Time synchronization using dynamic thresholds

• US 7,352,715 B2
• Filed: 10/25/2002
• Issued: 04/01/2008
• Est. Priority Date: 11/30/2001
• Status: Active Grant

• Current Assignee: Landis+Gyr Technology, Inc. (Landis+Gyr Group AG)
• Original Assignee: Cellnet Innovations, Inc.
• Inventors: Bengtsson, Johan, Ragle, Steven J., Salt, Metin, Paik, Ellen
• Primary Examiner(s): Jagannathan; Melanie
• Application Number:
  US10/280,448
• Publication Number:
  US 20030103486A1
• Time in Patent Office: 1,985 Days
• Field of Search: None
• US Class Current: 370/328
• 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

Methods and apparatus for time synchronization using dynamic thresholds. A method for synchronizing network elements includes receiving at a network element a time synchronization message sent from a master clock element. The network element includes an internal clock to be synchronized with a master clock of the master clock element. The method includes calculating a latency of the time synchronization message. The method includes estimating an adjustment that may be made to the internal clock in response to the time synchronization message. The method includes determining whether the latency calculated is less than the adjustment estimated. The method includes adjusting the internal clock when the latency calculated is less than the adjustment estimated.

Claims

1. A method for synchronizine network elements, the method comprising:

receiving at a network element a time synchronization message sent from a master clock element, the network element comprising an internal clock to be synchronized with a master clock of the master clock element, the time synchronization message comprising a time stamp specifying a time, as indicated by the master clock, when the master clock element sent the time synchronization message;

calculating a latency of the time synchronization message;

estimating an adjustment to be made to the internal clock in response to the time synchronization message;

determining whether the latency is less tan the adjustment on the assumption that the latency is imbalanced by

calculating an earliest current time of the master clock assuming that there is no latency and calculating a latest current time of the master clock by adding the calculated latency to the time stamp;

adjusting the internal clock when the latency calculated is less than the adjustment estimated.

2. The method of claim 1, wherein determining whether the latency calculated is less than the adjustment estimated comprises:

determining whether a current time as indicated by the internal clock is between the earliest and latest current time of the master clock.

3. The method of claim 1, wherein:

calculating the latency comprises calculating latency that varies from one instance of synchronization to another instance of synchronization; and

estimating the adjustment comprises estimating adjustment that varies from one instance of synchronization to another instance of synchronization.

4. A method for synchronizing network elements, the method comprising:

receiving at a network element a time synchronization message sent from a master clock element, the network element comprising an internal clock to be synchronized with a master clock of the master clock element;

calculating a latency of the time synchronization message;

estimating an adjustment to be made to the internal clock in response to the time synchronization message;

determining whether the latency calculated exceeds a threshold;

when the latency calculated does not exceed the threshold, applying a first set of criteria to determine whether the latency calculated is less than the adjustment estimated;

when the latency calculated exceeds the threshold, applying a second set of criteria to determine whether the latency calculated is less than the adjustment estimated; and

adjusting the internal clock when the latency calculated is determined to be less than the adjustment estimated.

5. The method of claim 4, wherein applying the first set of criteria comprises:

determining whether the master clock and the internal clock are sufficiently synchronized; and

determining whether adjustment to the internal clack will cause an error in the network.

6. The method of claim 5, wherein:

determining whether the master clock and the internal clock are sufficiently synchronized comprises determining whether a difference between a current time as indicated by the internal clock and the current time as estimated to be indicated by the master clock is less than five seconds; and

determining whether adjustment to the internal clock will cause an error in the network comprise determining whether there will be an error cause by adjusting the internal clock because a difference between the current time as indicated by the internal clock and the current time as estimated to be indicated by the master clock exceeds a preset threshold.

7. The method of claim 6, wherein the current time as indicated by the internal clock is a time on the internal clock when the network element received the time synchronization message.

8. The method of claim 4, further comprising sending a request message from the network element to the master clock element, the request message causing the master clock element to send the time synchronization message, and wherein calculating the latency comprises:

recording a time when the network element sent the request message;

recording a time when the network element received the time synchronization message sent from the master clock element; and

calculating a difference between the time when the network element sent the request message and the time when the network element received the time synchronization message sent from the master clock element.

9. A computer-readable medium on which is encoded program code for synchronizing a first clock of a network with a second clock of the network, the program code comprising:

program code for calculating a latency between the first clock and the second clock;

program code for estimating an adjustment of the first clock to synchronize the first clock with the second clock;

program code for using the latency to calculate an earliest current time for the second clock and a latest current time for the second clock, the calculation assuming that the latency is completely imbalanced;

program code for determining whether to adjust the first clock based on whether the latency calculated is less than the adjustment estimated for a first clock current time that is in between the earliest current time for the second clock and latest current time for the second clock; and

program code for adjusting the first clock.

10. The computer-readable medium on which is encoded program code of claim 9, further comprising program code for determining that the first clock is to be adjusted when:

the first clock and the second clock are not synchronized within five seconds; and

the adjustment will not cause an error in the network.

11. The computer-readable medium on which is encoded program code of claim 9, wherein:

the program code for calculating the latency between the first clock and the second clock comprises program code for calculating a time comprising the time interval elapsed while a first message travels from a second network element comprising the second clock to a first network element comprising the first clock.

12. The computer-readable medium on which is encoded program code of claim 9, wherein:

the program code for calculating the latency between the first clock and the second clock comprises program code for calculating a round trip time comprising the time interval elapsed while, a request message travels from the first network element comprising the first clock to the second network element comprising the second clock, the second element processes the request message, generates and sends a response message, and the response message travels from the second network element to the first network element.

13. The computer-readable medium on which is encoded program code of claim 9, wherein:

the network is a wireless network; and

the second clock is a master clock.

14. The computer-readable medium on which is encoded program code of claim 13, wherein:

the network is a radio frequency network.

15. A computer-readable medium on which is encoded program code for synchronizing a first clock of a network with a second clock of the network, the program code comprising:

program code for calculating a latency between the first clock and the second clock;

program code for estimating an adjustment of the first clock to synchronize the first clock with the second clock;

program code for calculating an earliest current time as indicated by the second clock and a latest current time as indicated by the second clock, the calculation assuming that the latency is completely imbalanced;

program code for determining that the latency calculated is less than the adjustment estimated when a current time as indicated by the first clock is in between the earliest and latest current times; and

program code for adjusting the first clock.