# A Comprehensive Survey of Recent Routing Protocols for Underwater Acoustic Sensor Networks
## 3 Routing Protocols for UWSNs
### EP-VIR-3 & BF-SPR-3
#### 3.2.2 Localization-free Routing Schemes without mobility consideration  
This section surveys the localization-free routing schemes proposed for static underwater sensor networks. Figure 16: Selection of relay node in EP VIR 3 and BF SPR 3. Figure 15: Information sharing strategy between one and multi-hop neighbors. EnOR  
Energy-aware opportunistic routing (EnOR) [61] aims to prolong the lifetime of the network by distributing the traffic load fairly across the network, thus avoiding node failure due to energy drainage. The protocol carries out routing in two steps. In step 1, a set of potential forwarders is determined based on a fitness value (Equation (19)), which is a function of the depth of the candidate forwarder relative to the sender, its residual energy level, and quality of the link between the sender and the candidate forwarder. In step 2, the sender transmits the packet along with the list of candidate forwarders. The list is sorted according to priority determined through the fitness value. The node with the highest priority is assumed to be the next forwarder. In order to regulate the transmission by candidate nodes, EnOR defines a parameter called \"packet holding time\" (Equation (20)), i.e., the time for which a candidate node will hold a packet before forwarding. Packet holding time is a function of the priority of the candidate node and the distance between the sender and the candidate node. This ensures that redundant transmissions by low priority nodes are avoided. EnOR achieves load balancing and increases the percentage of alive nodes by including the residual energy of nodes in the selection criteria for forwarders, thus ensuring that nodes at shallower depth levels do not rapidly run out of energy. However, communication overhead, incurred to inform neighbors about current energy levels and depths, may be a concern in the case of dense networks:  
\[F_{j}=P_{j}\times p\Big{(}d_{j},m\Big{)}\times\frac{RE_{j}}{RE_{initial}} \tag{19}\]  
\[TH(p)=\left\{ \begin{array}{ll}\frac{CR-d_{max}}{C},&if\ p=1\\ \frac{CR+p\times d_{max}}{C},&if\ p>1\end{array} \right. \tag{20}\]  
where in Equation (19) \(P_{j}\) is the probability of packet advancement by neighbor \(j\), \(p\Big{(}d_{j},m\Big{)}\) is the probability that a packet transmitted by node \(i\) will be delivered node \(j\). \(RE_{j}\) and \(RE_{initial}\) represent remaining and initial levels of energy of node \(j\) respectively. In Equation (20), \(CR\), \(d_{max}\), \(p\) and \(C\) denote communication range, max distance between the sending node and candidate forwarders, priority of a candidate forwarder based on its position in the packet header and speed of sound in water respectively. QA DFR AA & QA DFR TA  
Reference [62] proposes two enhanced versions of the DFR [63] protocol, namely QA DFR AA i.e., QoS-aware DFR with angle adaption, and QA DFR TA i.e., QoS-aware DFR with threshold adaption. These protocols are aimed at meeting the QoS requirement in a more dynamic fashion. In both the techniques, the sink node measures the end-to-end packet delivery ratios, and updates the sensor nodes through a hello message. The receiving sensor nodes compare the received value with the QoS requirement. If the end-to-end packet delivery ratio is less or more than what is dictated by the QoS requirement, the sensor node adjusts its flooding area, based on the gap between the two values. QA DFR AA adjusts the flooding area by adjusting its base angle (Figure 17). The new base angle is the function of the previous base angle and an adjustment value. The adjustment value in turn is a function of the QoS requirement and the packet delivery ration of each path. In contrast to QA DFR AA, QA DFR TA updates a threshold value, instead of the base angle. Moreover, QA DFR TA adjusts the flooding area locally, unlike QA DFR AA, which adjusts the flooding area all the way from source to destination. Besides the above mentioned two techniques, the authors also proposed a holding time technique for current DFR. Packet holding time is subjected to link quality.