# A Co-Design-Based Reliable Low-Latency and Energy-Efficient Transmission Protocol for UWSNs  
[PERSON]  
1 College of Computer Science and Technology, Jilin University, Changchun 130000, China;  
[EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (H.G. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (C.W.) [PERSON]  
1 College of Computer Science and Technology, Jilin University, Changchun 130000, China;  
[EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (H.G. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (C.W.) [PERSON]  
1 College of Computer Science and Technology, Jilin University, Changchun 130000, China;  
[EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (H.G. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (C.W.) [PERSON]  
1 College of Computer Science and Technology, Jilin University, Changchun 130000, China;  
[EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (H.G. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (C.W.) [PERSON]  
1 College of Computer Science and Technology, Jilin University, Changchun 130000, China;  
[EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (H.G. ); [EMAIL_ADDRESS] (X.W. ); [EMAIL_ADDRESS] (C.W.) [PERSON]  
2 Department of Computer Science and Engineering, Qatar University, Doha 2713, Qatar; [EMAIL_ADDRESS] (Department of Computer and Information Sciences, Temple University, Philadelphia, PA 19122, USA;  
[EMAIL_ADDRESS] (Correspondence: [EMAIL_ADDRESS] 2020; 202  
The discrepancies between underwater and terrestrial environments further complicate the design of data transmission protocols. The unavailability of long-distance optical communication leads to most underwater protocols relying on acoustic communication for data transmission. Although underwater acoustic communication has the properties of long acoustic propagation delay and high communication cost, it is possible to satisfy any two of the three above constraints by using separate designed routing or media access control (MAC) protocols. However, the separate design method cannot successfully deal with all three constraints simultaneously in UWSNs. In order to achieve reliability of transmission, MAC protocols allow the sender and receiver to handshake by sending small control packets (e.g., request-to-send (RTS)/clear-to-send (CTS) [2]) such as channel contention-based carrier sense multiple access/collision avoidance (CSMA/CA). Another common method is to collect information like time division multiple access (TDMA) [3] through a central node to manage the channel. The common feature of these methods is that each pair of nodes to be communicated will require at least one handshake. As for routing protocols, they improve the reliability by selecting multiple relay nodes, creating multiple copies of data or retransmission mechanisms. Sender-based routing protocols (e.g., PULRP [4]) let the sender choose a certain group of forwarding candidates from neighboring nodes as the next-hop. Before sending data packets, the sender broadcasts a control packet to its neighbors for choosing the next-hop. These methods all increase the amount of communication in the network in different ways. However, the transmission cost is very expensive in UWSNs compared with that of traditional MAC protocols in radio communication [5] and this cannot be ignored. In other words, it is difficult to reduce end-to-end delay and energy consumption to maintain reliability of transmission. In UWSNs, two main methods can reduce the end-to-end latency: reducing the numbers of any manner of communication and reducing the waiting time of nodes. For the first method, ALOHA-based UWAN MAC protocols [6] allow a node to transmit a data frame anytime at its will without any control packets. Receiver-based underwater routing protocols (e.g., depth-based routing (DBR) [7]) do not send any control packets for reservation before data transmission. Senders put their own information in the packet head and directly broadcast the data packet. Receivers utilize specific information (e.g., depth information) in the packet head to decide whether and when to relay this packet or not.