ROPA: A MAC Protocol for Underwater Acoustic Networks with Reverse Opportunistic Packet Appending

In most existing sender-initiated handshaking based underwater Media Access Control (MAC) protocols, only the initiating sender is allowed to transmit data packets to its intended receiver after the channel has been reserved; none of the potentially backlogged neighbors of the sender can transmit in the duration after the current handshake. Therefore, each of those neighbors must initiate their own handshakes, which incur additional overheads and potentially result in poor channel utilization. In this paper, we present a novel approach to increase the channel utilization by allowing a sender to invite its one-hop neighbors (appenders) to opportunistically transmit (append) their data packets. After the sender finishes transmitting its packets to its own receiver, it can immediately switch its role to receive the incoming appended data packets, which arrive in a packet train manner. This greatly reduces the relative proportion of time spent on control signaling. We refer to this MAC protocol as ROPA -- Reverse Opportunistic Packet Appending. From our extensive simulations and comparisons with existing protocols, we show that ROPA significantly increases the channel utilization and offers performance gains in terms of throughput and delay.