EP2893655A1 - Procédés et dispositifs de synchronisation d'horloge - Google Patents

Procédés et dispositifs de synchronisation d'horloge

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Publication number
EP2893655A1
EP2893655A1 EP12758592.5A EP12758592A EP2893655A1 EP 2893655 A1 EP2893655 A1 EP 2893655A1 EP 12758592 A EP12758592 A EP 12758592A EP 2893655 A1 EP2893655 A1 EP 2893655A1
Authority
EP
European Patent Office
Prior art keywords
time
clock
messages
client
delay
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP12758592.5A
Other languages
German (de)
English (en)
Inventor
James Aweya
Nayef AL SINDI
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Emirates Telecommunications Corp
Khalifa University of Science, Technology and Research (KUSTAR)
British Telecommunications PLC
Original Assignee
Emirates Telecommunications Corp
Khalifa University of Science, Technology and Research (KUSTAR)
British Telecommunications PLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Emirates Telecommunications Corp, Khalifa University of Science, Technology and Research (KUSTAR), British Telecommunications PLC filed Critical Emirates Telecommunications Corp
Publication of EP2893655A1 publication Critical patent/EP2893655A1/fr
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/06—Synchronising arrangements
    • H04J3/0635—Clock or time synchronisation in a network
    • H04J3/0638—Clock or time synchronisation among nodes; Internode synchronisation
    • H04J3/0658—Clock or time synchronisation among packet nodes
    • H04J3/0661—Clock or time synchronisation among packet nodes using timestamps
    • H04J3/0667—Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L43/00—Arrangements for monitoring or testing data switching networks
    • H04L43/08—Monitoring or testing based on specific metrics, e.g. QoS, energy consumption or environmental parameters
    • H04L43/0852—Delays
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L43/00—Arrangements for monitoring or testing data switching networks
    • H04L43/10—Active monitoring, e.g. heartbeat, ping or trace-route
    • H04L43/106—Active monitoring, e.g. heartbeat, ping or trace-route using time related information in packets, e.g. by adding timestamps

Definitions

  • the present invention relates to methods and devices for clock synchronization. It is particularly, but not exclusively, concerned with the alignment of slave clocks to a master clock using IEEE 1588 PTP with offset and skew correction.
  • IEEE 1588 PTP [1][2] is now one of the industry accepted protocols for delivering high accuracy time services in the sub-microseconds levels.
  • IEEE 1588 PTP is similar in concept to the Network Time Protocol (NTP) [3]. Both protocols distribute time messages over a packet network to time clients. NTP is ubiquitous and operates at the upper layers of the protocol stack and in its current form provides no better than millisecond levels accuracy.
  • IEEE 1588 PTP is a layer 2 protocol (although it can also be made to operate over higher protocol layers) with hardware timestamping capabilities to provide sub-microsecond accuracy. As a packet-based high accuracy synchronization protocol, IEEE 1588 PTP certainly has a number of advantages over GPS, which until now was the only available solution for high quality time synchronization.
  • GPS based satellite receivers 16 provide sub-100 nanosecond accuracies, and are often used where precision time synchronization is mission critical: in telecommunication, military, and aerospace applications. But improved accuracy comes at a cost. GPS receivers used for telecom networks synchronization have a much higher specification (high quality oscillators, high holdover capability accuracies, etc.) than those in the average portable satellite navigation system, plus they need all the correct interfaces and cabling to communicate with the teiecoms equipment. GPS based systems require outdoor antenna installations to assure a direct view of the sky to receive the low power satellite transmissions, which are not only an added expense but which create an extra burden on the physical infrastructure of the facility. For this reason, GPS is best suited to be used in a central location as the primary reference clock (PRC) for a telecom network with other technologies utilized to distribute synchronization and timing to remote locations.
  • PRC primary reference clock
  • GPS-based solutions also have poor or no reception in indoor and underground
  • GPS is also not suitable for devices with small footprints requiring time synchronization like wireless sensors.
  • Another concern is GPS is perceived outside the USA as an American controlled technology with no service guarantees (GPS is not autonomous), as a result its use as the primary source of timing is not favored by operators in other countries.
