# An Improved Forwarding of Diverse Events with Mobile Sinks in Underwater Wireless Sensor Networks
## 2 Background and Related Work
### Localization Based Routing Protocols  
In these protocols, position information of each node is calculated with some location finding algorithm. There have been many localization algorithms in literature [14; 15; 16; 17]. A modified version of DSR for underwater environments named as LASR (Location Aware Source Routing) was proposedin [18]. LASR is on demand routing protocol to cope with high latency, high probability of packet loss. It takes into account location and link quality metric during routing process. Vector Based Forwarding (VBF) was proposed in [19], which aims to increase network lifetime taking into account mobility of nodes. A hypothetical vector, which makes a virtual pipe of radius \(R\) from source to destination is assumed. Nodes within the range of vector are selected as candidates for forwarding process. EVBF (Efficient-VBF) [20] is an extension and improvement in VBF, in which \"desirable-ness\" factor \(\alpha\) and adaptation time \(T_{adaptation}\) are utilized in forwarding and selecting most suitable forwarder. This protocol is workable for both sink based queries and source generated queries. Another variant of this protocol is LE-VBF (Lifetime-Extended VBF) [21] in which desirableness is redefined employing position and energy of the deployed node. Routing pipe radius threshold influences on the efficiency of this protocol. VBF also requires localization information and extra equipment to measure Angle of Arrival (AoA) which is an overhead and causes extra energy consumption. Nodes closer to the axis of routing pipe become hot spot and their faster energy depletion causes void regions. In order to overcome the void area problems faced in VBF, Vector Based Void Avoidance (VBVA) is proposed in [22]. VBVA has capability to detect a void and bypass it on demand, hence it does not need to keep record of topology information. Void avoidance is realized using two mechanisms: vector-shift and back pressure. Void is detected if a node finds no neighbour with \"advance\" greater than its own \"advance\". Vector shift method allows nodes out of routing pipe to forward the packet and hence a shifted vector is formed. If a node cannot find neighbours to shift; it broadcasts a back pressure packet to reroute the packet with better path which has no void. However void detection process is only capable to detect it on the edge of the network and has no solution if void occurs in middle of networks due to unexpected deaths of intermediate nodes. Moreover backward flooding would cause extra over head especially in sparse networks. The performance of these protocols can be improved by identifying critical (i.e., cut-vertices) and non-critical nodes in the network using any of the algorithm such as [23]. Link quality based control and directional flooding protocol is proposed in [24] named as DFR (Directional Flooding based Routing protocol). Link quality is measured in terms of Expected Transmission Count (ETX) and flooding rather than established paths is employed during routing process. Flooding zone is locally formed based on link quality of neighbour nodes. If link quality is poor it involves more nodes in forwarding process otherwise few sufficient nodes are employed. It involves two type of angles: current angle of node \(i\); \(CA_{i}\) and reference angle of previous sender \(k\)\(RA_{k}\), in forwarding process. It also provide the mechanism for adaptive change and improvement in \(RA_{k}\) by each forwarding node. Geographically controlled flooding zone and directional (toward sink) forwarding based on link quality ensures better packet delivery. However, this protocol requires localization information which is itself a challenge in UWSN. Acquiring link quality information for each node increases control packet broadcasting and causes extra energy consumption. DFR does not have efficient mechanism for redundant packet avoidance.