| Wireless sensor networks is considered as one of the most influential 21 technology for 21st century, and also widespread concern has been a research hotspot in information technology in recent years. The nature and characteristics of wireless sensor networks is to integrate the data collection, processing and transmission, which greatly expanded the scope and manner of interaction between human and nature, has a broad application prospects.Time synchronization in wireless sensor networks is an important base technology, it's significant for node locating, data aggregation and other technologies to implement in sensor networks, because they are based on the sensor nodes in the network keep time synchronization state.At first, this paper introduce the present situation of the wireless sensor network time synchronization, and then a analysis the necessity, the system clock synchronization model, synchronous technology and its classification of the wireless sensor network time synchronization in detail. Then in-depth study of the principles of the various exiting synchronization mechanisms, then compared these algorithms in accordance with evaluation indicators of the wireless sensor network time synchronization algorithm.On this basis, among the wireless sensor network applications which are triggered by events and with some of the nodes synchronization, the traditional periodic time synchronization algorithm for the entire network synchronized will lead to unnecessary power consumption. Therefore, this paper proposes a time synchronization compensation algorithm based on event-triggered for such applications. This algorithm achieves synchronization through the statistics accumulated along the processing delay and propagation delay compensation. Moreover, implementation of this algorithm is present on XUTSN hardware platforms (which based on the CC2430 System on Chip) and the software platform (using TinyOS operating system). In process of the algorithm implementing, there are three major jobs of the work:(1) Porting TinyOS to XUTSN hardware platform. TinyOS is an open-source operating system for embedded system, due to realization by component-based model, it has a good cross-platform and efficiency, it's suited for the wireless sensor network applications which have relatively limited resources.(2) The TinyOS scheduling mechanism has been improved to achieve a non-preemptive earliest deadline first (NPEDF) scheduling policy. The FCFS scheduling mechanism of TinyOS2.x is not sufficient to meet real-time demanding applications like the time synchronization. This paper analyzes the FCFS task scheduling mechanism and explore several other scheduling mechanism, in consequent, implemented a kinds of non-preemptive earliest deadline first (NPEDF) task scheduling component in TinyOS. According to the experimental validation analysis, NPEDF scheduling mechanism can effectively enhance the TinyOS real-time performance.(3) Implemented the time synchronization compensation algorithm based on event-triggered in the customization and improved TinyOS.Finally, this article presents simulation experiments and actual experimental verification. The results show that the synchronization algorithm has good performance and energy efficiency when the interested nodes number and events frequency is low. |
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