  • IEEE 1588 PTP has gained traction in the industry as the protocol of choice for high quality synchronization.
  • Figure 1 shows a general view of some of the devices that require time synchronization in a packet network 12.
  • the time transfer protocol could be NTP or IEEE 1588 PTP, but the latter is the protocol of choice for telecom networks.
  • Synchronization is a key requirement in the telecommunications industry and this market has become a driving force in the development and evolution of synchronization solutions and standards.
  • wireless technologies like GSM, WCDMA (both frequency division duplexing (FDD) and time division duplexing (TDD) technologies
  • CD A2000 require frequency accuracy of 0.05 ppm (parts per million) at the air interface.
  • CDMA2000 requires time synchronization at the ⁇ 3 ps level ( ⁇ 10 ps worst case) and WCDMA TTD mode requires accuracy of ⁇ 1 .25 ps between base stations.
  • the accurate reference clock is typically derived from Time Division Multiplexing (TDM) interfaces (for GSM, WCDMA and other FDD based technologies) or from expensive GPS receivers located at the base station (for TDD based technologies).
  • TDM Time Division Multiplexing
  • the high level of synchronization provided by IEEE 1588 PTP could also be useful in time-of-arrival (TOA) and time-difference-of-arriva! (TDOA) based localization solution where one is interested in locating mobile nodes connected to a wireless network.
  • A/V Residential Audio/Video (A/V) Networking: Modern consumer electronics now gather, store, and transmit audio and video data in digital form and require adequate quality of service and synchronization for live streaming.
  • the 802.1 AS protocol which is based on IEEE 1588, provides timing and synchronization within bridged home A/V Ethernet LANs. The protocol allows high level timing information to be distributed to each media playback device, thus, providing them with a precise reference point for streaming multimedia. This synchronization technology, thus, enables a convergence to Ethernet as a viable consumer electronics interconnect.
  • IEEE 1588 PTP was originally designed to satisfy the need for synchronization in test and measurement systems but has now become beneficial in other areas. The challenge was to synchronize networked measuring devices with each other in terms of time so that they are able to record measured values and provide them with a precise system timestamp. Based on this timestamp, the measured values can then be correlated with each other.
  • WSNs have a wide range of applications including the monitoring of the status of industrial processes and equipment, environmental monitoring, infrastructure monitoring, surveillance, tracking, etc. But the efficient operation of a WSN depends on how well the various nodes are time synchronized. Time synchronization allows for the coordination among the nodes for power saving sleep/wake up modes, localization of sensor nodes and other sources, data fusion, object tracking, transmission scheduling among the nodes, and distributed communication and processing of data.
  • Billing services Billing services and similar applications must know the time accurately. Time based billing as in telecom networks rely on time synchronization. Mobile user billing and maintenance functions also rely on precise timing references.
  • Log file accuracy, auditing, and monitoring Tracking security breaches, network usage, or problems affecting a large number of components can be nearly impossible if timestamps in logs are inaccurate.
  • ® Network fault diagnosis and recovery Every aspect of managing, securing, planning, and debugging a network involves determining when events happen. Without time synchronization, it can be difficult or impossible to correlate the time sequence of events involving multiple network components, such as outages, bugs, security breaches, service problems, or network problems. Precise timestamping of data events through each element in the backhaul network facilitates the isolation and traceability of failures and outages.
  • File timestamps To reduce confusion in shared filesystems, it is crucial for the modification times to be consistent, regardless of what machine the filesystems are on. Sorting email and other network communications can also be difficult if timestamps are incorrect.
  • Access security and authentication (e.g., erberos): Many security and authentication protocols require accurate time synchronization. Applications such as cyptographic key management and secure document transmission may require using accurate, encoded timestamps which match unencoded time stamps to help assure document authenticity.
  • Directory services e.g., Active Directory, etc.
  • Scheduled operations e.g., cron jobs, network backups
  • the clock offset at a particular moment is the difference between the time reported by the time client (slave) and the "true” time as reported by the time server (master).
  • Clock Skew A clock's skew at a particular moment is the frequency difference (first derivative of its offset with respect to true time) between the client clock and the server clock.
