Time synchronization in wireless sensor networks (WSNs) is a fundamental issue for the coordination of distributed entities and events. Nondeterministic latency, which may decrease the accuracy and precision of time synchronization can occur at any point in the network layers. Specially, random back-off by channel contention leads to a large uncertainty. In order to reduce the large nondeterministic uncertainty from channel contention, we propose an enhanced precision time synchronization protocol in this paper. The proposed method reduces the traffic needed for the synchronization procedure by selectively forwarding the packet. Furthermore, the time difference between sensor nodes increases as time advances because of the use of a clock source with a cheap crystal oscillator. In addition, we provide a means to maintain accurate time by adopting hardware-assisted time stamp and drift correction. Experiments are conducted to evaluate the performance of the proposed method, for which sensor nodes are designed and implemented. According to the evaluation results, the performance of the proposed method is better than that of a traditional time synchronization protocol.
Wireless sensor networks are evolving from relatively undemanding applications to applications which have stronger requirements. The coordination of distributed entities and events requires time synchronization. Although a number of methods have been studied for WSNs, some applications require high precision time synchronization. Precision time synchronization enables a variety of extensions of applications. The IEEE 1588 precision time protocol (PTP) provides a standard method to synchronize devices in a network with sub-microsecond precision. This paper deals with precision time synchronization using IEEE 1588 over wireless sensor networks. Precision time synchronization using IEEE 1588 provides compatibility between heterogeneous systems in WSNs. This paper also presents experiments and performance evaluation of precision time synchronization in WSNs. Our result established a method for nodes in a network to maintain their clocks to within a 200 nanosecond offset from the reference clock of a master node.
As logistics has grown dramatically, container port terminals become heavily congested in the loading, unloading, and processing of containers into and out of port terminals currently. So, there is a growing need for a design of a location-based smart port terminal. In port terminals, especially, valuable time can be saved by recognizing precisely where the nearest available containers or mobile equipment can be found. A real time locating system (RTLS) is a novel technology which estimates and tracks mobile objects and personal. However, RTLSs in port terminals are likely to include a large of uncertainty, and could not find the location of assets because they can be easily damaged by malicious materials such as steel and water. This paper proposes an enhanced trajectory estimation method for the RTLS in port logistics environment. Basically, the RTLS for the port terminal estimates the location of mobile equipment which includes a large of packet loss and uncertainties. The proposed method corrects inaccurate location information and mitigates uncertainties occurred by the RTLS by adopting interpolation and the Kalman filter. In addition, this paper includes an instance of the RTLS for the port terminal and presents experimental results to prove the superiority of the proposed method.
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