| In 21 st century, countries in the world are paying special attention to the development and utilization of marine resources, China has developed the western development strategy go with the development of marine resources in this century. Therefore, a rational and efficient use of modern scientific and technological means to monitor the marine environment is a must for China to achieve maritime power necessary prerequisite. Using underwater acoustic sensors to achieve the monitoring of the marine environment is of great significance and broad prospects. Underwater sensor networks have advantages in monitoring the marine environment with the characteristics of high integration, real-time, self-organizing, low-power and low price. These characteristics enable large-scale deployment in a wide range of marine water and facilitate the achievement of different monitoring of the marine environment.At present, researching of underwater sensor networks is aimed at the physical layer, MAC protocols and network layer protocols. This paper describes the characteristics of the acoustic environment and underwater sensor networks, analyses the status of MAC protocols and introduces the two categories of MAC protocol for wireless sensor networks, including those based on competition and based on scheduling. Competition-based MAC protocol introduces the ALOHA protocol,802.11 wireless LAN applications, MAC protocols, S-MAC protocol, as well as T-MAC protocol, the competition-based protocol communication requires a lot of control information, These information may lead the problem as slowly networking and low communication efficiency. They don't appropriate to underwater sensor networks. The MAC protocols based on scheduling include TDMA, FDMA and CDMA. The TDMA-based MAC protocol which is time-division multiplexing does not requires a lot of control information. In case of synchronization between each node it can guarantee conflict-free and more applicable to underwater communications environment.This paper analyzes the impact of underwater acoustic communication quality, and then proposes a MAC protocol which is based on TDMA, according to characteristics of underwater acoustic communication. In the protocol the Sink node in the network or on land site can divide time slice region according each node's location, and then assigned time slice to every node. Sink node will broadcast the time-slice information to all the nodes in the network with flooding way. If each node determines its own time slice, it will work only within its own time slice and sleep in other time slice, thereby saving energy. Because the TDMA-based MAC protocol requires clock synchronization, this article also proposes a strategy for underwater acoustic communications. The network will set the synchronization cycle, at the beginning of each cycle Sink node will send the sync signal. Information transmission can be carried out three times to complete a synchronization process between every two nodes.For preliminary testing of the MAC protocol performance, we do the experiments based on LAN. The MAC protocol will be equipped with the experimental circuit board, and then the circuit board connects to PC through the serial port. By designing software we can simulate underwater communications environment in LAN. And then test the basic performance of this protocol.This article relies on 863 subjects "underwater sensor networks, key technology research of networking"。In order to further validate the performance of this protocol, we make MAC protocol of the underwater experiments with underwater sensor hardware platform. The experiments show that networking efficiency and energy consumption of the MAC protocol proposed in this paper has an obvious advantage. Finally, the contents of this article are summarized and presented for future work prospects. |
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