Showing papers on "Clock synchronization published in 2005"

Clock synchronization for wireless sensor networks: a survey

Abstract: Recent advances in micro-electromechanical (MEMS) technology have led to the development of small, low-cost, and low-power sensors Wireless sensor networks (WSNs) are large-scale networks of such sensors, dedicated to observing and monitoring various aspects of the physical world In such networks, data from each sensor is agglomerated using data fusion to form a single meaningful result, which makes time synchronization between sensors highly desirable This paper surveys and evaluates existing clock synchronization protocols based on a palette of factors like precision, accuracy, cost, and complexity The design considerations presented here can help developers either in choosing an existing synchronization protocol or in defining a new protocol that is best suited to the specific needs of a sensor-network application Finally, the survey provides a valuable framework by which designers can compare new and existing synchronization protocols

Time Synchronization and Calibration in Wireless Sensor Networks

Abstract: In this chapter, we review time synchronization and calibration for wireless sensor networks. We will first consider time synchronization in Sections 1.1–1.6, before turning to calibration in Section 1.7. We will show that time synchronization can be considered as a calibration problem and many observations about time synchronization can be transferred to calibration. In Section 1.1, we discuss applications of synchronized time in sensor networks, present challenges of sensor networks, and discuss why traditional synchronization approaches fail to meet these challenges. Section 1.2 presents models of sensor nodes, of hardware clocks, and of communication. Section 1.3 gives an overview of the various classes of synchronization. In Section 1.4, we present common synchronization techniques. Section 1.5 examines current synchronization algorithms. Section 1.6 presents common techniques for evaluating synchronization algorithms and selected evaluation results.

Estimating clock uncertainty for efficient duty-cycling in sensor networks

Abstract: Radio duty cycling has received significant attention in sensor networking literature, particularly in the form of protocols for medium access control and topology management. While many protocols have claimed to achieve significant duty-cycling benefits in theory and simulation, these benefits have often not translated to practice. The dominant factor that prevents the optimal usage of the radio in real deployment settings is time uncertainty between sensor nodes. This paper proposes an uncertainty-driven approach to duty-cycling where a model of long-term clock drift is used to minimize the duty-cycling overhead. First, we use long-term empirical measurements to evaluate and analyze in-depth the interplay between three key parameters that influence long-term synchronization - synchronization rate, history of past synchronization beacons and the estimation scheme. Second, we use this measurement-based study to design a rate-adaptive, energy-efficient long-term time synchronization algorithm that can adapt to changing clock drift and environmental conditions while achieving application-specific precision with very high probability. Finally, we integrate our uncertainty-driven time synchronization scheme with a MAC layer protocol, BMAC, and empirically demonstrate one to two orders of magnitude reduction in the transmit energy consumption at a node with negligible impact on the packet loss rate.

Method for self-synchronizing time between communicating networked systems using timestamps

Abstract: Nodes in a network include a pseudo-timestamp in messages or packets, derived from local pseudo-time clocks. When a packet is received, a first time is determined representing when the packet was sent and a second time is determined representing when the packet was received. If the difference between the second time and the first time is greater than a predetermined amount, the packet is considered to be stale and is rejected, thereby deterring replay. Because each node maintains its own clock and time, to keep the clocks relatively synchronized, if a time associated with a timestamp of a received packet is later than a certain amount with respect to the time at the receiver, the receiver's clock is set ahead by an amount that expected to synchronize the receiver's and the sender's clocks. However, a receiver never sets its clock back, to deter attacks.

Protocol and method for multi-chassis configurable time synchronization

Abstract: Systems and methods are disclosed for time synchronization of operations in a control system. Synchronization networks and devices are provided for transferring synchronization information between controllers in a distributed or localized control system, which is employed in order to allow operation of such controllers to be synchronized with respect to time. Also disclosed are synchronization protocols and hardware apparatus employed in synchronizing control operations in a control system.

