To meet the requirements of certain applications, an underwater sensor network must have several unique features which are similar to the free space optical wireless sensor network. When considering the network architecture or designing a sensor node, these features need to be kept in mind to meet the overall expectation. Here some of the important features are discussed, which are adopted from [65], and detailed explanation has been given, explaining how those criteria are met for proposed underwater optical wireless sensor network.
4.2.1 Low power consumption
In the underwater environment, the power consumption needs to be kept at a minimum level, because the device needs to be in water for a long period of time to monitor certain things, and it would not be possible for nodes to harvest energy from
water. As one of the objectives of an underwater sensor network is to keep minimum human involvement, it is crucial to design a power efficient system. To make the battery life longer, which would be the only power source underwater, low power devices need to be designed. When nodes do not communicate, they need to be turned off to save power. The designed transceiver which has been discussed in the previous chapter consumes very low current. A same way an energy efficient sensor node has been designed. For this reason, besides the power efficient hardware, an efficient communication protocol is also proposed, which is discussed in the following chapter.
4.2.2 Low cost
Sensor nodes are deployed densely underwater in a certain geographical area, so a node’s cost needs to be kept as low as possible. The cost of the custom designed optical wireless transceivers was kept very low by not using any external optics like lenses or filters, which are very expensive compared to common electronics and optoelectronics. In the same way, sensor node and gateway node need to be designed for low cost. Another aspect of cost is the maintenance cost of the deployed system. As the proposed network will be implemented underwater, the maintenance cost can be much higher compared to any land-based sensor network. For this reason, the network has to be made autonomous, which would not require much human intervention after deployment.
4.2.3 Network topology
The most common network topology which was previously being considered for free space sensor networks is the star topology. The problem of using this kind of topology is the physical coverage by a single network. Now a day’s, either mesh or
tree based topology is the realistic solution to cover a larger geographical network. As the communication range is a big issue for the underwater environment, so, multi- hop network architecture is considered for the proposed application. Details of the network topology are discussed in section 4-3.
4.2.4 Security
Security is a very important factor for underwater communication, as it could be mostly used for military applications, where types of data are very sensitive. To make the network secure, a visible light has been chosen as a communication carrier and line of sight communication links are deployed. In this process, data travel only in one direction, so no-one can receive the transmitted information from other directions. Security is ensured in the physical layer; also, higher layer encryption will be required for a more secured system.
4.2.5 Data throughput
Data throughput requirements for sensor networks are not crucial compared to other communication systems. Here data throughput means real data which are being sent. For communication to happen, other kinds of signal, such as channel allocation signals, MAC layer frames etc. also need to be used, which consumes bandwidth. But real data can be temperature, pressure, turbidity information, which is only a few bits long. Although optical wireless communication can ensure a very high data rate, it is not the focus of this work to make a high speed system. Rather, the approach is to extend the communication distance, using multi-hop network architecture. The proposed network supports data rate of 38.4kbps, which would be sufficient to support the proposed application.
In general, a wireless sensor network has a very low requirement of real time data transfer. In most cases, sensor networks do not send video or audio information which requires a certain level of Quality of Service (QOS). Therefore, the message latency requirement for a sensor network is very low, and this is also true for the underwater case.
4.2.7 Alignment between nodes
Like any other LOS optical wireless system, alignment has to be maintained accurately to get the maximum possible performance in the underwater optical wireless sensor network. Water as a medium is very harsh, and because of the water wave’s continuous movement, nodes may slightly drift to the other directions, which would cause communication failure. Thus, when sensor nodes are anchored in water, one has to ensure that they remain in the same direction all the time.