Time Synchronization in Heterogeneous Sensor Networks

Time synchronization is critical to sensor networks at many layers of its design and enables better duty-cycling of the radio, accurate localization, beamforming and other collaborative signal processing. While there has been significant work in sensor network synchronization, measurement based studies have been restricted to very short-term (few minutes) datasets and have focused on obtaining accurate instantaneous synchronization. Long-term synchronization has typically been handled by periodic re-synchronization schemes with beacon intervals of a few minutes based on the assumption that long-term drift is too hard to model and predict. Thus, none of this work exploits the temporally correlated behavior of the clock drift. Yet, there are incredible energy gains to be achieved from better modeling and prediction of long-term drift that can provide bounds on long-term synchronization error across a sensor network. Better synchronization can lead to significantly lower duty-cycles of the radio, simplify signal processing and can enable an order of magnitude greater lifetime than current techniques.We measure, evaluate and analyze in-depth the long-term behavior of synchronization skew and drift on typical Mica sensor nodes and develop an efficient long-term time synchronization protocol. We use four real time data sets gathered over periods of 12-30 hours in different environmental conditions to study the interplay between three key parameters that influence long-term synchronization - synchronization rate, history of past synchronization beacons and the estimation scheme. We use this measurement-based study to design an online adaptive time-synchronization algorithm that can adapt to changing clock drift and environmental conditions while achieving application-specified precision with very high probability. We find that our algorithm achieves between one and two orders of magnitude improvement in energy efficiency over currently available time-synchronization approaches.

As more and more innovative applications of wireless sensor networks emerge, the need for deploying heterogeneous sensors with different functionalities and capabilities rise rapidly. In this study, we consider a heterogeneous sensor network containing at least two types of sensors: powerful sensors and normal sensors. Powerful sensors have higher energy and computing capacities, and can communicate with normal sensors using the same network interface. Communications between powerful sensors may thus be facilitated by building an overlay on top of the normal sensors. The topology of the overlay is critical. The overlay must have a low diameter to efficiently support upper layer applications such as resource discovery. It must also consider the energy consumption of the relaying normal sensors. In this paper, we propose a distributed overlay formation protocol to achieve the above goals. Through simulation, we compare our protocol with two overlay formation protocols, fully connected graph and minimum spanning tree. The results show that our proposed protocol can achieve better performance both in message latency and energy consumption.

We present geometric broadcast for sensor and actor networks (GBSA), a novel broadcasting protocol for heterogeneous wireless sensor and actor networks. While broadcasting is a very energy-expensive protocol, it is also widely used as a building block for a variety of other network layer protocols. Therefore, reducing the energy consumption by optimizing broadcasting is a major improvement in heterogenous sensor networking. GBSA is a distributed algorithm where nodes make local decisions on whether to transmit based on a geometric approach. GBSA does not need any neighborhood information and imposes very low communication overhead. GBSA is scalable to the change in network size, node type, node density and topology. GBSA accommodates seamlessly such network changes, including the presence of actors in heterogeneous sensor networks. Indeed GBSA takes advantage of actor nodes, and uses their resources when possible, thus reducing the energy consumption by sensor nodes. Through simulation evaluations, we show that GBSA is very scalable and its performance is improved by the presence of actors. At best of our knowledge, GBSA is the first broadcast protocol designed specifically for heterogeneous sensor and actor networks.

Wireless sensor networks (WSN) have specific constraints and stringent requirements in contrast to traditional wired and wireless computer networks. Among these specific requirements, energy-efficiency is the most prominent. Sensor motes have finite amount of energy supply. In most WSN applications, hundreds or thousands of motes are deployed in an area (e.g., behind enemy lines or foliage) and there is no provision for battery recharging. Therefore, a fundamental constraint on WSN applications and services is to extend the network's lifetime by making the batteries run longer. In this paper we propose an energy-efficient time synchronization protocol, referred as ETSP, which gives significant improvement in energy consumption over existing time synchronization schemes. We first evaluate the existing reference broadcast synchronization (RBS) and time-sync protocol for sensor networks (TPSN), which are two prominent examples of receiver-receiver and sender-receiver synchronization methods. This performance analysis demonstrates that combination of RBS and TPSN can work better than using these techniques separately. In the proposed ETSP protocol, we identify conditions to toggle between RBS and TPSN based on a threshold value. We experimentally and mathematically derive an optimal value of the switching threshold for different sensor motes. In this way we effectively reduce the number of transmissions which results in saving energy. Moreover, we assess the performance of our protocol on the bit error traces collected over a real WSN test bed to cater for retransmissions overload for the time synchronization protocols under consideration.

Study of consensus-based time synchronization in wireless sensor networks

Distributed time synchronization is an important part of sensor networks. Due to the limited battery capacity of the sensor nodes, applicable schemes should concern the energy consumption of the networks. To achieve better efficiency, an event-triggered time synchronization scheme (ETTS) is presented, which aims at providing on-demand time synchronization in sensor networks. The scheme is composed of two phases: skew correction phase and estimation of offset phase. When timestamps from different clocks need to be compared, they can be reconciled after the event. ETTS removes the need to predict application requirements in advance, which means that the synchronization energy is only expended after an event has occurred. Compared to traditional schemes, for example, TPSN, simulation results show that ETTS achieves desired energy efficiency when the frequency of event is low.

