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Internet Time Synchronization: The Network Time Protocol
Abstract
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
... For the clock frequency recovery, each sensor node passively listens to any messages with timestamps either broadcasted (e.g., beacons) or unicasted (e.g., control messages to a specific node) from the head node and carries out asynchronous source clock frequency recovery described in [9], which is basically one-way clock frequency estimation. For the clock offset and delay estimation, a simple two-way message exchange procedure [11], [12] is used but in a reverse direction where the head node initiates the procedure and keeps track of the offsets between its reference clock and sensor node clocks; also, instead of dedicated, periodical synchronization message exchanges, we embed the synchronization "Response" messages of the two-way message exchange procedure in the measurement data report messages from sensor nodes in order to minimize the number of message transmissions. In this way we can move most of time synchronization operations to the head node and reduce the complexity and thereby power consumption of sensor nodes for time synchronization. ...
... The difference is that time translation occurs at sensor nodes just after receiving synchronization messages from a "third-party" node (e.g., a head node) in the post-facto synchronization, while, in the proposed scheme, the time of the occurrence is transmitted to the head node without waiting for a synchronization message and the time translation is done at the head node. As for the receiver clock skew compensation, the use of network time protocol (NTP) [11] was suggested in [17]; because local-clock resolution and skew are minimized by the controlfeedback design (e.g., based on phase-locked loop (PLL)), however, the NTP is not proper for low-power/complexity sensor nodes. In this regard the clock skew is handled by a oneway, low-complexity asynchronous SCFR scheme described in [9] for the proposed scheme. ...
... After some manipulations, we can obtain closed-form expressions forθ M L i (k) andR M L i (k) given in (10) and (11), respectively. ...
We consider energy-efficient time synchronization in a wireless sensor network where a head node (i.e., a gateway between wired and wireless networks and a center of data fusion) is equipped with a powerful processor and supplied power from outlet, and sensor nodes (i.e., nodes measuring data and connected only through wireless channels) are limited in processing and battery-powered. It is this asymmetry that our study focuses on; unlike most existing schemes to save the power of all network nodes, we concentrate on battery-powered sensor nodes in minimizing energy consumption for time synchronization. We present a time synchronization scheme based on asynchronous source clock frequency recovery and reverse two-way message exchanges combined with measurement data report messages, where we minimize the number of message transmissions from sensor nodes, and carry out the performance analysis of the estimation of both measurement time and clock frequency with lower bounds for the latter. Simulation results verify that the proposed scheme outperforms the schemes based on conventional two-way message exchanges with and without clock frequency recovery in terms of the accuracy of measurement time estimation and the number of message transmissions and receptions at sensor nodes as an indirect measure of energy efficiency.
... In the past two decades, there have been lots of interests in the distributed cooperative control [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], and [13], for multi-agent systems due to its potential applications in formation flying, path planning and so forth. Besides, the clock synchronization problems were also discussed in [31,32,33,34,35], which are very important to design distributed algorithms. Distributed average tracking, as a generalization of consensus and cooperative tracking problems, has received increasing attentions and been applied in many different perspectives, such as distributed sensor networks [14], [15] and distributed coordination [16], [17]. ...
... Since there exist differences between the local times of the agents, which may effect the distributed average tracking result, the clock synchronization is introduced in this paper. The clock synchronization problem has been solved in many existing papers such as [31,32,33,34,35]. With the help of the existing results on clock synchronization in [31,32,33,34,35], the first step before beginning computation is to set the local clock to synchronize the local times. ...
... The clock synchronization problem has been solved in many existing papers such as [31,32,33,34,35]. With the help of the existing results on clock synchronization in [31,32,33,34,35], the first step before beginning computation is to set the local clock to synchronize the local times. Thus, the boundary layer concept with clock synchronization devices plays a vital role to reduce the chattering phenomenon. ...
This paper studies the distributed average tracking problem for multiple time-varying signals generated by linear dynamics, whose reference inputs are nonzero and not available to any agent in the network. In the edge-based framework, a pair of continuous algorithms with, respectively, static and adaptive coupling strengths are designed. Based on the boundary layer concept, the proposed continuous algorithm with static coupling strengths can asymptotically track the average of multiple reference signals without the chattering phenomenon. Furthermore, for the case of algorithms with adaptive coupling strengths, average tracking errors are uniformly ultimately bounded and exponentially converge to a small adjustable bounded set. Finally, a simulation example is presented to show the validity of theoretical results.
... To achieve energy efficiency, these solutions minimize communication (radio "ON" time) such that the least number of messages are exchanged to achieve and maintain synchronization. Synchronization accuracy varies from a few seconds [11] to a few microseconds [5]- [7], depending on the type of protocol and application used. For sensors in real-time systems, the expected accuracy is in the order of microseconds [1]. ...
... do 8: n.time ← rcv().time 9: if n.synced() then 10: my_time ← n.time 11 and update their clocks. All nodes transmit in the discovery phase continuously while only the CH and CB nodes transmit beyond the election phase in a time-slotted manner as the synchronization among nodes improves along the state transitions. ...
... Generic Synchronization solutions: Historically, the Global Positioning System (GPS) or Network-Time Protocol (NTP) [11] has been used for time synchronization in networks. However, these protocols are not applicable for resource-constrained nodes. ...
Time synchronization of devices in Internet-of-Things (IoT) networks is one of the challenging problems and a pre-requisite for the design of low-latency applications. Although many existing solutions have tried to address this problem, almost all solutions assume all the devices (nodes) in the network are faultless. Furthermore, these solutions exchange a large number of messages to achieve synchronization, leading to significant communication and energy overhead. To address these shortcomings, we propose C-sync, a clustering-based decentralized time synchronization protocol that provides resilience against several types of faults with energy-efficient communication. C-sync achieves scalability by introducing multiple reference nodes in the network that restrict the maximum number of hops any node can have to its time source. The protocol is designed with a modular structure on the Contiki platform to allow application transitions. We evaluate C-sync on a real testbed that comprises over 40 Tmote Sky hardware nodes distributed across different levels in a building and show through experiments the fault resilience, energy efficiency, and scalability of the protocol. C-sync detects and isolates faults to a cluster and recovers quickly. The evaluation makes a qualitative comparison with state-of-the-art protocols and a quantitative comparison with a class of decentralized protocols (derived from GTSP) that provide synchronization with no/limited fault-tolerance. Results also show a reduction of 56.12% and 75.75% in power consumption in the worst-case and best-case scenarios, respectively, compared to GTSP, while achieving similar accuracy.
... In orthogonal frequency division multiplexing (OFDM)-based radio interfaces, precise synchronization of time and frequency among multiple users is critical to ensuring the integrity and reliability of data transmission [8]. Moreover, in contemporary computer and control networks, accurate synchronization is indispensable for preserving the safety and reliability of time-sensitive information [9]. However, achieving and maintaining precise clock synchronization in distributed wireless networks is particularly challenging in scenarios where access to the global positioning system (GPS) is restricted or unreliable, and atomic oscillators are unavailable [10]. ...
