network is reduced.
An increased bit rate for the backhaul network either requires more mesh access points or results
–
in a reduced SNR between mesh access points, limiting mesh reliability and interconnection.
– The mesh channel and bridge data rate (mesh backhaul bit rate) is set on each individual access
point. It is not a global setting.
Design Considerations
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