Impact of contention on performance of flows in multi-hop MAC protocol for sensor networks

Abstract

The MAC protocol in wireless sensor networks (WSNs) plays an important role in conserving energy and it generally adopts a duty cycling mechanism to eliminate idle listing energy at the cost of high delivery latency. The most effective method of bypassing the latency disadvantage is forwarding a packet over multiple hops in an operational cycle. The combination of duty cycling and multi-hop forwarding creates a new class of MAC protocol called multi-hop MAC, which has been proven to outperform other single-hop duty cycling protocols both in term of energy efficiency and latency. A packet in such a protocol is always relayed via multiple hops toward a sink in a cycle. The flow from a source node to the sink is affected by contention because of the sharing characteristic of wireless channels. We investigated what effect contention had on the flow capacity of Demand Wakeup MAC (DW-MAC), which is a state-of-the-art multi-hop MAC protocol. We also found that instead of using the original large contention window (CW), a DW-MAC's flow using a smaller CW could avoid unnecessary contention and use channels more efficiently. The analysis and resulting ns-2 simulation revealed that DW-MAC with the new values for CW achieves higher throughput, lower end-to-end latency, and greater energy efficiency.