# Real-Time Communication Support for Underwater Acoustic Sensor Networks +
## 4 System Model
### Routing Model  
In previous works, we have proposed a hierarchical propagation model [14] in which messages propagate in a tree from the root to the leaves, and later from the leaves to the root with nodes aggregating the messages from the sons before forwarding them to the next layer or father. In [43] the network was restructured as a mesh, and a routing algorithm based on the shortest path first (SPF) was proposed. Although valuable, these approaches do not consider the multipath problem and no fault-tolerance backup policy was proposed. The routing model we introduce in this paper takes advantage of the multipath effect, in order to provide a probabilistic recovery from transmission errors originated in unexpected disturbances. In this proposal we also have added an opportunistic routing based on the epidemic model. In [43] the SPF was used to determine the best path for a message; however, underwater transmission is so vulnerable to disturbances that using just one route jeopardizes the possibilities of success. Therefore a _multicast_ propagation is best suited in this case, since every message is forwarded from source to destination through different paths until it has been received. Deadlines associated to messages are used to discard those routes that are not feasible from the beginning. UW-ASN have a low traffic intensity and this allows the use of epidemic routing; otherwise, some nodes may be overloaded as they are in the middle of several paths. In the slot time allocation, the replicas coming from multipath propagation are usually discarded, but their presence should be considered in the schedule to avoid collisions. Therefore, a message that has been corrupted in the direct path can be recovered from a replica arriving later. Although different approaches are feasible to do this, for the sake of simplicity we have assumed that the message is read again. In the example represented in Figure 2, let us suppose that node \(a\) has to send a message to node \(e\), and there are several available paths. The SPF is \(a\)--\(d\)--\(e\), which has a worst case transmission delay of 18. However, if the link \(a\)--\(d\) is corrupted, the message is lost and the transmission fails. By using both \(a\)--\(d\) or \(a\)--\(b\), the system is more robust and even if \(a\)--\(d\) fails, the message may survive in the link \(a\)--\(b\). For this to be effective, neighbor nodes should overhear the activity in the network so they can detect a fallen link or a dead node. In the case of underwater acoustic channel, the probability of faults is significant. Therefore, to prevent the message lost, every node receiving a message will forward it. Although more energy is required to implement this kind of propagation, it is more robust. For instance, \(a\) sends a message to \(e\) through \(d\) with an epidemic policy. Node \(b\), listens to \(a\) transmission and forwards the message to \(e\) through \(c\). This message will also be received by \(a\) and \(d\), but as they already have the message, they will not forward the copy. The node \(e\) will receive a maximum of two copies of the same message. In the case that the path \(a\)--\(b\)--\(c\)--\(e\) requires more time than the deadline, the whole path is discarded saving energy and allowing other messages to use that path. In challenged networks, like UW-ASN, there is a high probability of failure in the transmissions. In WSN several protocols were proposed to provide alternative paths in case of nodes failures by \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Node**} & \multicolumn{5}{c}{**Time Slots**} \\ \cline{2-6}  & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline \(a\) & \(Tx(a)\) & & & \(Rx(b)\) & \(Rx(d)\) \\ \(b\) & \(Tx(b)\) & & & \(Rx(d)\) & \(Rx(a)\) & \(Rx(c)\) \\ \(c\) & & & \(Tx(c)\) & \(Rx(b)\) & \(Rx(e)\) \\ \(d\) & \(Tx(d)\) & & & \(Rx(b)\) & \(Rx(e)\) & \(Rx(a)\) \\ \(e\) & & \(Tx(e)\) & & \(Rx(d)\) & & \(Rx(c)\) \\ \hline \hline \end{tabular}
\end{table}