Underwater Routing Protocols: Analysis of Intrepid Link Selection Mechanism, Challenges and Strategies

The sensor nodes deployed in underwater environment has different routing mechanism in contrast to the terrestrial network. Getting underwater data on pollution detection, control of the ecosystem, marine mining, catastrophe avoidance and strategic surveillance thereby demands smooth packet transmission from dynamic nodes to base station encounters numerous challenges, out of which selecting best communication link between source and destination node is a key phenomenon of Underwater sensor network. The meticulous research has been conducted to search out the best link selection mythology of bodacious underwater routing protocol EnOR, SURS‐PES and USPF. The performance has been evaluated through NS2 simulation for packet delivery ratio, end-to-end delay, network lifespan and network energy consumption. and network energy consumption through NS2 simulator. The entire finding has revealed that USPF is remained


Introduction
It is a demanding task to develop routing protocols for underwater sensor networks (UWSNs), because they are highly hampered by poor acoustic connections. Traditionally, a UWSN consists of underwater sensor nodes, which are used to sense the atmosphere and unusual activities, and shore sonobuoys (sinks), that are responsible for gathering sensed data from the nodes. Optical and radio frequency interactions in such networks are generally deemed infeasible because optical signals suffer from extreme interference, while high-energy radio signals are easily absorbed due to high attenuation [1].
The acoustic channel was therefore seen as the only feasible system for wireless contact underwater.
organisms and water current, as it hits the sea, it reflects in multipurpose directions. Therefore, variations in sound speed are achieved, but a directional transmission can decrease this likelihood.
The multi-hop routing causes packet failure with an expected delay factor contributing to poor communication, so enough re-transmissions are needed to transmit packets reliably. Under these volatile and tough environments, the localized sensor nodes are completely battery-dependent and hard to replace, though replacement will increase high bits [4]. UWSN enhances the dimensions of exploration from underwater warfare to unforeseen and uncertain circumstances, such as ocean collisions, atmosphere and seismic warnings, the quality of pollutants, situational monitoring, sampling offshore and navigation. The UWSNs have given incredible success in the petroleum and gas business. Oceanographic data, mine identifications, underwater tracking and seafood items may also be collected. A submarine protocol specifies the size of a load and bit error packet. Not only does the network output degrade but also wastes resources through inadequate packet size selection [5]. The efficiency of the underwater network depends greatly on topology architecture, which is driven by a selection process for epitome relief nodes that increases the likelihood of transmission to the destination node. Intelligent topology has a very smaller energy consumption ratio than an insecure and less efficient topology.
The meticulously findings have been conducted in regards to the communication link selection process thereby considered three state of the art underwater routing protocols and their link establishment process has been analyzed by focusing the methodology which each protocol has adopted and through obtained results in terms of end-to-end delay, network throughput and energy consumption. At the end we came up with best link selection result adopted by one of these protocols. We came across some challenges still faced by the underwater routing and that needs to be considered for future findings.
There are two types of routing topologies underwater: Motion dependent topologies, and Coverage dependent topologies. The fixed or localized nodes are followed by a motion-based routing, whereas the coverage-based routing allows UWSN two dimensional and three dimensions [6].  Coverage based UWSN: It comprises primarily of two-dimensional architecture and three-dimensional architecture. The sensor nodes are anchored at the same depth for a two-dimensional topology and use the underwater bridge for contact that is responsible for capturing and transporting raw data to offshore station. The sink nods are set with a horizontal and vertical transceiver that captures sensed data from the nodes surrounding it. The sea is 10 km deep and the vertical transceiver is to be appropriately long, transmitting data to an offshore station while the horizontal transceiver performs the sensory node order for the acquisition of sensation data. The sinks are fitted with acoustic transceiver and wireless transmitter for the control of several parallel communications [7]. Underwater sensor nodes can have a direct connection via multi-hop path (relay node) to sink node or in-line link.
For a sink node far from the sensor node the power needed to send the packet will decrease by a value of over two of the distance [8], the direct link is an easy communication route but not a resourceefficient solution indeed. It also entails high transmitting capacity and is likely to will the network efficiency and the effect may be significant acoustic interference. A multi-shop communication strategy raises the latency factor as data is exchanged between intermediate nodes in the maze of the routing.
Furthermore, in terms of contact ability, two-dimensional underwater routing faces main challenges; selection of the water surface and the size. An improved version of a 2D UWSN is a three-dimensional underwater sensor network; the sensor nodes freely float at arbitrary water level to collect sensed data.
Adherence of the sensor nodes at the bottom is a more jingoistic strategy, so buoy retains the sensor nodes so draws into the surface of the water [9]. Depth of anchor nodes may be managed by changing the length of the cable. Some barriers to 3D undersea contact are obstructed, such as the sensor depth should be ingenuously modified to intelligently acquire sensed data and the network topology should be linked.
The article is structured as follows. Section II describes process of acoustic signal propagation. Section III addresses connection collection methodologies. Results and debate are summarized in section IV, and open challenges and conclusions were outlined in section V and VI respectively.

