# DOW-PR DOlphin and Whale Pods Routing Protocol for Underwater Wireless Sensor Networks (UWSNs)
## 2 Previous Work  
In this section, we first review some related works on routing protocols in Underwater Acoustic Sensor Networks (UASNs), and will discuss their pros and cons. Then, we discuss the differences between WDFAD-DBR and proposed DOW-PR routing protocol. The most important and recognized property of Wireless Sensor Network (WSNs) is Energy efficiency. There are many types of terrestrial WSN protocols i.e., Directed Diffusion [4], Two-Tier Data Dissemination [5], Gradient [6], Rumor routing [7], and Sensor Protocol Information via Negotiation (SPIN) [8], etc. The existing protocols designed for WSNs are not feasible for UWSNs because of multiple reasons that include high propagation delays, high mobility of nodes, severe multipath phenomena, acoustic signaling, etc. Numerous UWSNs protocols are proposed, and it includes, Depth Based Routing (DBR) [2], Vector Base Forwarding (VBF) [9], Focused Beam Routing (FBR) [10], Hop by Hop Vector Based Forwarding (HH-VBF) [11], Adaptive Hop by Hop Vector Based Forwarding (AHH-VBF) [12], Directional Flooding Routing DFR [13], Delay Tolerant Network DTN [14], etc. These networks have been proposed in the last few years for increasing energy efficiency in underwater WSNs. In UWSNs, some protocols are designed to achieved a higher efficiency in energy such as HH-VBF, AHH-VBF and DFR. Routing protocols for UWSNs are categorized into four classes. The first such class is that of Flooding Based Routing (FBR) protocols, which includes HH-VBF, FBR, DBR, H2-DAB, DFR, etc. The second is multipath based routing, which include protocols presented by [PERSON] et al. [15], [PERSON] et al. [16] and so on. Third is Clustered Based Routing protocols, which include Minimum-Cost Clustering Protocol (MCCP) [10], Distributed Underwater Clustering Scheme (DUCS) [17], and Hydro Cast [18], etc. The last class being that of Miscellaneous Routing protocols including ADAPTIVE, Information-Carrying based Routing Protocol (ICRP) [19], Phero-Trail and so on. Geographic based routing protocol such as VBF, HH-VBF requires a location information of the network where the nodes are deployed to send a packet to the centralized station from the source node. In VBF [9], a routing vector (a vector drawn between source and sink node) and a desirabeless factor (suitableness) is defined so that the node that is close to the virtual vector and also to the sink node will be selected as a forwarder. HH-VBF is proposed in contest of VBF. In HH-VBF, instead of using fixed virtual vector and fixed forwarding pipeline radius, it is redefined at each hop in the whole network lifetime. The reliability and PDR of the network is improved due to this amendment in VBF but at the cast of increase in communication overhead. In uneven distributed networks (i.e., sparse networks, where the number of nodes is less), the performance of HH-VBF is lower than that of VBF. The reason behind this drawback is the constant pipeline radius maintained throughout the network lifetime. In [12], [PERSON] et al. proposed AHH-VBF. In order to restrict the forwarding range and improve the PDR, AHH-VBF changes the width of pipeline, according to network nodes distribution, in order to control the forwarding area. Unlike HH-VBH, AHH-VBF can adaptively adjust the power level hop by hop according to the neighbor node distribution of local region. Therefore, energy consumption is reduced and lifetime of the network is increased. ESEVBF (Energy Scaled and Expanded VBF) [20] scales and expands the holding time with the residual energy of all forwarding nodes in the potential forwarding zone (PFZ). The ultimate goal achieved in this protocol is to reduce the duplicate packets due to the imbalance between the holding time difference and propagation delay. Consequently, it reduces the energy expenditure of the network along with considerable improvement seen in end-to-end delay. However, it does not provide any improvement in PDR compared to counterpart AHH-VBF.