Time synchronization attacks in sensor networks

Abstract: Time synchronization is a critical building block in distributed wireless sensor networks. Because sensor nodes may be severely resource-constrained, traditional time-synchronization protocols cannot be used in sensor networks. Various time-synchronization protocols tailored for such networks have been proposed to solve this problem. However, none of these protocols have been designed with security in mind. If an adversary were able to compromise a node, he might prevent a network from effectively executing certain applications, such as sensing or tracking an object, or he might even disable the network by disrupting a fundamental service such as a TDMA-based channel-sharing scheme. In this paper we give a survey of the most common time synchronization protocols and outline the possible attacks on each protocol. In addition, we discuss how different sensor network applications that are affected by time synchronization attacks, and we propose some countermeasures for these attack.

On maximum-likelihood estimation of clock offset

Abstract: Delay measurements from timing message exchanges between two clocks produce a maximum-likelihood estimator (MLE) of the clock offset when fixed delays in each direction are equal and unknown, and variable delays in each direction have an exponential distribution with an unknown mean. It is shown that the MLE corresponds to a previously proposed estimator of clock offset. The ML interpretation of the estimator provides further insight and motivation for its use.

System and method for clock synchronization over packet-switched networks

Abstract: Embodiments of the invention enable the synchronization of clocks across packet switched networks, such as the Internet, sufficient to drive a jitter buffer and other quality-of-service related buffering. Packet time stamps referenced to a local clock create a phase offset signal. A shortest-delay offset generator uses a moving-window filter to select the samples of the phase offset signal having the shortest network propagation delay within the window. This shortest network propagation delay filter minimizes the effect of network jitter under the assumption that queuing delays account for most of the network jitter. The addition of this filtered phase offset signal to a free-running local clock creates a time reference that is synchronized to the remote clock at the source thus allowing for the transport of audio, video, and other time-sensitive real-time signals with minimal latency.

Software and hardware prototypes of the IEEE 1588 precision time protocol on wireless LAN

Abstract: IEEE 1588 is a standard for precise clock synchronization for networked measurement and control systems in LAN environment. This paper presents the design and implementation of two IEEE 1588 prototypes for wireless LAN (WLAN). The first one is implemented using a Linux PC platform and a standard IEEE 802.11 WLAN with modifications to the network device driver. The second prototype is implemented using an embedded WLAN development board that implements the synchronization functionality using an embedded processor with programmable logic device (PLD) circuits. The measured results show that 1.1 ns average clock offset can be reached on HW based implementation, while Linux PC network driver enables 660 ns with a standard WLAN. Although WLAN is an extremely difficult environment for the synchronization, the results achieved with the prototype are fully comparable to those achieved with wired LAN implementations

Fault-tolerant cluster-wise clock synchronization for wireless sensor networks

Abstract: Wireless sensor networks have received a lot of attention recently due to their wide applications, such as target tracking, environment monitoring, and scientific exploration in dangerous environments. It is usually necessary to have a cluster of sensor nodes share a common view of a local clock time, so that all these nodes can coordinate in some important applications, such as time slotted MAC protocols, power-saving protocols with sleep/listen modes, etc. However, all the clock synchronization techniques proposed for sensor networks assume benign environments; they cannot survive malicious attacks in hostile environments. Fault-tolerant clock synchronization techniques are potential candidates to address this problem. However, existing approaches are all resource consuming and suffer from message collisions in most of cases. This paper presents a novel fault-tolerant clock synchronization scheme for clusters of nodes in sensor networks, where the nodes in each cluster can communicate through broadcast. The proposed scheme guarantees an upper bound of clock difference between any nonfaulty nodes in a cluster, provided that the malicious nodes are no more than one third of the cluster. Unlike the traditional fault-tolerant clock synchronization approaches, the proposed technique does not introduce collisions between synchronization messages, nor does it require costly digital signatures.

Algorithmic aspects of the time synchronization problem in large-scale sensor networks

Abstract: We study the time synchronization problem for large-scale wireless sensor networks in the high-density regime. Our interest in this problem arises from a sensor networking application, where a large number of power-constrained radio transmitters coordinate their access to a Gaussian multiple access channel to cooperate in generating a waveform stronger than any individual node would be able to generate. In a companion paper to this one, we study theoretical aspects of a time synchronization mechanism that is optimal in the limit of asymptotically high network densities. In this work we summarize those results, and explore practical implementation issues of that mechanism in the context of networks with large, but finite, numbers of nodes. Through simulations, we find that the synchronization mechanism performs very well for finite (and relatively small) networks, maintaining tight clock synchronization indefinitely.

