Research on MAC Protocol for Underwater Acoustic Sensor Networks

The underwater acoustic sensor network has been used in such wide areas as oceanographic data collection, pollution monitoring, disaster warning, offshore exploration, navigational assistant and tactical surveillance application. While the implementing of these applications lies in the key technology underwater acoustic communications technology.Therefore, it is a critically important mission for underwater acoustic network to design an underwater acoustic communication technology which can improve the throughput and reliability of network to the greatest extent, and at the same time, energy consumption and propagation delay can also be lowered effectively.  An underwater acoustic sensor network is a wireless network which is built with the aid of the sound signal. It consists of a series of sensor nodes and the equipment, between nodes by underwater acoustic channel connection. We put these sensors arranged in the monitoring area, to achieve long term underwater early warning of this area, target tracking, marine environmental factors monitoring and other tasks. Once the sensor nodes are deployed, it is often a few months, even a year. Because the sensor nodes are powered by batteries, the energy of the battery limits the service life of the underwater sensor network to a large extent. In addition, the narrow bandwidth, multipath phenomenon and high transmission delay also reduce the efficiency of the underwater acoustic communication network. Therefore, how to design an underwater acoustic communication network structure, so that it can maximally improve the network throughput and reliability, and can reduce the energy consumption and propagation delay, for underwater acoustic sensor networks is important task. Architecture of underwater Acoustic sensor networks Underwater Acoustic Sensor Networks(UAN) like most other networks, are organized in a hierarchical manner. However, with the OSI protocol stack model and the TCP/IP protocol architecture, the structure of the UAN is divided into three layers, the physical layer, data link layer and network layer. The architecture of the UAN, the OSI protocol stack model and the TCP/IP protocol are compared in  Engineering and Technology (ICASET 2016) (c) 2016. The authors Published by Atlantis Press 0181 Fig1. Comparison of UAN, OSI and TCP/IP Architecture The physical layer is the basis of effective communication between nodes, which is responsible for the modulation and demodulation of data. It converts the information into a binary bit stream transmitted over the channel,then detects the signal from the noise at the receiver, and then it is reduced to the original logic signal. The data link layer is responsible for error free transmission between adjacent nodes, which is composed of LLC layer and MAC layer. The network layer is mainly used to establish the topology structure of underwater acoustic network, route control, and reasonable flow and capacity control of the line, to ensure network reliability. As a result, the design of network topology has great influence on the performance and economy of the whole network, and it is the base of realizing all kinds of network protocols. Fig2. Multi Hop Network Topology Multi hop is a kind of multiple relay peer to peer topology network structure  The nodes in the network rely on the adjacent nodes to forward data, after several relay, finally to the main node, and then communicat with the shore based system by the master node.The coverage of the network is determined by the number of nodes, it can cover a large area, and the single node power consumption is small, so it is very suitable for the wide distribution of the sensor system in shallow water. The MAC protocol of the data link layer ensures reliable point to point and point to multipoint communication, establishes a link between the nodes, and ensures that all nodes can utilize the bandwidth fairly and efficiently. In the traditional wireless network, the main evaluation indexes are throughput, bandwidth utilization, fairness and delay etc. but for the underwater acoustic communication network, the power efficiency is the first, and sometimes even to sacrifice other aspects, to obtain a higher power efficiency. This means that the traditional MAC protocol is not suitable for underwater acoustic communication networks. In order to reduce the consumption of energy, underwater acoustic communication network protocol is usually used to "listen to sleep" When there is data to send and receive, the node will open the communication module to send or listen. If there is no data to send and receive, the node will control the communication module to enter the sleep state, thereby reducing energy consumption. In order to make the node in sleep do not miss the data, or reduce the excessive listenin, the neighbor nodes need to coordinate between listening and sleep cycle.  The protocol also uses the RTS CTS DATA ACK mechanism, RTS/CTS/DATA/ACK can only be used in the time slice of the start of transmission. Among them, the control package takes up a time slice, and the data packet can last a plurality of time slices. In Aloha CS protocol, If the node listens to the channel on which data is sent, it does not send data. otherwise the data can be sent directly. In the TDMA protocol, time is divided into several time slots, and the nodes can transmit data in their own time slots. MAC frame in Slotted FAMA is divided into four types,RTS,CTS,DATA and ACK. Among them, RTS,CTSand ACK is the control frame, DATA is data frame. Aloha CS and TDMAOnly have data frame. The control frame consists of frame type, source node ID ,destination node ID and cyclic redundancy check(CRC) as shown in  Fig 3.Structure Of the Control Frame Among them, the frame type occupies 2 bits. 00 means RTS frames, 01 means CTS frames, 10 means DATA frames, 11 means ACK frames.Counter means the data frame numberto be transmitted. The data frameconsists of frame type, source node ID ,destination node ID,dataand cyclic redundancy check, as shown in Figure 4.The length of data depends on the situation. Fig 4.Structure Of the Data Frame Figure 5 describes the underwater acoustic channel propagation model. From the transmitter (marked in the figure T) issued by the sound through the 5 different ways to reach the receiver (marked as R in the figure) Fig 5. Acoustic Propagation Model for Underwater Acoustic Channel 1) The sound wave emitted by the transmitter is reflected by the sea surface to the receiver, which is expressed as A; 2) The sound wave emitted by the transmitter is first reflected by the sea surface, and then through the seabed to reach the receiver, the symbol is B; 3) the direct sound wave, the mark is C; 4) The sound wave emitted by the transmitter is first reflected by the seabed, and then the though the sea surface reaches the receiver. The symbol is D; 5) the signal emitted by the transmitter reaches the receiver through the sea surface, which is marked as E. In this paper, the underwater sensor network is a multi hop and shallow sea network. Asynchronous half duplex mode was adopted for the communication between nodes in the network, 2016 6th International Conference on Applied Science, Engineering and Technology (ICASET 2016) (c) 2016. The authors Published by Atlantis Press 0183 channel band 10kHz, the control frame length 32bits, data frame length 512 bits (and therefore according to the definition of the data frame structure, the length of the data portion of the frame is 480bit) the transmission rate for 256bit/s. The network consists of 16 nodes, nodes are deployed in 4km 4km in the region, one of the sink nodes, multi hop network topology structure. The bit error rate of the underwater acoustic channel is fixed value, 10 5 bit rate, and the transmission rate of the underwater acoustic signal is 1500m/s. Fig 6.is network deployment diagram for simulation experiment.