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cNODE Network Transmits Sensor Data, Information Between Subsea Units
Multinode Configuration of Acoustic Modems Optimizes Network's Data Rate, Power Consumption and Robustness

By Jan Erik Faugstadmo
Vice President
Magne Pettersen
Research and Development Engineer
Underwater Navigation Subsea Division
Kongsberg Maritime
Horten, Norway

Kongsberg Maritime has put great effort into development of technology for underwater communication. Part of this program has been to develop an acoustic wireless network for communicating sensor data and control information between subsea units. This system utilizes a new range of transponders for positioning and data communication, named cNODE, in a network configuration.

Introducing acoustic communication to realize wireless functionality in water opens a new range of network applications. Underwater communication over long distances can be impossible due to ray-bending effects and difficult topography that disables line of sight between transmitter and receiver. A network solution can overcome this problem by relaying the messages between several nodes on the seabed. Real-time data from several subsea sensors can be captured with this network. Control information to remote and autonomous units can be sent through the network.

The design includes network self-configuration with automatic generation of multihop routing of the information flow. The network is also adaptable to the acoustic channel. The network can communicate with the outside world via the master network node, which has a hardwired link or a satellite connection to a control station. The cNODE units can be operated down to 4,000 meters' depth.

A cNODE network with five cNODE units was demonstrated in the Oslo Fjord at 200 meters' depth in December 2009.

Network Planning
For planning of a network layout, it is essential to have knowledge of the underwater acoustic channel. Understanding underwater acoustic propagation is a key to identifying efficient network deployment strategies and to design both point-to-point communication and networking algorithms. Sound propagation is complex and time-varying, and much effort has been put into modeling.

The final cNODE test setup. The master node is located onboard the vessel for this demonstration.
cNODE Network
The cNODE network is implemented on transponders that are used for acoustic positioning and telemetry purposes. cNODE was introduced to the market in 2010. It is available in a variety of models with different sizes, transducer types and battery capacity, from small compact units to large seabed units with long lifetimes. They are easy to handle and can be operated with Kongsberg Maritime's high-precision acoustic positioning (HiPAP) system.

Architecture. The network architecture is based on several stationary, self-supported nodes and one master node that has a hardwired connection to the operator control system. The nodes can be located at the seabed, in conjunction with subsea structures or other instrumentation modules. The master node is the connection point to the outside world. The HiPAP system can be used for positioning the nodes during deployment, communication to the node, setup, reading status and recovery. The network nodes can be used for navigation of remotely operated vehicles and autonomous underwater vehicles (AUVs).

Network Functionality. The nodes are self-configuring and do not need to have any prior knowledge of other nodes or location. The nodes will discover neighboring nodes during the network mapping 'flood' phase. Information gathered during flood is reported back to the master node. After a successful flood phase, the nodes will choose optimal transmission power and data rate to each of its neighbors.

The network is a central-based system, where all information streams either go out from or back to the master node. Routing information is generated in the master node, based on the network information gathered during the flood phase and distributed automatically. Data streams can either be initiated by the master node on request, or the nodes can be commanded to report back at certain intervals or during a specific event, such as a sensor input exceeding a threshold value or a detection of fish from an echosounder. Transferring large amounts of data requires a request-to-send/clear-to-send (RTS/CTS) handshake before a transfer can begin. Nodes that hear an RTS/CTS handshake not for themselves will remain quiet for a time set by the handshake. Data that does not fit into one single telemetry message are split up into fragments transferred as separate messages. These fragments are assembled in the receiving node, and missing fragments are then requested to be retransmitted. For multihop transfers, all fragments must be received by the current relay node before transmission to the next relay node is started with a new RTS/CTS handshake. An acknowledgment message is used as a handshake mechanism for single messages and individual fragments. A missing acknowledgment will trigger retransmission of the current message.

Nodes. The nodes are built on Kongsberg Maritime technology, which consists of hardware and software for acoustic communication and positioning. The electronics and mechanical design enables a modular product. The nodes can be built in several configurations, including as sensors or as a pure network communication node.

cNODE Demonstration Setup
A demonstration setup was arranged on the Nordområdenes Nye Nervesystem-Undervanns Trådløst Sensornettverk (NNN-UTS) project, which aims to develop technology for underwater sensor networks, employing acoustic communication to realize wireless functionality in water. Prior to this demo, several sea trials were carried out, first of all to test and improve the point-to-point communication between two nodes. This demo showed point-to-point communication, automatic generation of routing protocol, multihop of data communication, cyclic data transmission, cyclic and requested data communication, acoustic channel propagation modeling and sensor interfacing.

Rays with initial angles from -30° to 30° from a source at 180 meters' depth (four meters above the bottom). The receiver depths are 10 meters and 180 meters, as indicated with the dashed line.

The nodes were set up to be able to transfer one kilobyte of prerecorded seismic data, along with pressure and tilt data from interfaced sensors. The nodes where deployed at 190 meters' depth in the Oslo Fjord.

