# An Efficient Scalable Scheduling MAC Protocol for Underwater Sensor Networks +
## 5 Efficient Depth-Based MAC Protocol
### Overview of ED-MAC Protocol  
Efficient Depth-based (ED-MAC) is a reservation-based MAC protocol. It employs a duty cycle mechanism by assigning time slots to every individual node in the network in a distributed manner. The primary goal is to reduce the energy consumption by using a wakeup scheduling scheme; nodes are awake in some slots to transmit or receive data and are asleep over the remaining slots. The possibility of collision is very slim and limited to a very specific scenario. To remove even this slight chance, every slot is divided into a number of sub-slots. These sub-slots are selected randomly to avoid collision across the network. The sub-slots are also used to handle the newcomer nodes. ED-MAC trades off latency for energy efficiency and fairness and hence provides the flexibility to be utilised for various energy-critical applications. A summary of the notations used to describe our model is given in Table 1.  
Nodes in the network operate in three phases; namely initial, scheduling, and normal operational phase, as depicted in Figure 3. All network nodes operate asynchronously during each phase but share a common clock to start and end each phase together. To eliminate the effect of any clock drift that may occur over a long period of time, a guard time is also applied. In the initial phase, all the nodes randomly broadcast a few small beacons to discover their one-hop neighbouring nodes. The length of this phase, \(T_{b}\), is a predefined fixed value for all nodes, which is set at each node before deployment. The goal of the second phase, the scheduling phase, is to assign a unique slot to every node in the network. A timer is used at each node to prioritise slot reservation depending on the node depth; the greater the depth, the higher priority to reserve a slot. The length of the scheduling phase, \(T_{Delay}\) should be long enough to provide all nodes, from the seabed to water surface, with the opportunity to reserve a slot for themselves, but it is significantly shorter than that of the third phase. It should be noted that the initial and the scheduling phases only require to be repeated when the network topology changes. However, handling newcomers can allow ED-MAC to repeat these phases less frequently. The normal operational phase is divided into a number of rounds and each round is consisted of a number of slots. At each round, every node is aware of its own reserved slots and also the slots \begin{table}
\begin{tabular}{l l} \hline
**Terms** & **Definition** \\ \hline \(B_{p}\) & Beacon packet \\ \(D\)PL & Depth Priority List \\ \(D_{rate}\) & Offered traffic (packets/s) \\ \(M_{Depth}\) & The depth of the network (m) \\ \(N\) & underwater sensor node \\ \(N_{Depth}\) & A node depth in the network (m) \\ \(N\),\(ID\) & Node’s ID \\ \(N_{max}\) & Maximum number of nodes per neighbourhood \\ \(N_{s}\) & Number of slots \\ \(N_{s_{s}}\) & Number of sub-slots \\ \(N_{t}\) & Neighbouring table \\ \(R_{tr}\) & Transmission range (m) \\ \(S_{p}\) & Schedule packet \\ \(T_{b}\) & Predefined fixed value for the initial phase (s) \\ \(T_{d}\) & Propagation delay (s) \\ \(T_{Delay}\) & Predefined maximum delay (s) \\ \(T_{guard}\) & Guard time (s) \\ \(T_{r}\) & Length of each round (s) \\ \(T_{s}\) & Length of each slot (s) \\ \(T_{s_{s}}\) & Length of each sub-slot (s) \\ \(T_{sch}\) & Scheduling timer of each node (s) \\ \(T_{wake-up}\) & The wake-up times of a node (s) \\ \(U_{s}\) & Speed of sound in water (m/s) \\ \hline \end{tabular}
\end{table} 
Table 1: Notations. Figure 3: Timeline of ED-MAC protocol. reserved by its neighbouring nodes. Therefore, they can schedule to wakeup either to transmit their own data packet during the reserved slots or to possibly receive a data packet from a neighbouring node. They are asleep in other remaining slots when there is no data transmission or reception. This pattern is repeated during every round. The length of this phase, i.e., the number of rounds, depends on the topology changes due to node displacement or energy depletion.