# A Comprehensive Survey of Recent Routing Protocols for Underwater Acoustic Sensor Networks
## 3 Routing Protocols for UWSNs
### Location-Based Routing Schemes
#### EMGGR
This divides the network into sub-regions, each of which is assumed to be a cluster. Every cluster contains a CH, cluster coordinators, relay nodes, and ordinary sensor nodes (Figure 6). The cluster head collects data from nodes located in its cluster through relay nodes, and passes the data on to the cluster coordinators in the adjacent upper clusters. The same process is followed in each cluster, until the packet reaches the sink node(s). In ECBCCP, Cluster heads and relay nodes can store data. This helps in avoiding the retransmission of already received data. A newly received packet is compared with the already saved packets. In the case of a match, forwarding of the received packet is cancelled. Instead, a control message, (which is much smaller than the data packet), is sent to the sender to inform it about the current status. However, if the new data packet is an updated version of the previously stored data, the saved data is updated, and the newly arrived packet is forwarded. Relay nodes are selected based on a confidence value, which is a function of the link quality and hop count. In the case of a tie, i.e., two nodes having the same confidence value, the node with higher level of energy is selected as relay node. ECBCCP reduces unnecessary traffic by cancelling the transmissions of previously received data. However, the control messaging incurred for communication between different communicating entities and for informing the senders about status of their packets increases its communication overhead. Arep  
AREP [28] is designed to address the one-way communication problem that arises due to the asymmetry of links between senders and receivers in the underwater environment. The protocol achieves its task in four phases, termed: link state identification, greedy route establishment, void elimination, and routing update. Every node in the network builds up neighborhood tables during network initialization. The information in the tables is used during the link state identification stage to determine the symmetry between a node and its neighbors. This is followed by the greedy route establishment phase (Figure 7), during which the nodes that have bidirectional links with the sender are chosen as candidate relays. The node among the candidate forwarders that is nearest to the destination node is chosen as the forwarder. AREP eliminates the void problem with the help of notification messages that are sent from the void node to the upstream node. Route updates are made to deal with topology changes due to node mobility. If as a result of mobility, a void node is no longer a void node, the upstream nodes update their neighbor tables to include the previously void node. AREP improves delivery ratio by proposing a smart method for relay selection. Moreover, it adapts itself quickly to changes in network topology due to mobility. However, continuous table maintenance and updates incur relatively high communication overhead. Figure 5: Division of a network area into regions and sub-regions. (**a**) Division of field into regions; (**b**) Division of field into sub-regions. BLOAD  
BLOAD [29] focuses on resolving the energy hole problem by balancing energy consumption among nodes located at different distances from the sink node. It has three phases. Phase one deals with deployment. Nodes are deployed in multiple circular regions (also called coronas) around a sink node. Each corona has equal width, and each node in a corona has equal distance from the sink node. After deployment, load weights corresponding to every corona are calculated in the 2 nd phase. The load weights determine the transmission range allocation to nodes in different coronas to achieve balanced energy dissipation. In phase three, nodes transmit data using the assigned transmission range. As BLOAD solves the energy sink hole problem by allowing the nodes farther from sink to transmit with high transmission powers, it exposes deeper nodes to rapid energy drainage, as they transmit with high transmission power.