## 1 Introduction  
In recently years, more and more applications have appeared with the development of wireless communication network techniques [1; 2; 3; 4]. Underwater sensor networks are an emerging and promising network technique which has attracted considerable attention. In this paper, we present a time-slot based routing algorithm (TSR) by applying a series of improvements of the flooding protocol [5]. Conflict between nodes is avoided when they start to send packets only within their own time-slots, and they don't need to reply to their parents individually in the process of establishing routing, instead they directly broadcast the routing messages. Meanwhile, to save more time and energy to quicken routing establishing process, the packet could act as the ACK to reply to their parents. The network topology of underwater sensor networks of this paper searched is a planar centralized-tree construction. This construction has two advantages. One is easy to be extended. Tree construction can extend many branches and child branches which can be easily added into the networks. The other one is the convenience for isolating malfunctions. We can conveniently separate malfunctions from the rest of the system when nodes or routes in one branch breakdown. As Figure 1 shows, there is one underwater sink and m underwater sensor nodes. Every node presets an unique ID (1, 2, 3,..., m). The ID of the underwater sink is 0 and the parent node of the sink is \(-1\). All the nodes' ID will notify the underwater sink before deployment. The underwater sink connects with sensor nodes on land through wireless communication, and sensor nodes on land connect with a PC through serial ports. Thus, the PC can receive data which is collected by underwater sensor nodes through sensor nodes on land and send commands through sensor nodes on land as well as display the data on the screen and upload it to an Internet server which can be viewed and downloaded by researchers. Figure 1: Underwater sensor network model. Due to the fact the underwater sink undertakes more missions (e.g., generating and maintaining routes, distributing channels) than underwater sensor nodes, we have to provide more high-energy battery for underwater sink. Due to the fact TSR needs strict time synchronization, we assume each node has a strict default synchronization clock. \(\Delta t\) is minimum time for transmission of a packet between any two nodes with no conflict. \(T\approx k\Delta t\), k is revision coefficient, we set the default value is 1.5. We know all nodes' geographical location, and divide the time slot for each node based on its longitude and latitude. Before establishment of a route, the sink sends a time piece packet to a sensor node to notify which time slot they belong to. Sensor nodes only send packets in their own time slot. Through this method the control sensor nodes send packets in turns, and they can ensure the channel is unoccupied in their own time slot without overhearing the channel. Low spreading speed and high rates of missing code are big problems for UWSNs. Therefore, it is important to quickly establish a route tree process and save the energy of nodes in a large application scenario. If we use the traditional method to establish route through polling, we need to send packets to every node and wait for ACK. As a result, every node has to wait for a long time and wastes too much energy. Therefore, it is very important to use time appropriately and decrease the depletion of energy. In such cases, only by ensuring every node has an equal opportunity to send packets can decrease the waiting time. This paper presents TSR based on a flooding protocol to save waiting time. Nodes can send packets in their own time slot no matter if the last node is done sending or if it has received a waiting packet. The nodes broadcast the packets which they are received. Though this process we can decrease the amounts of packets and increase the rate of route construction. We will show the details in Section 2.