## 6 Performance Evaluations  
In this case study, we focus on the routing protocols that can be used to transport the collected BSN sensor data across the linear network using the DRN nodes. The presented protocols take advantage of the linear nature of the network to increase the robustness in reaction to failures, reliability, and efficiency of the routing and communication process.  
Communication between the DRN and DDN nodes is done using a multi-hop routing algorithm which functions on top of a MAC protocol such as Zigbee/IEEE 802.15.4. As mentioned earlier, the DRN nodes can play the role of Zigbee FFD nodes. The Zigbee routing mechanism for the FFD nodes normally uses the AODV routing protocol where the FFD nodes act as network routers. While the AODV routing protocol is relatively effective for a multi-dimensional sensor network with an unpredictable infrastructure, it incurs too much overhead in message exchanges related to route discovery and maintenance [32-35]. In this paper, we propose two routing protocols for DRN-to-DDN communication that take advantage of the special linear structure of the network to increase routing efficiency, and reliability. When a node fails, our proposed protocols either jump over the failed node by extending the node's transmission range, or redirect the message in the opposite direction to reach an alternative DDN node. This kind of adaptive reaction to node failures which is specific to WSNs with a linear structure is not available in AODV. Instead, in reaction to node or link failure AODV would reinitiate route discovery without extending the range of the node that is a neighbor of the failing node and would simply fail to find a new route to the target DDN node. After several failed route discoveries, which consume precious bandwidth and cause unnecessary delay, AODV will simply drop the message. AODV would not be aware of the fact that by simply extending the range of the neighboring node the target DDN node can be reached. Also, AODV would not be aware, that  
Figure 10: A representation of a three-level hierarchical sensor network design.  
another alternative route to another nearby DDN node can also be used to ultimately transmit the DRN data to the NCC. These important strategies are only possible due to the special linear topology and scalable hierarchy of the network. Furthermore, as indicated in [34], when the number of nodes in the network grows, the routing table which is necessary for the proper functioning of AODV simply grows too fast in a manner that is not desirable for sensors devices which usually have a relatively small memory. The same argument can be used to show why other similar _ad hoc_ and sensor network routing protocols would also not be very effective in this environment.  
With some basic modifications, the hierarchical model described in the previous section can be used to represent any one of the various architectures presented earlier. For example, the underwater nodes belonging to the underwater acoustic sensor network presented in Figure 4 can be considered DRNs, and the surface buoy nodes can be considered DDNs. In addition, each of the DRNs can collect the information to be relayed along the pipeline from multiple BSNs that are local to it. On the other hand, the wireless sensor network shown in Figure 5 can be considered as consisting of DRNs that relay the information among each other until a designated node transmits the collected information to the NCC. Similar modeling can be done for the other presented architectures. In this paper, we consider the network presented in Figure 4 as a case study. Simulation experiments were performed in order to verify the operation, and evaluate the performance of the proposed. Two strategies for using multi-hop routing of data messages through the DRNs to reach the nearest DDN were compared. The two algorithms deal differently with the failure of a next-hop DRN.