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Several existing communication systems can be seen in this section. This section is divided into: applications of underwater acoustics systems, underwater acoustic Ad-hoc networks, communication architecture and Sensor networks with autonomous underwater vehicles. In this section we attempt to show the several underwater systems, focusing on the Ad-hoc networks used in the underwater environment.
Currently there are many applications of underwater acoustics. The first steps into practical underwater acoustics applications were developed for military use. At the beginning of World War II the progress in electronics and in the radio industry was advance enough to build sonar, which played an important role for detecting the threat of German submarines which were destroying the allies' ships.
Passive military sonar is designed for detection, tracking and identification of submarines. They work at very low frequencies, between a few tens of hertz and a few kHz. The detection ranges are also longest at low frequencies, due to the smaller absorption losses. Modern passive sonar is characterized by the deployment of towed linear arrays, very long, able to efficiently detect and locate low-frequencies noise sources.
4.1.2 Underwater acoustics for civilian use
While the military industry develops underwater acoustic systems, private industrial was able to profit from development of underwater acoustics. Acoustics sounders quickly replaced the traditional lead line to measure the water depth below ship or to detect obstacles. These systems are indispensables tools for sea fishing, navigation and scientific monitoring of biomass. Other useful underwater system is Sidescan  sonar to obtain acoustics images from the seabed used for geology in the construction of seafloor maps, which are of relevant importance in the offshore oil industrial.
In the recent years techniques of acoustics monitoring have deployed to monitor the evolution of the average temperature of large ocean basins on a permanent basis, as a part of global climate studies.
The author of , says that underwater acoustic systems use a restricted variety of signals, chosen for their capacity to carry the information sought by the end-user in specific applications. He also explains that there are two main aspects to the good functioning of an underwater acoustic system:
The definition and use of the signal well suited to the objective, and to the environmental conditions know a priori.
The use in the reception chain of processing techniques combining the best performance achievable, and the level of complexity and cost compatible with the objectives of the system.
4.1.3 Underwater acoustics data transmission
Underwater acoustic data transmission applications are very varied. As mentioned before the military, naval and industrial domains, one uses them mainly:
For underwater vehicle control, either direct control of an Remotely Operated Vehicle (ROV), or management of an Autonomous Underwater Vehicle (AUV).
For communication between a submersible and the support vessel, or between 2 submarines, for standard communications and the transmission of measurement data.
For video images, to aid vehicles control and to examine underwater structures.
For communication between a vessel and an autonomous measurement station or an automated system, to get data available at the bottom without having to recover a submerged instrument physically.
For back-up safety systems.
For data transmission after geological survey or long term studies.
For voice communication between divers and a submersible.
For pollution monitoring and other environmental monitoring.
4.2 Underwater acoustic Ad-hoc networks
The actual work in underwater acoustic communication and networking is generating a huge amount of different systems with specific and custom protocols of communications for every new application. On the other hand currently, few standards exist for underwater communications, and modems from different vendors are in general incompatible. This leads to a lack in interoperability. So standardization became a relevant issue for underwater acoustic communication systems.
The authors of  propose a possible roadmap to standardization, which are prerequisites to reaching a state of ubiquitous underwater acoustic communications and networking. They also say that research in radio communications is mostly based on simulations using established models, but research in underwater acoustic communications is mostly based on sea trials, which are expensive and time-consuming.
Underwater Ad-hoc networks are envisioned to enable applications for oceanographic data collection, ocean sampling, environmental and pollution monitoring, offshore exploration, disaster prevention, tsunami and seaquake warning, assisted navigation, distributed tactical surveillance, and mine reconnaissance. To make these applications viable, there is a need to enable underwater communications among underwater devices. Underwater fixed/mobile nodes must possess self-configuration capabilities, i.e., they must be able to coordinate their operation by exchanging configuration, location and movement information, and to relay monitored data to an onshore station. So there is, in fact, significant interest in monitoring aquatic environments for scientific, environmental, commercial, safety, and military reasons.
While there is a need for highly precise, real-time, fine grained spatio-temporal sampling of the ocean environment, current methods such as remote telemetry and sequential local sensing cannot satisfy many application needs, which call for wireless underwater acoustic networking.
Under Water Acoustic Sensor Networks (UWASN)  consist of sensors/nodes that are deployed to perform collaborative monitoring tasks over a given region. UWASN communication links are based mainly on acoustic wireless technology, which poses unique challenges due to the harsh underwater environment, such as limited bandwidth capacity , temporary losses of connectivity caused by multipath and fading phenomena , high and variable propagation delays , and high bit error rates, among other things.
