Delay-Bounded Medium Access for Unidirectional Wireless Links
Ref: HURRAY-TR-060701 Publication Date: 1, Jul, 2006
Delay-Bounded Medium Access for Unidirectional Wireless LinksRef: HURRAY-TR-060701 Publication Date: 1, Jul, 2006
Wireless sensor networks (WSNs) perform collaborative processing and communication of sensor readings. The sensor readings are valid only during a certain time interval and hence it is necessary that a message reaches its destination node before a pre-specified deadline. Collaborative processing mandates that the network be connected whenever radio conditions permits so. This may require that a link from sensor node A to sensor node B is used although there is no link from B to A. Such links are called unidirectional.
We study medium access in wireless sensor networks where links may be unidirectional and messages have timing requirements. Three results are presented. First, we present a medium access control (MAC) protocol which replicates a message with carefully selected pauses between replicas, and in this way it guarantees that for every message at least one replica of that message is transmitted without collision. The protocol ensures this with no knowledge of the network topology and it requires neither synchronized clocks nor carrier sensing capabilities. Second, we propose schedulability analysis techniques for the protocol. Third, we implement the protocol and show experimentally that it reduces the number of lost messages (and deadline misses) significantly as compared to schemes with pauses of random duration between replicas. We believe these results are significant because (i) this protocol is the only one that supports schedulability analysis and is designed for unidirectional links and (ii) of all MAC protocols in the literature that support schedulability analysis, our protocol is the one that makes the least assumptions.