Maintaining synchronization of multiple data channels with a common clock signal

Abstract: Maintaining synchronization when sending/receiving multiple channels of data with a corresponding common reference clock signal. Synchronization signals (e.g., pulses) are generated periodically and the timing of channels is adjusted. In an embodiment, multiple sequences of parallel data elements are received on corresponding parallel data channels using a first common clock signal. Each sequence of parallel data elements is converted to a corresponding sequence of serial data elements. The serial data elements are transmitted on a corresponding serial channel using a serial clock as a common reference. A synchronization signal may be generated periodically with a time period of (the number of bits in each parallel data element x the time period of the serial clock), wherein ‘x’ represents multiplication operation. As the parallel data channels are synchronized in short intervals, synchronization is maintained.

Clock Synchronization for Wireless Sensor Networks : A Survey

Abstract: Abstract Recent advances in micro-electromechanical (MEMS) technology have led to the development of small, low-cost, and low-power sensors. Wireless sensor networks (WSNs) are large-scale networks of such sensors, dedicated to observing and monitoring various aspects of the physical world. In such networks, data from each sensor is agglomerated using data fusion to form a single meaningful result, which makes time synchronization between sensors highly desirable. This paper surveys and evaluates existing clock synchronization protocols based on a palette of factors like precision, accuracy, cost, and complexity. The design considerations presented here can help developers either in choosing an existing synchronization protocol or in defining a new protocol that is best suited to the specific needs of a sensor-network application. Finally, the survey provides a valuable framework by which designers can compare new and existing synchronization protocols.

A scalable and adaptive clock synchronization protocol for IEEE 802.11-based multihop ad hoc networks

Abstract: This paper studies the fundamental problem of clock synchronization in IEEE 802.11-based multihop ad hoc networks. Clock synchronization is important for power saving, network throughput and many basic operations of 802.11 protocols in a multihop ad hoc network (MANET). The scalability problem of 802.11 timing synchronization has been studied extensively in single hop ad hoc networks and good solutions are available. However these solutions do not perform well in the MANET environment. A few multihop solutions were proposed; but the performance is still not very good. The maximum clock offset is still over 200 mus for these protocols. In this paper, we propose an adaptive protocol through beacon transmission prioritization, frequency adjustment and construction of dominating set. The frequency adjustment is proved to be bounded. Simulation shows that we are able to control the maximum clock offset under 50 mus after protocol stabilization. The improvement is more than 400% over the current solutions with similar complexity. The new protocol also shows great long-term stability

Demand-based dynamic clock control for transaction processors

Abstract: A variable speed data processor includes a clock generator generating a plurality of clocks at different clock rates. Clock select circuitry synchronously selects one of the clocks as an output clock signal to data processing circuitry, based on a data activity indication. Activity logic generates the data activity indication based at least in part on the existence of data processing activity targeted to the data processing circuitry. When the data processing circuitry experiences bursty data processing activity, the clock rate can shift rapidly between the multiple clock rates, conserving power without substantially diminishing the availability of the data processing circuitry.

Method and apparatus for synchronizing internal state of frequency generators on a communications network

Abstract: A first level of control over operation of slave Digitally Controlled Frequency Selectors (DCFSs), such as DCOs or DDSs, may occur by periodic transmission of control words from the master clock to the slave clocks. To allow enhanced control over the output of the slave clocks, the frequency of the local oscillator used to generate the synthesized output of the master clock may also be conveyed to the slave clocks to allow a second level of control to take place. The second level of control allows the local oscillators at the slave clocks to lock onto the frequency of the master local oscillator to thereby allow the slave local oscillators to operate the slave DCFSs using the same local oscillator frequency. The first level of control synchronizes operation of the DCFSs while the second level control prevents instabilities in the local oscillators from causing long term drift between the slave and master clock outputs. Timestamps may be used to synchronize the master and slave local oscillators.

