# A Comprehensive Survey of Recent Routing Protocols for Underwater Acoustic Sensor Networks
## 3 Routing Protocols for UWSNs
### DMR & CoDMR  
Reference [67] proposes two routing protocols, delay minimization routing or DMR, and cooperative DMR or CoDMR. The aim of the two protocols is to reduce the delay induced due to long detour of packets in multi-hop routing protocols. To achieve this, DMR partitions the network into four equal parts. A sink node is deployed at the center of each part All of these sensor nodes in each part that can access their sink node directly send their packets to the sink node through one-hop forwarding. However, farther nodes send packets to the sink node through multi-hop forwarding. The authors argue that dividing the network into four parts and deploying a sink in each part will reduce latency, as compared to the case when data has to be routed through multiple hops to the sink nodes on the ocean surface. DMR can face reliability issues due to transmission of data over a single link. To cope with this, CoDMR is proposed. In CoDMR, the senor nodes that can access the sink nodes directly send their packets directly to the sink node. However, if the sink node is out of communication range of the sender, packets are sent through cooperation with a single relay (Figure 19). The node having lowest distance with respect to the sink node is considered as the destination node. The proposed schemes solve the long detour to surface sinks and the resultant latency, to some extent. However, the long detour issue due to multi-hop forwarding, which may occur inside partitioned parts of the network, is not addressed. CoDMR adds limited improvement in reliability by merging two copies of the same packet. However, the next hop selection criterion considers only distance from the sink node. The channel condition is ignored, which may create reliability issues, despite the merging of two copies of the same packet to improve reliability. GEDPAR & E2 EVHR  
Reference [68] proposes two routing techniques, GEDPAR and E2 EVHR. GEDPAR is basically a combination of GEDAR [69] and LMPC [70]. GEDPAR takes its layering scheme from LMPC. The nodes are deployed in layers of unequal interlayer distance. The size of a layer is inversely proportional to noise i.e., the size of the layer shrinks with increase in noise levels, and vice versa. Whenever a void region is encountered by a node, it increases its transmission range. However, if it cannot reach another node of smaller depth, even with the maximum possible transmission power, it resorts to depth adjustment (Figure 20). This scheme efficiently handles voids at the cost of higher energy consumption. E2 EVHR improves LMPC by finding a complete void-free route from source to destination, as opposed to hop-by-hop forwarding of LMPC. This is followed by routing multiple copies of a packet along multiple paths of a predefined binary tree structure. This improves end-to-end delivery of E2 EVHR. However this scheme has some demerits. As the packet travels on multiple paths in parallel, a lot of redundant transmissions are incurred that will result in high energy consumption. Moreover, the predefined paths may go stale, due to node mobility. relays, which are responsible for data forwarding, and cooperative relays, which carry out single-hop cooperative communication (Figure 21). Sensors nodes that want to transmit data independently select routing and cooperative relays. The relays are selected from the neighbors of the sender based on the link quality metrics, such as SNR and time of arrival. Moreover, the distance of the sender from the destination in terms of hop counts is also taken into account. Routing relays are selected based on propagation delays and the link's channel capacity, while the cooperative relay that has least propagation delay from the selected routing relay is chosen to decrease spread delay. As channel conditions play an important role in relay selection, channel asymmetry is taken into consideration to make routing decisions more reliable. The proposed scheme improves reliability even more, by incorporating a cooperative transmission scheme in every hop.