TIMING RECOVERY OVER PACKET NETWORKS

• US 20100296406A1
• Filed: 05/06/2010
• Published: 11/25/2010
• Est. Priority Date: 05/22/2009
• Status: Active Grant

• Current Assignee: Microsemi Semiconductor ULC (Microchip Technology Incorporated)
• Original Assignee: Zarlink Semiconductor Inc. (Microchip Technology Incorporated)
• Inventors: Rahbar, Kamran
• Granted Patent: US 8,483,244 B2
• Time in Patent Office: Days
• US Class Current: 370/252
• CPC Class Codes: G06F 1/14 Time supervision arrangements, e.g. real time clock
• CPC Class Codes: H04J 3/0664 unidirectional timestamps
• CPC Class Codes: H04J 3/0697 Synchronisation in a packet node
• CPC Class Codes: H04L 41/142 using statistical or mathematical methods
• CPC Class Codes: H04L 43/0858 One way delays
• CPC Class Codes: H04L 43/0864 Round trip delays
• CPC Class Codes: H04L 43/106 using time related information in packets, e.g. by adding timestamps
• CPC Class Codes: H04L 47/28 in relation to timing considerations
• CPC Class Codes: H04L 7/0008 Synchronisation information channels, e.g. clock distribution lines

Abstract

In a method of recovering timing information over packet networks, raw network delays are measured using timing packets sent between a transmitter and receiver. The expected delay is predicted using a minimum statistics adaptive filter to track local minima of measured time delays over a smoothing window. Only those incoming timing packets which meet a particular criterion relative to the expected delay within a smoothing window are selected, and a local clock is adjusted based on the measured timing delays from the selected timing packets.

Claims

1. A method of recovering timing information over packet networks, comprising:

measuring raw network delays from timing packets sent between a transmitter and receiver over a packet network;

predicting a delay estimate using a minimum statistics adaptive filter to track local minima of measured time delays over a smoothing window of L received timing packets, wherein a first variable represents the local minimum over L received timing packets, said first variable being updated after L received timing packets, and a second variable represents the minimum between the current value for the first variable and the current timing packet within the smoothing window, said second variable representing the current delay estimate, and wherein the value of L is adaptively varied based on estimated delay variations between the transmitter and receiver to change the tracking characteristics of the minimum statistics adaptive filter;

selecting those incoming timing packets within the smoothing window which meet a particular criterion relative to the current delay estimate; and

making frequency adjustments determined from the measured timing delays of the selected timing packets for a local clock at the receiver.

2. A method as claimed in claim 1, wherein the selected incoming timing packets are identified by determining the difference between the measured time delay and the delay estimate and comparing this difference with a threshold.

3. A method as claimed in claim 1, wherein the second variable is updated for each incoming timing packet within the smoothing window.

4. A method as claimed in claim 1, further comprising resampling the output of the minimum statistics adaptive delay filter to change the sampling rate.

5. A method as claimed in claim 1, wherein the frequency adjustments are filtered in a loop filter of a phase locked loop.

6. A method as claimed in claim 1, wherein the incoming timing packets are timestamped relative to the local clock on arrival and the measured delays are determined by comparing timestamps applied at the transmitter with timestamps applied at the receiver.

7. A method as claimed in claim 1, wherein the first variable is z1_min(k) and the second variable is z_min(k), and the following loop is implemented:

8. A timing recovery unit for recovering timing information over packet networks, comprising:

a block for measuring raw network delays from timing packets sent between a transmitter and receiver over a packet network;

a minimum statistics adaptive filter to track local minima of measured time delays over a smoothing window to predict a delay estimate over a smoothing window of L received timing packets, and wherein the minimum statistics smoothing filter is configured such that a first variable represents the local minimum over L received timing packets, said first variable being updated after L received timing packets, and a second variable represents the minimum between the current value for the first variable and the current timing packet within the smoothing window, said second variable representing the current delay estimate, and wherein the minimum statistics smoothing filter is configured to adaptively vary the size of smoothing window based on estimated delay variations between the transmitter and receiver to change the tracking characteristics of the minimum statistics adaptive filter;

a selection block for selecting those incoming timing packets within the smoothing window which meet a particular criterion relative to the expected delay; and

a phase locked loop for outputting a local clock at the receiver using frequency adjustments determined from the measured timing delays of the selected timing packets.

9. A timing recovery unit as claimed in claim 8, wherein the selection block determines the difference between the measured time delay of incoming timing packets and the predicted delay and compares this difference with a threshold to select the packets meeting the particular criterion.

10. A timing recovery unit as claimed in claim 8, wherein the second variable is updated for each incoming timing packet within the smoothing window.

11. A timing recovery unit as claimed claim 8, further comprising a resampling unit for resampling the output of the minimum statistics adaptive delay filter.

12. A timing recovery unit as claimed in claim 8, wherein the phase locked loop includes a loop filter that filters the frequency adjustments.

13. A timing recovery unit as claimed in claim 8, further comprising a timestamp block for timestamping the incoming timing packets relative to the local clock on arrival, and a subtraction block for determining the measured delays by subtracting the timestamps applied at the transmitter from the timestamps applied at the receiver.

14. A timing recovery unit as claimed in claim 8, wherein the minimum statistics adaptive filter implements the following loop: