Systems and methods for synchronization in asynchronous transport networks

• US 20040062278A1
• Filed: 09/30/2002
• Published: 04/01/2004
• Est. Priority Date: 09/30/2002
• Status: Active Grant

• Current Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Original Assignee: Lucent Technologies, Inc. (Nokia Corporation)
• Inventors: Szurkowski, Edward Stanley, Hadzic, Ilija
• Granted Patent: US 7,372,875 B2
• Time in Patent Office: Days
• US Class Current: 370/503
• CPC Class Codes: H03L 7/085 concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal H03L7/10 takes precedence; circuits for comparing the phase or frequency of two mutually-independent oscillations H03D13/00
• CPC Class Codes: H03L 7/148 said digital means comprising a counter or a divider
• CPC Class Codes: H04J 3/0641 Change of the master or reference, e.g. take-over or failure of the master
• CPC Class Codes: H04J 3/0667 Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays arrangements for monitoring round trip delays in packet switching networks H04L43/0864
• CPC Class Codes: H04J 3/0688 Change of the master or reference, e.g. take-over or failure of the master
• CPC Class Codes: H04L 69/28 Timers or timing mechanisms used in protocols
• CPC Class Codes: H04L 69/329 in the application layer [OSI layer 7]
• CPC Class Codes: H04L 9/40 Network security protocols

Abstract

Techniques for synchronizing the clock of a local telecommunications network connected to a remote clock source through an asynchronous transport network such as an Ethernet metropolitan area transport network. A basic holdover loop for retaining the current reconstructed clock frequency signal receives weighted corrections from an open loop and a network time protocol filter loop. The open loop measures data packet interarrival times on the local network and calculates a first reconstructed clock frequency signal. The network time protocol loop applies network time protocol to generate timestamps over the asynchronous transport network which are used to generate a second reconstructed clock frequency signal. The first and second reconstructed clock frequency signals are combined using dynamically adjusted weight factors and compared with the current reconstructed clock frequency signal to correct the latter which then synchronizes the clock of the local telecommunications network.

Claims

1. A system for synchronizing the clock source of a local telecommunications network connected to a remote clock source through an asynchronous transport network, comprising:

a holdover loop comprising a reference clock, a clock processor for producing a holdover loop signal representing a clock frequency, a comparator for comparing said holdover loop signal representing the clock frequency and a reconstructed signal representing the clock frequency, and means for converting said holdover loop signal representing a clock frequency into said clock frequency

2. The system of claim 1, comprising:

an open loop comprising a reference clock, a packet arrival detector, a packet interarrival time counter, and a first weighting unit, the open loop producing a final open loop signal representing the clock frequency.

3. The system of claim 1, comprising:

a network time protocol loop comprising a network time protocol module, an adder and a second weighting unit, the network time protocol loop producing a final network time protocol loop signal representing the clock frequency.

9. The system of claim 1, in which said holdover loop comprises a first filter.

10. The system of claim 1, in which the holdover loop comprises a digital to analog converter and a voltage-controlled oscillator.

11. The system of claim 1, in which the clock processor comprises means for comparing a signal from the reference clock with an initial holdover loop signal representing the clock frequency to produce said holdover loop signal representing the clock frequency.

12. The system of claim 1 in which the asynchronous transport network is an Ethernet metropolitan area transport network.

4. A system for synchronizing the clock source of a local telecommunications network connected to a remote clock source through an asynchronous transport network, comprising:

an open loop comprising a reference clock, a packet arrival detector, a packet interarrival time counter, and a first weighting unit, the open loop producing a final open loop signal representing the clock frequency.

5. The system of claim 4, comprising:

a network time protocol loop comprising a network time protocol module, an adder and a second weighting unit, the network time protocol loop producing a final network time protocol loop signal representing the clock frequency.

6. The system of claim 5, comprising:

an adder for combining the final open loop signal representing the clock frequency and the final network time protocol loop signal representing the clock frequency to produce a reconstructed signal representing the clock frequency.

13. The system of claim 4, in which said open loop comprises a second filter.

14. The system of claim 4, in which the open loop comprises a decimator.

15. The system of claim 4, in which the open loop comprises a moving average filter.

16. The system of claim 4, in which the packet interarrival time counter generates an initial open loop signal representing the clock frequency.

17. The system of claim 4 in which the asynchronous transport network is an Ethernet metropolitan area transport network.

22. The system of claim 13, in which said second filter is a finite impulse response filter.

7. A system for synchronizing the clock source of a local telecommunications network connected to a remote clock source through an asynchronous transport network, comprising:

a network time protocol loop comprising a network time protocol module, the network time protocol loop producing a network time protocol loop signal representing the clock frequency.

18. The system of claim 7, in which said network time protocol loop comprises a third filter.

19. The system of claim 7, in which said network time protocol module includes means for transmitting, receiving and analyzing timestamps.

20. The system of claim 7, in which said network time protocol module transmits timestamps in-band, transmits timestamps that represent the packet arrival time at the remote clock source, and implicitly derives all other timestamps.

21. The system of claim 7 in which the asynchronous transport network is an Ethernet metropolitan area transport network.

8. A system for synchronizing the clock source of a local telecommunications network connected to a remote clock source through an asynchronous transport network, comprising:

a holdover loop comprising a reference clock, a clock processor for producing a holdover loop signal representing a clock frequency, a comparator for comparing said holdover loop signal representing the clock frequency and a reconstructed signal representing the clock frequency, and means for converting said holdover loop signal representing a clock frequency into said clock frequency;

an open loop comprising a reference clock, a packet arrival detector, a packet interarrival time counter, and a first weighting unit, the open loop producing a final open loop signal representing the clock frequency;

a network time protocol loop comprising a network time protocol module, an adder and a second weighting unit, the network time protocol loop producing a final network time protocol loop signal representing the clock frequency; and

an adder for combining the final open loop signal representing the clock frequency and the final network time protocol loop signal representing the clock frequency to produce said reconstructed signal representing the clock frequency.

