# Real-Time Communication Support for Underwater Acoustic Sensor Networks +
## 1 Introduction  
The ocean is a source of life and energy that covers over two thirds of planet Earth, and about 80% of the whole live species are there. The life of the planet depends on resources that come directly or indirectly from the ocean, therefore it is important to take care of them. Many of these resources are fragile, e.g., UNESCO reports that half of the species living in the sea will be in danger of extinction within the next century [1]. These needs and opportunities in recent years have boosted research in underwater wireless sensor networks (UWSN), as a way to increase the feasibility of implementing systems that help us not only take care of the ocean and its resources, but also to identify hazardous events originating in the ocean that endanger the people living on the seashore [2]. The research in UWSN has been highly challenging due to the harshness of the submarine environment (i.e., the hostility of such a communication scenario) and the difficulties in transmitting information in the water [3]. These challenges refer mainly to the three first layers of the ISO/OSI model [4]. The communication scenario considered in this paper is oriented to cover a wide area in which several sensors are deployed. Once the sensors are anchored, the network topology is discovered and the clocks are synchronized. The nodes are in charge of sensing different parameters or capturing events. However, the data volume involved in the transmission is low. Messages are transferred in packets of at most 256 bytes including the header, and one packet is enough to send/receive all the information collected by the sensor. Another important issue is that the transmission speed is not high. Data rates are inversely proportional to the distance to cover, and as the network is deployed in medium to large areas, the transmission rate is set to 9600 bits/s according to recommendations of previous works [5; 6; 7]. Usual data transfers ratios are set between 5000 and 20,000 bits/s depending on the distance between nodes and available power. This communication scenario is typical in environmental monitoring or tsunami warning systems. The research results in radio-frequency electromagnetic signals to support underwater communication were not entirely successful, particularly in sea water, where the conductivity of the medium is high. Provided that these signals do not propagate well in such a medium, a huge amount of power is required to transmit messages even for short distances. On the other hand, the presence of moving particles and obstacles in the propagation medium hinders the use of optical carriers. Although this communication type can achieve high transfer rates in the order of GB per second with a blue laser diode, the distances between transmitter and receiver should be short [8], which is not suitable to support the application scenario addressed in this paper; i.e., wide area networks that have low density of nodes and use an any-to-any communication scheme. Although acoustic transmissions have been studied for decades [9; 10], just in the last decade it has been shown that they are best option to support underwater communication [11; 12], which has given room to develop UnderWater Acoustic Sensor Networks (UW-ASN). However, underwater acoustic transmissions also have several problems, e.g., the signal is attenuated while propagating, there are reflections in the bottom and surface, and currents and objects may introduce Doppler effects. The medium is so difficult to address that messages should be short, as long ones are more error-prone. Considering these characteristics, it is necessary to incorporate some kind of redundancy and fault tolerance to the communication process. Most ubiquitous systems used to perform submarine monitoring or detection of hazardous events in the sea (e.g., tsunamis or oil spills) need to count on real-time communication. This communication type requires not only that messages are transmitted properly, but also that they reach their destination before a particular deadline.