Z-MAC: Hybrid MAC for Wireless Sensor Networks

Z-MAC is a medium-access control (MAC) protocol designed for wireless sensor networks which combines the strengths of TDMA and CSMA while offsetting their weaknesses. Z-MAC uses an efficient TDMA channel reuse schedule from a distributed implementation of RAND, as a hint to enhance performance of CSMA, especially during high contention. In this scheme, the scheduling overhead is incurred mostly at the deployment time unless new nodes are added during operation. Like TDMA each node is statically assigned a time slot, but unlike TDMA a node can transmit in both its own time slot and other slots assigned to other nodes, but owners of the current time slot always have high priority in accessing the channel over the non-owners.  Therefore, under low contention where not all the owners have data to send, non-owners can "steal" time slots from owners. This contention resolution scheme has an effect of switching between CSMA and TDMA depending on contention, thus achieving the performance of CSMA under low contention (i.e. high channel utilization and low delays), and that of TDMA under high contention (i.e., high channel utilization, low contention and fairness). Z-MAC is also robust to topology changes and clock synchronization failures; in the worst case its performance always falls back to that of CSMA. Z-MAC uses CC and LPL features of B-MAC and is implemented in TinyOS.

[This work has been financially supported in part by NSF-NOSS 0435157]

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       Injong Rhee, Ajit Warrier and Lisong Xu (Technical Report TR-2005-21, Department of Computer Science, North Carolina State University, April 2005)

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