A Handshake-Based Low Delay MAC Protocol in Hybrid Topology for Urban Lake Monitoring

Underwater acoustic sensor networks (UASNs), which adopt acoustic communication, have been widely used underwater, such as in oceans and rivers, for monitoring applications. However, the UASNs designed for urban lake monitoring are facing challenges and are seldom regarded. Based on the analysis of the features of the urban lakes and the monitoring requirements, a UASN system is designed in this article. The node deployment strategy is discussed to balance the node density and transmission bandwidth. Then a hybrid topology is proposed to meet the monitoring requirements, in which the nodes on the water surface construct a 2-D ring topology, and the nodes underwater construct a 3-D grid structure. Based on this topology, a media access control (MAC) protocol-improved handshake-based low delay (i-HALO) is designed which improves the existing handshaking mechanism with a dynamic adaptive backoff algorithm to avoid unnecessary back off. As the result, the high delay problem is addressed and the “one handshake, multiple transmission” is realized. The channel access delay is analyzed theoretically and the formula of waiting time for a frame before sending is given. The performance of throughput and transmission delay under different node densities and frame lengths are simulated. The results indicate that a node having about 13 neighbors will have the best performance on throughput, and when the frame length is longer than 100 bytes, the throughput and delay will have fluctuation. In comparison with existing protocols, i-HALO has a better performance on throughput, delay, frame reception rate (FRR), and fairness.