## 5 Discussion  
SFAMA, ROPA, DSCT, and the proposed protocols are all handshaking based protocols, which have to content for the channel to transmit data. SFAMA is the only serial access protocol among the four protocols. Simulation results show that it achieves the worst performance in throughput, packet delivery rate, and average energy consumption. It is also worse than ROPA and DSCT in average end-to-end delay. This means that parallel transmission is an effective way to improve the performance of underwater acoustic networks. In ROPA, when nodes have data to send to the current source node, they can append their data transmission to the current data transmission according to the propagation delays. Therefore, ROPA allows concurrent transmission but sets a constraint to the source and destination nodes. On the other hand, DSCT supports multiple handshakes and concurrent transmissions for arbitrary source and destination nodes in one transmission cycle, which can utilize more concurrent transmission opportunity than ROPA. So DSCT is superior to ROPA in throughput, packet delivery rate, average end-to-end delay, and average energy consumption under different network loads and network sizes. The proposed protocol also supports multiple handshakes and concurrent transmissions as DSCT, except that the handshaking is implemented in a dedicated control channel by TDMA and the transmission rate of the data channels is reduced to maintain the same total transmission rate as the other protocols. The proposed protocol eliminates the control packet collision by using a content-free method for handshaking, therefore achieves significant improvements over DSCT in throughput, packet delivery rate, and average energy consumption when the packet size is properly selected. The major disadvantage of the proposed protocol is its large average end-to end delay. As shown in Figures 3 and 4, a transmission cycle in the proposed protocol consists of four pipeline stages. A message needs three pipeline stages to be passed from the source to its destination, i.e., the propagation delay of a message in a cluster is \(3\times T^{stage}\). After sending a message, the source and destination also need another pipeline stage for receiving or sending the ACK packet before processing the next message. Because \(T^{stage}\geq T^{TDMA}\) and \(T^{TDMA}\) is determined by the size of a cluster, larger cluster size will lead to longer \(T^{stage}\), as shown in Figure 9c. Therefore, the size of a cluster should be chosen carefully to keep the end-to-end delay in an acceptable range. Except the large average end-to-end delay, the proposed protocol can also be improved in several aspects. First, using TDMA makes the proposed protocol inflexible to the network size change. Additional mechanism should be adopted to handle the situation when a node join or leave the network. Second, while replication computation allows a flexible duration of the pipeline stage in a one-hop cluster, it cannot work so well in a multi-hop environment as in the single-hop situation because extra information exchange is needed to keep the inputs of the scheduling algorithm equal for two non-adjacent nodes, which is contradicted with the idea of replication computation. Therefore, the duration of the pipeline stage has to be fixed to the TDMA period in multi-hop networks, which might not be long enough to transmit all data packets in a transmission cycle so as to decrease the transmission efficiency of the proposed protocol. Third, in Equation (1), we assumed that if \(s_{i}\) sends out its data packet before \(s_{j}\), then the data packets from \(s_{i}\) will arrive at \(d_{i}\) and \(d_{j}\) earlier than the data packets from \(s_{j}\). This assumption sets a constraint on the packet arrival sequence at the destination node, which simplifies the question but leads to a sub-optimal solution. Further improvement can be expected by removing this constraint. The greedy algorithm is also sub-optimal, which can be improved by using more sophisticated optimal methods.