The network was laid out in order to cover both situations when the master node could reach all nodes and vice versa, and when multihop was necessary to obtain reliable and efficient communication. The layout was such that it would provoke some collisions in the network. Five nodes were deployed at the seabed. The nodes were positioned by the HiPAP system and located relative to each other. Four nodes were deployed with weight, floating collars and an acoustic release mechanism for easy retrieval. One node was deployed in a basket, and it contained one high-quality pressure sensor and one high-quality tilt sensor.

Network Testing
For Test A and Test B, the vessel with the master node was positioned in the center of the network, and a flood phase algorithm was run to map the network. After a successful flood phase, all the nodes sent flood reports directly to the master node. These reports included information about the individual nodes and all its neighbors. Routing paths to each node were then generated based on the flood reports.

In Test A, one kilobyte of seismic data was read directly from both node 9002 and 9006. The data packet was too large to fit into a single telemetry message and was split up into several fragments, which were all successfully merged in the master node. The received data were plotted in MATLAB for verification. Test A also included the reading of a simulated sensor transmitting 15 bytes every 15 seconds from node 9002. The sensor was given the command to start transmitting, and after 10 successful readings, the sensor reading was stopped on command.

Test B demonstrated multihop transfers. Node 9000 was commanded to communicate with the master node via node 9005 and vice versa. Node 9000 was connected to an external depth sensor which was read first, showing an absolute pressure of 20.291 bar (node 9000 was placed at a depth of 196.3 meters). Data packets of 150 bytes and one kilobyte were also read successfully with multihop transfer. The data rates were chosen adaptively by the nodes, based on acoustic channel parameters measured during the flood phase.

Test B also demonstrated the behavior of multihop transfers with interfering network traffic. Node 9002 was commanded to start transmitting 15 bytes every 15 seconds, and at the same time, one kilobyte of data was requested from 9000 via 9005. The whole one-kilobyte transfer from 9000 via 9005 up to the master node was successful, while 94 percent of the 15-byte packets were received.

For Test C, the vessel was moved to a different location before mapping the network again and reading the statistic logs from each node. The vessel was positioned 200 meters southeast of node 9006, and a new flood phase was run. This time the master node could only reach node 9006 directly. The other nodes in the network had to relay their flood reports via nodes closer to the master node than themselves. Multihop routing paths were automatically generated in the master node and distributed.

Statistic logs were read successfully through the network from nodes 9002, 9003, 9005 and 9006. The log from node 9000 was lost in the network. This log was instead read using direct communication during the retrieval of all the nodes at the end of the demonstration.

Analyzing the Acoustic Channel
The acoustic propagation model PlaneRay was used to model the acoustic sound field at the locations. This program is based on ray theory, which is the appropriate approach for the acoustic frequencies used in underwater communication. PlaneRay can handle depth-dependent sound-speed profiles and range-dependent bathymetry. The sound-speed profile was measured at the site during the demonstration; the water depth in the area was constant equal to 184 meters.

The sound-speed variation with depth is mainly governed by the water temperature and is therefore strongly seasonal-dependent. On the day of the demo, December 4, 2009, the temperature at the surface was 5.8° C, increasing to a maximum of 10.3° C at 60 meters' depth and then decreasing to 6.6° C at the bottom.

Two situations were covered in the analysis conducted onboard concurrently with the network demo: bottom-bottom and bottom-surface. The analyses was carried out by use of PlaneRay modeling software to investigate ray bending, multipath and propagation loss for the actual demo conditions.

In the bottom-bottom scenario, stable propagation conditions were expected due to transmission over a single path, up to a distance of 800 meters. At longer distances, there is multitude of refracted arrivals that may interfere and cause variable conditions. The structure of these refracted arrivals is also highly dependent to minor changes in the sound-speed at depths close to the bottom.

With both the source and receiver close to the boundaries at the bottom and surface, there are typically four arrivals at about the same time. This may cause severe degradations due to destructive interference, which is dependent of choice of modulation of the information-bearing signal. The problem can be reduced by increasing the node distance for the bottom and surface.

Conclusions, Future Work
After three years of intensive work on the project, a fully operational sensor network was demonstrated in December 2009.

The cNODE network can be set up by trained personnel within a relatively short timeframe. A network of six nodes can be deployed and set up within three to four hours. The sensor network does not require any external computers to be operational. Adaptive behavior optimizes the network with respect to power consumption, data rate and robustness.

The NNN-UTS project focused on a network protocol for stationary nodes. Future scenarios include networks operated with mobile network nodes and several AUVs that are partly members of the network.

More work can be done on the signal modulation technique, especially for horizontal communication and very shallow-water applications.

Jan Erik Faugstadmo is manager for the acoustic positioning activities at Kongsberg Maritime in Horten, Norway. Faugstadmo has been involved in research and development activities developing hydro-acoustic position reference and HiPAP systems.

Magne Pettersen is a research and development engineer working on signal processing technology and network protocols for underwater positioning and communication for Kongsberg Maritime.

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