A MAC-Layer Retransmission Algorithm Designed for the Physical-Layer Characteristics of Clustered Sensor Networks

We consider scenarios in which an event, such as a mobile node arriving to collect data or a data-request arriving at a node via the sensor network, initiates the collection of one packet of data from each node in the surrounding cluster of nodes. The node requesting the data broadcasts its identity and purpose to the cluster and assumes the role of clusterhead. This broadcast message is also used to synchronize all nodes in the cluster, which covers an area with a radius of several hundred meters, to within a few microseconds. We exploit this ability to synchronize the cluster to improve retransmission algorithms in CSMA/CA-based MAC protocols. The result is a retransmission algorithm that uses a synchronized, shared contention window (SSCW) that is easy to implement and results in fewer collisions than retransmission algorithms that use binary-exponential-backoff (BEB). A simple analytical approach is further proposed to optimize the size of the shared contention window. We show via analytical, numerical, and simulation results that an SSCW-based 802.11 DCF protocol performs significantly better in these scenarios we consider than both 802.15.4 (ZigBee) and BEB-based 802.11 DCF (WiFi). We also show that SSCW-based 802.11 is more fault-tolerant and fair than TDMA