  • Clock synchronization has received considerable attention over the last several years as the communication networks evolve from circuit-switched to ail-IP packet based networks. With this migration the challenge of frequency and time synchronization has surfaced.
  • the techniques in the state of the art differ by the assumed model and the estimated parameters.
  • the first group assumes that the two clocks differ by an offset.
  • the algorithms and techniques attempt to estimate only the offset between the clocks.
  • the reality is far from this model, however, and as a result the second group adopts a more realistic model where the clocks differ by an offset and a skew.
  • the skew is assumed constant in the duration of the estimation process, in order to achieve robust and accurate synchronization, advanced algorithms are needed that estimate the offset and skew simultaneously.
  • [8] proposes a median line-fitting technique which is a robust line-fitting technique.
  • the problem with line-regression algorithms is that they are usually not robust to presence of large outliers and thus the robustness is only valid for certain PDV models (e.g. Gaussian).
  • a simple skew and offset estimation method is proposed where timestamps are used to compute the average jitter and average inter-packet arrival time. Then the relative skew is computed as the ratio of the average jitter to the average inter-packet time.
  • IEEE 1588 PTP was defined [1 ][2] to synchronize distributed clocks across Ethernet and other packet based networks. It allows for synchronization of distributed clocks to sub- microsecond accuracy. IEEE 1588 PTP was designed as an improvement to current time
  • NTP network time protocol
  • IEEE 1 588 PTP which is now the industry accepted standard for synchronization, grew out of the need for greater accuracy synchronization over so packet networks, particularly Ethernet.
  • IEEE 1588 PTP relies on the use of hardware timestamped messages to synchronize one or more slave clocks (time client) to a master clock (time server). This process involves a message transaction between the master and slave where the precise moments of transmit /
  • Accurate time information is distributed hierarchically, with a grandmaster clock at the root of the hierarchy.
  • the grandmaster provides the time reference for one or more slave devices.
  • These slave devices can, in turn, act as master devices for further hierarchical layers of slave devices.
  • IEEE 1588 PTP also defines the descriptors that characterize a clock, the states of a clock and the allowed state transitions.
  • the standard defines network messages, fields and semantics, the datasets maintained by each clock and the actions and timing for all IEEE 1588 network and internal events.
  • the standard describes a suite of messages used for monitoring the system, specifications for an Ethernet-based implementation and conformance requirements and some implementation suggestions.
  • IEEE 1588 PTP relies on the transfer of PTP messages to determine clock and system properties and to convey time information. A delay measurement process is used to determine path delays, which are then accounted for in the adjustment of local clocks.
  • a master/slave hierarchy is created using the Best Master Clock (BMC) algorithm [1 ] to determine which clock has the highest quality clock (grandmaster clock) within the network.
  • BMC Best Master Clock
  • the BMC algorithm is then run continuously to allow clocks to adjust quickly to changes in network configuration and status. If the grandmaster clock is removed from the network or is determined by the BMC algorithm to no longer be the highest quality clock, the algorithm then redefines what the new grandmaster clock is and all other clocks are adjusted accordingly.
  • FIG. 2 shows the message flow process for a strictly peer-to-peer message transaction scenario. This figure illustrates the case where the master clock in a time server 10 is directly attached (or peered) to a slave clock in a time client 4 over a packet network 12. The slave clock derives its timing from the upstream master clock and then acts as a master clock for further downstream devices.
  • the main time synchronization related message types for this exchange involve the Sync, Foilow_Up, Deiay-Req, and Delay__Resp messages.
  • Other more complex message flow process where message traverse intermediate nodes are described in the IEEE 1588 PTP standard.
  • the master sends a Sync message to the slave and notes the time, ⁇ , at which it was sent.
  • the slave receives the Sync message and notes the time of reception, T 2 .
  • the master conveys to the slave the timestamp J ' , by either embedding the timestamp Ij in the
  • Sync message i.e., one-step clock mode which requires some sort of hardware processing for embedding the timestamp on-the-fly for highest accuracy and precision
  • embedding the timestamp ⁇ ] in a Follow_Up message i.e., two-step clock mode.
