An Energy-Efficient MAC Protocol for Underwater Sensor Networks

Underwater sensor networks have received growing interests recently [7], [5], [4], [1]. Compared with traditional techniques used in underwater activities such as scientific exploration and commercial exploitation, underwater sensor networks empower people to monitor or detect phenomena more accurately and timely in extended areas. The medium access control (MAC) protocol is very important to the final performance of underwater sensor networks. Different applications have different requirements on MAC protocols. In this paper, we aim to design a MAC protocol for the long term applications such as environment monitoring. These applications are not sensitive to the end-to-end delay and generate sporadic traffic unevenly distributed spatially and temporally. We focus on the dense sensor networks where neighboring nodes are located a few hundred meters apart and the bit rate of the underwater acoustic modem is around 10 kbps. The most important goal of MAC for such underwater sensor networks is to resolve data packet collision efficiently in terms of energy consumption. However, the characteristics of underwater sensor networks and the requirement for energy efficiency make the existing MAC protocols proposed for radio sensor networks unsuitable for underwater sensor networks. In underwater sensor networks, acoustic channel is used as the communication method. The propagation speed of sound in water is about 1500 m/s, which is 5 order of magnitude lower than that of radio. The low propagation speed results in a high propagation delay even for communication between two neighbors. The available bandwidth for an acoustic networks is typically less than 15 KHz, which is extremely narrow compared with radio networks. The low propagation speed makes the widely used collision avoidance method namely RTS/CTS, unfeasible in underwater sensor networks. As shown in[2], in order to completely avoid data packets collision, two conditions have to be satisfied: 1)The duration of RTS should be greater than the maximum propagation delay, and 2) The duration of CTS should be greater than that of RTS plus twice the maximum propagation delay plus the hardware transmit-to-receive transmission time. It is easily to satisfy the two conditions in terrestrial sensor networks, however, it is too expensive to do so in underwater sensor networks where the maximum propagation delay might be greater than 50 ms. The size of the control packets would be unacceptably large. Furthermore, the problem is made more complicated by the requirement for energy efficiency. In order to save energy, the sensor nodes have to sleep and wake periodically. In this paper, we address this problem under the energy constraint. We propose an energy efficient MAC protocol, reservation-based MAC protocol, R-MAC for underwater sensor networks. In R-MAC, we schedule the transmission of control packets and data packets on both the sender and receiver to avoid data packet collision completely. Moreover, R-MAC supports fair sharing of the channel among the competing senders. Additionally, we adopt a new ARQ technique, blockbased acknowledgment in R-MAC to improve the channel utilization.