Probabilistic Clock Synchronization Service in Sensor Networks
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Continuous clock synchronization avoids unpredictable instantaneous corrections of clock values. This is usually achieved by spreading the clock correction over the synchronization interval. In the context of wireless real time applications, a protocol achieving continuous clock synchronization must tolerate message losses and should have a low overhead in terms of the number of messages. The paper presents a clock synchronization protocol for continuous clock synchronization in wireless real time applications. It extends the IEEE 802.11 standard for wireless local area networks. It provides continuous clock synchronization, improves the precision by exploiting the tightness of the communication medium, and tolerates message losses. Continuous clock synchronization is achieved with an advanced algorithm adjusting the clock rates. We present the design of the protocol, its mathematical analysis, and measurements of a driver level implementation of the protocol on Windows NT.
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In the near future, small intelligent devices will be deployed in homes, plantations, oceans, rivers, streets, and highways to monitor the environment. These devices require time synchronization, so voice and video data from different sensor nodes can be fused and displayed in a meaningful way at the sink. Instead of time synchronization between just the sender and receiver or within a local group of sensor nodes, some applications require the sensor nodes to maintain a similar time within a certain tolerance throughout the lifetime of the network. The Time-Diffusion Synchronization Protocol (TDP) is proposed as a network-wide time synchronization protocol. It allows the sensor network to reach an equilibrium time and maintains a small time deviation tolerance from the equilibrium time. In addition, it is analytically shown that the TDP enables time in the network to converge. Also, simulations are performed to validate the effectiveness of TDP in synchronizing the time throughout the network and balancing the energy consumed by the sensor nodes.
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This paper presents lightweight tree-based synchronization (LTS) methods for sensor networks. First, a single-hop, pair-wise synchronization scheme is analyzed. This scheme requires the exchange of only three messages and has Gaussian error properties. The single-hop approach is extended to a centralized multi-hop synchronization method. Multi-hop synchronization consists of pair-wise synchronizations performed along the edges of a spanning tree. Multi-hop synchronization requires only n-1 pair-wise synchronizations for a network of n nodes. In addition, we show that the communication complexity and accuracy of multi-hop synchronization is a function of the construction and depth of the spanning tree; several spanning-tree construction algorithms are described. Further, the required refresh rate of multi-hop synchronization is shown as a function of clock drift and the accuracy of single-hop synchronization. Finally, a distributed multi-hop synchronization is presented where nodes keep track of their own clock drift and their synchronization accuracy. In this scheme, nodes initialize their own resynchronization as needed.
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Clock synchronization is a critical component in wireless sensor networks, as it provides a common time frame to different nodes.It supports functions such as fusing voice and video data from different sensor nodes, time-based channel sharing, and sleep wake-up scheduling, etc.Early studies on clock synchronization for wireless sensor networks mainly focus on protocol design.However, clock synchronization problem is inherently related to parameter estimation, and recently, studies of clock synchronization from the signal processing viewpoint started to emerge.In this article, a survey of latest advances on clock synchronization is provided by adopting a signal processing viewpoint.We demonstrate that many existing and intuitive clock synchronization protocols can be interpreted by common statistical signal processing methods.Furthermore, the use of advanced signal processing techniques for deriving optimal clock synchronization algorithms under challenging scenarios will be illustrated.
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Wireless ad-hoc sensor networks have emerged as an interesting and important research area in the last few years. The applications envisioned for such networks require collaborative execution of a distributed task amongst a large set of sensor nodes. This is realized by exchanging messages that are time-stamped using the local clocks on the nodes. Therefore, time synchronization becomes an indispensable piece of infrastructure in such systems. For years, protocols such as NTP have kept the clocks of networked systems in perfect synchrony. However, this new class of networks has a large density of nodes and very limited energy resource at every node; this leads to scalability requirements while limiting the resources that can be used to achieve them. A new approach to time synchronization is needed for sensor networks.In this paper, we present Timing-sync Protocol for Sensor Networks (TPSN) that aims at providing network-wide time synchronization in a sensor network. The algorithm works in two steps. In the first step, a hierarchical structure is established in the network and then a pair wise synchronization is performed along the edges of this structure to establish a global timescale throughout the network. Eventually all nodes in the network synchronize their clocks to a reference node. We implement our algorithm on Berkeley motes and show that it can synchronize a pair of neighboring motes to an average accuracy of less than 20ms. We argue that TPSN roughly gives a 2x better performance as compared to Reference Broadcast Synchronization (RBS) and verify this by implementing RBS on motes. We also show the performance of TPSN over small multihop networks of motes and use simulations to verify its accuracy over large-scale networks. We show that the synchronization accuracy does not degrade significantly with the increase in number of nodes being deployed, making TPSN completely scalable.
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The network time protocol (NTP), which is designed to distribute time information in a large, diverse system, is described. It uses a symmetric architecture in which a distributed subnet of time servers operating in a self-organizing, hierarchical configuration synchronizes local clocks within the subnet and to national time standards via wire, radio, or calibrated atomic clock. The servers can also redistribute time information within a network via local routing algorithms and time daemons. The NTP synchronization system, which has been in regular operation in the Internet for the last several years, is described, along with performance data which show that timekeeping accuracy throughout most portions of the Internet can be ordinarily maintained to within a few milliseconds, even in cases of failure or disruption of clocks, time servers, or networks.< >
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Abstract Clock parameters in wireless sensors experience slow changes due to low‐cost construction and environmental conditions. In this paper, a filter‐based distributed protocol, called FBP, is proposed to dynamically achieve clock synchronization for wireless sensor networks. The idea of FBP is derived from a first‐order filter which is robust to environmental noises. The proposed protocol is fully distributed, meaning that each node relies only on its local clock readings and reading announcements from its neighbouring sensor nodes. This will allow the proposed protocol applicable to large sensor networks. By applying FBP, the compensated clock skews can be bounded into a small steady‐state error. Numerical simulations show that the proposed protocol yields better performances in both convergence property and synchronization accuracy.
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To implement synchronization for the wireless sensor networks with indirect connectivity,the clock synchronization mechanism for wireless sensor networks was thoroughly analyzed.A kind of clock synchronization algorithm based on time-stamp scheme was presented.Then it was studied and evaluated in detail.Experiments indicated that the proposed clock synchronization algorithm based on timestamp was energy-effective,which was able to obtain a synchronization precision of one millisecond in order of magnitude,which can satisfy most of the practical applications of wireless sensor networks.
Self-clocking signal
Data synchronization
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