## 5 Hierarchical Sensor Network Design  
Although this paper provides several new architectures for underwater pipeline monitoring, it is noted that these architectures follow the linear sensor network model which we previously introduced in [31]. This general model can be adapted for the various categories of networks that were introduced. In this section, a brief description of this model is presented, and in a later section, simulation experiments performed to evaluate the performance of a specific implementation of this model will be offered. One of the important aspects of this hierarchical structure is its ability to take advantage of the linear structure to improve the reliability and robustness of the network in reaction to node failures. Even though the failures can have various causes, including intentional physical attacks, equipment failures, battery depletion (in case of battery-powered nodes), the network and corresponding message routing protocols are able to react quickly, autonomously, and efficiently to overcome the failures. In the above model, the following three types of nodes are defined. Each of these types of nodes has different functions to perform in the data collection, routing and final dissemination to the NCC.  
* _Basic Sensor Nodes_ (BSNs): These are the most common nodes in the network. Their objective is to perform the sensing function and communicate this information to the data relay nodes.
* _Data Relay Nodes_ (DRNs): These nodes serve as information collection nodes for the data gathered by the sensor nodes in their one-hop neighborhood. They also play a role in routing the data to the NCC. The distance between these nodes is determined by the communication range of the MAC protocol used.
* _Data Dissemination Nodes_ (DDNs): These nodes perform the function of delivering the collected data to the NCC. The technology used to transfer data from these nodes to the NCC can vary. For example, satellite cellular technology can be used. This implies that each of the DDN nodes would have such communication capability.  
The DDNs provide the network with increased reliability since the collected sensor data would not have to travel all the way along the length of the pipeline from the sensing source to the NCC. This distance is usually very long and can be hundreds of kilometers which would make it vulnerable to a large number of possible failures, unacceptable delay, higher probability of errors, and potential security attacks. The DDNs allow the network to pass on its sensor data simultaneously in a parallel fashion. Additionally, the distance between the DDNs is important and affects the reliability of the network. A small distance between the DDNs would increase the equipment cost of the network, as well as deployment and maintenance costs. On the other hand, a distance that is too large would decrease the reliability, security, and performance of the network. Figure 10 shows a graphic representation of the different types of nodes and their geographic layout.  
A hierarchical relationship exists between the various types of nodes in the sensor network. Multiple BSNs transmit their data to the nearest DRN. In turn, several DRNs transmit their data using a multi-hop strategy through the intermediate DRNs to the nearest DDN node. Finally, all DDNs transmit their data to the NCC. BSNs, DRNs, and DDNs can be completely wireless nodes that are equipped with batteries and used for underwater acoustic wireless sensor network or RF wireless sensor network architectures. BSNs, DRNs, and DDNs can be wired/wireless nodes and used for integrated wired/acoustic wireless sensor network and integrated wired/RF wireless sensor network architectures. In this case, the nodes will be equipped with rechargeable batteries that can be recharged from the wire. In addition, the wireless communication will be used only if there are some broken wires. Under normal conditions, the wire will be used for communication. In all cases, nodes can be connected to buoys on the sea surface for RF wireless communication.