An efficient Scheduling and Random Access Hybrid Medium Access Control protocol for Underwater Sensor Networks

Underwater Sensor Networks (UWSN) can collect ocean information and monitor ocean environment. Since the UWSN can only use sound waves to transmit messages, it has the following features as opposed to the wireless sensor network, which uses radio waves to transmit messages: long propagation delay, lower available bandwidth and multi-path problem. The sink node's broadcasting and collecting data from all the sensor nodes are routine works in the sensor networks, but many MAC protocols of sensor networks fail to consider the characteristics of underwater sensor networks' communications to design a more efficient MAC protocol. In this paper, we propose a cross-layered MAC protocol for UWSN named as Scheduling and Random Access Hybrid (SRH-MAC) protocol. In SRH-MAC channel is divided into several superframes with fixed length. Each superframe contains one to three periods. There is an event report period and may be a broadcast and a gathering periods in a superframe. During the broadcast and gathering periods, each sensor node broadcasts and gathers data in a predetermined time slot so as to avoid collisions and reduce idle waiting time. The data is received and transmitted from and to a predetermined neighboring source and destination nodes. The predetermined time slot, source, and destination nodes are set according to the broadcasting and gathering trees. During the event report period, the sensor node reports the sensed event through a random access protocol because events are difficult to predict. Simulation results have shown that the proposed SRH-MAC protocol performs better than existing protocols in terms of network throughput, broadcasting, and gathering latency.