Introduction

Mesh networks were born of the necessity to interconnect the many Access Points that are required in a large 802.11x deployment. Meshing is a radically different approach to WWAN service coverage. Instead of the traditional base station, placed in the center of the coverage area communicating with the scattered remote stations, every station may function as the base. The stations are placed such that they have a Line-of-Sight path to at least one other station, preferably several. Each station has routing capability and “awareness” of its neighbor stations. The message signal will be sent from station to station until it is carried back to the hub site, where traffic is aggregated onto a higher-bandwidth facility for backhaul to a switching center or interconnection to a carrier or the Internet. Hub sites must be located throughout the coverage area to balance the traffic loads so that routes through the network do not become traffic choked. As a message is hopped from station to station, processing delays are introduced. In large, wide area mesh networks, numerous hubs will be required in order to avoid excessive delay, also referred to as system latency. There are three basic mesh network designs currently in use: Single, Dual, and Mulit-Radio Mesh.

Single Radio Mesh

The most common today is a single radio mesh AP that supports local clients and forwards traffic wirelessly to other mesh Access Points. The same radio is used for access and wireless backhaul. This means every message packet must be repeated over the same channel and sent to at least one neighboring node. This task of packet forwarding generates significant traffic. The greater the number Access Points in the network, the higher the percentage of traffic-per-cell dedicated to forwarding. This can significantly reduce the amount of channel capacity available for user access. It is basically a bandwidth-limited system design and consequently does not scale very well. Given the bandwidth limitations, single radio mesh networks struggle to support features such as self-healing and redundancy. Optimizing the forwarding protocol won't solve the problem, because the basic capacity is already too low.

Dual Radio Mesh

A dual radio mesh AP has two radios operating on different frequencies in two different mesh topologies. One radio supports user access, while the other provides backhaul. A typical configuration uses 2.4 GHz Wi-Fi for local access and 5 GHz band wireless for backhaul. The access capacity is not impacted by the forwarding traffic since it’s done with a separate radio on a separate RF channel. This is a marked capacity improvement over single radio systems, but may still fall short of the required bandwidth for large, heavy-traffic networks. The reason is that the backhaul mesh is a shared network running the 802.11 MAC protocol. The backhaul radios contend for the channel and generate interference for one another, resulting in increased system latency and reduced system capacity as the network grows.

Multi-Radio Mesh

Multi-radio wireless mesh networks take separate access and backhaul a step further. The backhaul mesh isn’t a shared network. It is designed with multiple point-to-point links, with each link operating on independent channels. This backhaul design enables a performance similar to switched wired connections between the nodes and also allows you to run a custom protocol to optimize throughput. The performance of the multi-radio mesh is exponentially better than the dual or single radio approaches. It delivers more capacity and scales up as the size of the network increases. System capacity is now limited only by the amount of wired backhaul, and perhaps the size of your pocketbook.