  • the slave sends a Delay ... Req message to the master and notes the time, ⁇ , at which it was sent.
  • the master receives the Deiay_Req message and notes the time of reception, 7 ' 4 .
  • the master conveys to the slave the timestamp T 4 by embedding it in a Deiay_Resp message.
  • the use of FollowJJp messages eliminates the need to timestamp transmitted messages on the fly, thereby facilitating a simpler hardware implementation.
  • the slave After this message exchange the slave will have four timestamps ⁇ , , T 2 , 1 , T 4 ] from which it can determined both the network delay, d , (the time taken for messages to traverse the network link between the two nodes) and the slave offset, ⁇ , (time offset by which the slave clock leads or lags the master). Messages containing current time information are adjusted to account for their path delay, therefore providing a more accurate representation of the lime information conveyed. Under the assumption that the delays for the two paths are symmetric (the delay in one direction is the same as the delay in the opposite direction), the following relationships can be derived (see Figure 2):
  • the slave computes the fixed delay d and clock offset ⁇ , as follows:
  • the clock offset 0 can be used to align the local clock to the master's.
  • a key assumption here is that the message exchanges occur over a period of time so small that the offset ⁇ can be assumed constant over that period.
  • the accuracy of this link delay- measurement depends on both the symmetry of the one-way link delays and the accuracy of the timestamping process.
  • a complete IEEE 1588-based solution at a time client includes servo algorithms, filters, PTP- Clock based on hardware timer and direct timer access.
  • IEEE 1588 defines a wide range of synchronization capabilities except the clock synchronization mechanisms (servo algorithm, PLL, timers, etc.) to be used at the receiver (slave) to synchronize its local clock to the master.
  • Methods of clock adjustment implementation are not specified by IEEE 1588 - it only provides a standard protocol for the exchange of messages between clocks. The benefit of not specifying clock adjustment implementations, is to allow clocks from different manufactures to be able to synchronize with each other as long as they understand the messaging protocol. There are a number of factors that can cause two supposedly identical clocks to drift apart or lose synchronization.
  • An object of the present invention is to achieve accurate and robust synchronization, preferably over IEEE 1588, for critical applications that require stringent synchronization margins.
  • An exemplary aspect of the present invention provides a method of synchronizing a local dock in a time client to a master clock in a time server, the method including the steps of: transmitting messages carrying tirnestamps from the time server and from the time client; receiving the messages from the time server at the time client and extracting tirnestamps from said messages; receiving the messages from the time client at the time server and extracting tirnestamps from said messages; estimating the skew and offset of the local clock compared to the master clock; and adjusting the output of the local clock using said estimated skew and offset, wherein the step of estimating the skew and offset includes the sub-steps of: estimating the clock skew from the extracted tirnestamps over a predetermined observation period; calculating a plurality of delay observations for each message exchange between the time server and the time client in the observation period; determining, from said series of delay observations, one or more representative delay values for the observation window; and estimating the clock offset from the selected representative delay values.
  • a further exemplary aspect of the present invention provides a networked time system including a time server and at least one time client connected to the time server over a network, wherein: the time server includes a master ciock and a server control unit and transmits messages carrying timestamps from the master ciock; the time client includes a local clock and a client control unit and transmits messages carrying timestamps from the local clock, wherein: the server control unit is arranged to receive the messages from the time client and to extract the timestamps from said messages; and the client control unit is arranged to: receive the messages from the time server and to extract the timestamps from said messages; estimate the skew and offset of the locai clock compared to the master dock; and adjust the output of the local ciock using said estimated skew and offset, the client control unit being arranged to estimate the skew and offset of the local ciock by: estimating the clock skew from the extracted timestamps over a predetermined observation period; calculating a plurality
  • the time client comprising: a local dock; and a control unit, wherein the control unit is arranged to: receive messages carrying timestamps from the time server and to extract the timestamps from said messages; estimate the skew and offset of the local clock compared to the master ciock; and adjust the output of the local clock using said estimated skew and offset, the control unit being arranged to estimate the skew and offset of the local ciock by: estimating the ciock skew from the extracted timestamps over a predetermined observation period; calculating a plurality of delay observations for each message exchange between the time server and the time client in the observation period; determining, from said series of delay observations, one or more representative dela values for the observation window; and estimating the clock offset from the selected representative delay values.
