# A Novel Cooperative Opportunistic Routing Scheme for Underwater Sensor Networks
## 5 Experimental Results
### Results and Analysis  
In this section, we evaluate the performance of OVAR against those of VBF, HHVBF and VAPR in terms of packet delivery ratio, energy tax, end-to-end delay, average hop count and propagation deviation factor. Packet delivery ratio (PDR)This is defined as the ratio of the number of packets successfully received by the sink node to the number of packets generated by the source. The results for the packet delivery ratio at different node densities are shown in Figure 11. PDR is increased by increasing the number of nodes, because it reduces the size of void areas and also their number, In a dense network, more forwarding nodes have this chance to be placed in the routing path, and consequently, the PDRs of routing protocols converge to a high value. On the contrary, the majority of nodes in sparse networks are disconnected, which leads to lower PDR. OVAR always has higher PDR than that of other routing protocols (especially in a sparse network), because it inherently excludes all of the routes leading to a void area and enhances the packet delivery probability in each step towards the destination. However, in VBF and HHVBF protocols, packet failure is increased when the void area appears in their routing pipes. Furthermore, these protocols do not take into account the packet delivery probability as a criteria for forwarding nodes' selection. In VAPR, when the network is sparse, it has a better packet delivery ratio in comparison to that of HHVBF, but it falls below HHVBF by increasing the network density. It should be mentioned that VAPR is an efficient approach in a multi-sink architecture, while its performance is diminished in a single-sink architecture. Energy taxWe measure the energy tax in millijoules (mj) in terms of energy spent per node and per message to route a packet towards the destination (including energy for sending mode, receiving mode and idle mode). Figure 12 plots the energy tax for each protocol _versus_ the number of nodes. As can be seen, OVAR consumes lower energy than other protocols for the delivery of each packet to the sink. This is due to the fact that OVAR confines the forwarding candidates in a cluster (without hidden nodes), which can prevent redundant packet transmissions and collisions. However, the radius of the pipe in VBF and HHVBF has a great impact on the total energy consumption and packet delivery ratio. Selecting a large radius can involve more nodes in packet forwarding; however, it increases duplicated packets, which leads to more energy waste. On the other hand, a lower radius causes more packet failures. In contrast to our approach, using pipelines for opportunistic routing is not able to achieve  
Figure 11: Packet delivery ratio _vs_. node density. an appropriate trade-off between lower energy consumption and a higher packet delivery ratio. In sparse networks, the energy tax of VBF and HHVBF is high due to the low packet delivery ratio of them. Although VAPR does not have the duplicated packets problem, it still consumes more energy than OVAR. This is due to the fact that the number of forwarding nodes in the forwarding set is not adjusted according to the network density. VAPR also should periodically measure all distances to its neighbouring nodes to be used for forwarding set selection and forwarding timers' calculation, resulting in more energy consumption. In terms of normalised energy consumption, OVAR is highly efficient and achieves a high delivery ratio over its consumed energy. In dense networks (while the delivery ratio has almost reached the maximum), increasing the number of nodes has little contribution to the packet delivery ratio, but wastes energy. However, OVAR mitigates this energy waste by using a principled approach to modify the forwarding set. To achieve this, in dense networks, the number of forwarding nodes is slightly increased or held constant by OVAR, to control the energy dissipation.