Wireless sensor networks have a relatively stable topology except for the occasional failure of nodes or addition of new nodes. The path of traffic, being aggregated from a large number of end users, changes infrequently. Practically all the traffic in an infrastructure sensor network is either forwarded to or from a gateway, while in ad hoc networks or client sensor networks the traffic flows between arbitrary pairs of nodes. Wireless sensor networks were originally developed for military applications. Sensor networks are typically wireless. Over the past decade, the size, cost, and power requirements of radios has declined, enabling multiple radios to be contained within a single device, i.e., sensor node, thus allowing for greater modularity; each can handle multiple frequency bands and support a variety of functions as needed-such as client access, backhaul service, and scanning (required for high-speed handoff in mobile applications)-even customized sets of them.