Cisco Cisco Aironet 1522 Lightweight Outdoor Mesh Access Point デザインガイド

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.

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.

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.

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:

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.