Exploring the Design of an Asynchronous and Asymmetric MAC Protocol

In the design of a MAC-layer protocol for battery-powered wireless sensor networks, two powerful enhancements are asymmetry and asynchrony, both of which add flexibility. Asymmetry allows nodes to adapt their schedules to fit their roles, whether source, sink and/or forwarder. Asynchrony can grant nodes freedom to perform tasks when appropriate instead of waiting for an agreed upon time. These features are important for dealing with varied node workloads. If we let nodes with larger workloads consume more energy, these will be the first to deplete their batteries, causing problems for cooperative networks. For example, forwarders, which generally have higher workloads, may die first, creating partitions. This leaves remaining nodes unable to communicate, despite having energy available. In this paper, we present an overview of SEESAW, a protocol designed to provide balanced energy consumption, and explore its parameter design space. We also present SenSim, a sensor network simulator designed for fine-grained energy and time accounting of node radios. We validate SenSim by reproducing results from published MAC protocol implementations on Mica2 Motes. Finally, we explore the impact of clock instability on SEESAW.