John Heidemann

Understanding Spatio-Temporal Uncertainty in Medium Access with ALOHA Protocols

TitleUnderstanding Spatio-Temporal Uncertainty in Medium Access with ALOHA Protocols
Publication TypeConference Paper
Year of Publication2007
AuthorsA. Syed, W. Ye, B. Krishnamachari, and J. Heidemann
Conference LocationMontréal, Quebec, Canada

The goal of this paper is to gain deep understanding of how location-dependent propagation latency affects medium access control (MAC) by using ALOHA as a case study. MAC protocols in underwater acoustic networks suffer from latency that is five orders-of-magnitude larger than that in radio networks. Existing work on analyzing MAC throughput in RF networks, where the propagation latency is negligible, generally makes assumptions that render propagation latency irrelevant. As a result, only transmit time is considered as being uncertain in contention-based protocols. We introduce the spatial dimension of uncertainty that is inherent to varying locations of transmitters, resulting in unequal propagation latency to a receiver, where collision occurs. We show through simulation that the benefit of synchronization in slotted ALOHA is lost due to such latency. We propose a modification that adds guard bands to transmission slots to handle spatial uncertainty. We then perform simulation and first order analysis on this modified MAC to find its optimal operating parameters.