# A MAC Protocol to Support Monitoring of Underwater Spaces +
## 3 Description of the UWSO-TDMA Protocol  
Sensor networks like the one proposed in this paper are oriented to the data gathering. This implies that while there are several possible sources of data (sensor nodes), all of the data are transmitted towards a few sinks. This approach simplifies the data propagation, as it is not necessary to provide a path between all of the nodes in the network. In Section 3.1, the model used for the network is presented. By now, it is enough to know that as data captured by end nodes are transmitted towards the sink, intermediate nodes aggregate their own messages and also those provided by other nodes.  
The proposed protocol is based on the predictability of TDMA, rather than on the probabilistic CSMA/CA approach. The nodes use a particular slot within a frame, which is allocated in a partially distributed way, as will be later explained. This is a decentralized scheme where nodes are responsible for sharing the communication channel and the synchronization. In the case of the UWASN, the universal time coordinated (UTC) information is not available under the water; therefore, the synchronization source is usually a sonobuoy (SB) anchored in the region that is in charge of synchronizing the clocks of the underwater nodes, and it is used as a sink of sensed data. Frames repeat periodically. We assume that nodes generate information at a fixed rate \(T_{f}\), which is the frame period, and all messages have the same duration. Although one of the underwater nodes may be used instead of the SB as the synchronization phase, this node will not have access to the UTC, and we intend to have the network synchronized with the rest of the world.  
Since acoustic signals are attenuated, not all of the nodes listen to the SB synchronization slot. When a node is not within the SB transmission range, it is synchronized in successive phases by nodes already synchronized. To do this, the frame is divided into \(m\) sub-frames (\(SF\)). Within the first sub-frame, \(SF_{0}\), only the SB transmits. In the next sub-frame, \(SF_{1}\), the nodes listening to the SB can transmit and are used to synchronize nodes at two hops. In \(SF_{2}\), the nodes at two hops of the SB can transmit. With this hierarchical distribution, nodes in \(SF_{j}\) are synchronized by nodes in \(SF_{j-1}\). Each sub-frame has only one synchronizing node that is selected following the procedure described in Section 3.2.1. As nodes at a two-hop distance do not interfere, they may share the same time slot in the sub-frame, and different sub-frames originating in different synchronizing nodes may transmit simultaneously. The length of the sub-frames depends on the number of nodes being synchronized. As the SB is not aware of the network deployment, the frame length is not defined until all of the nodes of the network have been synchronized and are allocated to a particular sub-frame. Sub-frame \(SF_{0}\) has only one slot.  
In this paper, the use of only one SB is discussed. Although it is possible to think of a network with more than one SB, which means several roots in the tree, there is not a direct benefit in doing this, and the complexity of the system is incremented. The analysis is beyond the scope of this paper.