  • Figure 1 shows a general view of some of the devices that require time synchronisation in a packet network and has already been described
  • Figure 2 shows a genera! overview of synchronization with IEEE 1588 PTP using a series of message transactions between a master and its siave(s) and has already been described;
  • Figures 3a and 3b illustrate two variants of the common linear clock models used in clock synchronization analysis
  • Figure 4 shows the extension of the basic IEEE 1588 PTP protocol message exchange timing diagram to cover the case where clock offset and skew exists between the client and the server;
  • Figure 5 shows a clock model derived from a combination of Figures 3 and 4;
  • Figure 6 shows the architecture of a time client according to an embodiment of the present invention with a local clock and an estimation algorithm according to a further embodiment of the present invention.
  • Figure 7 shows the adjustments made to a free-running local clock to produce a
  • a first aspect of the present invention provides a method of synchronizing a local clock to a master clock by estimating the skew and offset of the local clock compared to the master clock and adjusting the output of the local clock accordingly,
  • a first aspect of the present invention preferably provides a method of synchronizing a local clock in a time client to a master clock in a time server, the method including the steps of: transmitting messages carrying timestamps from the time server and from the time client; receiving the messages from the time server at the time client and extracting timestamps from said messages; receiving the messages from the time client at the time server and extracting timestamps from said messages; estimating the skew and offset of the local dock compared to the master clock; and adjusting the output of the local clock using said estimated skew and offset, wherein the step of estimating the skew and offset includes the sub-steps of: estimating the clock skew from the extracted timestamps over a predetermined observation period; calculating a plurality of delay observations for each message exchange between the time server and the time client in the observation period; determining, from said series of delay observations, one or more representative delay values for the observation window; and estimating the clock offset from the selected representative dela values.
  • the messages are IEEE 1588 PTP messages. This allows the method to work using the timing protocol messages that are already exchanged in known format between a time server and a time client.
  • the messages transmitted from the time server may include IEEE 1588 Sync messages
  • the messages transmitted from the time client may include IEEE 1588 Deiay_Req messages. This allows the time server and time client to determine the delays in arrival of the messages in each direction and, on the assumption that the transmission delays in each direction are the same, estimate the effects of skew and offset of the local clock.
  • the estimation of the clock skew a is calculated as:
  • T 1IL are the timestamps applied by the time server to respectively the first and last Sync messages in the observation window
  • T S] 1 and T 2 are the times of receipt as recorded by the local clock on receipt of respectively the first and last Sync messages in the observation window
  • T 3 and T 3 are the timestamps applied by the time client to the first and last Delayford. Req messages in the observation window
  • T 4 and T 4 are the times of receipt as recorded by the master clock on receipt of respectively the first and last
  • Deiay_Req messages in the observation window are, over the observation window defined by ! ⁇ / ⁇ ! :
  • a is the estimated clock skew
  • ⁇ ⁇ is the timestamp applied by the time server to the rth Sync message in the observation window
  • T ? is the time of receipt as recorded by the local clock on receipt of the ih Sync message
  • T 3 is the timestamp applied by the time client to the rth Deiay___Req message
  • T 4 is the time of receipt as recorded by the master clock on receipt of the rth Delalinger.
  • the clock offset may be estimated as: ⁇ -— s ! .
  • the method of this aspect preferably further includes repeatedly performing the steps of transmitting, receiving, estimating and adjusting on a periodic basis, for example every time a particular group of message exchanges (such as the Sync, Follow Up, Delay Req and Delay_Resp message exchange) is completed, or after a predetermined number of such message exchanges.
  • a particular group of message exchanges such as the Sync, Follow Up, Delay Req and Delay_Resp message exchange
  • a second aspect of the present invention provides a networked time system in which a time ciient can estimate the skew and offset of a local clock compared to a master clock in a time server and adjust the output of a local clock using that skew and offset.
