Method and apparatus for probabilistic clock synchronization with interval arithmetic

• US 5,535,217 A
• Filed: 12/20/1994
• Issued: 07/09/1996
• Est. Priority Date: 12/20/1994
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Ng, Kenneth K. W., Cheung, Yiu M., Strong, Hovey R. Jr.
• Primary Examiner(s): Kizou, Hassan
• Application Number:
  US08/363,421
• Time in Patent Office: 567 Days
• Field of Search: 370/100.1, 370/103, 375/354, 375/356, 375/358, 395/200.19, 395/550, 340/825.14, 340/825.2, 340/825.21
• US Class Current: 709/248
• CPC Class Codes: G06F 1/14 Time supervision arrangements, e.g. real time clock
• 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

Abstract

A probabilistic dock synchronization scheme for synchronization of time docks between nodes on a communication network is disclosed, in which a round trip exchange of messages is used to establish that one time according to a first time scale falls between two times according to a second time scale. A time related to the two second time scale times, preferably midway between the two times, is used for synchronizing with the time according to the first time scale. Each time is given in terms of a time value and a plus-or-minus precision range, thereby defining an interval. Enhanced precision is achieved by computing a new precision range for the synchronized time based on an intersection between the intervals of the related time and the time according to the first time scale.

Claims

1. For use with a system including a node coupled to a communication network, the node maintaining an internal time representation, a method for synchronizing the internal time representation with a remote time representation maintained elsewhere in the system, each of the internal and remote time representations including an instantaneous time and a precision range about the instantaneous time, the method comprising the steps, executed by the node, of:

identifying a first time for the internal time representation of the node and a second time for the remote time representation, the precision range of the first and second times forming respective first and second time intervals, the synchronization to be performed based on the first and second time intervals; and

synchronizing the internal time representation, in terms of a new instantaneous time and a new precision range thereabout, the precision range thereabout being based on an intersection between the first and second time intervals.

2. A method as recited in claim 1, wherein:

the remote time representation is maintained by a remote time source elsewhere in the system; and

the step of identifying includes exchanging a predetermined sequence of messages between the node and the remote time source, the sequence of messages including a message bearing a time stamp from the remote time source.

3. A method as recited in claim 2, wherein the step of identifying further includes:

receiving the message beating the time stamp; and

determining the second time interval based on the time stamp.

4. A method as recited in claim 2, wherein the step of exchanging includes exchanging messages according to a probabilistic clock synchronization protocol.

5. A method as recited in claim 2, wherein the step of synchronizing includes:

identifying an intersection pattern between the first and second time intervals from a predetermined set of intersection patterns; and

synchronizing based on the intersection pattern.

6. A method as recited in claim 5, wherein:

the step of synchronizing based on the intersection pattern includes calculating a new instantaneous time and precision range in terms of the time values and precision ranges of the intervals of the messages exchanged in the step of exchanging, from one of a predetermined number of respective formulas, the formulas corresponding with the set of intersection patterns.

7. A method as recited in claim 1, further comprising the steps of:

determining a precision of synchronization which can be realized by synchronizing based on the identified first and second times;

comparing the precision determined in the step of determining with a predetermined threshold precision; and

,

if the precision determined in the step of determining is less precise than the predetermined threshold precision, then repeating the step of identifying, in order to identify a new first time for the internal time representation of the node and a new second time for the remote time representation, the precision range of the new first and new second times forming respective new first and new second time intervals, the synchronization to be performed based on the new first and new second time intervals.

8. A system comprising:

a communication network; and

a node coupled to the communication network, the node maintaining an internal time representation, the node including;

means for synchronizing the internal time representation with a remote time representation maintained elsewhere in the system, each of the internal and remote time representations including an instantaneous time value and a precision range about the instantaneous time, the means for synchronizing including;

means for identifying a first time for the internal time representation of the node and a second time for the remote time representation, the precision range of the first and second times forming respective first and second time intervals, the synchronization to be performed based on the first and second time intervals; and

means for synchronizing the internal time representation, in terms of a new instantaneous time and a new precision range thereabout, the precision range thereabout being based on an intersection between the first and second time intervals.

9. A system as recited in claim 8, further comprising:

a remote time source for maintaining the remote time representation; and

wherein

the means for identifying includes means for exchanging a predetermined sequence of messages between the node and the remote time source, the sequence of messages including a message bearing a time stamp from the remote time source.

10. A system as recited in claim 9, wherein the means for identifying further includes:

means for receiving the message bearing the time stamp; and

means for determining the second time interval based on the time stamp.

11. A system as recited in claim 9, wherein the means for exchanging includes means for exchanging messages according to a probabilistic clock synchronization protocol.

12. A system as recited in claim 9, wherein the means for synchronizing includes:

means for identifying an intersection pattern between the first and second time intervals from a predetermined set of intersection patterns; and

means for synchronizing based on the intersection pattern.

13. A system as recited in claim 12, wherein:

the means for synchronizing based on the intersection pattern includes means for calculating a new instantaneous time and precision range in terms of the time values and precision ranges of the intervals of the messages exchanged by the means for exchanging, from one of a predetermined number of respective formulas, the formulas corresponding with the set of intersection patterns.

