## 7 Discussion
The external nodes can be inexpensive sensor nodes with limited capabilities and scattered randomly. The external nodes can also be moving vehicles that need to communicate with the pipeline infrastructure. One example of using this type of communication is with maintenance boats or underwater robots that need to collect more information about the pipeline status. Another example is security vehicles that need to send special signals to the network not to report the movement of the vehicle to the NCC in case the pipeline infrastructure is equipped with motion sensors that automatically report approaching vehicles. Although the external nodes option can also be made available with completely wireless sensor networks for monitoring pipeline infrastructures, the existence of a wired network can provide higher communication bandwidth and reliability. A summary of the reliability issues for all discussed architectures is in Table 2.  
The hierarchical model presented in this paper employs a heterogeneous architecture with three types of nodes. It is worth noting that a homogeneous model can be used to perform the required monitoring and communication. In such a model, there would be only sensor nodes which are used to provide both sensing and routing services. Even though a self-organizing homogeneous model would have the following advantages: (1) Lower system cost; (2) Reduced installation complexity and time. The three-level heterogeneous model that we present has the following advantages: (1) Increased reliability: in the homogeneous network data messages would have to travel along the entire length of the network in order to reach the sink while in the three-level model, the messages are disseminated to the NCC by the DDN nodes at regular geographic intervals. Therefore, a disruption in the communication in one of the segments between two DDN nodes would not affect the transmission of messages in the other segments. This leads to higher overall system reliability and robustness; (2) Lower end-to-end delay. This is also due to the fact that in the heterogeneous system, the messages have to travel shorter distances before being disseminated to the NCC; (3) Reduced energy consumption (in Joules per byte) by the sensor nodes, which is critical for the important objective of longer network lifetime and maintenance. This is due to the fact that in the homogeneous network, the same data message would have to use many more hops to reach the destination. This requires significantly increased energy consumption at the individual sensors; (4) Increased bandwidth capabilities and improved performance. This is the case because, in the heterogeneous model, DDN nodes transmit data to the NCC in a parallel fashion. Therefore, the communication bandwidth of a particular DDN node is ultimately shared by a lower number of sensor nodes, which are in the segment that is serviced by that node. Consequently, while the homogeneous model can still be used in cases where the geographic area and length of the pipeline infrastructure is relatively shorter, most underwater pipeline systems stretch for very long distances, where the heterogeneous model would be better suited to meet typical system requirements of improved reliability and performance, reduced end-to-end delay, decreased energy consumption by the sensor nodes, and increased network lifetime.