AirSync Time Synchronization for Large-scale IoT Networks Using Aircraft Signals

Abstract : The prosperity of Internet of Things (IoT) brings forth the deployment of large-scale sensing systems such as smart cities. The distributed devices upload their local sensing data to the cloud and collaborate to fulfill the large-area tasks such as pollutant diffusion analysis and target tracking. To accomplish the collaboration, time synchronization is crucial. However, due to the long range and device heterogeneity, accurate time synchronization for a large-scale IoT network is challenging. Existing GPS or NTP solutions either require an outdoor environment or only have low and unstable accuracy. In this paper, we propose AirSync, a novel synchronization method that leverages the widely existed aircraft signals, ADS-B, to synchronize large-scale IoT networks with nodes even in indoor environments. But ADS-B messages have no time stamp and cannot provide a reference time. We leverage the continuity of aircraft movements to estimate the aircraft traveling time. Then devices that observe common aircraft moving segments can calculate their time offset. To obtain the time skew, we propose a combined aircraft linear regression method. We also design a transitive synchronization for devices that cannot observe common aircraft. We implement a prototype of AirSync and evaluate its performance in various real-world environments. The results show that AirSync can obtain the sub-ms accuracy.
 EXISTING SYSTEM :
 ? Existing synchronization systems that run in dedicated daemon processes would simply not be possible on such platforms. ? Many assumptions on which existing schemes are based no longer hold in this new area of WSNs. ? In timedivision based MAC layers, some form of synchronization already exists between radios, and can often be accessed by a synchronization process on the CPU. ? Many solutions exist for traditional networks and distributed systems. NTP, for example, has been widely deployed and proven effective and robust in the Internet.
 DISADVANTAGE :
 ? To overcome this problem, we leverage greedy algorithm to design a discarding mechanism, it searches for speed information from the same aircraft before and after the target position information, gradually expanding search until the first speed message is found or reaches the time threshold (10s). ? To address this problem, we develop a synchronization system, which includes a lightweight client, a new packet exchange protocol called SPoT and a scalable reference server. ? In this paper we consider the problem of synchronizing clocks in IoT devices with a remote reference source. ? Moreover, synchronization protocols treat this as a statistical variability problem, hence their filtering approaches require multiple samples to pick the best RTT.
 PROPOSED SYSTEM :
 These insights are key to our proposed approach for rate synchronization and varying the polling interval. • We proposed some design principles: use multiple, tunable modes of synchronization; do not maintain a global timescale for the entire network; use postfacto synchronization; adapt to the application, and exploit domain knowledge. • we propose that each node in an WSN maintain an undisciplined clock, augmented with relative frequency and phase information to each of its local peers. • Of course, sending synchronization packets during the sleep period defeats the purpose of sleeping, so we must consider frequency estimation as part of the time synchronization problem.
 ADVANTAGE :
 ? In order to evaluate the performance of AirSync, we conduct experiments in the real world. ? To evaluate the performance of AirSync, we first evaluate AirSync time error in a short range by using node B and node C. ? In the real world, nodes may work in different scenarios, we try to simulate these scenarios to evaluate AirSync performance. ? There are some advantages of combining multiple aircraft: ? (1) When the number of matched packets from one aircraft gets smaller, combining information from multiple aircraft can take full use of matched packets. ? (2) One aircraft is hard to cover entire time horizon, combining aircraft can extend the coverage of information on the time horizon which can promote the accuracy of time skew estimation.

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