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28 st / March 2016 www.sea-technology.com Underwater Surveillance System Architecture In our USS architecture, we have four different types of nodes: underwater sensor node, surface station, onshore station and surface buoy. The underwater sensor nodes have less battery energy than the surface buoy, and they are cheap nodes that cannot directly communicate with the surface or onshore station. Surface buoys are powerful nodes that can gather data from ordinary sensor nodes and forward to the surface or onshore station through radio signals. The surface and onshore stations are command centers that gather and interpret data from surface buoys. In our architecture, multiple surface buoys are deployed. Underwater sensor nodes can deploy in two different ways: random and structure-based. The random method is easier to deploy. In this method, hundreds of underwater sensor nodes are randomly deployed in a specifc underwater en- vironment. This doesn't ensure full coverage of the environ- ment or connectivity among sensor nodes. The structure-based method is more complex. S. M. Naz- rul Alam and Z. J. Haas have proposed a structured-based method to achieve full coverage and full connectivity among sensor nodes, based on an octahedron placement strategy, which ensures full coverage and full connectivity while mini- mizing the number of nodes required for surveillance. DBR Depth-based routing (DBR), proposed by Yan H. et al, uses a greedy approach to deliver data packets to the des- tination sink nodes at the water surface. A localization-free routing protocol means each node in the network does not need full dimensional location information. Only local depth information per node is required. Depth information can be achieved with a cheap depth sensor. Each intermedi- ate sensor node forwards the data packets from the deeper sensor nodes to shallower sensor nodes, and each sensor node makes a decision on packet forwarding based on the depth of the previous sender and its own depth. When a sen- sor node receives a data packet, it retrieves the depth of the packet's previous hop, which is embedded in the received packet. Then it compares it with its own depth. If a node's depth is shallower than a received depth, then this node is qualifed to forward the packet. Otherwise, it discards the packet. Until the data packet reaches the sink node, this process will be repeated. D uring the last two decades, underwater wireless sensor networks (UWSNs) have attracted the attention of many researchers. Underwater wireless sensor networks enable a wide range of aquatic applications, such as assisted navi- gation, tactical surveillance, water pollution monitoring, oceanographic data collection, offshore exploration, and disaster prevention. Unlike terrestrial sensor networks, UWSNs have many constraints. Since acoustic signals can be propagated to sev- eral kilometers in water, UWSNs use acoustic signals in the physical layer for transmitting data. The speed of acoustic signals in water is low (approximately 1,500 m/s). Also,the speed of acoustic signals in water isn't constant and depends on water pressure, humidity and salinity. Therefore, propa- gation delay in UWSNs is long and variable. The available bandwidth in UWSNs is severely limited and is typically less than 15 kHz. Moreover, due to multipath and fading problems, the bit error rate is high and temporary losses of connectivity can occur. One of the important applications of UWSNs involves underwater surveillance systems (USS). A USS consists of many underwater sensor nodes spread over a given environ- ment to collect data, e.g., on marine life, contaminants and submarine presence, and report data via multihop routes to a sink node or distant command center. Usually, USS is applied for economic and tactical purposes. These applica- tions are critical. Therefore, to achieve reliability and eff- ciency, satisfying quality-of-service (QoS) parameters and using traffc engineering methods are key. QoS Parameters In USS, the most important QoS parameters are: cov- erage—the optimal placement strategy to achieve full coverage of the environment and maintain connectivity among sensor nodes; energy effciency—prolonging the USS life cycle for typically long missions; low end-to-end delay—USS monitoring must be in real time, so low end- to-end delay is vital; and high packet delivery ratio—a high packet delivery ratio and effcient routing protocol ensure that a USS can continue its task with a high net- work load. One of the major tools in underwater traffc engineering is routing, an essential function for energy sav- ing, low end-to-end delay and traffc management in a USS, where the main role of the routing protocols is discovering and maintaining the optimal routes. Underwater Surveillance System Routing Protocols Comparison of Underwater Wireless Sensor Network Routing By Reza Mohammadi • Dr. Reza Javidan