Synchronization in wireless sensor networks using Bluetooth

Abstract: Wireless sensor networks (WSNs) can take advantage of versatility, completeness, and low prices of standard wireless protocols; Bluetooth as we show later is a candidate suitable for WSNs. The fusion of data collected over a WSN is just an evident application of time synchronization. Bringing together these two issues, we find that synchronization using standard protocols poses an important drawback. In this paper, we present a simple method that allows clock synchronization in Bluetooth WSNs, down to few microseconds.

Self-stabilization of byzantine protocols

Abstract: Awareness of the need for robustness in distributed systems increases as distributed systems become integral parts of day-to-day systems. Self-stabilizing while tolerating ongoing Byzantine faults are wishful properties of a distributed system. Many distributed tasks (e.g. clock synchronization) possess efficient non-stabilizing solutions tolerating Byzantine faults or conversely non-Byzantine but self-stabilizing solutions. In contrast, designing algorithms that self-stabilize while at the same time tolerating an eventual fraction of permanent Byzantine failures present a special challenge due to the “ambition” of malicious nodes to hamper stabilization if the systems tries to recover from a corrupted state. This difficulty might be indicated by the remarkably few algorithms that are resilient to both fault models. We present the first scheme that takes a Byzantine distributed algorithm and produces its self-stabilizing Byzantine counterpart, while having a relatively low overhead of O(f′) communication rounds, where f′ is the number of actual faults. Our protocol is based on a tight Byzantine self-stabilizing pulse synchronization procedure. The synchronized pulses are used as events for initializing Byzantine agreement on every node’s local state. The set of local states is used for global predicate detection. Should the global state represent an illegal system state then the target algorithm is reset.

Communication system and synchronization method thereof

Abstract: A clock signal of a master clock of a sender is transmitted to a receiver through a classical channel and is returned from the receiver. The clock signal is transmitted with strong light from a sender-side quantum unit to a receiver-side quantum unit through a quantum channel. A sender-side synchronization section establishes phase synchronization between the clock signal returned from the receiver and the clock signal detected by the sender-side quantum unit, and generates a calibration clock signal. At the receiver as well, a receiver-side synchronization section establishes phase synchronization between the clock signal detected from the classical channel and the clock signal detected by the receiver-side quantum unit, and generates a calibration clock signal.

A compatible and scalable clock synchronization protocol in IEEE 802.11 ad hoc networks

Abstract: This paper studies the scalability and compatibility problems of clock synchronization in IEEE 802.11 ad hoc networks. The scalability problem of 802.11 timing synchronization has been recognized and studied by researchers in the field, but the proposed solutions are not meeting industry expectation. The compatibility issue is not well investigated by the research community yet. The compatibility issue is very important and practical because of the large deployment base of 802.11 networks. In this paper, we try to address both issues. We propose a simple, compatible protocol without any change of beacon format. The frequency adjustment is proved to be bounded and the maximum clock offset is controlled under 20 /spl mu/s. It is a significant improvement over the current results in the field. The current solutions with similar complexity can only control the maximum clock offset around 125 /spl mu/s for compatible solutions and 50 /spl mu/s for non-compatible protocols.

Clock synchronization in powerline networks

Abstract: Although most aspects of clock synchronization are well investigated, most studies focus on high-speed, highly reliable and topologically stable networks. For the synchronization of a distributed system networked using powerline, however, essential modifications are needed in order to cope with the peculiar behaviour of power line as a communication medium. One of these special properties is the non-stable network topology caused by load balancing of the energy suppliers; another is the non-symmetric communication delay, as opposed to the common symmetry assumption in most state-of-the-art synchronization protocols. Finally, it is well understood that high-accuracy clock synchronization needs a very tight coupling of the protocol to the physical layer. This paper investigates a concept for synchronization in a powerline network and describes an approach which is taking advantage of a special clock synchronization hardware working closely together with a powerline physical layer.

