# Real-Time Communication Support for Underwater Acoustic Sensor Networks +
## 2 Related Work  
Underwater acoustic sensor networks have been proposed as a convenient way of supporting ubiquitous applications, ranging from biological, geophysical and environmental monitoring, to intrusion detection and also military uses that demand underwater communication. Radio, microwave, optical, and acoustic communication channels have been developed, though each technology has its own advantages and limitations. for instance, radio and microwave communications can provide high data transfer rates, but due to the seawater conductivity, they suffer from strong attenuation that increases with the carrier frequency, and demands a transmission power in the order of tens of kilowatts to cover tens of kilometers [8]. The German \"Goliath\" transmitter, developed during World War II, was capable of outputting up to 2 Mega-Watts of power to reach submarines in the Indian Ocean [16]. However, with new antenna designs, radio waves within the frequency range of 1-20 MHz can propagate over distances up to 100 m (at rates beyond 1 Mbps), using dipole radiation with transmission powers in the order of 100 Watts. Optical waves provide data rates higher than radio waves, but they are subject to attenuation due to the turbidity and dispersion of light produced by particles suspended in the sea water. Recent developments in blue laser diodes enable bit rates up to 7.2 GHz, but in very short distances limited to 6.8 m [8]. In counterpart, acoustic waves reach long distances in the order of a few kilometers, with a moderate power consumption at the expense of a low data bandwidth. There are numerous research works that address different aspects of a UW-ASN, and here we mention those that in our opinion are the most relevant. It is important to note that in most cases, these research works are theoretical proposals that have been validated by analytical methods, simulations, and small scale experiments. The literature also reports experiments in real-world scenarios, which usually involve message exchange between two or three nodes. These experiments are oriented to provide solutions at the physical layer (e.g., they are for acoustic modem design or channel evaluation) and not at the higher layers (e.g., the link, network or transport layers). The UW-ASN have several aspects to be considered that have not yet been universally defined or standardized, such as, the physical layer transmission mode, the protocols in the link, network and transport layers [6; 7; 17; 18; 19]. The literature shows four main approaches for medium access control (MAC): Frequency Division Multiple Access (FDMA), Carrier Sense Multiple Access (CSMA), Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA). FDMA divides the carrier frequency into sub-bands assigning each one to an individual user among the neighboring nodes. Due to the limited bandwidth of underwater acoustic channels and its vulnerability to a multipath, FDMA is considered not entirely suitable [20] for sea underwater networks, especially in shallow water. However, some solutions for special topologies and node deployment restrictions use FDMA for UW-ASNs [21; 22]. CSMA is a contention protocol in which the nodes should sense the presence of the carrier in the channel before accessing it. As underwater acoustic signal propagation delay is long, the carrier sense cannot indicate the real status of the channel, thus a node may start its transmission and detect it after the collision. To avoid this, nodes lock the channel with previous request to send (RTS) and clear to send (CTS) messages. Protocols derived from this are known as Multiple Access Collision Avoidance (MACA). This however reduces the throughput [23; 24; 25]. CDMA based MAC schemes have promising results and researchers focus on controlling the transmission power at all sensor nodes, in order to deal with the near-far problem and save energy. In [26] a hierarchical tree-topology is considered, where the nodes in same hierarchical level are multiplexed by means of different orthogonal codes.