# An AUV-Aided Cross-Layer Mobile Data Gathering Protocol for Underwater Sensor Networks +
## 4 Description of Our Proposed Cross-Layer Mobile Data-Gathering Protocol
### AUV Discovery Phase
Thus, sensors in different clusters with the same frame and slot can simultaneously transmit their packets without collisions. The MAC challenges and issues, such as hidden or exposed node problem, spatial-temporal uncertainty, and the near-far effect, are therefore addressed in CLMD [29]. After obtaining the frame and slot information from the initial two phases, each sensor knows its operational times as well as neighboring transmitting times. Each round repeats this pattern until further changes are required by the central scheduler due to energy depletion of CHs, or topology changes as sensors may move with the water current (if they are not anchored). Thus, the operational phase length depends on the number of rounds before rescheduling. The offered traffic, \(\lambda\), indicates the number of sent packets per second (p/s) for each node. Assuming the offered load is identical for all nodes, the length of each round, \(T_{R}\), can be calculated by reversing the offered traffic. Thus, the length of \(T_{R}\) is given by \((T_{R}=\frac{1}{\lambda})\). The shorter \(T_{R}\) is obtained with a higher \(\lambda\) and vice versa. A longer \(T_{R}\) can create more opportunity for nodes to sleep. \(T_{R}\) is divided by two to include two equal-size frames as \((T_{F}=\frac{T_{R}}{2})\) and each \(T_{F}\) contains a number of slots, \(N_{s}\), with the same size. Let \(D_{max}\) be the maximum node degree found within the whole network. The value of \(D_{max}\) depends on the network topology and density. The number of slots can be given

Figure 3: Timeline of our cross-layer proposed protocol. by \((N_{s}=D_{max})\) to have enough time slots for the members of each cluster. The length of a slot, \(T_{S}\) is calculated by

\[T_{S}=\frac{T_{F}}{D_{max}}, \tag{16}\]

while:

\[T_{S}\geq T_{delay}+G_{t}. \tag{17}\]

The slot length should be long enough to compensate for the high propagation delay in UWSNs. A small guard time, \(G_{t}\), needs to be considered within the slot time to obtain zero packet loss due to the possibility of any conflict among sensors. The propagation delay, \(T_{delay}\), is calculated by \((T_{delay}=\frac{R_{tr}}{V_{s}})\), where \(V_{s}\) and \(R_{tr}\) are the velocity of sound in water and the transmission range of each sensor, respectively. ## 5 Experimental Results

In this section, we first discuss the simulation set-up of our protocol in the Aqua-Sim underwater simulation [31]. We then evaluate the most important metrics such as packet deliver ratio (PDR), network lifetime, goodput, energy consumption, and probability of collision. We also compare our proposed protocol with another cross-layer protocol (i.e., integrating the routing protocol in [6] with the MAC protocol in [32]). We finally present and analyze the simulation evaluation. ### Simulation Setups

This study implemented CLMD in Aqua-Sim, an NS-2 based simulator for underwater sensor networks [31]. Simulations were performed with the following parameters, unless otherwise noted. The underwater communication channel described in Section 3 is used in our simulation. The sending, receiving, and idle powers are equal to 50 W, 0.158 W, and 0.008 W, respectively. The receiving power threshold and the transmission power are set to 10 \(dB\)\(re\)\(\upmu\)\(Pa\), and 105 \(dB\)\(re\)\(\upmu\)\(Pa\), respectively. Each sensor has initial energy equals to \(E_{0}=40,000J\). The data packet size is equal to 1024 bits while the bit rate is set to 10 \(kbps\). The packet generation rate is 0.01 packet per second. The transmission range of each sensor is set to 100 m. The number of sensors ranges from 100 to 500, and sensors are uniformly deployed in a 1000 m \(\times\) 1000 m field with a fixed depth at 300 m. There is one static sink at the corner of the network topology with (0, 500, 0) coordinates. The AUV travels at a depth of 250 m while its speed is 4 m/s.