  • a second aspect of the present invention preferably provides a networked time system including a time server and at least one time client connected to the time server over
  • the time server includes a master clock and a server control unit and transmits messages carrying tirnestamps from the master clock
  • the time ciient includes a local clock and a ciient control unit and transmits messages carrying tirnestamps from the local clock
  • the server control unit is arranged to receive the messages from the time client and to extract the tirnestamps from said messages
  • the client control unit is
  • the client control unit being arranged to estimate the skew and offset of the local clock by: estimating the clock skew from the extracted tirnestamps over a predetermined observation period;
  • the messages are IEEE 1588 PTP messages. This allows the networked time system to use the timing protocol messages that are already exchanged in known format between a time server and a time client.
  • the messages transmitted from the time server may include IEEE 1588 Sync messages
  • the messages transmitted from the time client may include IEEE 1588 Delay_Req messages. This allows the time server and time client to determine the delays in arrival of the messages in each direction and, on the assumption that the transmission delays in each direction are the same, estimate the effects of skew and offset of the local clock. in one embodiment the estimation of the clock skew a is calculated as:
  • T 1 and T L are the timesfamps applied by the time server to respectively the first and last Sync messages in the observation window;
  • T 2i 1 and T ZL are the times of receipt as recorded by the local clock on receipt of respectively the first and last Sync messages in the observation window;
  • T 3i 1 and T 3,L are the timestamps applied by the time client to the first and last Delay_Req messages in the observation window;
  • T 4 and T 4 are the times of receipt as recorded by the master clock on receipt of respectively the first and last
  • ⁇ is the esiimated clock skew
  • ⁇ ⁇ is the timestamp applied by the time server to the fth Sync message in the observation window
  • T 2 j is the time of receipt as recorded by the local clock on receipt of the Hh Sync message
  • T 3ii is the timestamp applied by the time client to the ;lh Delay_Req message
  • T 4:i is the time of receipt as recorded by the master clock on receipt of the it Delay ... Req message.
  • the control unit may select the representative delay values as the minimum values of X i and I, from within the observation window, i.e. X se , - mini: , and Y s - min i? .
  • the client control unit preferably adjusts the output of the local clock on a periodic basis.
  • message exchanges are performed regularly between server and client.
  • the server control unit and the client control unit preferably repeatedly carry out the steps of transmitting, receiving, estimating and adjusting on a periodic basis, for example every time a particular group of message exchanges (such as the Sync, Fol!owJJp, Delay ,. Req and Delay ... Resp message exchange) is completed, or after a predetermined number of such message exchanges.
  • a particular group of message exchanges such as the Sync, Fol!owJJp, Delay ,. Req and Delay ... Resp message exchange
  • the local clock includes a free-running oscillator which supplies pulses to a free- running counter.
  • Embodiments of this second aspect may include some, ail or none of the above described optional or preferred features.
  • a third aspect of the present invention provides a time client having a local clock which is arranged to estimate the offset and skew of that clock compared to a master clock in a time server and to adjust the output of the local clock using the estimated offset and skew.
  • a third aspect of the present invention preferably provides a time client communicabiy coupled to a time server having a master clock over a network, the time client comprising: a local clock; and a control unit, wherein the control unit is arranged to: receive messages carrying timestamps from the time server and to extract the timestamps from said messages; estimate the skew and offset of the local clock compared to the master clock; and adjust the output of the local clock using said estimated skew and offset, the control unit being arranged to estimate the skew and offset of the local clock by: estimating the clock skew from the extracted timestamps over a predetermined observation period; calculating a plurality of delay observations for each message exchange between the time server and the time client in the observation period; determining, from said series of delay observations, one or more representative delay values for the observation window; and estimating the clock offset from the selected representative delay values.
  • the messages are IEEE 1588 PTP messages.
  • the time client may use the timing protocol messages that are already exchanged in known format between a time server and a time client.
  • the messages transmitted from the time server may include IEEE 1588 Sync messages, and the messages transmitted from the time client may include IEEE 1588 Delay Req messages. This allows the time server and time client to determine the delays in arrival of the messages in each direction and, on the assumption that the transmission delays in each direction are the same, estimate the effects of skew and offset of the local dock.
