Demo Abstract: Synchronization using Inhibitory and Excitatory Coupling: From Theory to Practice Wasif Masood1 , Johannes Klinglmayr 1 , Istv´an Feh´erv´ari 1 , Thomas Watzl, and Christian Bettstetter 1
1,2
Institute of Networked and Embedded Systems, University of Klagenfurt, Austria 2 Lakeside Labs GmbH, Klagenfurt, Austria Email:
[email protected]
Abstract—Solutions for time synchronization based on coupled oscillators operate in a self-organizing and adaptive manner and can be applied to various types of dynamic networks. In this demo, we present the actual implementation of our guaranteed self-organizing synchronization algorithm on packet based networks like wireless sensor networks as well as on iPhone devices (by using acoustic signals). The algorithm is purposed for pulse couple oscillators and exploits the interactions between them to guarantee synchronization on arbitrary network topologies. It supports the design of scalable, minimal control toter and energy efficient communication protocols for fully distributed synchronization.
I. S YNCHRONIZATION A LGORITHM In recent years, self-organizing synchronization has received more and more attention within the field of wireless communications and mobile computing. This technique offers advantageous properties such as scalability, adaptability, and low computational efforts, which are specifically beneficial for distributed systems. Whereas it is easy to spot self-organizing synchronization phenomena in nature [1], it is surprisingly difficult to ensure self-organizing synchronization for technical environments. The mechanism behind pulse coupled oscillators [2] has gained considerable attention in scalable time synchronization protocols for large-scale wireless networks (see, e.g., [3], [4], [5], [6], [7] and references therein). According to this approach, each network node has a phase value which increases with time. As soon as it hits a predefined threshold, it ends its cycle, emits a pulse/syncword and resets its phase back to zero. Reception of such a pulse may result in phase update with the goal that all nodes end up in the same phase and are thus synchronized in time. One of the most distinct impacts of such an approach is to minimize the information needed to synchronize since the underlying signals contain no encoded information [8]. Verification of these effects are mainly given via simulation results; theoretical results for guaranteed synchronization could be given for specific environments [2], [6]. But for applications in real world environments, the theoretical results are highly curtailed due to the practical constraints like random individual pulse delays, unreliable communication, or not fully connected and dynamically changing network topologies. Moreover, as pointed out in [9], the synchronization precision is also constrained since the elements cannot align their internal clocks better than the occurring delay spread.
In order to cope with the aforementioned constraints, specific coupling strategy with a theoretical solution can be given in [7]. In this theoretical work, the authors combine the inhibitory and excitatory coupling together with stochastic pulse interactions. According to the proposed algorithm, each node transmits a pulse with a certain probability, 0 < psend < 1 as soon as its phase hits a certain threshold. Upon reception, each receiving node adjusts its phase either by reducing it towards 0 or by exciting it towards the phase threshold. No updates are performed if the pulses are received within a refractory period of the phase. This results in a guaranteed networkwide synchronization for all (connected) network topologies with arbitrary initial conditions. II. D EMONSTRATION In this demonstration, we will show the authenticity of the synchronization algorithm [7] initially presented for pulse coupled oscillators, by implementing it on real hardware nodes and iPhone devices. In order to validate the synchronization approach we implement this synchronization scheme in a test bed and give a proof of concept. This realization supports further exploitation of the coupling strategy, and presents it in such a way that it visualizes the underlying coupling idea. By demonstrating the individual adjustments of each entity an observer gets an intuitive picture of how the synchronization strategy works. By appending wooden pendula to the microcontrollers, the step-wise synchronization processes of the technical devices are visualized by the corresponding gradual movement of the pendula arms. In another realization, we demonstrate the synchronization process with mobile phones which are coupled via audio signals. An eventually synchronized beeping of the phones is reached via the collection of individual phones. In this demonstration, we intend to do the following: 1) Blinking LEDs: We will use blinking lights on sensor nodes (see Fig.1a, we have 100 pieces of Z1 nodes but we will use them as per the feasibility of the space) to show the synchronization among them. Attendees will be allowed to randomly reset the nodes and experience the emergence of synchronization. 2) Wooden Pendula: In order to provide a better understanding of the entire phenomenon, we visualize the process using pendula (see Fig.1b). These are the custom designed
(a) TelosB Nodes
(b) Wooden pendula
(c) The BUZZflies application
Fig. 1. (a) A snapshot of TelosB nodes blinking simultaneously as a demonstration of the synchronization process. (b) Custom designed wooden pendula attached with TelosB sensor nodes. There synchronized to-and-fro motion will show how well the nodes are synchronized. (c) A snapshot showing BUZZflies application in action. One can start/reset the application by touching the on-screen button.
