Kasim Sinan Yildirim

Time Synchronization Based on Slow-Flooding in Wireless Sensor Networks

The accurate and efficient operation of many applications and protocols in wireless sensor networks require synchronized notion of time. To achieve network-wide time synchronization, a common strategy is to flood current time information of a reference node into the network, which is utilized by the de facto time-synchronization protocol Flooding Time-Synchronization Protocol (FTSP). In FTSP, the propagation speed of the flood is slow because each node waits for a given period of time to propagate its time information about the reference node. It has been shown that slow-flooding decreases the synchronization accuracy and scalability of FTSP drastically. Alternatively, rapid-flooding approach is proposed in the literature, which allows nodes to propagate time information as quickly as possible. However, rapid flooding is difficult and has several drawbacks in wireless sensor networks. In this paper, our aim is to reduce the undesired effect of slow-flooding on the synchronization accuracy without changing the propagation speed of the flood. Within this context, we realize that the smaller the difference between the speeds of the clocks, the smaller the undesired effect of waiting times on the synchronization accuracy. In the light of this realization, our main contribution is to show that the synchronization accuracy and scalability of slow-flooding can drastically be improved by employing a clock speed agreement algorithm among the sensor nodes. We present an evaluation of this strategy on a testbed setup including 20 MICAz sensor nodes. Our theoretical findings and experimental results show that employing a clock speed agreement algorithm among the sensor nodes drastically improves the synchronization accuracy and scalability of slow-flooding.

External Gradient Time Synchronization in Wireless Sensor Networks

Synchronization to an external time source such as Coordinated Universal Time (UTC), i.e., external synchronization, while preserving tight synchronization among neighboring sensor nodes may be crucial for applications such as determining the speed of a moving object in wireless sensor networks. However, existing time synchronization protocols in the literature, which can be used for external synchronization, poorly synchronize neighboring nodes. On the other hand, the only protocol that aims at optimizing the synchronization error between neighboring nodes is lack of a mechanism which synchronizes sensor nodes to a reference node, and hence, it cannot provide external synchronization. Therefore, there is a lack in the literature of a time synchronization protocol, which can be used by applications demanding both external synchronization and tight synchronization among neighboring nodes. In this paper, we answer the question of whether it is possible for sensor nodes to synchronize to a reference node while they optimize the clock skew between their neighboring nodes at the same time. Within this context, we present a novel time synchronization protocol, namely External Gradient Time Synchronization Protocol (EGSync). In EGSync, each sensor node synchronizes to a reference node by using time information flooded by this node, as well as synchronizes to its neighboring nodes by employing the agreement algorithm. We implemented EGSync on the MICAz platform using TinyOS and evaluated it on a testbed setup including 20 sensor nodes. We present the experimental results on our testbed and the simulation results for networks with larger diameters and densities.

Drift estimation using pairwise slope with minimum variance in wireless sensor networks

Time synchronization is mandatory for applications and services in wireless sensor networks which demand common notion of time. If synchronization to stable time sources such as Coordinated Universal Time (UTC) is required, employing the method of flooding in order to provide time synchronization becomes crucial. In flooding based time synchronization protocols, current time information of a reference node is periodically flooded into the network. Sensor nodes collect the time information of the reference node and perform least-squares regression in order to estimate the reference time. However, least-squares regression exhibits a poor performance since sensor nodes far away from the reference node collect the time information with large deviations. Due to this fact, the slopes of their least-squares line exhibit large errors and instabilities. As a consequence, the reference time estimates of these nodes also exhibit large errors.This paper proposes a new slope estimation strategy for linear regression to be used by flooding based time synchronization protocols. The proposed method, namely Pairwise Slope With Minimum Variance (PSMV), calculates the slope of the estimated regression line by considering the pairwise slope between the earliest and the most recently collected data points. The PSMV slope is less affected by the large errors on the received data, i.e. it is more stable, and it is more computationally efficient when compared to the slope of the least-squares line. We incorporated PSMV into two flooding based time synchronization protocols, namely Flooding Time Synchronization Protocol (FTSP) and PulseSync. Experimental results collected from a testbed setup including 20 sensor nodes show that PSMV strategy improves the performance of FTSP by a factor of 4 and preserves the performance of PulseSync in terms of synchronization error with 40% less CPU overhead for linear regression. Our simulations show that these results also hold for networks with larger diameters and densities.

Adaptive control-based clock synchronization in wireless sensor networks

This paper presents PISync, a novel distributed synchronization algorithm based upon a Proportional-Integral (PI) controller for Wireless Sensor Networks (WSNs). PISync synchronizes each sensor node by applying a proportional feedback (P) and an integral feedback (I) on the relative synchronization error with respect to the received reference time which allow to simultaneously compensate both clock offset and frequency differences. We highlight the benefits of this approach in terms of improved steady state error and scalability as compared to least-squares based time synchronization, and we also propose an on-line adaptive strategy for the design of the integrator gain to further improve performance. We present practical flooding-based and fully-distributed protocol implementations of the PISync algorithm and show through real-world experiments that it has considerably better performance over FTSP, the de-facto time synchronization protocol in WSNs, in terms of both rate of convergence and steady-state error with the additional advantage of minimal resource requirement.