... This method minimizes large fluctuations, balancing real-time data with historical trends, resulting in a more consistent and reliable time offset, with observed improvements in practical cases. The frequency skew is calculated as (θ m,k − θ m,k−1 )/T s ; however, inherent uncertainties in the PTP protocol-even with the application of the Kalman fil-VOLUME , 9 This article has been accepted for publication in IEEE Open Journal of Instrumentation and Measurement. This is the author's version which has not been fully edited and content may change prior to final publication. ...
The integration of radar sensing and imaging capabilities into future integrated sensing and communication (ISAC) networks enables advanced use cases, including autonomous vehicle navigation, real-time health monitoring, and smart city management. However, ultra-precise time and frequency synchronization is crucial for unlocking the full potential of such networked ISAC systems. In this paper, a novel real-time wireless time and frequency synchronization scheme is developed and fully implemented on a high-end radio frequency system-on-chip field-programmable gate array (FPGA) platform. The excellent performance and robustness of the proposed solution in practical applications are demonstrated. It is evidenced that the recursive nature of the Kalman filter is well-suited to the dynamic capabilities of FPGA-based simultaneous synchronization. Observed values obtained through the precision time protocol (PTP) are iteratively refined, thus effectively compensating for uncertainties encountered during a synchronization packet exchange. Due to the deterministic processing time inherent in the FPGA, the proposed synchronization method achieves exceptional precision, with clock offset deviations in the nanosecond range and clock rate deviations limited to only a few parts per billion, even across considerable distances between the network nodes.
... If (λcle, ecle) = Qd.last // last event in Qd A time server, such as a NTP server [29], can be deployed to the system for synchronizing the clock of event senders and recipients, which can improve the performance of the system. ...
... Reliably send QUERY_REPLY to ri 28. Else 29. ...
With the development of mobile technology, mobile virtual worlds have attracted massive users. To improve scalability, a peer-to-peer virtual world provides the solution to accommodate more users without increasing hardware investment. In mobile settings, however, existing P2P solutions are not applicable due to the unreliability of mobile devices and the instability of mobile networks. To address the issue, a novel infrastructure model, called Virtual Net, is proposed to provide fault-tolerance in managing user content and object state. In this paper, the key problem, namely object state update, is resolved to maintain state consistency and high interaction responsiveness. This work is important in implementing a scalable mobile virtual world.
... Existen dos planteamientos para la sincronización, el primero es por medio de un reloj lógico y el segundo por medio de un reloj físico. El primer planteamiento para la solución del problema de sincronización en la comunicación entre eventos de máquinas diferentes fue propuesto por Lamport (1978), Fidge (1988) y Mattern (1988) le sumaron un mecanismo de etiquetado de tiempo para seguir la pista del orden causal y, Singhal y Kshemkalyani (1992) se encargaron de eficientar este mecanismo; en el segundo planteamiento destacan las aportaciones de Gusella y Zatti (1987) , Cristian Flaviu (1989), Marzullo y Owicki (1983), Mills (1985aMills ( , 1985bMills ( , 1985cMills ( , 1988Mills ( , 1989aMills ( , 1989bMills ( , 1991Mills ( , 1992Mills ( , 1996Mills ( , 2010aMills ( , 2010b y el estándar IEEE 1588 (2002,2008,2019,2021). ...
... En forma general, Mills (1991) indica que el sistema NTP consta de una red de servidores de tiempo primarios y secundarios, clientes y rutas de transmisión, el servidor de tiempo principal se sincroniza directamente con una fuente de referencia principal UTC -Tiempo Universal Coordinado, y el servidor secundario deriva la sincronización, posiblemente a través de otros servidores secundarios mediante las rutas de red. En circunstancias normales, la sincronización del reloj se determina utilizando sólo los servidores y las rutas de transmisión más precisas y confiables, de modo que las rutas de sincronización reales generalmente asumen una configuración jerárquica con las fuentes de referencias primarias en la raíz y servidores de precisión decrecientes hacia las hojas. ...
El objetivo de la investigación es proporcionar una comprensión profunda de los conceptos y algoritmos aplicados para la sincronización de relojes entre los nodos de un sistema distribuido (SD). En el presente artículo se describen los principales algoritmos relacionados con la solución al problema de la discrepancia de tiempo en los relojes de un sistema distribuido débilmente acoplado. Se parte de la sincronización lógica propuesta por Lamport, que establece una base teórica para la coordinación temporal en sistemas distribuidos, hasta las aportaciones de Mills, implementadas en las versiones del Protocolo de Tiempo de Red (NTP) utilizado actualmente en Internet y el Protocolo de Tiempo de Precisión (PTP) estandarizado por el IEEE, que permite una mayor precisión en la sincronización. La metodología utilizada incluye una revisión exhaustiva de la literatura, analizando tanto algoritmos centralizados como descentralizados. Se abordan las propuestas de Gusella y Zatti, que introducen enfoques innovadores para la sincronización de relojes, así como el algoritmo de Cristian Flaviu, conocido por su simplicidad y efectividad. Los resultados obtenidos demuestran que cada algoritmo tiene ventajas específicas dependiendo del escenario y los requisitos del sistema distribuido en cuestión.
... This implies that we assume all timestamps in persistent signals are synchronized within a certain precision. We only consider application scenarios where the counted wall clock time, synchronized using the network time protocol [30], provides sufficient precision. ...
... This implies that we assume all timestamps in persistent signals are synchronized within a certain precision. We only consider application scenarios where the counted wall clock time, synchronized using the network time protocol [30], provides sufficient precision. ...
Context: Many systems require receiving data from multiple information sources, which act as distributed network devices that asynchronously send the latest data at their own pace to generalize various kinds of devices and connections, known as the Internet of Things (IoT). These systems often perform computations both **reactively** and **retroactively** on information received from the sources for monitoring and analytical purposes, respectively. Inquiry: It is challenging to design a programming language that can describe such systems at a high level of abstraction for two reasons: (1) reactive and retroactive computations in these systems are performed alongside the execution of other application logic; and (2) information sources may be distributed, and data from these sources may arrive late or be lost entirely. Addressing these difficulties is our fundamental problem. Approach: We propose a programming language that supports the following features. First, our language incorporates reactive time-varying values (also known as signals) embedded within an imperative language. Second, it supports multiple information sources that are distributed and represented as signals, meaning they can be declaratively composed to form other time-varying values. Finally, it allows computation over past values collected from information sources and recovery from inconsistency caused by packet loss. To address the aforementioned difficulties, we develop a core calculus for this proposed language. Knowledge: This calculus is a hybrid of reactive/retroactive computations and imperative ones. Because of this hybrid nature, the calculus is inherently complex; however, we have simplified it as much as possible. First, its semantics are modeled as a simple, single-threaded abstraction based on typeless object calculus. Meanwhile, reactive computations that execute in parallel are modeled using a simple process calculus and are integrated with the object calculus, ensuring that the computation results are always serialized. Specifically, we show that time consistency is guaranteed in the calculus; in other words, consistency can be recovered at any checkpoint. Grounding: This work is supported by formally stating and proving theorems regarding time consistency. We also conducted a microbenchmarking experiment to demonstrate that the implemented recovery process is feasible in our assumed application scenarios. Importance: The ensured time consistency provides a rigorous foundation for performing analytics on computation results obtained from distributed information sources, even when these sources experience delays or packet loss.
