# A Novel Cooperative Opportunistic Routing Scheme for Underwater Sensor Networks
## 5 Experimental Results
### Results and Analysis
Average end-to-end delayThis criteria measures the average delay time taken from the moment of the creation of packets at the source until being received by the sink for all of the successfully-received packets. We take into account the propagation delay, transmission delay and holding time of packets for calculating the end-to-end delay. The average end-to-end delay for each protocol is plotted in Figure 13. The average end-to-end delay for all protocols decreases by increasing the number of nodes, because the forwarding node can find more qualified nodes in its neighbourhood. The latency of OVAR is very small in comparison to other protocols, because packets almost use the optimal path towards the sink with the least possible transmissions. However, in VBF and HHVBF, nodes with better progress towards the sink may be located at the outside of the pipe, and ignoring them can increase the latency. Moreover, VBF and HHVBF only give higher priority to the nodes that are close to the virtual vector (which is drawn from the source or sender to the sink) and not necessarily the nodes with a lower hop count distance to the sink. On the other hand, VAPR imposes more delay on packets in comparison to OVAR, because it is not flexible enough to forward the packet in any direction and only can maximise expected packet advancement upward or downward (according to the selected direction). This feature of VAPR increases its delay despite the use of reachability information. Furthermore, in OVAR, each node can hold a packet with less average holding time (by setting a lesser amount of \(T_{Delay}\)) due to the fact that candidate nodes are closer to each other on average. However, the  
Figure 12: Energy consumption per message _vs._ node density. predefined maximum delay) of VBF and HHVBF is obviously longer than that of our method because of the different ways of the selection and prioritization of the forwarding nodes. VAPR also takes advantage of the ACK mechanism, which increases its delay. In OVAR, the number of collisions and retransmissions reach the least amount possible, and this improves the packet delivery time. However, in VBF and HHVBF, forwarding candidates may be located at different sides of the pipe, and because of the hidden nodes, collisions will be increased at the receiver. As a result, only the packets that avoid the collisions (by using the back-off process) can successfully be delivered to the sink. Thus, the latency of VBF and HHVBF is increased by increasing the number of retransmissions due to the existence of hidden nodes in the pipe. On the other hand, the VAPR approach is not reliable enough to completely remove all of the hidden nodes from the forwarding set. This is due to the fact that VAPR constructs the forwarding set based on the estimated distances of two hops' connectivities, which is not sufficiently precise in comparison to the OVAR approach. Average hop countThis shows the average number of hops on the routing path from the source to the sink. Involving as few as possible nodes to deliver a packet is a desirable factor. An efficient routing protocol should find its path toward the destination with the least possible number of relay nodes (e.g., minimum number of transmissions). Ideally, a packet should not be directed toward a void area. However, this is not always possible for some algorithms, like VBF and HHVBF. Figure 14 shows that the number of hops to reach the destination is varied with changing the node density. In sparse scenarios, the shortest line between the source and sink is not always covered by some nodes, and consequently, a longer path, with more number of hops, should be taken to reach the destination. However, as the network becomes denser, a lesser number of hops is on average required; the chance of finding the routes with the least number of hops is also higher. As can be seen, OVAR can deliver packets to the destination by involving lower intermediate nodes due to its global view of the network topology and its flexibility to change the forwarding direction.