Underwater Acoustic Signal Propagation Mechanism
A highly dynamic undersea environment and acoustic communication is often subject to variable factors which require an acoustic channel's bandwidth to remain and to only stay on the frequency and distance of a sensor node. The communicating underwater varies shallow and deep by ocean division. Slightly shallow water had high temperature, a multipath effect, a surface disturbance, and a broad pause in propagation that eventually adversely affected the output of sound signals; whereas a deep-water sea had the same history with specific measurements [10]. Table 1 describes the main features of shallow and deep water.
Several malicious dissemination components are evaluated in sequence as pragmatically.

A. Factors affecting underwater Accoustic Communication
Underwater environments are defined as major propagation factors that influence acoustic communication: i. Path loss: the diffusion effect indirectly raises the underwater temperature resulting in vacillated movement between the sensor nodes and signal power. The lack of direction is further split into three parts.
a. Geometric Spanning Error: the sound wave is distant but unchanged from frequency [11], where spheric spreading losses arise in deep water, creating a cylindrical lack of stretching in low water.
b. Signal Attenuation: Attenuation is based on frequency and distance between the nodes [12]. That is since acoustic energy is transformed into another form of energy such as heat energy.
c. Dispersion: This happens by shifting angles of acoustic waves. Varying wind speeds cause surface roughness to increase the decline of the dispersion barrier, ultimately causing a delay in transmission and loss of power in subsequent communications [13].  Table   2. Generally, all underwater routing approaches have local and global considerations whereas a global approach requires a high overhead computation relative to the local one, and the source node in the network thereby preserved [16]. Moreover, picking of next forwarding node is typically accomplished by dividing the opportunistic routing into two classes: location-base and location-free routing. The location dependent routing picks the forwarding node by taking in account the distance or depth metric up to the node.

Link Selection Methodologies
The link selection methodologies of three prominent underwater routing protocols are extensively been investigated through their network architecture and then performance has been analysed in regards of packet delivery ratio, end-to-end delay, network lifespan and network energy consumption.
The findings are discussed as under.
A. Energy Balancing Routing Protocol for Underwater Sensor Networks (EnOR): Rodolfo W et al [16], discussed one of the immutability problems relating to the priority level of transmission of nodes., resulting in balanced power usage and extended lifespan of UWSN network. It rotates the priority Link Selection methodology: A beacon packet is periodically transmitted by each underwater sensor node. The lightning packet comprises the identity of the sender, the remaining information on its energy and its size. Algorithm 1. provides the procedure for selecting the best connection.
Using i as a sensor node with a data packet to be sent while maintain the neighbouring table as Ni.
The node analyses its adjacent nodes to choose the most suitable nodes to be forwarded (lines 2-7).
For this function, only if a neighboring node advances towards the surface sonobuoys can a candidate node be considered. (Lines 3-6). A difference between the current I node sender depth and j in P j = depth(i) -depth(j) is used to measure a packet advancement for a neighbor j. The fitness of the neighbor j is then determined (line 5). Using connection reliability, packet progress and remaining energy to assess the suitability of a nearby node. This is measured in line 5 and thus as Eq. (1).
where P j > 0 represents packet advancement of node j; p(d j , m) is an estimation of data packet of m between node i and node j; Ej rem shows remaining potency of j; and Ej init is the initial value of potency of j.
The nodes allowed the connection to be selected and sorted by fitness value (line 8). Finally, from the potential nodes the relation set is calculated. A limited link set can lead to low reliability of the link. At the other side, a wide connecting set may also damage the query, because it takes a long time. The possible node connections are applied to the whole collection until the required link reliability γ is reached.