  • the estimation of the dock skew a is calculated as:
  • T 1 and T are the timestamps applied by the time server to respectively the first and last Sync messages in the observation window: T 2i 1 and T 2iL are the times of receipt as recorded by the local clock on receipt of respectively the first and last Sync messages in the observation window; T 3 and T J:L are the timestamps applied by the time client to the first and last Delay_Req messages in the observation window; and T 4 and T 4:L are the times of receipt as recorded by the master clock on receipt of respectively the first and last
  • the delay observations are, over the observation window defined by and ! ' 4"
  • ⁇ is the estimated clock skew
  • T 1tl is the timestamp applied by the time server to the fth Sync message in the observation window
  • T 2 j is the time of receipt as recorded by the local clock on receipt of the in Sync message
  • T 3ii is the timestamp applied by the time client to the ;lh Delay__Req message
  • T 4:i is the time of receipt as recorded by the master clock on receipt of the it Delay salt.
  • the control unit may select the representative delay values as the minimum values of X i and I, from within the observation window, i.e. X se , - min i: , and Y s - min i? .
  • the control unit preferably adjusts the output of the local clock on a periodic basis.
  • the control unit preferably repeatedly carries out the steps of transmitting, receiving, estimating and adjusting on a periodic basis, for example every time a particular group of message exchanges (such as the Sync, Foilow JJp, Delay Fleq and Deiay . choir Resp message exchange) is completed, or after a predetermined number of such message exchanges.
  • the local clock includes a free-running oscillator which supplies pulses to a free- running counter.
  • Embodiments of this third aspect may include some, all or none of the above described optional or preferred features.
  • Figures 3a and 3b illustrate two variants of the common linear clock models used in clock synchronization analysis.
  • a is a very small number expressed in parts per million (ppm) or parts per billion (ppb).
  • the fractional part of ⁇ i.e. a-1
  • ppm parts per million
  • ppb parts per billion
  • Figure 4 extends the basic IEEE 1588 PTP protocol message exchange timing diagram (e.g. as illustrated in Figure 2 and described above) to cover the case where clock offset and skew exists between the client and the server.
  • Figure 4 highlights the various contributions the clock skew makes on the clock offset and system delay as shown in the lower diagram compared to the upper diagram in which the dock demonstrates offset only.
  • the two figures in Figure 4 become identical when there is zero skew between the clocks.
  • a free-running local clock is used at the client 14.
  • the free running clock is used for timestamping and for synthesizing an image of the server clock (synchronized local clock) for the time client ( Figure 6).
  • the timestamps indicated in Figure 5 at the client are with respect to this local clock. Timestamps at the client are captured according to the client's free running local oscillator as depicted in Figure 6.
  • ⁇ (0 R - al , ) denotes the initial reference offset that has to be estimated so that the client can align its clock to that of the server.
  • T L T 4L ⁇ T, + a(T 4L -r 4>1 )
  • FIG. 6 shows a time client according to an embodiment of the present invention and which, when connected to a time server having a master clock over a network (particularly a packet network) forms a networked time system according to a further embodiment of the present invention.
  • the time client is shown in schematic form with the main functional blocks of the synchronization technique according to a further embodiment of the present invention.
  • Local dock 81 is made up from a local free-running oscillator 80 which feeds pulses at a rate inversely proportional to the frequency of the oscillator to a free-running counter 82 which produces the local clock time.
  • the control unit 70 carries out a series of functions in conjunction with PIP messages to/from a time server (not shown) over a network (not shown). Firstly, timestamps from the local clock 81 are applied to outgoing PTP messages and timestamps from the master dock in the time server are extracted from incoming PTP messages by the timestamp detection & generation function 72.
  • the free running local oscillator is used together with the estimated clock parameters to synthesize a synchronized local clock which is an estimated image of the server clock.
  • a synchronized local clock which is an estimated image of the server clock.
  • the estimated clock skew and offset can then be used by the client to align its clock to the server's as illustrated in Figure 7.
  • Figure 7 shows how the local time C(t) produced by the local free running oscillator 80 and
  • the free-running counter 82 is adjusted by the skew adjustment factor ⁇ 0 and the
  • computer system includes the hardware, software and data storage devices for embodying a system or carrying out a method according to the above described
  • a computer system may comprise a central processing unit (CPU), input means, output means and data storage.