... The Network Time Protocol (NTP) [14] and Precision Time Protocol (PTP) [15,16] are widely used time synchronization protocols, which transmit time information from a master clock through Ethernet packets. The synchronization principle of the NTP and PTP involves recording timestamps between the master clock and the slave clock, exchanging timestamps, and calculating the time offset between the slave clock and the master clock using four timestamps, thereby correcting the slave clock to achieve time synchronization between the two devices. ...
In datacenter networks, it is necessary to determine whether the path is congested according to the one-way delay of packets. The accurate measurement of one-way delay depends on the high-precision time synchronization of the source device and destination device. We have proposed a time synchronization method based on timestamp mapping, combined with in-band network telemetry technology to obtain the packet send timestamp and receive timestamp on devices. The results show that the maximum synchronization error is 19 ns, and the standard deviation is 7.8 ns with a 100 ms time synchronization period and offset adjustment strategy. The proposed time synchronization method achieves outstanding synchronization accuracy and stability.
... A frequent limitation is that, considering the sophisticated models for the mass market clock synchronization, it is difficult to retrieve accurate estimation of the parameters needed to initialize the models. In WSN, IoT devices typically need only millisecond-level precision, thus they usually rely on the network time protocol (NTP) (Mills, 1991). For higher performance, the precision time protocol (PTP) can be used (IEEE, 2008). ...
To ensure the authenticity of navigation data, Galileo Open Service navigation message authentication (OSNMA) requires loose synchronization between the receiver clock and the system time. This means that during the period between clock calibrations, the receiver clock error needs to be smaller than a pre-defined threshold, currently up to 165s for OSNMA. On the other hand, relying on the PVT solution to steer the receiver clock or correct its bias may not be possible since this would depend on the very same signals we intend to authenticate. This work aims to investigate the causes of the frequency accuracy loss leading to clock errors and to build a model that, from the datasheet of a real-time clock (RTC) device, allows to bound the error clock during a certain period. The model's main contributors are temperature changes, long-term aging, and offset at calibration, but it includes other factors. We then apply the model to several RTCs from different manufacturers and bound the maximum error for certain periods, with a focus on the two-year between-calibration period expected for the smart tachograph, an automotive application that will integrate OSNMA.
... We achieved clock synchronization using NTP with Chrony [35], which provides synchronization accuracy within a few dozen milliseconds. This level of precision is adequate for our experiments, as the physical environment does not change significantly within such a short time frame. ...
We introduce CSI-Inpainter, a novel approach for obstacle removal using Wi-Fi channel state information (CSI). This method harnesses CSI data to reconstruct obscured visual elements, regardless of lighting conditions. Extensive empirical evaluation in both office and industrial settings demonstrates the effectiveness of CSI-Inpainter’s exceptional ability to identify and reconstruct occluded segments, outperforming traditional baselines and our RSSI-based work, RF-Inpainter in terms of visual quality. Our findings emphasize the superiority of CSI data over RSSI for providing richer visual information and underscore the critical role of optimal sensor placement and data fusion from multiple CSI sensors in enhancing the performance. CSI-Inpainter represents a significant advancement in obstacle removal for various applications like surveillance, offering new insights into the integration of wireless sensing and visual scene recovery, expanding the potential applications of Computer Vision in real-world environments.
... The time synchronisation of a distributed network such as a smartphone fleet is challenging. For synchronizing clocks in a distributed network, the Network Time protocol (NTP) was introduced and normalised in 1985 and is used to synchronise computer clocks over internet 35 . For better performance, the Precision Time Protocol (PTP) 36 was later introduced. ...
Smartphones are widespread objects that have been used as physics sensors for the general public thanks to their availability, high connectivity and built-in sensors. Here, we present the use of a fleet of smartphones to create a distributed network of time-synchronized sensors. We first evaluate the sensors quality in the laboratory and then describe the network configuration that allows the remote control of an entire fleet. Finally, we present two test cases that use the smartphone fleet for physical field measurements. By this study, we show that this approach paves the way for large-scale field scientific studies.
... In other words, the local clocks at the transmitter and receiver, which are used to timestamp events for transmitting and receiving update packets, are typically assumed to be perfectly synchronized. However, in real-world systems, achieving precise clock synchronization is challenging due to hardware imperfections, network delays, and other contributing factors [9], [10]. For example, in distributed systems where each node relies on its local clock rather than a global clock, discrepancies between clocks can result in inconsistent timestamps during data exchanges [11]. ...
In this paper, we address the problem of timely delivery of status update packets in a real-time communication system, where a transmitter sends status updates generated by a source to a receiver over an unreliable channel. The timestamps of transmitted and received packets are measured using separate clocks located at the transmitter and receiver, respectively. To account for possible clock drift between these two clocks, we consider both deterministic and probabilistic drift scenarios. We analyze the system's performance regarding the Age of Information (AoI) and derive closed-form expressions for the distribution and the average AoI under both clock drift models. Additionally, we explore the impact of key system parameters on the average AoI through analytical and numerical results.
... There are multiple synchronization solutions for network devices. Network Time Protocol (NTP) is one of the oldest protocols in the field and can achieve millisecond accuracy which is far from 100 nanoseconds requirement [4]. Another method is distributed Primary Reference Time Clock (PRTC) which GNSS receivers are distributed to each 5G network device. ...
Synchronization plays an extremely critical factor in a 5G network system as coordinating transmission between 5G cells occurs at very high speed. Precision Time Protocol defined by IEEE 1588 standard is a popular ethernet-based synchronization mechanism between telecommunication devices because of its sub microsecond accuracy. However, maintaining a low time error under high network usage conditions is very challenging. In this article, a synchronization system based on FPGA is developed and implemented for the 5G Radio Unit to cope with such scenarios where time error is raised under high network load. The prototype based on the proposed solution is able to maintain the time error under 100 nanoseconds at under 4Gbps network usage on a 10Gbps link. From 4Gbps and above, the time error did exceed the 100 nanoseconds standard defined by ORAN for 5G fronthaul network but the system still showed a significant improvement compared to the original design.
... Since distributing a single clock to all devices in a network is resource-consuming, thus hardly feasible especially with wireless connections, it is not surprising that clock synchronization is a very relevant and well studied topic, that dates back to the dawn of computing systems [19], [11], but where innovations are still being introduced nowadays [21], [1]. ...