B. Design of Shrewd Underwater Routing Synergy
Using Porous Energy Shells (SURS-PES) [17]: To transmit the data packet from source to sink-node, authors used a newly-developed link with residual energy and depth detail. In an area where energy usage has direct impact as the entire active underwater nodes rely on batteries and when costeffective data packets are delivered, no charge or replacement steps are taken becomes a crucial factor. The authors utilized a shrewd link selection mechanism, if a link is less than or equivalent to 50% shaky, after broadcasting of a sensor node the destination node is checked, and the destination node is returned to the source node, adding some unusable porous energy shell to strengthen a link from 5% to full 90, and then transmitting it to the target.

Link Selection methodology:
The link quality inspection has been taken through link reparation mechanism that is depicted in Figure 2. sensor node, a, broadcasts the packet, p, with substantial information such as depth, ID, and residual energy towards neighboring nodes, i.e., b, c, and d. The source node, Na is broadcasting the packet towards neighbors, upon receiving this packet node b includes the necessary information and sends it back as Nbp' to node a.
When a duplicate node a is attached to the required energy shells, the packet multizes again to node b as Na2p, in a trivial time t, The grain of the final relation is measured as shown in Eq. (2). eventually, the optimal link is being obtained holding energy utilization Eap, Ebp' and Ea2p respectively thereby remains unchanged thereupon Eq. (3), The probability of connection status from 50 to 90 percent updates in due course.
There is exhaustive study of the contact connections between node a and others. Therefore, there is a stipulated connection quality control, which records the hop links are hit by more than 50% and which links are more stable than 50% at all. Unlike the consistency of the connection between the source node a and b, the connections to the node a and d are more than 50 percent stable, but not up to 90 percent stable. The suggested approach (SURS-PES) therefore takes account of the hop connection between node a or b for the more secure packet transmission, i.e. up to 90 percent.

C. USPF: Underwater shrewd packet flooding mechanism through surrogate holding time
The authors [18] developed a shrewd data forwarding mechanism by taking three unique steps in regards to link selection and packet holding time namely called surrogate holding time. They implemented an angled approach in order to boost the distribution of data packets and to revitalize the life of the network. No single process consists of three stages, from source to sink. Forwarder Hop Angle (FHA) and Counterpart Hop Angle (CHA) are litigated for inclusion of data packets in the first phase of the same transmission field. If a value of FHA is equal to or higher than CHA, the packet produced will be in the same zone of transmission otherwise it would claim that the packet has other maverick. The next step selects the best relayscale node by again using the Additive-Rise and Additive-Fall method in three states connection consistency with prefix values. Ultimately, the third stage offers the exorbitant overhead fistula a definitive solution; the package holding time is built to avoid the risk of a packet loss.

