# Real-Time Communication Support for Underwater Acoustic Sensor Networks +
## 7 Simulations
### End-To-End Delay Analysis
If we consider any other routing model, most of the nodes that are in the middle of several paths may be overloaded. In order to demonstrate this, we have compared tree multi-hop routing-algorithms: Epidemic, Shortest-Path-First (SPF), and UnderWater Opportunistic Routing based [42] (UWOR-based). * Epidemic: This is the proposed flooding routing scheme. Aiming to deal with message loss (due to high noise and interference of underwater scenarios) and link degradation (due to water currents and the sensor-nodes movements, the topology changes frequently). * UWOR-based [42]: This is a simplified version of the proposal called \"Unicast without ReTx\". It uses the proposed metric \(EEL|success(. )\) to find a single forwarder by taking into account both one-hop reliability and end-to-end latency, but in these simulations the T-Lohi MAC protocol has  
Figure 9: End-to-end message delays. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Metric** & **None** & **Uniform 10** & **Uniform 2** & **Pareto 10** & **Pareto 2** \\ \hline packet delivery ratio (PDR) & 1.0 & 0.998 & 0.918 & 1.0 & 0.916 \\ network goodput ratio & 1.0 & 0.998 & 0.918 & 1.0 & 0.916 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Path usage. been changed by TDMA, and also some minor changes to compute the \(EEL|success(. )\) metric had been done. * Shortest-Path-First (SPF): This is a classical routing schema that we assume as a baseline. Table 11 shows Packet-Delivery Ratio (PDR) and network-goodput ratio of both routing algorithms, considering no link-failures and the Pareto 2 approach. There we can observe that, with no link-failures, all messages were delivered (PDR = 1.0) and their deadlines were satisfied (PDR matches network goodput ratio). Unfortunately, this is not a realistic scenario in UW-ASN. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{**Pareto 2**} \\ \hline \hline
**Message** & **Path** & **Usage (\%)** \\ \hline \hline  
On the other hand, when we compare Epidemic, UWOR-based, and SPF routing considering a link-failure model (Pareto 2), we can observe that PDR is below 1. Notice that this is quite good for the Epidemic-routing approach (PDR = 0.94), but significantly poor for the SPF schema (PDR = 0.72), and UWOR-based schema (PDR = 0.87). Notice also that, in both cases, the deadline of messages is satisfied (network goodput ratio matches PDR). Figure 10 shows empirical cumulative distribution-function of end-to-end message delays for these routing strategies, with and without link-failures. The results indicate that SPF has better end-to-end delays compared to the Epidemic and UWOR-based strategies. In this case, the large number of Epidemic messages fills the queues of the nodes, thus delaying the message transmission. However, its PDR is significantly better. Therefore, this is reduced to a trade-off decision between end-to-end delay and packet-delivery ratio. The harshness of the submarine environment makes the UW-ASN highly challenging, and underwater communications are susceptible of losing messages. Hence, our goal is that messages reach destination nodes, therefore, we choose the Epidemic approach since it prioritizes PDR over delay (within RT deadlines). Any other routing algorithm that transmits an unique message and uses a unique path, put in risk the communication given the rate of message lost in such a communication medium is usually high. In this case, the use of epidemic routing improves the transmission capability without increasing considerably the message distribution cost (in terms of energy); this is because the model uses TDMA, and therefore only the assigned slots are used in the transmission.