Method and devices for time transfer using end-to-end transparent clocks

Patent metadata

Field	Value
Publication number	US-9671822-B2
Application number	US-201414566990-A
Country	US
Kind code	B2
Filing date	Dec 11, 2014
Priority date	Dec 11, 2014
Publication date	Jun 6, 2017
Grant date	Jun 6, 2017

Abstract

This invention relates to methods and devices for time transfer. The invention has particular application in the alignment of slave clocks to a master clock and dealing with packet delay variations. In embodiments of the invention, the slave clock uses the residence times measured by end-to-end transparent clocks to compensate for clock synchronization errors that arise due to variability in message transfer delays. Embodiments provide a simple linear approximation technique and a Kalman filter-based technique for estimating offset and skew of the slave clock.

First claim

The invention claimed is:

1. A method for estimating the skew and offset of a slave clock in a slave device in relation to a master clock in a master device, the slave device and the master device being connected by a network, the method including the steps of:
exchanging timing messages between the master device and the slave device over the network, each timing message passing through at least one end-to-end transparent clock;
recording timestamps which are the times of the sending and receiving said messages according to the respective clocks;
recording the residence times, r ftotal,n and r rtotal,n , which are the residence times of the nth packet in the forward and reverse directions respectively, of each message passing through the or each transparent clock;
estimating the skew and offset of the slave clock using the said timestamps and said residence times; and
synchronizing the output of the slave clock to the master clock using said estimated offset and skew; and
wherein the step of estimating the skew and offset estimates the skew α as the average of the skew α 1 calculated in the forward direction from master to slave and the skew α 2 calculated in the reverse direction from the slave to the master:

α
1

=

(

T

1
,
n

-

T

1
,

n
-
1

)

+

(

r

ftotal
,
n

-

r

ftotal
,

n
-
1

)

(

T

2
,
n

-

T

2
,

n
-
1

)

-
1

⁢

&

α
2

=

(

T

4
,
n

-

T

4
,

n
-
1

)

+

(

r

rtotal
,
n

-

r

rtotal
,

n
-
1

)

(

T

3
,
n

-

T

3
,

n
-
1

)

-
1

,

 wherein
T 1,n is the time of departure of the nth Sync message as measured by the master clock;
T 2,n is the time of receipt as recorded by the slave clock on receipt of the nth Sync message;
T 3,n is the time of transmission of the nth Delay_Req message as recorded by the slave clock;
T 4,n is the time of receipt of the nth Delay_Req message as recorded by the master clock; and
r ftotal,n and r rtotal,n are the residence times of the nth packet in the forward and reverse directions respectively, of each message passing through the or each transparent clock,
estimates the offset θ as

θ
=

(

T

Assignees

• Khalifa Univ Of Science Tech And Res
• British Telecomm
• Emirates Telecommunications Corp
• Khalifa Univ Of Science Tech And Res

Inventors

• Aweya James

Classifications

• H04J3/0667

  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 title

• G06F1/10

  Distribution of clock signals {, e.g. skew} · CPC title

• G06F1/12 Primary

  Synchronisation of different clock signals {provided by a plurality of clock generators} · CPC title

• H04L69/28

  Timers or timing mechanisms used in protocols · CPC title