wooden devices operated via servo motors, which we will operate using our Z1/TelosB nodes. The nodes will communicate over the wireless channel as usual and will project the current state of their phases by adjusting the to-and-fro motion of the pendulum attached. The steady convergence of their to-and-fro motion will give a more refine pictorial view to the attendees. 3) iPhone devices: For iPhone devices, we have designed a small application named ’BUZZflies’ (freely available at Apple App Store [10], see Fig.1c), to demonstrate synchronization using acoustic signals. In this application each phone periodically generates a beep and adjusts its period after overhearing the same sound from its neighbours until the point they all beep synchronously. In order to eliminate background noise and prevent any false positives, we have applied the Goertzel Filter [11] and have employed real-time signal processing. Attendees will be welcome to reset the applications on different devices and will witness that they eventually converge to synchrony by beeping simultaneously. ACKNOWLEDGEMENT This work was performed as part of the research cluster Lakeside Labs. Funding was obtained as follows: ERDF/KWF grant 20214 |21532| 32604 (RELAY), FFG grants 825893 (ROSSY) and 2305537 (EVOSOS). R EFERENCES [1] S. H. Strogatz, SYNC: The Emerging Science of Spontaneous Order. Hyperion, 2003. [2] R. E. Mirollo and S. H. Strogatz, “Synchronization of pulse-coupled biological oscillators,” SIAM J. Appl. Math., vol. 50, pp. 1645–1662, 1990. [3] R. Mathar and J. Mattfeldt, “Pulse-coupled decentral synchronization,” SIAM Journal on Applied Mathematics, vol. 56, no. 4, pp. 1094–1106, Aug. 1996. [4] Y.-W. Hong and A. Scaglione, “A scalable synchronization protocol for large scale sensor networks and its applications,” Selected Areas in Communications, IEEE Journal on, vol. 23, no. 5, pp. 1085 – 1099, may 2005. [5] A. Tyrrell, G. Auer, and C. Bettstetter, “Emergent slot synchronization in wireless networks,” IEEE Transactions on Mobile Computing, vol. 9, pp. 719–732, 2010. [6] J. Klinglmayr and C. Bettstetter, “Self-organizing synchronization with inhibitory-coupled oscillators: Convergence and robustness,” ACM Trans. Auton. Adapt. Syst., vol. 7, pp. 30:1–30:23, 2012.
[7] J. Klinglmayr, C. Kirst, C. Bettstetter, and M. Timme, “Guaranteeing global synchronization in networks with stochastic interactions,” New Journal of Physics, vol. 14, no. 7, p. 073031, Jul. 2012. [8] R. Pagliari and A. Scaglione, “Scalable network synchronization with pulse-coupled oscillators,” IEEE Transactions on Mobile Computing, vol. 10, no. 3, pp. 392 –405, march 2011. [9] H. Kopetz, Real-time systems Design Principles for Distributed Embedded Applications. Kluwer Academic Publishers, 2003. [10] I. Feh´erv´ari. (2010) Buzzflies. [Online]. Available: https://itunes.apple. com/us/app/buzzflies/id402295450 [11] G. Goertzel, “An algorithm for the evaluation of finite trigonometric series,” The American Mathematical Monthly, vol. 65, no. 1, pp. 34–35, Jan 1958.