Link Selection methodology:
The link quality of forwarding node considering P and the neighbor nodes has been explored using Additive-Rise and Additive-Fall methods [33] that shrewdly makes the adjustment to the states of the Forwarding hop angle values as illustrated in Figure 3.
The aforementioned three steps are described as under: Step 1: The forwarder node p changes the route by producing more αi packets to exploration more sparsely when the Connection status (Sh_L) is shaky or slanting compared to the prefix value (Prefix_v) with next nodes.
Step 2: If the connection state (St_L) is secure and hence meets the prefix property (Prefix_v), packet forwarding takes place without any obstacles.
Step 3: At a time if the connection state (Nr_L) is regular, but not ready for transmission due to certain salinity consequences, certain energy packets with additional shell have to go ahead and, for this reason, only fewer nodes are involved in transfers.
With relation quality only from forwarder to neighboring nodes, the flood zone is modified. Eq. (4) points, increasing node temporally updates the threshold value. A stronger connectivity also offers a slight delay. Throughout the reparation of the hop angle of the counterpart, a flood zone is never impacted by a nullity, since the hop angle is complex in hop by hop form. However, any relay node is aware of the hop angle of counterparts in the nodes around, which seem to preclude nodes from engaging in the forwarding process.

Results And Discussion
After through discussion regarding link selection procedure by the aforementioned underwater routing protocols. The results in packet delivery ratio, end-to-end delay, and network throughput are being analyzed by conducting NS2 simulation sessions. The different sensor nodes between 100 to 400 have been deployed in 3D region with 800 m × 800 m ×800 m dimensions and rest of the simulation parameters are given in Table 3. B. End-to-end delay: The average amount of time is defined as the end-to-end interval for all data packets obtained with performance in the sink node. For all protocols, the overall end-to-end delay has been reported to be decreased with the increasing number of nodes. Infact, this happens as the source seeks more eligible nodes that can orward packets to the corroborated surface sink as depicted in Figure 5.
It can be observed that EnOR still has a large end-to-end delay as compare to others because due to fixing of high priority for rotating transmission relay nodes, in addition because of packet holding duration that depends on the nodes priority thereby it has added the greatest gape among sender and the sink node, that is why this delay approximately remained larger than even to the SURS-PES.

Open Challenges
The extensive findings regarding communication link selection mythologies and its impact on various underwater routing factors have been presented which unveiled numerous hidden aspects of this amazing field and many open challenges are still need to address and left behind out of which some are highlighted as.
The noise and reduction channel models are methodological in UWSNs. This field is already available for theoretical or numerical research and study of new models. Furthermore, prototype models may be built to represent the characteristics of the medium underwater.
With water flows, sensor nodes shift, continuously shifting their locations [19]. This often shifts their place details. It is a challenging task in finding the sensor nodes. Node movement often includes frequent new locations changes. This causes uncertainty and energy usage, since these knowledges has to be shared at nodes.
The UWSN routing protocols use the network layer to supply data packets to water from below. The protocols listed in this survey consider network layer. The network layer may be combined with the MAC layer to reduce packet waiting time in sensor node queues to increase network performance. That would therefore reduce disturbance when sending packets and hence the energy usage. That is because intrusion contributes to the destruction of packets, and missing packets are recycled, so extra energy is absorbed.

Conclusion
The objective of reviewed communication link selection methodologies by underwear state of the art protocols EnOR, SURS-PES and USPF is to overcome the data routing challenges. It is essential to enhance the link selection mechanism for better and maximum packet delivery yield. The EnOR protocol uses the spinning transfer priority stage uses the available capacity, stability and advancement of the packet to the forwarding relay nodes. While SURS-PES is utilizing a resurrect link factor with residual energy and depth data for the selection of best link and thus forward the packet to the sink node from the source node. The USPF introduced new technique for link selection mechanism which based on directional flooding by considering two angles namely Forwarder Hop Angle (FHA) and Counterpart Hop Angle (CHA) with special packet holding time. The performance has been evaluated in relation to packet delivery ratio, end-to-end delay, network lifespan and network energy consumption through NS2 simulator. The entire finding has revealed that USPF is remained Data packet delivery ratio v/s number of nodes End to end data packet delivery ratio v/s number of nodes Overall network operational duration Energy utilization by entire network v/s number of nodes