ENERGY EFFICIENT MEDIUM ACCESS CONTROL FOR WIRELESS SENSOR NETWORKS

A wireless sensor network designates a system composed of numerous sensor nodes distributed over an area in order to collect information. The sensor nodes communicate wirelessly with each other in order to self-organize into a multi-hop network, collaborate in the sensing activity and forward the acquired information towards one or more users of the information. Applications of sensor networks are numerous, ranging from environmental monitoring, home and building automation to industrial control. Since sensor nodes are expected to be deployed in large numbers, they must be inexpensive. Communication between sensor nodes should be wireless in order to minimize the deployment cost. The lifetime of sensor nodes must be long for minimal maintenance cost. The most important consequence of the low cost and long lifetime requirements is the need for low power consumption. With today's technology, wireless communication hardware consumes so much power that it is not acceptable to keep the wireless communication interface constantly in operation. As a result, it is required to use a communication protocol with which sensor nodes are able to communicate keeping the communication interface turned-off most of the time. The subject of this dissertation is the design of medium access control protocols permitting to reach a very low power consumption when communicating at a low average throughput in multi-hop wireless sensor networks. In a first part, the performance of a scheduled protocol (time division multiple access, TDMA) is compared to the one of a contention protocol (non-persistent carrier sensing multiple access with preamble sampling, NP-CSMA-PS). The preamble sampling technique is a scheme that avoids constant listening to an idle medium. This thesis presents a low power contention protocol obtained through the combination of preamble sampling with non-persistent carrier sensing multiple access. The analysis of the strengths and weaknesses of TDMA and NP-CSMA-PS led us to propose a solution that exploits TDMA for the transport of frequent periodic data traffic and NP-CSMA-PS for the transport of sporadic signalling traffic required to setup the TDMA schedule. The second part of this thesis describes the WiseMAC protocol. This protocol is a further enhancement of CSMA with preamble sampling that proved to provide both a low power consumption in low traffic conditions and a high energy efficiency in high traffic conditions. It is shown that this protocol can provide either a power consumption or a latency several times lower that what is provided by previously proposed protocols. The WiseMAC protocol was initially designed for multi-hop wireless sensor networks. A comparison with power saving protocols designed specifically for the downlink of infrastructure wireless networks shows that it is also of interest in such cases. An implementation of the WiseMAC protocol has permitted to validate experimentally the proposed concepts and the presented analysis.