Accurate and energy-efficient clock synchronization is an enabler for many applications of Wireless Sensor Networks. A fine-grained synchronization is beneficial both at the system level, for example to favor deterministic radio protocols, and at the application level, when network-wide event timestamping is required. However, there is a tradeoff between the resolution of a WSN node's timekeeping device and its energy consumption. The Virtual High-resolution Timer (VHT) is an innovative solution, that was proposed to overcome this tradeoff. It combines a high-resolution oscillator to a low-power one, turning off the former when not needed. In this paper we improve VHT by first identifying the jitter of the low-power oscillator as the current limit to the technique, and then proposing an enhanced solution that synchronizes the fast and the slow clock, rejecting the said jitter. The improved VHT is also less demanding than the original technique in terms of hardware resources. Experimental results show the achieved advantages in terms of accuracy.
... There are many clock synchronization solutions in the literature that can be used for our scheme (e.g., [5], [6]). For instance, Network Time Protocol (NTP) is a low-cost solution whose accuracy ranges from hundreds of microseconds to several milliseconds ( [7]). ...
Sufficiently strong security and privacy mechanisms are prerequisite to amass the promising benefits of the IoT technology and to incorporate this technology into our daily lives. This paper introduces a novel approach to privacy in networks, an approach which is especially well matched with the IoT characteristics. Our general approach is based on continually changing the identifying attributes of IoT nodes. In particular, the scheme proposed in this work is based on changing the IoT nodes' IP addresses, and because the changing patterns of the IP addresses appear random to a non-intended observer, an adversary is unable to identify the source or destination of a particular transmission. Thus, packets that carry information generated by a particular node cannot be linked together. The scheme offers additional security benefits, including DoS mitigation, is relatively easy to implement, and requires no changes to the existing networking infrastructure. We discuss the details of the implementation of the scheme and evaluate its performance.
... Variants of the Network Time Protocol (NTP) [11] and the Precision Time Protocol (PTP) [12] constitute the most popular methods for time reference and synchronization in wired networks [13]. The emergence of a variety of wireless networks during the past decade has led to the development of wireless time-synchronization protocols and localization schemes. ...
Clock synchronization is ubiquitous in wireless systems for communication, sensing and control. In this paper we design a scalable system in which an indefinite number of passively receiving wireless units can synchronize to a single master clock at the level of discrete clock ticks. Accurate synchronization requires an estimate of the node positions. If such information is available the framework developed here takes position uncertainties into account. In the absence of such information we propose a mechanism which enables simultaneous synchronization and positioning. Furthermore we derive the Cramer-Rao bounds for the system which show that it enables synchronization accuracy at sub-nanosecond levels. Finally, we develop and evaluate an online estimation method which is statistically efficient.
... The telehaptic nodes are time-synchronized using Network Time Protocol (NTP) [Mills 1991]. The end-to-end delay encountered by a telehaptic packet received is thus calculated as the difference between the time of reception and the timestamp of the earliest haptic sample embedded in the received packet. ...
Telehaptic applications involve delay-sensitive multimedia communication between remote locations with distinct Quality of Service (QoS) requirements for different media components. These QoS constraints pose a variety of challenges, especially when the communication occurs over a shared network, with unknown and time-varying cross-traffic. In this work, we propose a transport layer congestion control protocol for telehaptic applications operating over shared networks, termed as dynamic packetization module (DPM). DPM is a lossless, network-aware protocol which tunes the telehaptic packetization rate based on the level of congestion in the network. To monitor the network congestion, we devise a novel network feedback module, which communicates the end-to-end delays encountered by the telehaptic packets to the respective transmitters with negligible overhead. Via extensive simulations, we show that DPM meets the QoS requirements of telehaptic applications over a wide range of network cross-traffic conditions. We also report qualitative results of a real-time telepottery experiment with several human subjects, which reveal that DPM preserves the quality of telehaptic activity even under heavily congested network scenarios. Finally, we compare the performance of DPM with several previously proposed telehaptic communication protocols and demonstrate that DPM outperforms these protocols.
... In this scenario, many packets are sent from a server containing the true time (usually a server attached to an atomic clock) and the average delay is estimated, leading to an accurate time estimate. The most famous of these algorithms is the Network Timing Protocol (NTP) [20]. This fails to help in the GM case of timing noise, because the packets cannot be altered themselves. ...
Side channels have become an essential component of many modern information-theoretic schemes. The emerging field of cross technology communications (CTC) provides practical methods for creating intentional side channels between existing communications technologies. This paper describes a theoretical foundation for one such, recently proposed, CTC scheme: Ghost Modulation (GM). Designed to modulate a low-data-rate message atop an existing network stream, GM is particularly suited for transmitting identification or covert information. The implementation only requires firmware updates to existing hardware, making it a cost-effective solution. However, GM provides an interesting technical challenge due to a highly asymmetric binary crossover erasure channel (BCEC) that results from packet drops and network delays. In this work, we provide a mathematical description of the signal and channel models for GM. A heuristic decision rule based on maximum-likelihood principles for simplified channel models is proposed. We describe an algorithm for GM packet acquisition and timing synchronization, supported by simulation results. Several well known short block codes are applied, and bit error rate (BER) results are presented.
... Precision Time Protocol (PTP) [1][2][3]6] and Network Time Protocol (NTP) [22] are the most widely used time synchronization protocols in various networks. PTP is specifically designed for industrial environments that involve control systems and is ideal for distributed systems because of its minimal bandwidth requirements and low processing overhead. ...
Time synchronization is a critical component in network
operation and management, and it is also required by Ultra-
Reliable, Low-Latency Communications (URLLC) in next-
generation wireless systems such as those of 5G, 6G, and
Open RAN. In this context, we design and implement AraSync
as an end-to-end time synchronization system in the ARA
wireless living lab to enable advanced wireless experiments
and applications involving stringent time constraints. We
make use of Precision Time Protocol (PTP) at different levels
to achieve synchronization accuracy in the order of nanoseconds. Along with fiber networks, AraSync enables time synchronization across the AraHaul wireless x-haul network
consisting of long-range, high-capacity mmWave and microwave links. In this paper, we present the detailed design
and implementation of AraSync, including its hardware and
software components and the PTP network topology. Further,
we experimentally characterize the performance of AraSync
from spatial and temporal dimensions. Our measurement
and analysis of the clock offset and mean path delay show
the impact of the wireless channel and weather conditions
on the PTP synchronization accuracy.
... In a data center network, the most appropriate placement of a white-box switch is Top-Of-Rack (TOR). As line rate packet transmission has growingly become a critical aspect of any modern networking infrastructure, the necessity of precise time synchronization within TOR white-box switches has become paramount [3]. In modern networking architectures, accurate timekeeping is essential for various critical functions, including packet scheduling, network security, performance monitoring, and compliance with regulatory requirements. ...
White-box switches have emerged as a pivotal component in modern networking architectures due to their flexibility, cost-effectiveness, and programmability. Within a data center network, achieving precise time synchronization within these switches is imperative for various critical functions such as packet scheduling, network security, and compliance with regulatory requirements. Conventional time synchronization protocols, like the Network Time Protocol (NTP), fail to meet the stringent time synchronization demands of the white-box switches because they are software-based. In contrast, the Precision Time Protocol (PTP), a hardware-based solution, offers sub-microsecond accuracy and low-latency time synchronization. However, despite the advantages of PTP, integrating it into white-box switches may pose challenges due to additional hardware requirements. This paper elaborates on a study conducted to understand the role of white-box switches and their precise time synchronization in data center networking, as well as a comparative measurement between NTP and PTP. Ultimately, this research contributes to a nuanced understanding of the role of PTP in modern white-box switches and lays down decision-making processes regarding time synchronization protocols in networking deployments.
