Tham khảo tài liệu 'smart wireless sensor networks part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Time Synchronization of Underwater Wireless Sensor Networks 289 The profile manager PM will also help decide the resynchronization interval for a particular sensor cluster. As we discussed above the confidence of data provided by neighbor nodes settle on whether the data packet could be accepted by the existing sensor cluster a subsystem of the whole underwater network. In overall view higher acceptance rate stands for higher utilization of censured data. If most of sampled data packets are dropped due to accuracy requirement Ả it does not reduce the utilization of censuring data but also dries out power supply since underwater is more energy consuming. The criterion of switching the node s mode from transferring data to resynchronization is determined by the data packet acceptance rate. Profile manager creates a global table called Global Confidential Table GCT aiming to record the accept data packet ratio. The GCT is a one dimension fixed size table which marks 1 standing for acceptance of data packet. Default value is 0 which means the packet does not meet the Ả requirement. The protocol defines a threshold R as the number of acceptance data packets in GCT shown in Fig 4. If ratio of acceptance data packets to table size is below R the profile manager will stop the node receiving data and start resynchronization until local clock accuracy reaches requirement formula 4 and 5 . The upper GCT in Fig. 4 shows that the ratio is higher than the threshold and the lower one means that the cluster needs to be resynchronized. Threshold 6 T T T T T T T size 10 Threshold 6 T T T T T size 10 Fig. 4. Global Confidential Table The whole process flow is shown in Fig 5. Because there is no bidirectional neighbor relationship for every two nodes each node maintains the relative clock drift in its own acoustic range a cluster for profiling data. On the other hand adjacent sensors clusters must have overlap. The overlap plays the role to keep the whole relative clock as close as .
