## 3 Routing Protocols for UWSN
### Protocols Facturing Node Mobility
#### 3.1.1 Vector Based Forwarding (VBF) and Hop-by-Hop VBF (HH-VBF)
Its data delivery rate does not depend on the stability on the neighborhood rather on the network density. In a dense network it gives a decent performance in delivery rate, average delay and energy consumption. Even though VBF has some drawbacks. In VBF, routing path as well as the routing nodes are restricted by the virtual routing pipe. This pipe is extended up to the surface destination, originating from the source. In a sparse network, it may happen that no node lies within the predefined routing path, then it will not transmit the data to the sink even if it happens to have other paths outside of the routing pipe. It will decrease the packet delivery ratio drastically. As the node distribution is uneven in underwater environment, it is very troublesome finding the right radius threshold. To overcome its short comings [PERSON] et al. proposed an improved protocol in which authors preferred per hop routing pipe for individual forwarder over the single pipe connecting sink and the source. It is called hop-by-hop VBF [51]. In this, every intermediate forwarder determines the pipe direction depending on its own position and the position of the neighboring nodes and the sink. So HH-VBF can always find a route in sparse region where there is a less number of neighboring nodes. Eventually HH-VBF gives better packet delivery ratio than VBF, especially in sparse regions. Yet, its routing pipe radius threshold can degrade its performance. Besides, it gives more signal overhead than VBF due its hop by hop forwarding. Figure 3: Routing Procedure of Vector Based Forwarding (VBF) [38]. 1.2. Depth Based Routing (DBR), Depth Based Multi-Hop Routing (DMBR) and Energy-Efficient DBR (EEDBR)  
Knowing the exact location of the sensor nodes is very difficult and also a critical factor of the underwater networking. [PERSON] et al. presented a protocol where only the height of the sensors is necessary instead of the full-dimensional location information. This information can be achieved just by attaching a depth sensor with the nodes. These depth sensors are not that costly [52]. Several data sinks should be floating at the top of water body. This ultimately collects the data from the nodes deployed at different level of water.A node from the bottom of the water-body collects the data which act as the source. At first a node compares its height with the height of the prior sender from the bottom surface. Then it distributively takes a decision on packet forwarding. When the height of the node is higher than that of the sender, it considers itself to be qualified forwarder and will forward the data. Otherwise it just discards the packet because a node closer to the surface was its prior sender. This procedure continues until the packet reaches the surface. Reception of the packets by any of the surface sinks is considered a successful transmission. As the nodes closer to the surface is always chosen as a forwarding node, it consumes less energy and travels less distance. Figure 4 depicts the routing technique of DBR where S is the source. The range of the source S covers the neighboring nodes n1, n2 and n3. Here, a larger circle around source and forwarders depicts the transmission range of the source. Source broadcasts a data packet with its location information. Every node falling in the range of S receives the data to compare the depth of node S from the surface its own depth. Node n3 is discarded as its depth from the surface is higher than that of S. Here, both the nodes n1 and n2 are qualified forwarder and n2 is selected as the next sender as it is closer to the surface. Now, n1 just drops the packet and n2 broadcast the data in the previous way to find the next sender. This is done recursively until the packet finds any of the surface sinks. The authors introduced a global parameter called depth threshold to control the number of nodes needed to send the data packet. A node forwards data only when it finds that difference of its own depth and earlier hop's depth are greater than that of threshold value.