Abstract
Time (or clock) synchronization is a large and vital field of research, as synchronization is a precondition for many applications. A few example applications are distributed data acquisition, distributed databases, and real-time communication. First, this survey paper introduces the research area of time synchronization and emphasizes its relation to other research areas. Second, we give an overview of the state-of-the-art of time synchronization. Herein, we discuss both established protocol and research approaches. We analyze all techniques according to three criteria: used estimation algorithm, achievable synchronization accuracy, and the experimental conditions. In our opinion, this analysis highlights potential improvements. The most important question in this survey is as follows: which estimation method can be used to achieve which accuracies under which conditions? The intention behind this is to identify estimation methods that are particularly worth considering, as these already achieve good results in the wireless area but have not yet been examined in the wired area (and vice versa). This survey paper differs from other surveys in particular through the consideration of wireless and wired synchronization and the focus on estimation algorithms and their achievable accuracy.
Highlights
In order to improve the comprehensibility of this survey paper, we provide a few examples of common time synchronization estimators
We examine approaches based on Kalman filters (KF)
The approach is examined analytically and simulatively and achieves an accuracy below 1 ms, whereby this depends on the number of hops, the skew, and the delay
Summary
There are many applications of time synchronization in the IoT (Internet of Things) and IIoT (Industrial Internet of Things). The sensor nodes communicate, e.g., only for a short time period and otherwise remain in a sleep mode to save energy [7,8] Another application of time synchronization is distributed data acquisition. E.g., temperature, electron density, and plasma properties [10] These requirements apply to similar large-scale experiments such as the particle accelerators at CERN (European Organization for Nuclear Research) [11]. Another example of distributed data acquisition as an application of time synchronization is the interconnection of several radio telescopes in order to achieve a significantly higher resolution. Time synchronization is important for distributed databases to ensure consistency as well as to optimize throughput and latency [5]
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