## 3 Sensor Network Architectures for Monitoring Underwater Pipelines  
This section develops a number of sensor network architectures that can be used to monitor underwater pipelines. In this section we discuss these architectures and show their advantages, disadvantages, and challenges. These networks can be used to allow a remote facility to detect and to report the positions of any leakage, defect, or risk. One of the main differences between the networks used for pipelines and other networks is that the network needed for pipeline applications is structured in a line where all sensor nodes are distributed on that line. This characteristic enforces some reliability challenges in monitoring pipeline infrastructures. The reliability challenges are linked directly with the reliability of the network connecting these sensor nodes. Having a reliable network is one of the main conditions of having a reliable monitoring system for pipelines.  
Different network architectures are developed in this paper for reliable communication in pipeline systems. These architectures are based on wired networks, wireless networks, or a combination of wired and wireless networks. These architectures are evaluated based on three reliability factors:  
1. _The Connectivity of the Network_: since the pipeline network extends linearly, it is important for the network to be continuously connected to transfer information from the sensor nodes distributed across the pipeline to the Network Control Center (NCC), that provides the main control for the underwater pipeline network, and also to transfer control commands from the NCC or other control points to the actor and sensor nodes.
2. _The Continuity of Power Supply_: pipeline networks will not be able to operate unless there is a sufficient power supply available continuously. Power is needed not only to operate the network but also to operate the sensor and actor nodes.
3. _The Physical Network Security_: pipelines are usually considered important infrastructures. Devices or networks monitoring these infrastructures must be physically secured. Otherwise, the monitoring systems will fail easily and the pipelines will not be appropriately protected.  
These important reliability factors are chosen in this paper based on the characteristics of the targeted environment. First, existing and planned underwater pipelines expand for tens and hundreds of kilometers in unattended and technically challenging environments. In this type of environments, it is not easy to find and fix network faults. Fixing network faults underwater may take a very long time during which the pipelines will be fully or partially unmonitored. Networks in this type of environments are structured linearly where all sensor and actor nodes are distributed along a somewhat single line. These devices will not be able to communicate with the NCC unless the network connectivity along the line is available. Different sensor network architectures can provide different levels of connectivity in the normal case and with the existence of network faults. Generally, it is important to maintain some level of connectivity even with occurring network faults. Second, sensor, control, and other network related devices on the used need power to operate and the network expands on very long distances thus maintaining the availability of power to operate the network is an important factor for maintaining its reliability. Some network architectures can provide continuous power supply for its components while others rely on batteries, which limit the life of the network. Third, as this type of networks is in unattended environments, the reliability of the networks depends on the physical protection of the network components. Different sensor network architectures have different degrees of physical protection, thus, different levels of reliability. Some network architectures can be completely hidden while others have some components that are physically visible, which makes them susceptible to attacks or accidents.