# A MAC Protocol to Support Monitoring of Underwater Spaces +
## 1 Introduction  
The ocean covers 71 percent of the Earth's surface and contains 97 percent of the planet's water, yet more than 95 percent of the underwater world remains unexplored. UNESCO states that over 90 percent of the Earth's habitable space is within the ocean, and near 80 percent of all life is under the ocean surface. There are quite dramatic projections stating that by the year 2100, more than half of the marine species may stand on the brink of extinction [1]. This shows only some of the several reasons that we have to deploy underwater technology that helps us study several phenomena that affect our lives and the health of our planet. In this scenario, the underwater sensor networks could provide valuable information that helps us address some of these challenges [2]. However, underwater radio communications are practicallyunfeasible, especially in the ocean, because radio signals suffer a strong attenuation, limiting the effective communication to a few meters. Acoustic signals instead are capable of traveling for long distances, depending on the power used for the transmission and the physical characteristics of the medium. Therefore, the communication feasibility depends on several variables, such as the carrier communication frequency, the point-to-point distance, the chemical composition of the water, the topology of the seafloor, the temperature of the water, the depth of sensor placement and the spreading pattern [3]. Underwater acoustic sensor networks (UWASN) have several challenges that should be considered for a successful implementation. The first one is related to the lossy nature of the channel. Although acoustic signals can propagate well beneath the water, they suffer an important attenuation due to the absorption; and the signal amplitude attenuation is proportional to the distance and the frequency of the signal. There are some other factors contributing to the attenuation, like the scattering and reverberation in the surface, because wind moves the reflection point in the surface. There is also reflection in vessels that may introduce a Doppler effect in the signal. Moreover, the geometric spreading of the signal produces path loss in the transmission; regardless, we are using spherical (that is common in deep water) and cylindrical spreading (that is more common in shallow water) [4]. The second challenge to address refers to the nature of the acoustic signal used for transmitting the information. There are several man-made noises present in the ocean and especially in the surf zone, where it is usual to have a higher density, vessel traffic, sport activities and city noise. Another source of noise is provided by the environment itself; i.e., waves, winds, rains, animals and even seismic noise that may interfere with data transmission in the acoustic sensor network. The third challenge is related to the multi-path propagation of the signals. Different signal paths may interfere between each other by means of the inter-symbol interference. The vertical links usually have little dispersion, so it is not common to have multi-path interference; however, in horizontal channels, the spread may be significant, and therefore, different signal paths may interfere among them. The fourth challenge to be addressed is the energy consumption. Underwater sensor networks are battery powered, which also represents a major concern during its operation. The energy consumption during standby, data sensing and data reception is low in commercial modems, but in data transmission, the consumption depends on the distance that the signal should travel. The relationship between transmission power versus distance follows a quadratic function. One way to save the energy in these networks is by transmitting at higher baud rates for short periods of time. On the other hand, transmissions through long distances require high power and lower baud rates; thus, high data volumes result in being incompatible with large distances under the power consumption point of view.