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www.sea-technology.com March 2016 / st 29 this node is in the routing pipe and is qualifed to forward the data packet. In VBF, multiple routes might be used at the same time, which helps handle the problem of packet losses and node failures. VBF is not suitable for sparsely deployed underwater networks and is sensitive to the routing pipe ra- dius threshold. HH-VBF Hop-by-hop vector-based forwarding (HH-VBF), pro- posed by Nicolaou N. et al, overcomes VBF problems and increases robustness. HH-VBF is an enhanced version of VBF, using the same concept of virtual routing pipe. But unlike VBF, HH-VBF defnes per-hop virtual pipe for each forwarder. In HH-VBF each sender sensor node must be aware of its own location and the location of the sink node and makes a decision about the pipe direction based on its current location. In order to enhance routing performance, there is no necessity to increase the pipe radius beyond the transmission range because in HH-VBF the maximum pipe radius is the transmission range. In sparsely deployed un- derwater networks, HH-VBF produces better data delivery ratio and is less sensitive to the routing pipe radius thresh- old compared to VBF. HH-VBF has more signaling overhead than VBF, too. FDBR The fuzzy depth-based routing protocol (FDBR), pro- posed by Mohammadi R. and Javidan R. to overcome DBR problems, increases energy effciency and decreases the holding time in DBR. FDBR uses fuzzy logic to improve DBR, with the goal of determining the value of holding time in each sensor node. To calculate the value of hold- ing time, FDBR considers the residual energy of the receiver node, depth difference of the receiver node and the previ- ous forwarder node, and the number of hops traveled by the received packet. This mechanism leads to less end-to-end delay and more energy saving. VBF Vector-based forwarding (VBF), proposed by Xie P. et al, is the frst geographic-based routing protocol designed for underwater sensor networks. In VBF, each sender sensor node must be aware of its own location and the location of the sink node. VBF defnes a routing vector from the source to the sink. Data packets are forwarded along the routing vector. When a sensor node receives a data packet, it cal- culates its distance to the routing vector based on the loca- tion of the predecessor node embedded in the header of the packet. If the distance is less than a predefned threshold, "One of the important applications of UWSNs involves underwater surveillance systems (USS)."