Increasing Mesh Availability
A wireless mesh cell has similar properties to the cells used to create a cellular phone network. The technology may define the maximum size of the cell; smaller cells can be created to cover the same physical area, providing greater availability or capacity. This is done by adding RAPs to the cell. Just as in the larger mesh deployment, the decision is whether to use RAPs on the same channel, as shown in Figure 8-16, or use different channels, as shown in Figure 8-17. The addition of RAPs into an area adds capacity and resilience to that area.
Figure 8-16 Two RAPs per Cell with the Same Channel
Figure 8-17 Two RAPs per Cell on Different Channels
Multiple RAPs
Before deploying multiple RAPs, the purpose for deploying these RAPs needs to be considered. If additional RAPs are being considered to provide hardware diversity, they should be deployed on the same channel as the primary RAP. The reason for this is to minimize the convergence time in a scenario where the mesh transfers from one RAP to another. When planning RAP hardware diversity, the 32 MAPs per RAP limitation should be remembered.
If the additional RAPs are being deployed primarily to provide additional capacity, the additional RAPs should be deployed on a different channel from its neighboring RAPs to minimize the interference on the backhaul channels.
When adding a second RAP on a different channel, channel planning or RAP cell splitting can be used to reduce the extent of potential collision domains. Channel planning allocates different non-overlapping channels to RAPs in the same collision domain to minimize the collision probability. RAP cell splitting is a simple, yet effective, way to reduce the collision domain. Instead of deploying one RAP with omni-directional antennas in a mesh network, two or more RAPs with directional antennas can be deployed. These RAPs are collocated but operate on different frequency channels, thus dividing a large collision domain into several smaller ones that operate independently.
If the Wireless Mesh bridging features are being used with multiple RAPs, these RAPs should all be on the same subnet to ensure that a consistent subnet is provided for bridge clients.
If you build your mesh with multiple RAPs on different subnets, MAP convergence times can increase in the event of a fail over as the MAP has to failover to another RAP on a different subnet and DHCP for an appropriate IP address. One way to limit this from happening is to use different BGN for segments in your network that are separated by subnet boundaries. In segmenting in this manner, MAPs do not associate with a RAP on a different subnet and you avoid slow convergence issues and the expense of the higher availability offered by the additional RAPs.
Hope that helps.
