## 4 Simulations and Results  
The simulations were implemented by Aqua-Sim [28], which is an NS-2 based simulator for underwater sensor networks and developed at the Underwater Sensor Network (UWSN) Lab at the University of Connecticut. In the simulations, the nodes are randomly located in a square area \(1.5\times 1.5\) km\({}^{2}\) and 500 m deep. The single-hop static network is tested in the simulations, i.e., all nodes in the network are static and can reach the other nodes directly. In the network initialization stage, each node measures the round-trip time with other nodes, and broadcasts this information to the others to obtain an inter-nodal propagation delay table. The acoustic propagation speed is set to 1500 m/s. Transmission rate is set to 1200 bps. Control packet size is 10 bytes and the data packet size varies as required. The powers of sending, receiving, and idle states are set to 20, 1 and 0.5 Watt, respectively. Data packets generation for each node follows a Poisson distribution, and the destination of each packet is selected randomly with equal probability. Channel-related packet loss is ignored, i.e., packet loss only occurs due to packet collisions. A two channel network is tested for the proposed protocol, where one channel for control packet transmission and another channel for data packet transmission. The ratio of the control and data channel's transmission rates is set to 0.7, while the total transmission rate of the two channels maintains 1200 bps as the other protocols.  
In the simulations, the throughput, packet delivery rate, average end-to-end delay, and average energy consumption of the proposed method are compared with the slotted floor acquisition multiple access (SFAMA), reverse opportunistic packet appending (ROPA) [14] and distributed scheduling based concurrent transmission (DSCT) [18] protocols with different network loads, data packet sizes and node numbers. SFAMA is a typical serial access protocol while the others are all parallel access protocols. In ROPA, concurrent transmissions will occur when other nodes have data to send to the source node. DSCT supports multiple handshakes and concurrent transmissions for arbitrary source and destination nodes in one transmission cycle, which can utilize more concurrent transmission opportunities than ROPA. 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 channel is reduced. The throughput, packet delivery rate, average end-to-end delay, average energy consumption and normalized offered load are defined as [29]:  
* [noitemsep,topsep=0 pt]
* Throughput: the average number of data bits successfully received by the intended destinations per second (in bit/s).
* Packet delivery rate: the ratio of the number of data packets successfully delivered at the intended destinations to the total number of data packets generated (dimensionless).
* Average end-to-end delay: the average time interval between generation and successful delivery of data packets at the intended destinations (in ms).
* Average energy consumption: the ratio of the total energy consumption to the number of data packets successfully received by the intended destinations (in Joule/packet or J/pkt).
* Offered load: the average number of data packet generated per second for each node (in pkt/s).