## 4 Experimental
### Simulation Experiments with MAC Protocols and the Wake-Up Modem  
Given the scenario constraints when the underwater nodes have to remain operative and unattended during long periods of time and the network load is not periodically and uniformly distributed, it was decided that ALOHA and MACA-like protocols were more appropriate. These protocols do not need time synchronization and can adapt to the number of contending nodes by using the backoff mechanism. The interested reader can find a more comprehensive study on protocol selection in [12].  
The specific protocols chosen for this study are ALOHA-CS, MACA [13], FAMA [14] and T-Lohi [15]. These protocols have been adapted (when possible) to take advantage of the modern wake-up capabilities.  
The operation of the sensor nodes is as follows. From the beginning, all nodes remain in a low-power state, and whenever a node needs to send a packet, a wake-up signal is initially sent, and the packet follows immediately afterward.  
The Bellhop propagation model was used for the simulations. It was fed with the SSPand bathymetry data, obtained using the WOSS API [16], for the Spanish Mediterranean coast near Burriana, where the application is to be deployed. The modem consumption parameters are shown in Table 4, and the transmission speed was set to 1,000 bps [8].  
A scenario of 100 \(\times\) 100 meters was tested with 10, 50 and 100 nodes randomly deployed. Only one sink was placed at the center of the scenario. Each scenario was simulated several times in order to achieve a confidence interval of \(\pm 1\) with a confidence level of \(95\%\). All simulations were seeded using  
Figure 8: Increasing the RX time by \(m=4\) and \(m=5\) times.  
the number 1330703057, independent replications were performed advancing the run number [17] and the simulation stop time was set to \(30\) minutes.  
All traffic was directed to the sink node and was generated by a Poisson distribution with an average packet inter-arrival time of \(0.6\) seconds, which ensured that the network was working under saturation. Each packet was generated by this function and assigned to a source node using a random uniform distribution.  
To obtain the best possible results for each protocol regardless of the backoff algorithm used, a set of simulations was performed for each protocol by varying the backoff time from \(0.5\) seconds to \(20\) seconds. The best results in terms of packet delivery ratio are shown in the following figures and specified in Table 5. T-Lohi has no backoff time specified, since it is fixed in the protocol specifications [15].  
The wake-up modem has two wake-up modes [8, 9], a tone or broadcast mode and a selective mode. The tone mode wakes every node that receives the wake-up signal, while the selective mode only wakes the intended receiver. Only the MACA protocols learn, and T-LOHI Tone protocols use the tone or broadcast mode. However, T-LOHI always uses the tone mode during the contention period to decide who is going to be awarded the channel, although the data packet can be sent using either mode.  
The results of the simulations follow for each of the implemented protocols using and not using acknowledgments.