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Wireless mesh networks boost reliability

By Bob Jordan, special to Network World
November 10, 2003 12:11 AM ET
This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.

Network World - Wireless networks have long embraced a centralized model that holds the potential for bottlenecks, latency and a single point of failure. But wireless mesh networks are emerging as an alternative to wireless switching. Mesh networks distribute intelligence from switches to access points by incorporating a grid-like topology.

The development of this topology parallels the architecture evolution in the computer industry. First, computing environments were stand-alone mainframes; these were followed by client/server and then peer-to-peer. Network architecture inevitably will evolve to a distributed, dynamic wireless architecture.

A mesh network allows nodes or access points to communicate with other nodes without being routed through a central switch point, eliminating centralized failure, and providing self-healing and self-organization. Although decisions on traffic are made locally, the system can be managed globally.

Wireless expansion

Today's wireless LAN mesh networks use standards-based 802.11a/b/g, but they can be extended to any radio-frequency technology, such as UltraWideband or 802.15.4 Zigbee. Because network intelligence is contained within each access point, no centralized switches are needed - just intelligent access points with network processors, switching capability and system software.

For a network to intercommunicate in a mesh topology, the nodes' self-discovery features first must determine whether they are to serve as access points for wireless devices, as backbones for traffic coming from another node, or a combination of roles.

Next, the individual nodes locate their neighbors using discovery query/response protocols. These network protocols must be parsimonious so they don't add much overhead to the traffic - that is, they cannot require more than 1% to 2% of available bandwidth.

Once the nodes recognize one another, they measure path information such as received signal strength, throughput, error rate and latency. These values must be communicated among the neighbor nodes, but this information must not take up very much bandwidth. Based on these signal values each node then selects the best path to its neighbors so the optimum quality of service is obtained at any given moment.

The network-discovery and path-selection processes run in the background, so that each node maintains a current list of neighbors and frequently recomputes the best path. If a node is taken off the network - for maintenance, rearrangements or failure - the adjacent nodes quickly can reconfigure their tables and recompute paths to maintain traffic flow when the network changes. This self-healing attribute, or failover, sets mesh topologies apart from hub-and-spoke networks.

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