## 4 Medium Access and Network Layer Protocols
### Medium Access Layer Protocols
#### 4.1.2 User Implementation of Tone Lohi Protocol  
An energy-efficient protocol called Tone Lohi (T-Lohi) [20] has been developed for an underwater acoustic network of short-range sensors [6; 20]. This protocol ensures stable access to the transmission medium even under conditions of low power from the on-board transmitter and limited energy resources. The T-Lohi protocol [20] incorporates a channel redundancy mechanism to prevent message collisions during transmission. Additionally, the protocol requires specialized equipment with a wake-up function based on a tone signal. This approach effectively resolves conflicts related to channel redundancy without significantly increasing energy consumption. The T-Lohi protocol employs specific mechanisms to transition equipment into power-saving mode and to wake up or request network resources using a distinct tonal acoustic signal. This approach can lead to substantial energy savings. There are three distinct implementations of this protocol. ST-Lohi (Synchronized T-Lohi): this implementation prioritizes energy-efficient transmission. According to the modeling presented in the same work, the protocol achieves efficiency within 3% of the maximum value. aUT-Lohi (Aggressive Unsynchronized T-Lohi): this variant allows for the maximum bandwidth utilization among the three implementations, with a channel utilization of approximately 50%. cUT-Lohi (Conservative Unsynchronized T-Lohi): this implementation focuses on reliable data transmission with minimal loss. It ensures efficient use of the transmission medium while maintaining stable bandwidth and high energy efficiency. The T-Lohi protocol [6] relies on the utilization of short messages (tones) to reserve a communication channel. It also permits competitive requests for channel access from other network nodes (as depicted in Figure 7). As a result, this medium access control (MAC) level protocol ensures stable bandwidth and low power consumption. Figure 7 schematically depicts four devices: A, B, C, and D. Device B has previously allocated a channel for transmitting information and has sent packets to devices A, C, and D, effectively blocking other devices from transmitting. Consequently, only device A will have the chance to exit the Backoff/Blocking state during the third competition round and data transfer. The protocol operates based on the following principle: when a node have data to transmit, it notifies other devices and enters the contention round (CR). Each device that receives a tone enters the \"Backoff\" state for a duration w, where w is a normally distributed random value within the range from 0 to the number of applicants (CTC). Consequently, the device that exits the \"Backoff\" state earlier or one that was not a part of it reserves the channel, initiating the next competitive round for the remaining devices. The protocol data frame's structure is depicted in Figure 8. Each frame comprises a backup period (RP), followed by a block of payload data. Within each RP, a series of negotiation rounds occurs until one node successfully reserves a channel. To identify collisions and estimate the number of competitors, a collision resolution mechanism is employed, considering both spatial and temporal uncertainties as well as the significant latency in the data packet delivery. Based on the number of incoming tones, applicants for the channel (CTC) are tallied. When the device has detected a tone, it enters the \"Backoff\" state for a duration of w:  
\[W=U\times CTC\text{, where }U\text{$\sim$Uniform (0,1)}.\]  
If, after the competition round (CR) expires, the device has not entered the \"Backoff\" state, it reserves a channel for transmitting its data packet and exits the CR. The period between competition rounds is calculated as follows:  
\[CR=T_{max}+T_{tone},\]  
where \(T_{max}\) is the longest packet transmission time in the environment, and \(T_{tone}\) is the transmission time of a short message (tone). The algorithm of the T-Lohi protocol is presented in Appendix A, Algorithm A2.