Source localization is an important application of wireless sensor networks (WSNs). Many types of sensors can be used for source localization, e.g. range-only sensors, bearing-only sensors and time-of-arrival (TOA) sensors, etc. It is well known that the relative sensor-source geometry can significantly affect the performance of any particular localization algorithm. Existing works in the literature mainly deal with the geometry analysis for a single type of sensors. However, in real applications, different types of sensors may be utilized for source localization simultaneously. Hence, in this paper, we consider the optimal sensor placement problem in heterogeneous sensor networks, where two types of sensors are deployed for source localization. Relative optimal sensor-source configurations with the minimum number of sensors for source localization, are identified under the D-optimality criterion with potential extensions to a general case. Explicit characterizations of optimal sensor-source geometries are given for hybrid range-only and bearing-only sensors as well as hybrid bearing-only and TOA sensors, respectively.

Packet collision in underwater wireless sensor networks (UWSNs) can create inconsistent time stamps leading to inaccurate time synchronization in underwater sensor networks. Most extant time synchronization techniques do not model for the probability of packet collision in UWSNs. This paper proposes a novel non-homogenous Poisson process (NHPP)-based back-off technique that estimates probability of packet collision in UWSN over a set period of time and allows sending of data packets only when probability of collision in a network is low. Any inconsistent time stamps are further identified and removed using Mahalanobis distance technique before linear regression is run. The proposed method performs better in extensive simulations as compared to two similar time synchronization techniques in literature.KeywordsUWSNsNon-homogenous Poisson processPacket collisionTime synchronization

A holistic sensor network design for energy conservation and efficient data dissemination

In this work, we propose a small world model to design Heterogeneous Sensor Networks (HSNs). The proposed model takes into account the communication pattern of this network to create shortcuts directed to the monitoring node, decreasing data communication latency. The endpoints of these shortcuts are nodes with more powerful communication range and energy reserves to support the long communication range. However, this kind of communication causes a high interference in the wireless channel. For this, the proposed model uses the multi-interface and multi-channel capability of the MAC layer to reduce the number of collisions during data communication. Simulation results showed that with just a few powerful nodes, a wireless sensor network can be tuned into a HSN with small world features. Also, when the shortcuts are directed to the monitoring node and assigned to a different wireless channel, the number of collisions and the data communication latency are reduced.

Time Synchronization is one of the problems and still ignored problem in area of wireless sensor network (WSN). After reviewing the existing literatures, it is found that there are few studies that combinely address the problem of energy conservation, clustering, routing along with minimizing the errors due to time synchronization in sensor network. Therefore, this manuscript presents a delay-based routing which considers the propagation delay to formulate a mechanism for delay compensation in large scale wireless sensor network. The prime goal of this technique is to jointly address energy problems, time synchronization, and routing in wireless sensor network. The outcome of the proposed study was found to posse’s minimized communication overhead, minimized synchronization errors, lower energy consumption, and reduced processing time when compared with the existing standards of time synchronization techniques.

Fault tolerant aggregation in heterogeneous sensor networks

Time synchronization is an important part of any distributed networked embedded system. It is essentially the process of achieving and maintaining common time base among all network nodes of the system. This task is quite challenging for the systems or applications like sensor networks, since these systems are highly resource constrained, yet need to process time-sensitive data in collaborative manner. Though many protocols have been suggested for terrestrial sensor networks (RSB, TPSN, FTSP and LTS) and they perform reasonably well, very few protocols (THSL, Tri-message) have been suggested for the high-latency underwater acoustic networks, since achieving time-synchronization for high-latency networks is even more challenging issue. Here we describe a very simple extension and implementation of Tri-message time synchronization protocol for multi-hop Underwater Acoustic Sensor Network (UASN) on the miniature test-bed setup.

Information fusion helps telemonitoring systems to better unify data collected from different sensors. This paper presents last improvements made on a telemonitoring system aimed at enhancing remote healthcare for dependent people at their homes. The system is based on SYLPH, a novel platform based on a service-oriented architecture approach over a heterogeneous Wireless Sensor Networks infrastructure to create intelligent environments. Because of SYLPH, the system allows the interconnection of several sensor networks from different wireless technologies, such as ZigBee or Bluetooth. Furthermore, the SYLPH platform can be executed over multiple wireless devices independently of their microcontroller or the programming language they use.

Time synchronization is essential for several ad-hoc network protocols and applications, such as TDMA scheduling and data aggregation. In this paper, we propose a clustering- based time synchronization framework for multi-hop sensor net- works. We assume that relative node synchronization is sufficient, i.e., consensus on one time value is not required. Our goal is to divide the network into connected synchronization regions (nodes within 2-hops) and perform inter-regional synchronization in O(LLSync) × Niter time, where O(LLSync) denotes the complexity of the underlying low-level synchronization technique (used for single hop synchronization), and Niter denotes the number of iterations where the low-level synchronization protocol is invoked. We propose two novel fully-distributed protocols, SYNC-IN and SYNC-NET, for regional and network synchro- nization, respectively, and prove that Niter is O(1) for both protocols. We exploit the tradeoff between rapid convergence (and consequently energy-efficiency) and perceived accuracy. Our framework does not require any special node capabilities (e.g., being GPS-enabled), or the presence of reference nodes in the network. Our framework is also independent of the particular clustering, inter-cluster routing, and low-level synchronization protocols. We formulate a density model for analyzing inter- regional synchronization, and evaluate our protocols via extensive simulations.