... Precision Time Protocol (PTP) [1][2][3]6] and Network Time Protocol (NTP) [22] are the most widely used time synchronization protocols in various networks. PTP is specifically designed for industrial environments that involve control systems and is ideal for distributed systems because of its minimal bandwidth requirements and low processing overhead. ...
Time synchronization is a critical component in network operation and management, and it is also required by Ultra-Reliable, Low-Latency Communications (URLLC) in next-generation wireless systems such as those of 5G, 6G, and Open RAN. In this context, we design and implement AraSync as an end-to-end time synchronization system in the ARA wireless living lab to enable advanced wireless experiments and applications involving stringent time constraints. We make use of Precision Time Protocol (PTP) at different levels to achieve synchronization accuracy in the order of nanoseconds. Along with fiber networks, AraSync enables time synchronization across the AraHaul wireless x-haul network consisting of long-range, high-capacity mmWave and microwave links. In this paper, we present the detailed design and implementation of AraSync, including its hardware and software components and the PTP network topology. Further, we experimentally characterize the performance of AraSync from spatial and temporal dimensions. Our measurement and analysis of the clock offset and mean path delay show the impact of the wireless channel and weather conditions on the PTP synchronization accuracy.
... However, due to the inherent variations in clock accuracy among different devices, it was essential to synchronize their timestamps accurately to obtain reliable metrics in our experiments. To achieve this, we utilized the Network Time Protocol (NTP) [36], a widely adopted networking protocol specifically designed for clock synchronization in distributed networks. ...
The lack of trust is one of the major factors that hinder collaboration among Internet of Things (IoT) devices and harness the usage of the vast amount of data generated. Traditional methods rely on Public Key Infrastructure (PKI), managed by centralized certification authorities (CAs), which suffer from scalability issues, single points of failure, and limited interoperability. To address these concerns, Decentralized Identifiers (DIDs) and Verifiable Credentials (VCs) have been proposed by the World Wide Web Consortium (W3C) and the European Union as viable solutions for promoting decentralization and "electronic IDentification, Authentication, and trust Services" (eIDAS). Nevertheless, at the state-of-the-art, there are no efficient revocation mechanisms for VCs specifically tailored for IoT devices, which are characterized by limited connectivity, storage, and computational power. This paper presents EVOKE, an efficient revocation mechanism of VCs in IoT networks. EVOKE leverages an ECC-based accumulator to manage VCs with minimal computing and storage overhead while offering additional features like mass and offline revocation. We designed, implemented, and evaluated a prototype of EVOKE across various deployment scenarios. Our experiments on commodity IoT devices demonstrate that each device only requires minimal storage (i.e., approximately 1.5 KB) to maintain verification information and most notably half the storage required by the most efficient PKI certificates. Moreover, our experiments on hybrid networks, representing typical IoT protocols (e.g., Zigbee), also show minimal latency in the order of milliseconds. Finally, our large-scale analysis demonstrates that even when 50% of devices missed updates, approximately 96% of devices in the entire network were updated within the first hour, proving the scalability of EVOKE in offline updates.
... For larger plants or a geographically distributed IoT, clock distribution is not feasible anymore and network synchronization protocols are used instead [2]. Network Time Protocol (NTP) [3,4] is a widely used protocol for synchronizing clock and time between clients and a clock server. The NTP clock server reads accurate Universal Time Coordinated (UTC) from an authoritative clock source such as an atomic clock or GPS. ...
Proper timing synchronization is important when data from sensors are acquired by different devices. This paper proposes a simple but effective solution for System on Chip (SoC) architectures that integrates a general-purpose Field Programmable Gate Array (FPGA) with a CPU. The proposed approach relies on a network synchronization protocol implemented in software, such as Network Time Protocol (NTP) or Precision Time Protocol (PTP), and uses the FPGA to generate a clock reference that is maintained in step with the synchronized system clock. The clock generated by the FPGA is obtained from the FPGA oscillator via appropriate fractional clock division. Clock drift is avoided via a software program that periodically compares the FPGA and the system counters, respectively, and adjusts the fractional clock divider in order to slightly adjust the FPGA clock frequency using a Proportional Integral controller. A specific implementation is presented on the RedPitaya platform, generating a 1 MHz clock in step with the NTP synchronized system clock. The presented system has been used in a distributed data acquisition system for fast transient recording in the neutral beam test facility for the ITER nuclear fusion experiment.
... Since three different devices (i.e., two sensing devices and the video recorder) with different internal clocks were used, we needed a clock synchronization mechanism [46] on all devices. To achieve that, we used an NTP-based solution [47], [48], where the devices synchronized their internal clocks using Google's NTP server 5 . ...
Physical performance tests aim to assess the physical abilities and mobility skills of individuals for various healthcare purposes. They are often driven by experts and usually performed at their practice, and therefore they are resource-intensive and time-demanding. For tests based on objective measurements (e.g., duration, repetitions), technology can be used to automate them, allowing the patients to perform the test themselves, more frequently and anywhere, while alleviating the expert from supervising the test. The well-known Timed Up and Go (TUG) test, typically used for mobility assessment, is an ideal candidate for automation, as inertial sensors (among others) can be deployed to detect the various movements constituting the test without expert supervision. To move from expert-led testing to self-administered testing, we present a mHealth system capable of automating the TUG test using a pocket-sized smartphone or a wrist smartwatch paired with a smartphone, where data from inertial sensors are used to detect the activities carried out by the patient while performing the test and compute their results in real time. All processing (i.e., data processing, machine learning-based activity inference, results calculation) takes place on the smartphone. The use of both devices to automate the TUG test was evaluated (w.r.t. accuracy, reliability and battery consumption) and mutually compared, and set off with a reference method, obtaining excellent Bland-Altman agreement results and Intraclass Correlation Coefficient reliability. Results also suggest that the smartwatch-based system performs better than the smartphone-based system.
... Alternatively, time synchronization can be achieved by means of communication [45]. For example, one may utilize the Network Time Protocol [46] that relies on a master clock or decentralized approaches that rely on, e.g., the heartbeats [47]. ...