High precision clock synchronization according to IEEE 1588 : implementation and performance issues

Abstract: A high precision time base is important for distributed systems in measurement and automation applications. The Precision Time Protocol (PTP) specified in IEEE 1588 [1] is able to synchronize distributed clocks with an accuracy of less than one microsecond. It is applicable in multicast capable network technologies such as Ethernet LANs. The mechanism combines high accuracy and fast convergence with low demand on clocks and on network and computing capacity. The paper outlines the application areas of precise time synchronization. The operation of the mechanism as specified in IEEE 1588 is explained and the factors contributing to precision are analyzed. A real and portable implementation is presented. Its performance is evaluated with respect to synchronization behaviour, accuracy and stability under real condi-

Failure detection with booting in partially synchronous systems

Abstract: Unreliable failure detectors are a well known means to enrich asynchronous distributed systems with time-free semantics that allow to solve consensus in the presence of crash failures. Implementing unreliable failure detectors requires a system that provides some synchrony, typically an upper bound on end-to-end message delays. Recently, we introduced an implementation of the perfect failure detector in a novel partially synchronous model, referred to as the Θ-Model, where only the ratio Θ of maximum vs. minimum end-to-end delay of messages that are simultaneously in transit must be known a priori (while the actual delays need not be known and not even be bounded). In this paper, we present an alternative failure detector algorithm, which is based on a clock synchronization algorithm for the Θ-Model. It not only surpasses our first implementation with respect to failure detection time, but also works during the system booting phase.

Magnetic recording medium, magnetic recording/reproducing apparatus, and stamper for manufacturing magnetic recording medium

Abstract: A magnetic recording medium includes a servo area that has a preamble area where a magnetic portion and a non-magnetic portion for clock synchronization are formed; and a data area where user data is written into. The magnetic portion is different in an occupancy to the preamble area from the non-magnetic portion.

An energy aware coverage-preserving scheme for wireless sensor networks

Abstract: How well a large wireless sensor network can be monitored or tracked while keeping long live is a challenging problem known as the energy aware coverage preserving. Several coverage solutions have been introduced based on node scheduling and quality coverage. Node scheduling based solutions usually rely on global clock synchronization and/or time delays to resolve conflicts when determining what nodes should be turned-off to save energy. If these time delays cannot be calculated accurately blind areas might emerge jeopardizing the network coverage quality. Other challenges to node scheduling based solutions include finding optimal wakeup strategies that avoid waking up more nodes than necessary; and keeping connectivity and coverage of the network while optimizing the number of nodes. This paper extends the coverage calculation method proposed by Tian and Georganas, referred here as C-PNSS scheme, and describes a novel distributed solution based on local information exchange without the uncertainty of self-schedule algorithms. A Decision algorithm and a new node wakeup scheme were devised to overcome existing problems in actual schemes. We implement our optimal coverage-preserving scheme (OCoPS) as an extension of LEACH. A set of simulation experiments was performed to evaluate OCoPS performance when compared to LEACH and C-PNSS schemes. The results indicate that our solution outperforms C-PNSS by over 20% on network lifetime and by over 25% on network lifetime when the coverage rate is higher than 80%. LEACH is outperformed by nearly over five times on network lifetime. The experimental results also show that our coverage scheme based on our extended coverage calculation method effectively limits the on-duty node number when compared to both LEACH and C-PNSS.

Graduated delay line for increased clock skew correction circuit operating range

Abstract: Clock synchronization and skew adjustment circuits are described that utilize varying unit delay elements in their delay lines in either a graduated or a stepped unit time delay arrangement, allowing a reduced circuit implementation and improved lock characteristics. These graduated or a stepped unit time delays allow reduction in the number of the fine unit delay elements of the delay lines by placing a fine delay element granularity at the most critical timings to sense and adjust for the portion of the clock signal time period that are high speed or critical. This allows clock synchronization and skew adjustment circuits to be implemented in an optimized manner that exhibits a reduced overall circuit size and power consumption, while having improved lock characteristics over a wide range of frequencies.

