Underwater wireless sensor networks are significantly different from terrestrial sensor networks in which sound is mainly used as the communication medium. This paper reviews the main challenges caused by the underwater environment for the development of efficient under water sensor networking solutions and introduces several novel medium access control (MAC) protocols with power control to increase efficiency and save on energy.
Underwater wireless sensor networks (UWSN) has received growing interest recently. It has many applications potentially in ocean graphic data collection, water quality monitoring, surveillance purposes and national security purposes. UWSN consist of a certain number of sensors that interact to send data to the onshore or surface sink node and perform collaborative tasks. Different applications have different reliability requirements. Thus data reliability is one of the most important requirements for data transmission. However, it would be a challenging task to assure reliable data delivery between the sensor nodes and the sink node, because the network performance is subject to the limitations and constraints of the complex underwater environment.
In this paper review, the main challenges of the UWSN MAC protocol design are introduced. Then the related works of existing reliable MAC protocols for UWSN are reviewed and summarized.
II. Challenges of MAC protocol for UWSN
Compared to radio waves used in terrestrial sensor networks, which would have very high attenuation underwater, acoustic waves has better propagation characteristics in water, making it the preferred method for underwater communications. Hence acoustic channel is used as a link for communication in underwater sensor networks.
The propagation speed of sound in water is about 1500 m/s, which is 5 orders of magnitude lower than that of radio. The low propagation speed results in a high propagation delay (about 0.67ms/m) even for communication between two neighbors, while the delay in terrestrial is only 0.33ns/m. The high propagation delay increases the probability of collisions and highly degrades the throughput of the protocols.
In additional, sound propagation underwater is severely affected by transmission loss, noise, and reverberation, temporal and spatial variability of the channel. Transmission loss and noise are the principal factors determining the available bandwidth, range, and signal-to-noise ratio. The background noises bring difficult to the signal detection, which require wider range control of transmission power. In underwater environment, the networks suffer from high bit error rate and significant signal attenuation which depends on frequency, so the available bandwidth of acoustic channels is much narrower compared with that of RF channels, which is typically less than 15 KHz, resulting in low data rate for underwater communication, which can only provide bit rates in the order of tens or hundreds. This can create...