## 4 Simulations and Results
### Performance Under Different Network Loads  
First SFAMA, ROPA, DSCT, and the proposed protocols are tested under different network loads with the node number of 10 and the data packet size of 400 bytes. The simulation results are shown in Figure 7.  
Figure 7a shows the throughputs of all protocols under the offered loads from 0.3 to 5.0. It can be seen that SFAMA gives the lowest throughputs among the four protocols due to its serial transmission mode. ROPA has a similar throughput as SFAMA when the offered load is below 1.5, but becomes higher when the offered load increases. This is because that the opportunity for other nodes to send data to the source node increases as the offered load rises, which increases the concurrent transmission probability in ROPA accordingly. DSCT gives the second highest throughput in the simulation.  
It is superior that ROPA because it imposes less constraints on the source and destination nodes and supports arbitrary multiple successful handshakes in a transmission cycle, which increases the concurrent transmission opportunity. The proposed method achieves the best performance. It reaches its maximum throughput around 4700 bit/s, which is significantly higher than DSCT and ROPA. This is because that the proposed method can support arbitrary multiple successful handshakes in a transmission cycle as DSCT, while eliminates the control packet losses due to collision by using a collision-free handshaking method. The pipeline parallel transmission also helps to increase the throughput of the proposed protocol. It can also be noticed that the throughput of the proposed method reaches its maximum quickly in Figure 7a. It is because that the throughput of the proposed method will become saturated when the duration of data transmission is equal or larger than the handshaking period.  
Figure 7b shows the delivery rates of the four protocols. Because there is no control packet loss due to collision and almost all transmission requests can be satisfied, the delivery rate of the proposed method is nearly 100% in the simulations. The delivery rates of the other three protocols decrease as the offered load increases. DSCT gives higher delivery rates than ROPA because it increases the concurrent transmission opportunity. The SFAMA is inferior to the three parallel transmission protocols.  
Figure 7c shows the average end-to-end delays of the four protocols. It can be seen that the average end-to-end delay decreases as the transmission parallelism increases for SFAMA, ROPA, and DSCT, where DSCT gives the lowest average end-to-end delay while SFAMA the highest among the  
Figure 7: Performance of the slotted floor acquisition multiple access (SFAMA), reverse opportunistic packet appending (ROPA), and distributed scheduling based concurrent transmission (DSCT) protocols and that of the proposed method under different network loads. (**a**) throughput; (**b**) packet delivery rate; (**c**) average end-to-end delay; and (**d**) average energy consumption.  
three protocols. The proposed protocol shows stable but larger average end-to-end delays under different offered loads than SFAMA, ROPA, and DSCT. It is because that the TDMA handshaking introduces a fixed delay to the transmission, which is usually larger than the normal handshaking delays and cannot be shortened by the pipeline technique. It is the price for eliminating the collision of control and data packets.  
Figure 7d shows the average energy consumption of the four protocols. By eliminating the collision of control and data packets, the proposed protocol achieves the lowest and stable average energy consumption among the four protocols. The average energy consumption also decreases as the transmission parallelism increases for SFAMA, ROPA, and DSCT because higher transmission parallelism can reduce the handshake attempts for a data transmission, which will lead to fewer handshake packets wasted.