# Real-Time Communication Support for Underwater Acoustic Sensor Networks +
## 3 Initial Design Decisions for Implementing UW-ASN  
Applications based on UW-ASN are being deployed with different purposes as indicated in Section 1. These networks require to count on a gateway to conventional networks, which can be a buoy in the surface or a fiber link connected to some point in the coast. In most cases, the usual topology of a UW-ASN is based on a star or tree, where the root sensor (e.g., a sonobuoy) is the master and all the other nodes act as slaves. Although useful, this approach centralizes the communication and restricts the information flow, overloading certain nodes with much more traffic than others, since they have to aggregate the messages from downstream nodes [14]. This centralized model makes the system error-prone, since a failure in a root node or in the links to it, can produce a system failure. In this sense, using a mesh network has several advantages because various nodes may act as gateways to traditional networks, and the information may flow through different paths. Besides, the possibility of sharing information between any pair of nodes contributes to the development of distributed applications or information processing, by increasing thus the computational power of the network and improving its performance. The integration and collaboration between nodes of a UW-ASN need to count on solutions to share data in a peer-to-peer scheme. Supporting this type of interaction allows the applications to implement distributed data storage, fault tolerance, rapid data sharing, and early warning propagation in the application layer [46]. Similar to the regular wireless sensors networks, the most common MAC protocols for UW-ASN can be grouped into two classes: contention (CSMA and MACA) and schedule (FDMA, CDMA, TDMA). In the first one, nodes transmit whenever they are able to lock the shared channel. For real-time messages this approach is not useful, since message delays may be unbounded. In the second approach, the FDMA is not useful as explained in the previous section, CDMA is promising and TDMA has been widely accepted. Although TDMA may introduce an important latency, the worst case delay may be computed and the behavior of the network (in terms of time) is predictable. TDMA can be divided into two modes. The first one considers that each node sends a broadcast to every node within transmission range. In this case, if a node has several messages to transmit to different destinations, it has to wait for equal number of frames. In the second mode, a per-message TDMA is computed in such a way that the slot for sending a message from node \(a\) to node \(b\) is defined, and also the moment at which node \(b\) receives the message from node \(a\). In this case, each message has a particular slot to be sent and received at destination, and the nodes may wait for the proper instants to become active. Using TDMA in UW-ASN has several advantages since, e.g., each node transmits and receives in specific time slots (or time windows), where the channel is free to transport messages without collisions with other nodes in the network, or it is able to receive messages from specific nodes. The sequence of slots is periodic, and each set is named frame. In a frame, all nodes in the network transmit and receive without interference from other nodes. The slot allocation is an NP-hard problem [47; 48]; however, in this paper we propose two solutions to address such a problem: one based on ILP techniques (like branch-and-cut) and another based on evolutionary meta-heuristics (like genetic algorithms). Provided that in a mesh network the routing is important, as it is the access to the transmission channel, we propose to use epidemic routing to address this challenge. The harshness of underwater communications requires to employ all possible strategies to reduce the probability of failure. Therefore, replicating the messages in several paths, increments the chances of achieving a successful transmission.