# 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
The packet delivery ratio for various network densities has been shown in Figure 4. CLMD has delivered a higher number of data packets to the destination compared to other similar schemes because of using a cross-layer design to limit the number of collisions. However, CMDG only uses a CSMA MAC protocol which is not optimal to be used in such a complex network. In the case of AUV_PN, no relay hop bound is considered to limit the number of hops between the CH and its members while it increases the chance of packet failure by transferring the packets over a longer distance. On the other hand, in AEERP, the chance of void occurrence between CH and its member increases when the network is sparse. Thus, in sparse scenarios, AEERP suffers from the lowest packet delivery ratio. The energy efficiency of the models has been shown in Figure 5. CLMD consumes less energy because of bounding the packet transmissions to one hop and also reducing the number of collision by using an efficient MAC protocol. The CMDG consumes higher energy than CLMD because it does not benefit from a cross-layer design like CLMD to reduce the number of collisions. The energy consumption in AUV_PN is higher than CLMD because of using CHs only for obtaining the list of PNs and not involving them in data collection. In this way, the number of nodes involving in data collection is lower than the actual value. In sparse networks, the void problem in AEERP increases energy consumption while the lack of relay hop bound in dense networks still wastes the energy. Figure 4: Packet delivery ratio vs. node density. The network lifetime of the proposed schemes has been shown in Figure 6. CLMD has higher network lifetime because of assigning sensors to a higher number of CHs in a uniform way, and the packet retransmission is highly reduced by utilizing an efficient MAC protocol. In CMDG, the network lifetime is higher than CLMD because of higher number of packet retransmissions due to using a CSMA MAC protocol. AUV_PN has a lower number of clusters which reduces the network lifetime. There is also no uniform assignment to the CHs in AEERP which can result in a shorter lifetime. The latency of the sensor networks may be crucial in some applications. The latency depends on the tour length. The tour length also depends on different factors such as single-hop or multi-hop relay strategies, fixed tour or an adaptive tour, energy constraints, and reliability. In AEERP, the AUV travels a short path which is fixed; however, it increases the energy consumption and packet failure. The tour constructed by AUV_PN is not optimal as it crosses over itself. Sensor members in AUV_PN also need to transmit packets to the CH with a higher power to decrease the tour length. The tours established

Figure 5: Energy consumption vs. node density. Figure 6: Network lifetime vs. node density. by CMDG and CLMD schemes are optimized using the 2-opt algorithm. In order to conduct a fair comparison for the time-critical applications, all these parameters need to be considered. In the case of an emergency, there are several ways to decrease the latency. One way is to form the cluster heads with multi-hop neighbors (instead of one-hop neighboring nodes). In this way, the number of cluster heads is reduced and subsequently the AUV tour is shortened. However, there is a trade-off between latency and energy. If the latency is decreased by aggregating the data in fewer CHs, it will increase the energy consumption because the packets should be relayed in multi-hops. #### 5.3.2 The Impact of Mac Layer on the Cross-Layer Protocol

In this set of simulation scenarios, the impact of sensor density on the performance of CLMD with DL-MAC, CLMD with ED-MAC, and CLMD with UWAN-MAC are examined. The number of sensors varies from 50 to 300, while other parameters are fixed. The simulation results for the packet delivery ratio, energy per packet, and network lifetime are shown in Figures 7- 9, respectively. Figure 7 illustrates the PDR as a function of number of nodes.