## 5 Experimental Part  
### The Hardware  
For the development and testing of user software intended to address tasks in the field of networked communication, an emulator of modern networks was utilized, represented as an online service, providing access to a multitude of emulated modems. The user \"places\" them within a virtual space, thus defining the geometry of the communication network, which can be altered at any given moment or initially set as continuously evolving. For instance, this could entail the emulation of modem networks carried by AUVs. The EviNS software framework was executed on an Odroid H2 single-board computer (Figure 10).  
The emulated modems adhere to a predefined message format in the form of AT Commands. All commands commence with the prefix \"AT\", where \"AT\" signifies \"Attention code\". Thus, the device receives a signal indicating that a command follows. Command symbols and/or alphanumeric values conclude the command string.  
Example of a simple sent message:  
AT*SEND,<length>,<destination address>,<data>,  
Figure 10: A single-board computer with installed EviNS.  
where length is the length of payload (user information), destination address is the hydroacoustic address of a recipient, and data is the payload (user information), to work with such a message format, and a parser and a message generator are implemented and developed in the software framework. Tests are conducted for two interaction scenarios: between two modern nodes exchanging data in a P2P connection and for a group of modems interconnected within a network using network layer protocols. The network in the emulator comprises six nodes, the layout of which is depicted in Figure 11. The network nodes are positioned at distances ranging from 0.5 to 4 km from each other, with depth ranging from 14 to 15 m. The emulated modems are situated at depths of 12-13 m. Signal reception conditions are favorable, with the emulated bit error probability being 0.001. Hence, nearly all losses observed below are attributed to data packet collisions when multiple modems operate within the network.  
In the tests, one of the network nodes acts as a data source, generating and sending packets to the four most distant nodes in the network at random intervals, once every 15 s for a duration of 600 s.  
During protocol stack testing, the transmission of a payload array across the network occurs in the zone of indirect modem interaction (mediated). Specifically, nodes M3, M4, M2, and M1 within the network depicted in Figure 11 interact with each other.  
The test packet consists of a message containing information in string format:  
<n_p>,<depth>,<temp>,<volt>  
where n_p is the number of transmitted packets, temp is the environment temperature, and volt is the modem operating voltage.  
The total size of each packet is 20 bytes, and 50 packets are transmitted. Data exchange occurs both with and without the utilization of network protocols, and after which, the metrics that are assessed are as follows:  
1. Time taken to transmit a data packet between two devices in P2P mode and across the network;
2. Probability of successful transmission of data packets between two devices in P2P mode and across the network.  
Furthermore, experiments were conducted on UC&NL uWave modem units [4] (Figure 12a), with the layout scheme depicted in Figure 12c. Additionally, experiments under analogous conditions were conducted using physical (non-emulated) modem units  
Figure 11: The network topology.  
from the company Evologics (Figure 12b). However, due to the limited number of hydroacoustic modems, testing in the physical environment was conducted only for the link layer protocols.