# A Co-Design-Based Reliable Low-Latency and Energy-Efficient Transmission Protocol for UWSNs
Obviously, these two methods cannot guarantee delivery reliability. For the second method, nodes in a centralized-based MAC need to wait for instructions from the central node, and nodes in receiver-based routing need to wait for information from other receivers. In order to reduce the waiting time, this will increase the probability of packet conflicts and thus reduce reliability. The sources of underwater energy consumption are mainly the number and duration of transmissions. That is, the fewer the transmissions, the lower the energy consumption. At present, ALOHA-based UWAN MAC protocols are undoubtedly the most ideal. Receiver-based protocols reduce energy consumption to a certain extent without additional control packets. However, numerous packets traveling through the network will result in higher energy consumption. The holding time mechanism in DBR and other receiver-based routing protocols can alleviate this problem. Nevertheless, there exists a special case: two non-adjacent nodes receiving packets from a same sender successfully may render the holding time mechanism invalid. Considering this situation, the solution of the multiple copy problem is confronted with new challenges. As discussed earlier, these two kinds of protocol cannot meet the constraint of reliability. Combining the above points, we found that the reliability of transmission requires time and energy to ensure There is also a certain degree of contradiction between time and energy. In terrestrial wireless sensor networks (TWSNs), the layered structures like MAC and routing facilitate the development, design and troubleshooting of each layer, and can also prevent changes made to one layer affecting other layers. This design concept is based on the speed of optical communication, and the overhead of small control packets in each layer can be ignored. But in UWSN, transmission resources are very scarce and it is advantageous to have control packets on every layer. If we use only one control packet to complete the function of two layers together, this would improve the QoS of UWSN effectively. In addition, the sharing of information between different layers may also reduce the number of communications. Thus, it is worth developing the co-design method underwater. Co-design in this paper means that routing and MAC work together and share their information with each other. That is, they implement the functionality of MAC and routing jointly. The information from the MAC process provides sufficient information and can be obtained without communicating with neighbors. Latency and energy consumption can be further reduced while maintaining reliability. With the help of routing information, certain strategies can be used during the routing phase to improve the probability of handshake success in MAC. In the routing, the sender can choose the next-hops that have fewer competitors. The acquisition of neighbor information requires additional topology update packets. However, unlike control packets, these topology update packets can be transmitted when the channel is idle so as to affect the transmission as little as possible. Moreover, having neighbor information in the routing phase can effectively alleviate multi-copy issues, thereby reducing energy consumption. In such a case, the co-design method will reduce latency and energy consumption without loss of reliability. This paper proposes a reliable low-latency and energy-efficient co-design transmission protocol (CDTP) for UWSNs. With the help of the co-design mechanism, the protocol exploits fewer control packets to reduce end-to-end latency and improve the reliability of transmission effectively. At the same time, it alleviates the multi-copy problem to save energy successfully. In the network initialization phase, all nodes will obtain their neighbor information. After the initialization phase, nodes utilize the information in the MAC process (such as the _doppler scale shift_[8]) to estimate the relative speed between nodes and further estimate when their neighbors are out of range.