14. A system as recited in claim 8, further comprising:

means for determining a precision of synchronization which can be realized by synchronizing based on the identified first and second times;

means for comparing the precision determined by the means for determining with a predetermined threshold precision; and

,

means, operable if the precision determined by the means for determining is less precise than the predetermined threshold precision, for repeating operation of the means for identifying, in order to identify a new first time for the internal time representation of the node and a new second time for the remote time representation, the precision range of the new first and new second times forming respective new first and new second time intervals, the synchronization to be performed based on the new first and new second time intervals.

15. A node, for use with a system including a communication network and for coupling thereto, the node maintaining an internal time representation, the node comprising:

means for synchronizing the internal time representation with a remote time representation maintained elsewhere in the system, each of the internal and remote time representations including an instantaneous time and a precision range about the instantaneous time, the means for synchronizing including;

means for identifying a first time for the internal time representation of the node and a second time for the remote time representation, the precision range of the first and second times forming respective first and second time intervals, the synchronization to be performed based on the first and second time intervals; and

means for synchronizing the internal time representation, in terms of a new instantaneous time and a new precision range thereabout, the precision range thereabout being based on an intersection between the first and second time intervals.

16. A node as recited in claim 15, wherein:

the remote time representation is maintained by a remote time source elsewhere in the system; and

the means for identifying includes means for exchanging a predetermined sequence of messages between the node and the remote time source, the sequence of messages including a message beating a time stamp from the remote time source.

17. A node as recited in claim 16, wherein the means for identifying further includes:

means for receiving the message beating the time stamp; and

means for determining the second time interval based on the time stamp.

18. A node as recited in claim 16, wherein the means for exchanging includes means for exchanging messages according to a probabilistic clock synchronization protocol.

19. A node as recited in claim 16, wherein the means for synchronizing includes:

means for identifying an intersection pattern between the first and second time intervals from a predetermined set of intersection patterns; and

means for synchronizing based on the intersection pattern.

20. A node as recited in claim 19, wherein:

the means for synchronizing based on the intersection pattern includes means for calculating a new instantaneous time and precision range in terms of the time values and precision ranges of the intervals of the messages exchanged by the means for exchanging, from one of a predetermined number of respective formulas, the formulas corresponding with the set of intersection patterns.

21. A node as recited in claim 15, further comprising:

means for determining a precision of synchronization which can be realized by synchronizing based on the identified first and second times;

means for comparing the precision determined by the mans for determining with a predetermined threshold precision; and

,

means, operable if the precision determined by the means for determining is less precise than the predetermined threshold precision, for repeating operation of the means for identifying, in order to identify a new first time for the internal time representation of the node and a new second time for the remote time representation, the precision range of the new first and new second times forming respective new first and new second time intervals, the synchronization to be performed based on the new first and new second time ;

intervals.

22. A computer program product, for use with a processor serving as a node of a system including nodes and a communication network coupled between the nodes, the node maintaining an internal time representation, for directing the node to synchronize the internal time representation with a remote time representation maintained elsewhere in the system, each of the internal and remote time representations including an instantaneous time and a precision range about the instantaneous time, the computer program product comprising:

a recording medium;

means, recorded on the recording medium, for directing the node to identify a first time for the internal time representation of the node and a second time for the remote time representation, the precision range of the first and second times forming respective first and second time intervals, the synchronization to be performed based on the first and second time intervals; and

means, recorded on the recording medium, for directing the node to synchronize the internal time representation, in terms of a new instantaneous time and a new precision range thereabout, the precision range thereabout being based on an intersection between the first and second time intervals.

23. A computer program product as recited in claim 22, wherein:

the remote time representation is maintained by a remote time source elsewhere in the system; and

the means for directing to identify includes means, recorded on the recording medium, for directing the node to exchange a predetermined sequence of messages between the node and the remote time source, the sequence of messages including a message bearing a time stamp from the remote time source.

24. A computer program product as recited in claim 23, wherein the means for directing to identify further includes:

means, recorded on the recording medium, for directing the node to receive the message bearing the time stamp; and

means, recorded on the recording medium, for directing the node to determine the second time interval based on the time stamp.

25. A computer program product as recited in claim 23, wherein the means for directing to exchange includes means, recorded on the recording medium, for directing the node to exchange messages according to a probabilistic dock synchronization protocol.

26. A computer program product as recited in claim 23, wherein the means for directing to synchronize includes:

means, recorded on the recording medium, for directing the node to identify an intersection pattern between the first and second time intervals from a predetermined set of intersection patterns; and

means, recorded on the recording medium, for directing the node to synchronize based on the intersection pattern.

27. A computer program product as recited in claim 26, wherein:

the means for directing to synchronize based on the intersection pattern includes means, recorded on the recording medium, for directing the node to calculate a new instantaneous time and precision range in terms of the time values and precision ranges of the intervals of the messages exchanged by the means for directing to exchange, from one of a predetermined number of respective formulas, the formulas corresponding with the set of intersection patterns.

28. A computer program product as recited in claim 22, further comprising:

means, recorded on the recording medium, for directing the node to determine a precision of synchronization which can be realized by synchronizing based on the identified first and second times;

means, recorded on the recording medium, for directing the node to compare the precision determined by the means for directing to determine with a predetermined threshold precision; and

,

means, recorded on the recording medium, operable if the precision determined by the means for directing to determine is less precise than the predetermined threshold precision, for repeating operation of the means for directing to identify, in order to identify a new first time for the internal time representation of the node and a new second time for the remote time representation, the precision range of the new first and new second times forming respective new first and new second time intervals, the synchronization to be performed based on the new first and new second time intervals.