There has been a growing interest in monitoring underwater mediums for scientific exploration, commercial exploitation, and attack protection as it contributes for human well being. Industries are increasingly interested in technologies like wireless sensor networks. Under water sensor network consists of a variable number of sensors and vehicles that are deployed to perform collaborative monitoring tasks over a given area. Efficient information exchange via wireless communication using physical waves as the carrier among nodes in an underwater sensor network.
Underwater Acoustic Sensor Network Vs Terrestrial Network
1) Communication method - Terrestrial sensor networks employ electromagnetic waves but in underwater network is relied on physical means like acoustic sounds to transmit the signal.
2) Protocols - Due to distinct network dynamics, existing communication protocols for terrestrial networks are not suitable for underwater environment.
3) Cost - Terrestrial networks are becoming inexpensive but underwater sensors are still expensive devices.
4) Deployment - While terrestrial sensor networks are densely deployed, in underwater, the deployment is generally more sparse.
5) Power - The power needed for acoustic underwater communications is higher than in terrestrial radio communications.
6) Memory - Underwater sensors need to have large memory compared to terrestrial sensors.
7) Node Mobility - In case of terrestrial networks nodes mobility can be predicted whereas in the underwater networks prediction of mobility of the node is difficult.
8) Spatial Correlation - Readings taken from terrestrial networks with sensors are often correlated but this is not the case in underwater networks.
Unique characteristics of Underwater Acoustic Sensor Network
1) Communication media - Acoustic communication is the most versatile and widely used technique in underwater due to low attenuation in water.
2) Transmission loss - The attenuation is mainly provoked by absorption due to conversion of acoustic energy into heat. The geometric spreading refers to the spreading of sound energy as a result of the expansion of the wave fronts.
3) Noise - The man-made noise is mainly caused by machinery noise and shipping activity, while the ambient noise is related to hydrodynamics and to seismic and biological phenomena
4) Multipath Propagation - The Doppler spreading generates two effects: a simple frequency translation and a continuous spreading of frequencies.
5) Doppler spread - Multipath propagation may be responsible for severe degradation of the acoustic communication signal, since it generates ISI.
6) High delay - The propagation speed in the UW-A channel is five orders of magnitude lower than in the radio channel.Large propagation delay of 0.67 s/km.
Challenges in Underwater Acoustic Sensor Network
- The available bandwidth is severely limited
- The underwater channel is severely impaired, especially due to multipath and fading
- Propagation delay is five orders of magnitude higher than in Radio Frequency (RF) terrestrial channels, and variable
- High bit error rates and temporary losses of connectivity(shadow zones) can be experienced
- Battery power is limited and usually batteries can not be recharged, also because solar energy cannot be exploited
- Underwater sensors are prone to failures because of fouling and corrosion
Underwater Acoustic Sensor Network Architecture
- Two-dimensional Underwater Sensor Networks - For ocean bottom monitoring
- Three-dimensional Underwater Sensor Networks - For ocean-column monitoring
- Sensor Networks with Autonomous Underwater vehicles - For underwater explorations
Sensor nodes are anchored to the bottom of the ocean with deep ocean anchors. By means of wireless acoustic links, underwater sensor nodes are interconnected to one or more underwater sinks (UW-sinks). UW-sinks are equipped with two acoustic transceivers, horizontal and vertical transceiver. The surface station is equipped with multiple acoustic transceivers, one for each UW-sink deployed. It is also endowed with a long range RF or satellite transmitter to communicate with the onshore sink (OS-sink) or to a surface sink (s-sink). Sensor nodes float at different depths in order to observe a given phenomenon. The possible solution to achieve different depths would be to attach each UW-sensor node to a surface buoy, by means of wires. Multiple floating buoys may obstruct ships navigating on the surface.They may also be easily detected and deactivated by enemies in military settings.
Overview of Networking Protocols
Medium Access Control Protocols - Existing terrestrial MAC solutions are unsuitable for this environment. MAC solutions are mainly focused on carrier sense multiple access (CSMA) or code division multiple access (CDMA). FDMA is not suitable for the underwater environment. TDMA shows a limited channel utilization efficiency. Two spread-spectrum physical layer techniques, namely,DSSS and FHSS.
Routing Protocols - Provoke a large signalling overhead to establish routes for the first time and each time the network topology is modified. Each device can establish a path to any other node in the network, which may not be required in underwater networks. Proactive protocols may not be suitable for underwater networks.
Transport-Layer Protocols - A transport-layer solution specifically designed for the underwater environment should. Correctly handle shadow zones by predicting losses of connectivity and also interfacing with the routing layer. A cross-layer design, by violating a strictly layered architecture, especially in a harsh environment such as underwater. Cross-layer solution enable the efficient use of the scarce resources such as bandwidth and battery energy.