# A Co-Design-Based Reliable Low-Latency and Energy-Efficient Transmission Protocol for UWSNs
## 6 Simulation and Performance Evaluation
### Comparison with VBF, DVOR and DBR  
Now we compare CDTP with DBR [7], DVOR [14] and VBF [22]. In the set of simulations, we set the MAC protocol to R-MAC [35] for DBR, DVOR and VBF. For CDTP, we do not set any MAC protocol. The MAC protocol is preset in Aquasim [33]. As we discussed in the previous chapters, an ALOHA-based protocol cannot be used well in the previous protocol. During the simulation, we try to deploy the ALOHA protocol in DBR, DVOR and VBF. However, the sink node cannot receive any packets in our simulation settings. CDTP utilizes the co-design method to avoid conflicts successfully. Meanwhile, CDTP can also utilize R-MAC but with little delay and energy.  
From Figure 7, we notice that the packet delivery ratio of CDTP and VBF cannot reach one without enough nodes. This is because there may be isolated nodes in a sparse network that are unable to find the next-hop. With the increase of nodes in the network, this phenomenon will be significantly reduced as shown in the figure. As for DBR and DVOR, the void region in an extremely sparse network makes the delivery ratio lower than CDTP. However, the delivery ratio of these two protocols reaches 1 faster than CDTP and VBF. The closer the delivery ratio gets to 1, the more reliable it proves to be. Packets in CDTP are always forward to lower layers, however in sparse networks some nodes may have neighbors but not lower-layer neighbors.  
According to Figure 8, we observe that the multi-copy packets ratio of CDTP and VBF is close to one. In other words, almost all packets are successfully transmitted to the sink without redundancy. Another point to note is that in DBR and DVOR, the packet delivery ratio increases as the number of nodes increases. The multi-copy packets ratio is more than 4 when 500 sensor nodes exist in the system. Solicet, for each packet, at least three other identical data packets are transmitted between nodes and eventually transmitted to the sink. This phenomenon illustrates that receivers in DBR and DVOR may be non-adjacent and will not be prevented from forwarding by the holding time mechanism. Simulation results have shown that selection strategies in CDTP can avoid this problem and thus reduce the occurrence of this phenomenon. Fewer multi-copies means less energy consumption.  
The total energy consumption for different numbers of sinks are shown in Figure 9. We observe that CDTP has a higher overall energy efficiency when compared with other protocols. As the figure shows, the total energy consumption of VBF is always higher than CDTP. The reason is that in VBF, once a source node initiates a transmission, it first sets up a coordinate system originated at itself and floods a data ready packet into the network. Conversely, in the case when the numbers of nodes are 200 and 250, the total energy consumption of DBR is slightly lower than CDTP. This is because the percentage of energy consumption in network topology initialization and maintenance operations in CDTP is greater when there are fewer nodes. However, the multi-copy problem decreases as the number of nodes increases, allowing us to save energy. Combining the two points above, CDTP eventually performs well in respect of total energy consumption.  
Figure 10 shows that CDTP achieves the lowest end-to-end delay among the three protocols. Compared to VBF, the end-to-end delay of CDTP shows a significant reduction. This is because VBF tries to find the shortest path from the source node to the sink along the virtual vector between them. As mentioned before, the sink floods a data ready packet before it initiates a transmission. Then, in the process of searching for the shortest path, the node holds the packet for a time interval to wait for a better forwarding option. These strategies increase the transmission delay of the packets. Compared to DBR and DVOR, CDTP transfers the calculation of holding time