Comparing underwater MAC protocols in real sea experiments

Underwater acoustic networks (UANs) have drawn significant attention from both academia and industry in recent years. Even though a number of underwater MAC protocols have been proposed and studied based on simulations and theoretical analysis, few work has been conducted to test these protocols in multi-hop real sea experiments. Due to the complex multipath environment, fast varying acoustic channel and heterogenous link condition, it is difficult for existing network simulators to evaluate the performance of MAC protocols in the real world. This paper presents the results of a multi-hop sea experiment comparing three representative MAC protocols: random access based UWAloha, handshaking based SASHA, and scheduling based PMAC. From the experiments, we identified several problems that have never been well studied before, such as heterogeneous packet delivery, temporal and spatial transmission range uncertainty, multi-hop interference and delayed data transmissions. Discussions are provided based on the new discoveries, in hopes of giving some meaningful insights into the practical MAC design for real multi-hop networks.