Real-time operating environment for networked control systems

Abstract: This paper discusses the real-time aspects of networked control systems' (NCSs) operating environments. An open-loop unstable magnetic-levitation (Maglev) test bed was constructed and used to develop an NCS with a real-time application interface (RTAI) operating environment. A client-server architecture on a local area network (LAN) was developed with the network communication based on the user datagram protocol (UDP). The implementation of an event-driven server and a time-driven client presented in this paper facilitates a simple timing scheme that does not require clock synchronization between the client and the server. A novel prediction scheme involving the multiple-step-ahead generation of control signals is used to maintain system stability in the presence of excessive time delays and packet losses in the communication network. The current system can compensate for up to 20% data-packet losses without losing stability with the Maglev real-time-control test bed in the communication network.

System and method for synchronizing audio-visual devices on a power line communications (PLC) network

Abstract: A method and apparatus for synchronizing streaming media devices within a PLC network. Output synchronization errors exceeding ˜30 ms become noticeable when multiple streaming media devices are outputting an audio stream. The present invention provides a system and method for isochronously sending periodic reference clocks from a master device to client devices coupled to the PLC network. The client devices set their clocks based on the reference clock. In addition the clients adjust their system clock time base in response to the average divergence of the system clock with the reference clock, or a count of the number of clocks between beacon frames. In this way the client clock is adjusted to closely track the server clock so that synchronization is maintained between each of the devices. Streaming audio shared between servers and client devices is thus output across the network with high fidelity due to the accurate synchronization.

Brief announcement: gradient clock synchronization in sensor networks

Abstract: We examine gradient clock synchronization [1], where the difference between any two network nodes' clocks is required to be upper-bounded by a non-decreasing function of their distance: A node has to be synchronized better to nearby nodes than to faraway nodes. We look at the gradient property in a system model that is typical for sensor networks, provide a lower bound for the achievable synchronization quality in our model, and discuss its relation to the bound in the model from [1]. Time information is degraded by clock drift, and it cannot be communicated without loss due to delay uncertainties. Upper bounds on drift and delay lead to lower bounds on the synchronization error. For instance, the worst-case error between two nodes with delay uncertainty D is in Ω(D). The central theorem of [1] states that in a network with maximal delay uncertainty D, the error between two nodes with constant delay uncertainty is in Ω(log D log log D), i.e. the error grows with D although the delay uncertainty between the two nodes remains constant. The system model in [1] assumes unbounded communication frequency. The upper-bounded drifts and delays are unknown to the nodes, and the lower bound is derived by letting an adversary modify them. We use a system model that we consider more appropriate for sensor networks. Clock drifts are still bounded, but we assume also the communication frequency to be bounded; in sensor networks , communication is expensive in terms of energy, and hence infrequent communication is desirable. As a consequence of infrequent communication, we neglect delay uncertainties and eliminate them from our analysis, i.e. communication occurs in zero time. This is reasonable: As the frequency of communication decreases, the uncertainty due to clock drift increases, while the uncertainty due to message delays remains constant. Two particular characteristics of sensor nodes further strenghten the case for the dominance of the drift: On the one hand, time-stamping on sensor nodes can be done at a low level, such as in the MAC layer, leading to a small delay * For a full version of this paper, see [2]. uncertainty. Recent algorithms reduce it to a few microseconds, e.g. by using packet streams or reference broadcasts. On the other hand, sensor nodes typically employ inexpensive oscillators with drifts of up to 100 ppm. A numeric example: If the delay uncertainty is 1 µs and the clock drift's absolute value …

Adaptive and dynamic data synchronization system for managing data and inventory

Abstract: A system and method for adaptive and dynamic synchronization includes a data synchronization controller which enables synchronization of a plurality of different data types between a main computer and one or more remotely disposed computer elements. The controller includes an orchestrator which responds to requests for data synchronization for components in accordance with predetermined policies maintained by a policy management system. A synchronization interface is controlled by the orchestrator in accordance with the policies to select a synchronization engine to service requests for synchronization of different data types from the orchestrator. Synchronization may be handled for on demand and/or for on schedule requests for synchronization in the policy-based system and method.