4.3 Communication architecture
A reference architectures for UWASN has been studied [1, 2 , 42, 43] for deployment of underwater communications in the recent years, but still the underwater Ad-hoc network topology is an open research issue in itself that needs further analytical and simulative investigation from the research community.
The network topology is in general a crucial factor in determining the energy consumption, the capacity and the reliability of a network. Hence, the network topology should be carefully engineered and post-deployment topology optimization should be performed, when possible.
It is important that the deployed network be highly reliable, so as to avoid failure of monitoring missions due to failure of single or multiple devices. For example, it is crucial to avoid designing the network topology with single points of failure that could compromise the overall functioning of the network. The network capacity is also influenced by the network topology. Since the capacity of the underwater channel is severely limited.
Also [2, 42, 43] introduce reference architectures for two-dimensional and three-dimensional underwater acoustics networks, and presenting several types of fixed and mobile nodes which can enhance capability of underwater Ad-hoc networks.
4.3.1 Two-dimensional underwater Ad- hoc network
A 2-D underwater Ad-hoc network, it is a set of underwater nodes which are anchored to the sea floor, organized in a cluster-based architecture, but able to move due to anchor drift or disturbance from external effects.
Underwater nodes are interconnected to one or more Under Water Gateways (UWG) by means of acoustics links. Since data are not stored in the underwater node, data loss is prevented as long as isolated node failures can be circumvented by reconfiguring the network
The UWGs are equipped with two acoustic transceivers. The first one, is a long-range vertical transceiver (of up to 10 Km) used by the UWG to relay collected monitored data from the sea floor network to a surface station, the second one is a horizontal transceiver used by the UWG to establish communication with nodes in order to send commands and configurations to them, and to collect monitored data from the sea floor network.
The surface station is able to handle multiple parallel communications with the deployed UWGs using a multi-connection acoustic transceiver, and with a long-range radio transmitter and/or satellite transmitter, which is needed to communicate with an onshore station and/or offshore ship.
The underwater nodes are able to connect to UWG via direct links or through multi-hop paths.
Although direct link connection is the simplest way to create an underwater wireless Ad-hoc network, it may not be the most energy efficient solution [40, 44]. Like in terrestrial Ad-hoc networks , where information of a node is relayed by intermediate nodes until it reaches its final destination. A 2-D underwater Ad- hoc network architecture may be implemented using the same above principle, by creating intra-cluster communication or extended to inter-cluster communication as a measure to support a failed UWG or an overloaded cluster/node. However a custom routing protocol may be required to handle this functionality.
In Figure 6, there is an illustration of a 2-D underwater Ad-hoc network architecture, which can be support a mobile underwater Ad-hoc device, like ROV or AUV.
Fig. 6. Architecture for 2-D underwater ad-hoc network.
4.3.2 Three-dimensional underwater Ad-Hoc Network
A 3-D underwater network, it is a set of fixed and mobile underwater nodes which float at different depths to perform cooperative tasks. A wide scope of tasks can be performed by this depth network, it may be used for surveillance applications or monitoring and sampling of the 3D ocean environment in order to detect and observe unusual ocean phenomena, like bioââ‚¬"geochemical processes, water streams and pollution that cannot adequately detected or observed by means of a sea floor network.
An easy and quick deployment of a 3-D underwater network is attaching each node to a surface buoy, by means of retractile cables, in order to adjust the depth of each node . However, having multiple floating buoys may obstruct commercial fishing, ships navigating water sports, or any other activity that takes place on the surface. Furthermore, floating buoys are vulnerable to weather and tampering or pilfering.
One approach to resolve the above issue is to anchor each node device to the sea floor and equip it with a floating buoy that can be inflated by a pump, which pushes the node towards the sea surface. The depth of the node can then be regulated by adjusting the length of the retractile cable that connects the node to the anchor, by means of an electronically controlled engine that resides inside of the node. In Figure 7 there is an illustration of this kind of architecture, which has the same communication hardware of 2-D underwater Ad-hoc network architecture.
There are a couple of considerations concerning to a 3-D underwater network deployment:
Node coverage: Nodes should collaboratively regulate their depth in order to achieve 3D coverage of the ocean column.
Communication coverage: In 3D underwater networks, nodes should be able to relay information to the surface station via multi-hop paths, using for this fixed nodes or mobiles nodes like ROV/AUV. So, network devices should coordinate their depths in such a way that the network layer is always active and running.
Fig. 7. Architecture for 3D underwater Ad-hoc network.
4.4 Sensor networks with autonomous underwater vehicles
4.4.1 Actual Systems
There are few systems currently for underwater Ad-hoc acoustic networks. One of them is AquaNodes . It is a group of underwater nodes with dual communication and support for sensing and mobility, where they can be deployed in lakes, rivers, and even the ocean.
