Tomac – Real-time Message Ordering in Wireless Sensor Networks Using the Mac Layer

The ordering of messages according to a given time measure is a largely discussed topic. Nevertheless, wireless or wired systems considering the order of messages do not use the access layer to accomplish this task. In this paper, we discuss the use of non-destructive bit-wise arbitration on the MAC layer for real-time message ordering in wireless sensor networks. With this access method, it is possible to use the measure of elapsed waiting time of a message as an input parameter to the channel access. Through the use of adaptive priorities, messages carrying older information get higher priorities and can therefore gather access first. This hard real-time message ordering mechanism realizes message ordering for one-hop distance mesh topologies. Additionally it is an ideal basis for message-ordering transport protocols. We discuss the system in the parameter context of WiFi (IEEE 802.11a) and Zigbee (IEEE 802.15.4) and successfully implemented the access method on a wireless sensor network.