23. A method of synchronizing the clock source of a local telecommunications network connected to a remote clock source through an asynchronous transport network, comprising steps of:

providing a holdover loop signal representing a clock frequency and a reconstructed signal representing the clock frequency;

comparing said holdover loop signal representing the clock frequency against said reconstructed signal representing the clock frequency to produce a signal representing the clock frequency;

converting said signal representing the clock frequency into said clock frequency; and

providing said clock frequency to synchronize a clock within said local telecommunications network.

24. The method of claim 23 comprising steps of:

calculating interarrival times of data packets at said local telecommunications network; and

using said interarrival times to generate said reconstructed signal representing the clock frequency.

25. The method of claim 23 comprising steps of:

running a network time protocol over said asynchronous transport network to generate timestamp data;

using said timestamp data and said holdover loop signal representing the clock frequency to calculate a signal representing the clock frequency; and

adding a nominal clock frequency signal to said signal representing the clock frequency, to yield said reconstructed signal representing the clock frequency.

31. The method of claim 23 comprising the step of filtering said signal representing the clock frequency.

32. The method of claim 23 comprising steps of:

comparing a reference clock signal with an initial holdover loop signal representing the clock frequency to produce said holdover loop signal representing the clock frequency.

26. A method of synchronizing the clock source of a local telecommunications network connected to a remote clock source through an asynchronous transport network, comprising steps of:

calculating interarrival times of data packets at said local telecommunications network; and

using said interarrival times to generate a first reconstructed signal representing a clock frequency.

27. The method of claim 26 comprising steps of:

running a network time protocol over said asynchronous transport network to generate timestamp data;

using said timestamp data to calculate a signal representing the clock frequency; and

adding a nominal clock frequency signal to said signal representing the clock frequency, to yield a second reconstructed signal representing the clock frequency.

28. The method of claim 27 comprising steps of:

applying a first weighting factor to said first reconstructed signal representing the clock frequency to yield a final reconstructed open loop signal representing the clock frequency;

applying a second weighting factor to said second reconstructed signal representing the clock frequency to yield a final reconstructed network time protocol loop signal representing the clock frequency;

controlling said first and second weighting factors so that their sum always equals one, so that the first weighting factor begins as equal to one and tends thereafter toward zero, and so that the second weighting factor begins as equal to zero and tends thereafter toward one;

using said final reconstructed open loop signal representing the clock frequency and said final reconstructed network time protocol loop signal representing the clock frequency to calculate a combined reconstructed signal representing the clock frequency;

converting said combined reconstructed signal representing the clock frequency into said clock frequency; and

providing said clock frequency to synchronize a clock within said local telecommunications network.

33. The method of claim 26 comprising the step of filtering said first reconstructed signal representing the clock frequency.

34. The method of claim 26 comprising the step of downsampling said first reconstructed signal representing the clock frequency.

35. The method of claim 26 comprising the step of subjecting said first reconstructed signal representing the clock frequency to moving average filtration.

29. A method of synchronizing the clock source of a local telecommunications network connected to a remote clock source through an asynchronous transport network, comprising steps of:

running a network time protocol over said asynchronous transport network to generate timestamp data;

using said timestamp data to calculate a signal representing the clock frequency; and

adding a nominal clock frequency signal to said signal representing the clock frequency, to yield a reconstructed signal representing the clock frequency.

36. The method of claim 29 comprising the step of filtering said signal representing the clock frequency.

37. The method of claim 29 comprising running said network time protocol over said asynchronous transport network to generate timestamp data by transmitting, receiving and analyzing timestamps.

38. The method of claim 29 comprising running said network time protocol over said asynchronous transport network to generate timestamp data by transmitting timestamps in-band, transmitting timestamps that represent the packet arrival time at the remote clock source, and implicitly deriving all other timestamps.

30. A method of synchronizing the clock source of a local telecommunications network connected to a remote clock source through an asynchronous transport network, comprising steps of:

providing a holdover loop signal representing a clock frequency and a reconstructed signal representing the clock frequency;

comparing said holdover loop signal representing the clock frequency against said reconstructed signal representing the clock frequency to produce said holdover loop signal representing the clock frequency;

calculating interarrival times of data packets at said local telecommunications network;

using said interarrival times to generate a first reconstructed signal representing the clock frequency;

applying a first weighting factor to said first reconstructed signal representing the clock frequency to yield a final reconstructed open loop signal representing the clock frequency;

running a network time protocol over said asynchronous transport network to generate timestamp data;

using said timestamp data and said holdover loop signal representing the clock frequency to calculate a second signal representing the clock frequency;

adding a nominal clock frequency signal to said second reconstructed signal representing the clock frequency;

applying a second weighting factor to said second reconstructed signal representing the clock frequency to yield a final reconstructed network time protocol loop signal representing the clock frequency;

controlling said first and second weighting factors so that their sum always equals one, so that the first weighting factor begins as equal to one and tends thereafter toward zero, and so that the second weighting factor begins as equal to zero and tends thereafter toward one;

using said final reconstructed open loop signal representing the clock frequency and said final reconstructed network time protocol loop signal representing the clock frequency to calculate said reconstructed signal representing the clock frequency;

converting said holdover loop signal representing the clock frequency into said clock frequency; and

providing said clock frequency to synchronize a clock within said local telecommunications network.