Systems and methods of clock synchronization between devices on a network

Patent metadata

Field	Value
Publication number	US-9912465-B2
Application number	US-201615138172-A
Country	US
Kind code	B2
Filing date	Apr 25, 2016
Priority date	Jul 20, 2011
Publication date	Mar 6, 2018
Grant date	Mar 6, 2018

Abstract

An exemplary method of synchronizing a master clock and a slave clock comprises transmitting a plurality of packets between a master device and a slave device, calculating a first skew between a first pair of the plurality of packets at the slave device and a second skew between the first pair at the master device, calculating a ratio between the first skew and the second skew, providing a slave clock frequency correction to the slave device, calculating a first packet trip delay using a time that the master device initiates sending a packet to the slave device, a time the master device receives a response from the slave device, a corrected time the slave device receives the packet, and a corrected time the slave device initiates sending the response, calculating a first offset based on the first packet trip delay, and providing the first offset to the slave device.

First claim

The invention claimed is:

1. A method of synchronizing a master clock and a slave clock, the method comprising:
transmitting a first packet and a second packet from a master device with a master clock to a slave device with a slave clock;
determining a first time when the master device sends the first packet and a second time when the master device sends the second packet;
determining a third time when the slave device receives the first packet and a fourth time when the slave device receives the second packet;
determining a fifth time when the slave device sends a third packet to the master device and a sixth time when the slave device sends a fourth packet to the master device;
determining a seventh time when the master device receives the third packet and an eighth time when the master device receives the fourth packet;
calculating a first skew based on a difference between the second time and the first time, and a second skew based on a difference between the fourth time and the third time;
calculating a third skew based on a difference between the eighth time and the seventh time, and a fourth skew based on a difference between the sixth time and the fifth time;
calculating an RMS ratio based on the first skew and the second skew;
calculating an RSM ratio based on the third skew and the fourth skew;
comparing the RMS ratio to the RSM ratio;
based on whether the comparison of the RMS ratio and the RSM ratio satisfies a condition, determining whether to trust a packet trip delay;
calculating the packet trip delay either based on the first time, the third time, a time that the slave device initiates sending a response to the first packet, and a time that the master device receives the response to the first packet from the slave device, or based on the second time, the fourth time, a time that the slave device initiates sending a response to the second packet, and a time that the master device receives the response to the second packet from the slave device;
calculating an offset between the master clock and the slave clock based on the packet trip delay; and
providing the offset to the slave device to enable the slave device to make a correction to the slave clock.

2. The method of claim 1 , wherein the offset is based on a difference between the RMS ratio and the RSM ratio.

3. The method of claim 1 , wherein the condition includes determining whether a difference between the RMS ratio and the RSM ratio is less than a predetermined threshold.

4. The method of claim 1 , wherein the time that the slave device initiates sending the response to the first packet is the fifth time, and the time that the master device receives the response to the first packet from the slave device is the sixth time.

5. The method of claim 1 , wherein the time that the slave device initiates sending the response to the second packet is the seventh time, and the time that the master device receives the response to the second packet from the slave device is the eighth time.

6. The method of claim 1 , further comprising determining the packet trip delay is a minimum packet trip delay of multiple packet trip delays before the step of calculating the offset.

7. The method of claim 1 , wherein the calculating the packet trip delay comprises

RTD
=

(

T
4

-

T
1

)

-

(

T
3

-

T
2

)

×
R

2

wherein T 4 is the time that the master device receives the response to the first packet or the second packet, T 1 is the time that the master device sends the first packet or the second packet to the slave device, T 3 is the time that the slave device sends the response to the first packet or the second packet to the master device, T 2 is the time that the slave device receives the first packet or the second packet, and R is the ratio between the first skew and the second skew.

8. The method of claim 1 , wherein the calculating the offset comprises offset=T 2 −T 1 −RTD wherein T 2 is the time that the slave device receives the first packet or the second packet from the master device, T 1 is the time that the master device sends the first packet or the second packet to the slave device, and RTD is the packet trip delay.

9. The method of claim 1 , wherein the calculating the offset comprises offset=T 3 −T 4 +RTD wherein T 3 is the time that the slave device sends the response to the first packet or the second packet to the master device, T 4 is the time that the master device receives the response to the first packet or the second packet from the slave device, and RTD is an averaged packet trip delay.

10. A system for synchronizing a master clock and a slave clock, the system comprising:
a communication module configured to transmit a first packet and a second packet from a master device with a master clock to a slave device with a slave clock;
a frequency rate module configured to:
determine a first time when the master device sends the first packet and a second time when the master device sends the second packet,
determine a third time when the slave device receives the first packet and a fourth time when the slave device receives the second packet;
determine a fifth time when the slave device sends a third packet to the master device and a sixth time when the slave device sends a fourth packet to the master device;
determine a seventh time when the master device receives the third packet and an eighth time when the master device receives the fourth packet;
calculate a first skew based on a difference between the second time and the first time, and a second skew based on a difference between the fourth time and the third time;
calculate a third skew based on a difference between the eighth time and the seventh time, and a fourth skew based on a difference between the sixth time and the fifth time;
calculate an RMS ratio based on the second skew and the first skew;
calculate an RSM ratio based on the fourth skew and the third skew;

a rate correction module configured to compare the RMS ratio to the RSM ratio and, based on whether the comparison of the RMS ratio and the RSM ratio satisfies a condition, determine whether to trust a packet trip delay;
a packet trip delay module configured to calculate the packet trip delay either based on the first time, the third time, a time that the slave device initiates sending a response to the first packet to the master device, and a time that the master device receives the response to the first packet from the slave device, or based on the second time, the fourth time, a time that the slave device initiates sending a response to the second packet to the master device, and a time that the master device receives the response to the second packet from the slave device; and

Assignees

• Aviat Us Inc

Inventors

• Mihelic Janez

Classifications

• G06F1/10

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

• 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

• H04L7/0033 Primary

  Correction by delay · CPC title

• H04L43/0852

  Delays · CPC title

• G06F1/12 Primary

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