  • the data storage may comprise RAM, disk drives or other computer readable media.
  • the computer system may include a plurality of computing devices connected by a network and able to communicate with each other over that network.
  • the methods of the above embodiments may be provided as one or more computer programs or as computer program products or computer readable media carrying a computer program which is arranged, when run on a computer, to perform the method(s) described above.
  • computer readable media includes, without limitation, any medium or media which can be read and accessed directly by a computer or computer system.
  • the media can include, but are not limited to, magnetic storage media such as floppy discs, hard disc storage media and magnetic tape; optical storage media such as optical discs or CD-ROMs; electrical storage media such as memory, including RAM, ROM and flash memory; and hybrids and combinations of the above such as magnetic/optical storage media.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)
  • Electric Clocks (AREA)

Abstract

Cette invention porte sur des procédés et des dispositifs de synchronisation d'horloge. L'invention utilise en particulier la norme IEEE 1588 avec correction de décalage et de biais. Selon des modes de réalisation de l'invention, le protocole de précision temporelle (PTP) IEEE 1588 est utilisé pour échanger des estampilles temporelles entre un serveur de temps et un client, à partir desquelles le client peut estimer le décalage et le biais d'horloge. Selon des modes de réalisation de l'invention, une horloge non asservie au niveau du client est dotée d'une technique d'estimation basée sur les estampilles temporelles issues de l'échange de messages PTP IEEE 1588 entre les horloges du serveur et du client. Le décalage et le biais issus du processus d'estimation peuvent être combinés avec l'horloge non asservie locale afin d'obtenir une horloge locale synchronisée qui est une image précise de l'horloge maître.
EP12758592.5A 2012-09-04 2012-09-04 Procédés et dispositifs de synchronisation d'horloge Withdrawn EP2893655A1 (fr)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9749717B2 (en) 2015-01-30 2017-08-29 Scientific Drilling International, Inc. Collaborative telemetry
TWI618432B (zh) 2016-02-01 2018-03-11 財團法人資訊工業策進會 頻率校正裝置及方法
CN109005557B (zh) * 2018-09-26 2021-12-07 中兴通讯股份有限公司 一种时延对称性测量方法、装置和系统
JP7439474B2 (ja) * 2019-11-25 2024-02-28 富士電機株式会社 プログラマブルコントローラシステムおよびモジュール
CN111835449A (zh) * 2020-07-30 2020-10-27 山东超越数控电子股份有限公司 一种基于ptp协议的局域网高精度时钟同步方法及装置
CN114079525A (zh) * 2020-08-19 2022-02-22 华为技术有限公司 时钟同步方法和装置
CN114157377B (zh) * 2021-11-30 2023-06-27 广东电网有限责任公司江门供电局 一种配电终端实时时钟同步方法、同步系统及配电终端
CN114237020B (zh) * 2021-12-10 2023-09-26 合肥兆芯电子有限公司 计时器校准方法与电子装置
CN115134034B (zh) * 2022-07-06 2023-07-25 中国人民解放军国防科技大学 一种云边端虚实结合仿真时间同步方法及系统

Family Cites Families (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6327274B1 (en) 1998-09-15 2001-12-04 Nokia Telecommunications, Inc. Method for estimating relative skew between clocks in packet networks
US6661810B1 (en) 1999-03-19 2003-12-09 Verizon Laboratories Inc. Clock skew estimation and removal
US6957357B2 (en) 2000-10-30 2005-10-18 International Business Machines Corporation Clock synchronization with removal of clock skews through network measurements in derivation of a convext hull
US7051246B2 (en) 2003-01-15 2006-05-23 Lucent Technologies Inc. Method for estimating clock skew within a communications network
US7688865B2 (en) 2005-09-09 2010-03-30 International Business Machines Corporation Method and system for clock skew and offset estimation
US8275087B2 (en) * 2008-12-19 2012-09-25 Alcatel Lucent Frequency synchronization using first and second frequency error estimators

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO2014037684A1 *

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