The next generation of spacecraft technology is anticipated to enable novel applications, including onboard processing, machine learning, and decentralized operational scenarios. Although several of these applications have been previously investigated, the real-world operational limitations associated with actual mission scenarios have been only superficially addressed. Here, we present an open-source Python module called PASEOS, capable of modelling operational scenarios involving one or multiple spacecraft. It considers several physical phenomena, including thermal, power, bandwidth, and communications constraints, and the impact of radiation on spacecraft. PASEOS can be run as a high-performance-oriented numerical simulation and/or in a real-time mode on edge hardware. We demonstrate these capabilities in three scenarios: one in real-time simulation on a Unibap iX-10 100 satellite processor, another in a simulation modelling an entire constellation performing tasks over several hours, and one training a machine learning model in a decentralized setting. While we demonstrate tasks in Earth orbit, PASEOS also allows deep space scenarios. Our results show that PASEOS can model the described scenarios efficiently and thus provide insight into operational considerations. We show this by measuring runtime and overhead as well as by investigating the constellation's modelled temperature, battery status and communication windows. By running PASEOS on an actual satellite processor, we showcase how PASEOS can be directly included in hardware demonstrators for future missions. Overall, we provide the first solution to holistically model the physical constraints spacecraft encounter in space. The PASEOS module is available online with extensive documentation, enabling researchers to incorporate it into their studies quickly.
... However, the processing time overhead makes the solution unsatisfactory for the real-time performance requirement [22]. For network synchronization, Precision Time Protocol (PTP) and Network Time Protocol (NTP) are proposed to perform synchronization between multiple sensors [23]. Though network synchronization synchronizes the sensors to capture data with respect to common reference clock source rather than the sensor's internal clock. ...
LiDAR and camera are crucial perception sensors that provide complementary information for object detection in autonomous driving vehicles. However, the fusion of sensor data to achieve efficient object detection requires accurate calibration and precise time synchronization between the sensors. While calibration ensures the geometric relationship between the sensors, and time synchronization ensures that data from both sensors corresponds to the same moment in the real world. Even though various methods have been introduced to achieve accurate calibration between LiDAR and camera, time synchronization between sensors remains relatively unexplored. Poor synchronization between sensors caused by incorrect time stamping process significantly affects data fusion. Therefore, in this paper we present sensor time synchronization for LiDAR and camera data fusion approach, especially in autonomous driving vehicles. It also explores various techniques involved in establishing synchronization between LiDAR and camera. Subsequently, we propose a hardware level time synchronization system, including automatic hardware trigger signal delay estimation to precisely match LiDAR and camera trigger time. Furthermore, validation experiments were conducted to assess the accuracy of the proposed synchronization system in various driving scenarios accompanied by detailed experimental analysis.
... Preceding works such as Sundial [4] have also showcased the difficulty in managing fault tolerance for synchronized real-time clocks. For the majority of systems using wall-clock time as their global clock, synchronization implies exchanging timestamps [3,18]. Techniques such as TrueTime [5] and White Rabbit [19] attempt to reduce the latency uncertainty, and thus the time-uncertainty bounds, from milliseconds in TrueTime to sub-nanosecond in White Rabbit. ...
We introduce logical synchrony, a framework that allows distributed computing to be coordinated as tightly as in synchronous systems without the distribution of a global clock or any reference to universal time. We develop a model of events called a logical synchrony network, in which nodes correspond to processors and every node has an associated local clock which generates the events. We construct a measure of logical latency and develop its properties. A further model, called a multiclock network, is then analyzed and shown to be a refinement of the logical synchrony network. We present the bittide mechanism as an instantiation of multiclock networks, and discuss the clock control mechanism that ensures that buffers do not overflow or underflow. Finally we give conditions under which a logical synchrony network has an equivalent synchronous realization
The JT-60SA project succeeded in commissioning test toward the first plasma in 2023 under a framework: the Satellite Tokamak Programme of the Broader Approach Agreement between EU and Japan [1]. JT-60SA is a tokamak type magnetic confinement device with a superconducting magnet system[2] developed and operated at the Quantum Science and Technology institute (QST) in Japan. The magnet system consists of 18 Toroidal Field coils, 6 Equilibrium Field coils, and 4 modules of Central Solenoids, and all the coils are superconducting magnets. In the first commissioning, during 2020 - 2021, JT-60SA has experienced a discharge incident at the EF1 magnet[3]. As a conclusion of the investigation of the incident, it turned out that the insulation resistance of the magnet system was insufficient. Although the insulation was partly reinforced, the voltage holding under accidentally arising Paschen conditions was not improved enough for the recent operation (OP-1, 2023)[4]. To prevent the magnet system from discharging during vacuum degradation, a vacuum monitoring system must be prepared to detect vacuum degradation and ramp down the magnet current immediately in case of unexpected vacuum degradation. In this article, the development of the vacuum monitoring system, operational results, and the future plan of the system will be described.
We are moving toward a distributed, international, twenty-four hour, electronic stock exchange. The exchange will use the global Internet, or internet technology. This system is a natural application of multicast because there are a large number of receivers that should receive the same information simultaneously. The data requirements for the stock exchange are discussed. The current multicast protocols lack the reliability, fairness, and scalability needed in this application. We describe a distributed architecture together with a reliable multicast protocol, a modification of the RMP protocol, that has characteristics appropriate for this application. The architecture is used in three applications: In the first, we construct a unified stock ticker of the transactions that are being conducted on the various physical and electronic exchanges. Our objective is to deliver the the same combined ticker reliably and simultaneously to all receivers, anywhere in the world. In the second, we construct a unified sequence of buy and sell offers that are delivered to a single exchange or a collection of exchanges. Our objective is to give all traders the same fair access to an exchange independent of their relative distances to the exchange or the loss characteristics of the international network. In the third, we construct a distributed, electronic trading floor that can replace the current exchanges. This application uses the innovations from the first two applications to combine their fairness attributes.
We consider the model of communication where wireless devices can either switch their radios off to save energy, or switch their radios on and engage in communication. We distill a clean theoretical formulation of this problem of minimizing radio use and present near-optimal solutions. Our base model ignores issues of communication interference, although we also extend the model to handle this requirement. We assume that nodes intend to communicate periodically, or according to some time-based schedule. Clearly, perfectly synchronized devices could switch their radios on for exactly the minimum periods required by their joint schedules. The main challenge in the deployment of wireless networks is to synchronize the devices' schedules, given that their initial schedules may be offset relative to one another (even if their clocks run at the same speed). We significantly improve previous results, and show optimal use of the radio for two processors and near-optimal use of the radio for synchronization of an arbitrary number of processors. In particular, for two processors we prove deterministically matching upper and lower bounds on the number of times the radio has to be on, where n is the discretized uncertainty period of the clock shift between the two processors. (In contrast, all previous results for two processors are randomized.) For processors (for any ) we prove is the lower bound on the number of times the radio has to be switched on (per processor), and show a nearly matching (in terms of the radio use) randomized upper bound per processor, with failure probability exponentially close to 0. For our algorithm runs with at most radio invocations per processor. Our bounds also hold in a radio-broadcast model where interference must be taken into account.
This manuscript presents novel techniques for identifying the switch states, phase identification, and estimation of equipment parameters in multi-phase low voltage electrical grids, which is a major challenge in long-standing German low voltage grids that lack observability and are heavily impacted by modelling errors. The proposed methods are tailored for systems with a limited number of spatially distributed measuring devices, which measure voltage magnitudes at specific nodes and some line current magnitudes. The overall approach employs a problem decomposition strategy to divide the problem into smaller subproblems, which are addressed independently. The techniques for identifying switch states and system phases are based on heuristics and a binary optimization problem using correlation analysis of the measured time series. The estimation of equipment parameters is achieved through a data-driven regression approach and by an optimization problem, and the identification of cable types is solved using a Mixed-Integer Quadratic Programming solver. To validate the presented methods, a realistic grid is used and the presented techniques are evaluated for their resilience to data quality and time resolution, discussing the limitations of the proposed methods.