Design and Implementation of a Software Presenting Information in DVB Subtitles in Various Forms

Subtitle data carried by MPEG-2 transport stream includes texts of dialogs in bitmap graphics format. However, this data is not reachable for the blind audience. In addition, carrying extra compatible data with subtitle packets can be helpful for testing subtitle software automatically and for presenting live content (i.e. latest news or special advertisements). In this paper, we propose a method for converting subtitle data into text format by recognizing characters using neural networks. The converted data can be used for blind television users and testing subtitle software automatically. We also define a new subtitle segment for carrying test comparison data and a new subtitle page for representing live content. We present our implementation on Linux platform.

Adaptive Proportional–Integral Clock Synchronization in Wireless Sensor Networks

In this paper, we present a novel control-theoretic time synchronization algorithm, named PISync for synchronizing sensor nodes in wireless sensor networks (WSNs). The PISync algorithm is based on an adaptive proportional–integral controller. It applies a proportional feedback (P) and an integral feedback (I) on the local measured synchronization errors to compensate the differences between the clock offsets and the clock speeds. We present practical flooding-based and fully distributed protocol implementations of the PISync algorithm, and we provide theoretical analysis to highlight the benefits of this approach in terms of improved steady-state error and scalability as compared with existing synchronization algorithms. We show through theoretical analysis, real-world experiments, and simulations that PISync protocols have better or comparable performance over existing protocols in the WSN literature in terms of rate of convergence and steady-state error with the additional advantages of requiring minimal CPU overhead, memory allocation, and code footprint independent of network size and topology, and of employing blind communication.

Robust and Efficient Self-Adaptive Position Tracking in Wireless Embedded Systems

Apart from static deployments, sensor nodes in Wireless Sensor Networks (WSNs) are unaware of their location information. In order to estimate their actual or relative positions with respect to other nodes, they are required to self-localize themselves by collecting information from their environment. However, due to the high dynamism and the noise introduced by the WSN environment, self-localization procedures are not straightforward and they may require quite sophisticated algorithmic techniques to satisfy precision requirements of the WSN applications. Among the self-localization procedures in the literature, the ones based upon the technique of trilateration are easy to implement and efficient in terms of resource requirements. On the other hand, their performance is fragile against environmental dynamics. Besides, even though multilateration based procedures are reported to be more robust, their practicability in WSNs seems questionable due to their high resource requirements. In this paper, our objective is to develop a practical self-localization procedure for WSNs that puts away the fragility against noisy ranging measurements in an efficient manner. To that end, we take a different approach to self-localization procedure and treat it as a search process during which sensor nodes find their relative positions without knowing the actual correct values. We present a novel trilateration-based self-localization procedure by exploiting a robust and efficient search technique, named Adaptive Value Tracking (AVT), that finds and tracks a dynamic searched value in a given search space through successive feedbacks. We evaluate this procedure on a real test bed setup and show that our approach to self-localization is efficient, robust to environmental dynamics and adaptive in the sense of reacting to position changes.

Efficient and Discrete Gradient Synchronization

Gradient clock synchronization is a particular synchronization scheme in distributed systems that requires neighboring nodes to be more tightly synchronized than far away nodes. Up until now, two gradient clock synchronization algorithms have been proposed in the literature which are optimal in terms of worst-case synchronization error among neighboring nodes. In this article, we focus on these algorithms and reveal their drawbacks: The first algorithm requires continuous decision making, which makes it unsuitable for discrete computing systems. Although the second one is a discrete algorithm, it performs computation at every tick of the clock which increases its computational complexity drastically. In addition, both algorithms share the drawback of increasing memory requirements with the network density. Considering these drawbacks, we devise a new discrete gradient clock synchronization algorithm whose communication and computation events are synchronized with clock tick events. The proposed algorithm is lightweight in terms of computational overhead since its computation steps are simple and they are not performed at each clock tick. Moreover, it has constant space complexity that is independent from the network density.

A lightweight method for time synchronization in wireless sensor networks

A distributed time synchronization protocol whose objective is to provide a common notion of time between the sensor nodes is mandatory for many applications in wireless sensor networks. This paper presents Simple Time Synchronization (STS) protocol whose main objective is to achieve networkwide synchronization with microsecond precision using a simple and lightweight method. STS achieves synchronization by fast flooding synchronization messages which are periodically broadcasted by a reference node and uses a simple averaging strategy for drift compensation. STS is implemented on Iris platform using TinyOS and evaluated on a testbed setup including 16 sensor nodes. Experimental results show that the prototype implementation of STS outperforms Flooding Time Synchronization Protocol which is the de facto protocol for time synchronization in wireless sensor networks.