Therefore, one of them would rebroadcast the message to the second cell of the grid, on channel \"11\" (Figure 1), making use of its transmitter. This process (SITOR-B) would be repeated from cell to cell until the message reaches the cell in which the ship being called is located, and that ship sends back the appropriate message. The return message from the ship to be identified to the TCC, passing through the different repeaters that have contributed to this message evolving from each cell to the next one, is no longer an open message. The return response will follow the inverse path to that of the initial message, passing through each of the vessels that had acted as a repeater. It is a point-to-point message (SITOR-A); therefore, listening to this message will require a receiver equipped with DSC. Since each ship will be called directly, it requires receiver \"D\" (Figure 2). In the operation of this system, there are two main factors that play a decisive role: wait times and repeater-ship determination. Wait times are important since a certain message calling a ship is heard until it acts as a repeater if there is no response. It is also important  
Figure 2: Block diagram of the first system proposed for installation on board (AVISOMEF). The ship is assumed to be in the first cell, which is located to the left of the grid depicted in Figure 1.  
to select the best ship to act as the repeater. As we have already commented in previously presented literature, current technology can provide solutions to these problems. A delay protocol was proposed when acting as a repeater, in which the geographical distance to the central point of each cell minimizes response times. After a previously established time, if no ship responds as the recipient of the message from the TCC, those located in the cell in which the message has been broadcast can act as repeaters, but the closer a ship is to the geographic point indicated above, the shorter the time of response imposed in the protocol. This is perfectly feasible as the system incorporates data from geographic information systems installed on board. To maintain safety at sea [11] when vessels are far away from the coast, it is mandatory to establish a communication system capable of overcoming distance limitations of the frequency band used. AVISOMEF is a system that allows for the identification of vessels without using satellite communications, overcoming the range limitations of medium frequency communication. Communication between vessels and shore using satellites has been a very useful system to communicate with vessels far away from the coast [8; 12; 13] but it is associated with high costs. Some studies have proposed the integration of satellite systems with non-satellite systems that have less coverage but that are cheaper or free [14; 15; 16]. These hybrid systems can overcome the coverage limits of non-satellite systems thanks to the use of satellite systems. Other proposals only use non-satellite technics. In [17], a new net layer protocol in an OSI model based on AIS was proposed. This new protocol uses VHF in its physical layer, so it needs less distance between vessels than do MF systems such as AVISOMEF. In other studies, WiMAX was proposed as a physical layer of the system [18; 19]. WiMAX is a non-satellite communication system with a lower associated cost than satellite systems with a maximum range between 40 and 70 km: a range that can be overcome with AVISOMEF. This paper studies the possibility of using AVISOMEF as a physical layer in a non-satellite maritime communication network. It is necessary to check if the maritime traffic is sufficient to cover the needs of AVISOMEF and, therefore, of the new maritime communications network.