In mobile environments where recording devices and subjects are in motion, integrating data collected from multiple devices requires precise location and time information. Given that high-precision satellite positioning technology provides centimeter-level accuracy and that movement speeds in mobile environments are around several 10 m/s, the required time accuracy is under 1 millisecond. However, achieving this time accuracy with commonly used devices is not typically feasible. This paper describes a basic architecture to realize time synchronization with less than one millisecond error with an independent recorder using high-precision timing pulses (1-PPS signal) output by a GNSS receiver. Next, we propose a method to precisely identify the image capture time using an optical beacon combining multiple point light sources called GNSS Clock Beacon (GCB). The time of image capture can be determined from GCB images with an accuracy less than or equal to the exposure duration. Finally, we describe an example implementation of a mobile recording system that can be mounted on a motorcycle, which can record time-synchronized data and video with high accuracy using multiple data loggers and video equipment.
As the number of electronic control units (ECUs) in vehicles continues to grow, exchanging physical components between original equipment manufacturers (OEMs) and ECU suppliers presents logistical challenges, impeding the pace of development and leading to accumulation of costs. In this work, we introduce a conceptual framework that enables remote testing of geographically dispersed ECUs over the Internet. Pertinent to the Internet of Things (IoT) sphere, where interconnection of distributed components is hampered by the inherent challenges of latency, we propose a hybrid synchronous methodology that combines asynchronous test management with time synchronization mechanisms to mitigate the delay impact within the distributed environment. Additionally, we discuss challenges and prospects associated with this approach.
The NIST Automated Computer Time Service (ACTS) is a telephone time service designed to provide computers with telephone access to time generated by the National Institute of Standards and Technology at accuracies approaching 1 ms. Features of the service include automated estimation by the transmitter of the telephone-line delay, advanced alert for changes to and from daylight saving time, and advanced notice of insertion of leap seconds. The ASCII-character time code operates with most standard modems and computer systems. The system can be used to set computer clocks, and simple hardware can also be developed to set noncomputer clock systems.
To a client of a loosely-coupled distributed system, one of the simplest services is a time service. Usually the client simply requests the time from any subset of the time servers making up the service, and uses the first reply. Issues that need to be considered in other services, such as connection establishment or client authentication, need not be considered in a time service. The simplicity of this instruction, however, misrepresents the complexity of implementing such a service.
Packet switching is store-and-forward by nature. Network delay is a therefore a critical performance measure for packet-switching communications. A catenet is a system of packet-switched communication networks interconnected via gateways [Cerf 78]. The catenet "link" delays are thus variable. Their measurement, the measurement of delays across member networks of a catenet, becomes important for catenet investigations. An effective way to measure catenet delays is by means of packet header...
The Fuzzball is an operating system and applications library designed for the PDP11 family of computers. It was intended as a development platform and research pipewrench for the DARPA/NSF Internet, but has occasionally escaped to earn revenue in commercial service. It was designed, implemented and evolved over a seventeen-year era spanning the development of the ARPANET and TCP/IP protocol suites and can today be found at Internet outposts from Hawaii to Italy standing watch for adventurous applications and enduring experiments. This paper describes the Fuzzball and its applications, including a description of its novel congestion avoidance/control and timekeeping mechanisms.
This paper presents a unified mathematical model for studying the behavior of various network synchronization techniques, e.g., plesiochronous or independent clocks, hierarchical master-slave, delay-compensated and uncompensated network models are presented in the form of a set of nonlinear space-time matrix equations in which network interconnection matrices are manifested. Each equation in the set characterizes the diffusion of phase (time) and frequency generated at each network node. The matrices of this model define the topological structure of the network. The time-frequency model is used to characterize the timefrequency stability of the network clock ensemble. Expressions for the steady-state network frequency and the time differences between nodal clocks are derived and compared for plesiochronous, delay compensated and uncompensated networks. A network frequency stability measure is introduced, evaluated and performance comparisons are made for specific network configurations. Finally, we illustrate the steady-state frequency stability achievable when M subnetworks are connected to form a larger network.
The paper contains a proposal and an analysis of a hybrid method for synchronizing the clocks in a digital switching network to a more select group of "master clocks." The scheme is a hybrid of two well-known techniques, "mutual synchronization" and "masterslave," offering certain unique advantages. It allows different synchronizing disciplines for the masters (such as the toll switching machines) and the remaining subnetwork (composed, for example, of local switches). We begin by considering an idealized model containing only one master with constant clock frequency. The behavior of the controlled frequencies in the subnetwork is described by linear delay-differential equations with the delays determined by distances separating switches. We show that in all cases all clocks in the subnetwork approach in steady state the master clock frequency. Considerable emphasis is placed on the rate of synchronization, as determined by the principal root, the root with largest real part, of the characteristic function of the differential equations. The idealized model is extended in stages. First we consider the effects of many masters with constant but possibly nonidentical frequencies and show that the steady state frequencies in the subnetwork are narrowly bounded. Next, we allow clock drift and the master clocks to fluctuate about nominal centers with given tolerances. An expression for the residual steady-state departures from perfect synchronization shows the tradeoff between transient and stead-state behavior. Finally, two illustrative examples are numerically solved.
The generation of a fault-tolerant global time base with known accuracy of synchronization is one of the important operating system functions in a distributed real-time system. Depending on the types and number of tolerated faults, this paper presents upper bounds on the achievable synchronization accuracy for external and internal synchronization in a distributed real-time system. The concept of continuous versus instantaneous synchronization is introduced in order to generate a uniform common time base for local, global, and external time measurements. In the last section, the functions of a VLSI clock synchronization unit, which improves the synchronization accuracy and reduces the CPU load, are described. With this unit, the CPU overhead and the network traffic for clock synchronization in state-of-the-art distributed real-time systems can be reduced to less than 1 percent.
This paper gives two simple efficient distributed algorithms: one for keeping clocks in a network synchronized and one for allowing new processors to join the network with their clocks synchronized. The algorithms tolerate both link and node failures of any type. The algorithm for maintaining synchronization will work for arbitrary networks (rather than just completely connected networks) and tolerates any number of processor or communication link faults as long as the correct processors remain connected by fault-free paths. It thus represents an improvement over other clock synchronization algorithms such as [LM1,LM2,LL1]. Our algorithm for allowing new processors to join requires that more than half the processors be correct, a requirement which is provably necessary.
