# An Improved Forwarding of Diverse Events with Mobile Sinks in Underwater Wireless Sensor Networks
Value of depth threshold is always less than the transmission range. * Forwarding Functions: Forwarding function is a routing metric which can be dependent on different parameters of nodes, like absolute depth, depth difference, residual energy, transmission range and depth threshold. Different forwarding functions have been defined in literature and included in details in the next sections. * Holding Time: Holding time is assigned to each node to avoid redundant transmissions and collision. It is the time during which a node holds the packet and avoids any transmission. Underwater environments are very diverse in nature. A very wide range of living and non living activities take place in the same oceanic area. Generally, this diversity increases as we move deeper into the ocean. There can be many diverse applications in which severity of the sensed value of different nodes may not be the same. The data generated by some nodes may be of more importance than others. This stemness can be of many types like, in terms of delay criticality or in terms of size of data generated. Hence the costly deployment of UWSN can not be dedicated for sensing only a single type of occurring events. These diverse applications require the routing for each type of sensed event. So it is necessary to provide an underwater network solution which considers the diverse events occurrence and appropriate response to that diverse characteristic (different delay criticality associated with different occurring events). Our contribution in this paper is enlisted in following steps. * We have considered a diverse environment in which multiple types of events are occurring in the sensing field. These events are named as critical, very critical and normal. Critical and very critical events are delay sensitive in nature so forwarding to surface sink is planned for these types of event using respective forwarding functions in different depth regions. Normal events are sent to the mobile sinks (AUV: Autonomous Underwater Vehicle or CN: Courier Node) patrolling in the network. * Forwarding functions defined in literature use different parameters in different depth regions. However the values of different forwarding functions must be comparable with each other to ensure the steady flow of packets. Analysis on the range of previously defined forwarding function reveals the incomparable ranges of different forwarding functions. Hence the forwarding functions are redefined to ensure their comparable values in different depth region for the steady flow of packets. * Mobile sinks are deployed to collect the data from the nodes which has sensed normal events. Hence mobility of these nodes is influenced by the distribution and rate of data generation of normal nodes. We have proposed two different mobility schemes for the mobile sinks. The rest of the paper is organized as follows: In Section 2 we have provided an extensive literature review about the protocols especially available for underwater sensor networks. The underwater channel model is explained in Section 3. Section 4 consists of problem statement and motivation in which we have identified a problem in previously defined forwarding functions by analyzing their ranges. We also present motivation for event segregation in subsection of this section. Event segregation approach, network initialization and node deployment, improvements in terms of forwarding function and holding time and mobility of courier nodes are explained in the subsections of Section 5. Performance evaluation and result discussion are explained in Section 6. Finally conclusion and future work is presented in Section 7.