# A MAC Protocol to Support Monitoring of Underwater Spaces +
## 6 Evaluation Process  
In this section, we evaluate the performance of the MAC proposal using the application scenario introduced in the previous section. We compare the general behavior of our proposal to the one of the MACA-U protocol [26]. For this evaluation process, we assume that each node has an acoustic range equal to 500 m; the carrier frequency is set to 40 kHz [27] and the transmission data rate to 9600 bits/s. Each slot takes 167 ms for 160 bytes. Figure 13 shows the frame structure after the initialization phase for the network topology represented in Figure 12. As can be seen, there are seven slots used. The complete frame is transmitted in 1.167 s.  
Frames can start with different periodicity, for example every 10 s. In this case, the power demand (i.e., energy consumption) of the communication system in the node will be slightly more than 10%. After the initialization phase, a message from the farthest node (in our case, Node 7) requires three frames to reach the SB or 30 s. The ACK message is issued in three steps. First, Node 6 acknowledges the reception of the message to Node 7 and in the next frame; then, Node 1 acknowledges Node 6 in the following frame; and finally, the \(SB\) acknowledges the reception of the message from Node 1. If the frame reverse order is implemented, the message will reach the SB in just 10 s. In steady state, it is possible to send a message every 10 s, receiving the acknowledgment in the following 10 s, and the general utilization ratio of the network is close to 10%.  
Figure 15: Network deployment at the seaside.  
In the case of the MACA-U protocol, the nodes can transmit whenever they have information available, and they can lock the access to the acoustic channel, by issuing a RTS-CTS message with the next hop in the network. In this case, a timer is set with both, the time needed to send the message and the maximum delay for the message to reach the next node. Considering that the underwater acoustic signal propagates at 1.5 km/s, the time needed to reach a node in the limit of the transmission range is equal to 0.333 s; therefore, the maximum round trip \(\tau_{\text{max}}\) is equal to 0.667 s. The same time slot is kept for comparison purposes at 0.167 s. In case Node 7 has a message to send, it will send an RTS message and wait for \(\tau_{\text{max}}\,+\,t_{slot}\,=\,0.834\) s. If in this time, this node receives a CTS message, then it gains access and transmits its message. So, the total time needed by Node 7 to send its message to the SB is 3 s without collisions. However, Node 7 may be delayed by collisions with Nodes 5 and 6. In this case, and assuming only one collision with each one, the message from Node 7 will only be transmitted to Node 6 after 3 s. Node 6 can be delayed by collisions with Node 1 and 7. Therefore, considering again only one collision with each one, the message needs another 3 s to reach Node 1. In the last hop, Node 1 may have collisions with Nodes 6, 2 and the SB, and like in the previous cases, the message will be delayed for another 4 s. The end-to-end transmission time for a message originating in Node 7 is the sum of the partial delays, i.e., 10 s.  
This simple example shows that the complexity of a collision avoidance algorithm for the transmission of information in networks with important delays may compromise its performance severely. Another important aspect is that the delay is not deterministic, showing great variations according to the network load. Instead, once the slot selection process is done, the TDMA approach guarantees a constant latency and transmission delay, making the message propagation predictable.