We are interested in an alternative model, which we call the environmental adaptation programming paradigm. We use the term local node to distinguish the node in which we are principally interested. We consider the neighhours of a local node to be those other nodes which have a direct communications link with the local node. In environmental adaptation programming, a local node monitors the current status of its nelghbours, and then adjusts its own state so as to adapt, or to maintain an equilibrium. It periodically informs its neighbours of its own state, and expects the neighbours to inform the local node of their state. The failure of a nelghbour to inform the local node of its state is in itself information about the state of the neighbour. We also consider environmental adaptation programming an interesting alternative to the usual synchronization methods used in shared memory multiprocessors. Since each node (i.e. processor) only inspects the memory written to by another processor, there is no multiple update problem, and therefore no need for costly spin locks to control memory access. We recognize, however, that not all parallel computing problems are well suited to the environmental adaptation model. The distributed clock synchronization problem. A number of papers ([2], [3], [5], [6], [7]) have discussed the synchronization of clocks over a distributed system. The solutions have been based on the "Byzantine agreement problem" which derives from the ancient problem of the Byzantine Generals. We have chosen to use the clock synchronization problem as a basis for programming experiments to investigate environmental adaptation programming. The Byzantine agreement papers have developed algorithms which guarantee that the clocks will maintain synchronization, provided that less than n/3 nodes fail (in a distributed network with n nodes). These algorithms function by periodically sending messages to one another, and resynchronizlng clocks using information conveyed in these messages. Since in each period, every node must send a message to each of the other nodes in the system, these algorithms required that in ea~ period the total number of messages to be sent ~uld be proportional to n (amounting to n messages per node). Indeed the n may be optimistic for in practical systems each node is directly
To a client of a loosely-coupled distributed system, one of the simplest services is a time service. Usually the client simply requests the time from any subset of the time servers making up the service, and uses the first reply. Issues that need to be considered in other services, such as connection establishment or client authentication, need not be considered in a time service. The simplicity of this interaction, however, misrepresents the complexity of implementing such a service.
This paper describes an experiment to obtain an upper bound on the accuracy that can be expected when attempting to synchronize the system clock of one computer with the system clock of a physically remote computer. We used one computer in the U.K. and a number of computers in the U.S., linked by a complex datagram network, to provide a worst-case environment. This paper gives some reasons for wanting to synchronize clocks and the relationship to previous work. The experimental problems are discussed and the results analyzed.
Algorithms are described for maintaining clock synchrony in a distributed multiprocess system where each process has its own clock. These algorithms work in the presence of arbitrary clock or process failures, including “two-faced clocks” that present different values to different processes. Two of the algorithms require that fewer than one-third of the processes be faulty. A third algorithm works if fewer than half the processes are faulty, but requires digital signatures.
The concept of one event happening before another in a distributed system is examined, and is shown to define a partial ordering of the events. A distributed algorithm is given for synchronizing a system of logical clocks which can be used to totally order the events. The use of the total ordering is illustrated with a method for solving synchronization problems. The algorithm is then specialized for synchronizing physical clocks, and a bound is derived on how far out of synchrony the clocks can become.
Introducción a los conceptos fundamentales de circuitos utilizados en los receptores y transmisores modernos, haciendo énfasis en el análisis y diseño de los circuitos de comunicaciones empleando simulación por computadora.
A probabilistic method is proposed for reading remote clocks in
distributed systems subject to unbounded random communication delays.
The method can achieve clock synchronization precisions superior to
those attainable by previously published clock synchronization
algorithms. The method can be used to improve the precision of both
internal and external synchronization algorithms. The approach is
probabilistic because it does not guarantee that a processor can always
read a remote clock with an a priori specified precision; however, by
retrying a sufficient number of times, a process can read the clock of
another process with a given precision with a probability as close to
one as desired. An important characteristic of the method is that, when
a process succeeds in reading a remote clock, it knows the actual
reading precision achieved. The use of the remote clock reading methods
is illustrated by presenting a time service which maintains externally
(and, hence, internally) synchronized clocks in the presence of process,
communication, and clock failures
With respect to network timing, the introduction of SONET
equipment into the public network offers both problems and
opportunities. Wide-scale deployment of SONET equipment and the use of
SONET line timing for reference distribution will eventually provide
improved timing performance in terms of both wander and jitter. One of
the main motivations for developing the SONET architecture was to
establish standards for interconnecting optical transmission equipment
from different suppliers. In the process of establishing the SONET
standard other (e.g. synchronization) technical issues were addressed.
The article compares the existing (asynchronous) network architecture
with the new (synchronous) architecture in terms of their respective
performance in transporting the clock rates of tributary digital
signals. Of primary interest is the timing quality of a first level
digital signal such as the North American DS1 signal or the ITU-T
(formerly CCITT) El signal. Although the new architecture offers
eventual promise in solving numerous timing and interfacing problems for
such tributary signals, the new equipment must necessarily co-exist and
inter-operate with the existing facilities. Thus, another major topic of
the article concerns the performance of interim networks with mixtures
of asynchronous and synchronous multiplexing
Existing fault-tolerant clock synchronization protocols are shown to result from refining a single clock synchronization paradigm. In that paradigm, a reliable time source periodically issues messages that cause processors to resynchronize their clocks. The reliable time source is approximated by reading all clocks in the system and using a convergence function to compute a fault-tolerant average of the values read. The performance of a clock synchronization algorithm based on the paradigm can be quantified in terms of the two parameters that characterize the behavior of the convergence function used: accuracy and precision.
A specification of the Internet protocol (IP) timestamp option DARPA Network Working Group Rep. RFC-781, SRI Int
- Su
Su, " A specification of the Internet protocol (IP) timestamp option, " DARPA Network Working Group Rep. RFC-781, SRI Int., May 1981. 28, pp. 1245-1259, Aug. 1980.
User datagram protocol DARPA Network Working Group Rep. RFC-768
- J Postel
J. Postel, " User datagram protocol, " DARPA Network Working Group Rep. RFC-768, U.S.C. Inform. Sci. Inst., Aug. 1980.
DARPA Network Working Group Rep. RFC-792
Defense Advanced Research Projects Agency, Internet Control Message
Protocol, DARPA Network Working Group Rep. RFC-792, U.S.C.
Inform. Sci. Inst., Sept. 1981.
Blair Time and Frequency Theory and Fundamentals
- D W Allan
- J E Gray
- H E Machlan
DARPA Network Working Group Rep
Defense Advanced Research Projects Agency, Internet Protocol, DARPA
Network Working Group Rep. RFC-791, U.S.C. Inform. Sci. Inst., Sept.
1981.
Network synchronization of random signals
- W C Lindsay
- A V Kantak
W. C. Lindsay and A. V. Kantak, "Network synchronization of random
signals," IEEE Trans. Commun., vol. COM-28, pp. 1260-1266, Aug.
1980.
On the accuracy and stability of clocks synchronized by the network time protocol in the Internet system
[23] -,
"Internet time synchronization: The network time protocol,"
DARPA Network Working Group Rep. RFC-1129, Univ. Delaware,
Oct. 1989.
[24] -,
"On the accuracy and stability of clocks synchronized by the
network time protocol in the Internet system," ACM Comput. Commwl.
Rev., vol. 20, no. 1, pp. 65-75, Jan. 1990.