Understanding spatio-temporal uncertainty in medium access with ALOHA protocols

The goal of this paper is understand 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. In this paper, we investigate the spatial dimension of uncertainty that is inherent to varying locations of transmitters, resulting in unequal propagation latency to a receiver. We show through simulation that the benefit of synchronization in slotted ALOHA is completely lost due to such latency. To handle spatial uncertainty, we propose a modification that adds guard bands to transmission slots. We then perform simulation and first-order analysis on this modified MAC to find its optimal operating parameters. Our simulation and analytic results suggest that shorter hops improve throughput.