Underwater sensor networks (USNs) are characterized by the limited bandwidth and long propagation delay because they use radio waves to transmit data packets. The characteristics pose challenges to the design of medium access control (MAC) protocol. In particular, handshaking based MAC protocols are not appropriate in USNs because of the large delay in exchanging control and data packets. In this paper, we propose a new MAC protocol, called node grouping based multiple data transmission (NG-MDT) protocol to improve performance in USNs. In the proposed protocol, nodes are grouped in a distributed manner according to the distance between themselves and the destination node. When a node obtains a channel access right, it selects one node in the next group. And then, it transmits its own data packet, including the address of the selected node. The selected node also selects another node in its next group, and transmits its data packet including the address of the selected another node. This process is repeated until the last group is reached. In the proposed protocol, multiple nodes transmit their data packets consecutively with one handshaking for channel reservation. Performance evaluation is conducted using simulation, and confirms that the proposed protocol significantly outperforms the previous protocol in terms of throughput, end-to-end delay, and frame success ratio.
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