Improvement of Medium Access Control Protocol to Reduce Latency in Ubiquitous Wireless Sensor Networks

In several ubiquitous sensor networks, sensor nodes often face contention for medium access and service. Unfortunately, traditional carrier sense multiple access with collision avoidance (CSMA/CA) protocols such as the IEEE 802.11 Distributed Coordination Function (DCF) do not handle such constraints adequately, and this weakness finally leads to an increase in the latency and degradation of the throughput as the scale of a network is increased. Therefore, we present a more efficient method for medium access in a real-time ubiquitous sensor network. The proposed MAC protocol is similar to conventional CSMA/CA protocols, except that it does not use a time-varying contention window from which a node randomly selects a transmission slot. To reduce the latency for the delivery of event data from sensor nodes, a fixed-size contention window with a non-uniform probability distribution of transmitting in each slot is selected. Through a simulation using ns-2, a widely used network simulation package, it is shown that the proposed method can reduce the latency considerably as compared to conventional IEEE 802.11 MAC protocols for a sensor network with up to 256 nodes. It is also shown that the proposed MAC scheme realizes a latency similar to that realized by a decentralized CSMA-based MAC protocol for real-time ubiquitous sensor networks that are sensitive to latency.