The systems consist of fixed and mobile robots that are dually networked, optically with an optical modem implemented using green light, for a point-to-point transmission at 330kb/s and acoustically for an acoustic modem for broadcast communication over ranges of hundreds of meters at 300b/s, using TDMA as communication protocol.
Each node has built-in a camera and sensors for temperature, pressure, and inputs for water chemistry sensors.
US Navy underwater wireless network development is following a concept of operations called Seaweb [48, 49]. The system uses through-water acoustic modems to interconnect a scalable quantity of underwater network nodes, linking them to a gateway node typically located at the sea surface. The gateway node is equipped with some form of radio modem permitting bidirectional real-time digital communications between the underwater Seaweb domain and distant command centers.
Seaweb networking, shown in Figure 8, provides acoustic ranging, localization, and navigation functionality, and thereby supports the participation of mobile nodes, including submarines and collaborative swarms of AUVs.
The system has proven to be effective in shallow waters such as the Intracoastal Waterway and in waters up to 300 meters deep off the coasts of Nova Scotia(Canada), San Diego, Long Island and Florida (USA). It has been demonstrated in the Pacific and Atlantic Oceans, in the Mediterranean and Baltic Seas, in Norwegian fjords, and under the Arctic ice shelf.
Fig. 8. Seaweb Underwater Networks
220.127.116.11 The Autonomous Oceanographic Sampling Network (AOSN)
The Autonomous Ocean Sampling Network (AOSN) is a US project from The Monterey Bay Aquarium Research Institute (MBARI) , the systems is very similar to SeaWeb, which uses a new robotic type of AUVs with advanced ocean models to improve the ability of humans to observe and predict the ocean. The AOSN system includes data collection by smart and adaptive platforms and sensors that relay information to a shore in near real-time, where it is assimilated into numerical models, which create four dimensional fields and predict future conditions. Also the system has the ability to predict physical properties of the ocean, such as temperature and current, as well as biological and chemical.
4.4.2 Cluster for Underwater Ad Hoc Vehicles (proposal)
A multi-cluster communication systems for Ad-hoc mobile underwater acoustic networks , it is a proposal system which consists of a varying number of vehicles that are required to perform collaborative tasks over a given area. To do so, vehicles must be able, at a minimum, to coordinate their operation by exchanging location and movement information by using a multiple access scheme based on clustering which provides efficient scalability by spatial reuse of channel resources.
The performance on the systems is evaluated in terms of measures of connectivity, successful transmission rate, average delay and energy consumption. An important issue discussed in the proposal system is the cluster size, which is determined by the maximal number of nodes per cluster for which the network connectivity is maximized.
Underwater communications networks have became an important field of investigation for many research groups in the recent years. In this chapter the main concepts in underwater Ad hoc communications have been analyzed. We have presented an overview of the state of the art in underwater Ad hoc communications networks, and the potential use of this new kind of systems mainly used by military, naval and industrial domains.
Firstly, the main problems related to transmission have been shown. We have described the most used technologies in underwater communications. Acoustic communication is the most versatile and widely method used in underwater environments due to the low attenuation of sound in water. On the other hand, the use of acoustic waves in shallow waters may be adversely affected by temperature gradients, ambient background noise and multipath propagation due to reflection and refraction. The much slower speed of acoustic propagation in water, about 1500 m/s compared with electromagnetic and optics waves, is another limiting factor for effective communication and networking. Still, the best technology for underwater communications is acoustic.
Then, the Ad hoc network protocols have been described. There are several ad hoc network protocols, for this reason they have been divided into groups. Each group uses a way to create and maintain their routing tables, besides having an architecture and/or topology. According to the references the reactive protocols are most commonly used. Within this group we could highlight the DSR and AODV protocol. The underwater communications use this type of protocols, because they do not need save the routes for a long time. They create their routes when they need to transmit some data to another node. In the case of using a hierarchical architecture, the cluster-oriented routing protocols are most used.
Finally, the current applications have been exposed. At the present, the work in underwater acoustic communication and networking is generating a huge amount of different systems, sometimes incompatible to operate between different manufacturers. So standardization has become a relevant issue for these systems. Furthermore, there are other crucial factors in underwater acoustic network to be considered, these are: energy consumption, capacity and reliability of the network, which need be studied in depth to deliver a proper Quality of Service.
Underwater Ad-hoc networks are envisioned to enable applications for oceanographic data collection, ocean sampling, environmental and pollution monitoring, in others. To make these applications viable, it is required a communication among underwater fixed and mobile nodes, by creating a 2-D or 3-D underwater Ad-hoc network able to relay monitored data to a surface or onshore station.