# A MAC Protocol to Support Monitoring of Underwater Spaces +
## 4 Application Example  
Figure 10 presents an example with a possible distribution of nodes in a UWASN. The SB is the center of the red circle that marks the communication range of the SB, which we set to be 500 m. As can be seen, two nodes may be in direct contact with the SB, but not between them (e.g., Nodes 1 and 2). Both nodes synchronize with the SB. As they have almost the same delay to the SB, we assume Slots 1 and 2 are allocated to Nodes 1 and 2 in the \(SF_{1}\), respectively. In the next hop area (dotted red circle), there are three more nodes. Like in the previous case, even if they have a similar delay to the SB, that is within the second hop, they have different synchronizing nodes and, thus, data paths. Node 2 synchronizes Nodes 3 and 4 in \(SF_{2}\), and Node 4 synchronizes Node 5 in \(SF_{3}\).  
The slot allocation is trivial for this example. A message starting in Node 5 may need to go through Nodes 4 and 2 before reaching the SB. Instead, Node 3 is within range of Node 2, so messages beginning in Node 3 pass through Node 2 before reaching the SB. This example shows that even if Nodes 3, 4 and 5 are within a distance of two communication radios from the SB, the amount of hops needed to reach such a node is different for each one.  
During this phase, nodes build the network topology to determine the path towards the SB. Each node in the system is allocated to a slot in the frame, and it informs about its depth, delay to the SB (if it is within one hop), the accumulated delay and the path towards the SB. In the example shown in Figure 10, Node 4 informs about the delay to the SB and the path through Node 2. In that way, Node 5 can determine its own delay to the SB and path.  
In Figure 10, the Node 5 is the last node, and its messages have to go through Nodes 4 and 2 to reach the SB. In this case, Node 4 has to aggregate the information of Node 5 in its own message, and in the same way, Node 2 aggregates the information from Nodes 4 and 5 in its own message. Figure 11 extends the example incorporating new nodes. As the networks is re-initialized periodically, Node 1 will discover the presence of Node 6 and this last one the presence of Node 7 that is also discovered by Node 5. Let us assume that the delay between Nodes 6 and 7 is smaller than the delay between Nodes 5 and 7. Under these considerations, Node 1 synchronizes Node 6, and this last one synchronizes Node 7. In this case, the path from 6 to the SB is through Node 1. However, as can be seen, Node 7 listens to Nodes 5 and 6. Node 5 is in the third sub-frame (SF), but Node 6 is in the second one. The aggregation process is made only by nodes in the previous SF. When there is more than one node in the previous SF that listens to the messages from lower SF nodes, only one node aggregates and  
Figure 10: Case 1: deployment and transmission ranges.  
forwards the information to the next SF. The other nodes keep a copy of the information as a backup, in case there is a failure in the transmission.  
Figure 12 presents the topology of the UWASN represented in Figure 11. The dotted lines represent the different sub-frames used in the transmission of messages. As can be seen, if Node 6 disappears, the network will be reconfigured, and Node 7 will be in a new sub-frame behind Node 5. In the same way, if Node 4 is turned off, Node 5 will send its messages through Node 7. Figure 13 illustrates the slot allocation in the example frame.  
Figure 11: Case 2: deployment and transmission ranges.  
Figure 12: Topology of the network in Case 2.  
Figure 13: Slot allocation in the example frame.