Efficient Broadcast MAC for Underwater Networks

I. PROBLEM DESCRIPTION Underwater sensor networks present a unique set of challenges due their long propagation delays, low data rates, and high transmit power. These require a thorough redesign of medium access control (MAC) functionalities. Here, we focus on the important case of a broadcast MAC, where all neighbors of a node are the intended receivers (not to be confused with network-layer broadcast). This is an important networking primitive, as it forms the basis of many higher layer services such as neighbor discovery, route establishment, self-localization, flooding, etc. The goal is to make this broadcast MAC protocol reliable and energy-efficient. Existing MAC protocols for underwater rely on mechanisms and handshakes that only work for unicast [2], and not for broadcast. Specifically, unicast handshakes suffer from what is known as the ‘reply storm’ problem. On the other hand, MAC broadcasting solutions developed for terrestrial RF networks [4] are not suitable in underwater scenarios. The reason is that the underwater environment has large distancedependent propagation delays, precluding techniques such as time-based priorities or fixed-delay time slotting. Also, transmit power dominates in the underwater case, and minimising overhead and retransmissions becomes crucial.