## 3 Sensor Network Architectures for Monitoring Underwater Pipelines
### Integrated Wired/Acoustic Wireless Sensor Networks  
As we can see from the previous sections the main reliability challenges in pipeline sensor networks communication systems are network connectivity, continuity of power supply, and physical security of the network. To solve all these issues, we are proposing in this section new network architecture for underwater pipeline communication systems. The architecture in this system consists of multiple point-to-point segments as shown in Figure 6. These segments link the system nodes.  
Nodes are either sensor or actor nodes. Each node is connected to a acoustic transceiver and a wired network interface. Sensor nodes also consist of processor, memory and storage units. The nodes are connected through wireless acoustic and wired links. Wires are used for networking and for transferring power to the nodes. Unlike completely wireless nodes in the wireless or underwater acoustic sensor network architectures, nodes in this architecture have rechargeable batteries which are charged by the received power through the connected wires. The power can be provided for this network architecture using the techniques used for wired network architecture as discussed in Section 3.1.  
Neighborhood nodes can communicate either using wired or acoustic communication. The transceivers in the normal case are turned off and the wired network is used for communication. Therefore the connectivity of the network is through the wired links in the normal case. Each node periodically checks the status of the right side of the network wire by sending echo messages to the neighboring nodes on the right. Each node also periodically checks the status of the left side network wire by receiving/replying to the echo messages received from the neighboring node on the left. A break of a wired link between two nodes can be discovered by the left node when it does not receive replies for the echo messages it just sent. The break can be discovered by the right node if there are no echo messages received from the left node. When both nodes discover the break, they will activate  
Figure 5: RF Wireless sensor network.  
their transceivers and communicate through the acoustic link. This wireless acoustic link between the two nodes can provide connectivity for the pipeline network and sensed information can be still transported through the network as shown in Figure 6. The nodes that discovered the break will report it along with the location information to the NCC for immediate maintenance. If an intermediate node is disconnected from the left and from the right, the node can operate temporarily using the rechargeable battery until the wire breaks are fixed.  
Link breaks due to faulty nodes can be recovered by using a wider transmission range in which each node can communicate using the wireless links with multiple nodes on the left and multiple nodes on the right as discussed in Section 3.2. Discovered faulty nodes can be also directly reported to the NCC. The network connectivity will remain even with multiple breaks on multiple segments occurring while any node faults or wire breaks will be discovered and reported for maintenance. In addition, with the availability of rechargeable batteries, the power constraint issue is terminated. Ordered ID codes can be used for identifying and positioning nodes along the pipeline.  
One of the main disadvantages of this architecture is that with a single wire cut, the overall bandwidth of the network will drop to the bandwidth of the replacement acoustic communication channel. However, unlike with the wired sensor network solution, the communication bandwidth will not drop to zero. The limited available bandwidth can be used to report the position of the faults as well as other critical information until the damage is fixed.