Methods and systems for determining optimal packets

Patent metadata

Field	Value
Publication number	US-2018145863-A1
Application number	US-201615358315-A
Country	US
Kind code	A1
Filing date	Nov 22, 2016
Priority date	Nov 22, 2016
Publication date	May 24, 2018
Grant date	—

Abstract

This invention relates to timing message selection techniques that can be used in conjunction with a clock recovery mechanism to mitigate the effects of packet delay variation on timing messages exchanged over a packet network, particularly when seeking to synchronize the time of a clock in a slave device to that of a master clock. The selection techniques allow the identification of optimal or minimally-delayed timing messages which can subsequently be used in timing synchronisation. Embodiments of the invention provide techniques which identify optimal timing messages in both forward and reverse directions which are then processed to form composite timing messages which are used in a frequency estimation algorithm. Timing messages selected by the methods of the invention are particularly useful in phase synchronization between the master and slave clocks.

First claim

1 . A method of estimating the phase offset between a master clock in a server and a slave clock in a client, the server and the client being in communication over a network, the method including the steps of:
within a time window of predetermined duration, exchanging timing messages between the server and the client and recording timestamps which are the times of sending and of receipt of those messages according to the master and slave clocks; determining, from those timestamps, at least one first timestamp pair, which is the times of sending and receipt associated with the least delayed timing message sent from the server to the client in the window; determining, from those timestamps, at least one second timestamp pair, which is the times of sending and receipt associated with the least delayed timing messages sent from the client to the server in the window; generating, from the first and second timestamp pairs at least one composite timing message, which includes said first timestamp pair and said second timestamp pair; using said composite timing messages to estimate the phase offset between the master clock and the slave clock.

2 . A method according to claim 1 wherein said estimation of the phase offset includes operating a Kalman filter using the timestamps of said timing messages, and includes the steps of:
initialising the Kalman filter by operating it using the measured timestamps of said timing messages until the estimates of the phase offset and skew are estimated to within a predetermined accuracy; and then
repeatedly for each time window:
operating the Kalman filter in prediction only mode to estimate the instantaneous phase offset based on the state vector derived from the composite timing message or messages from the previous time window;
generating the composite timing message or messages for the current time window; and
at the end of each time window, updating the Kalman filter with the values from the composite timing message or messages from the time window just ended.

3 . A method according to claim 1 further including the steps of:
calculating, for each of said least delayed timing messages, an estimate of the phase offset; and
calculating, for each of said composite timing messages, an estimated mean phase offset which is the geometric mean of: the phase offset calculated for the timing message associated with said one of said first plurality of timestamps and the phase offset calculated for the timing message associated with said one of said second plurality of timestamps.

4 . A method according to claim 3 , further including the step of calculating, as the time associated with each estimated mean phase offset, the geometric mean of the times of sending or receipt of the timing messages from which the estimated mean phase offset is derived.

5 . A method according to claim 4 wherein said estimation of the phase offset includes operating a Kalman filter using the timestamps of said timing messages, and includes the steps of:
initialising the Kalman filter by operating it using the measured timestamps of said timing messages until the estimates of the phase offset and skew are estimated to within a predetermined accuracy; and then
repeatedly for each time window:
operating the Kalman filter in prediction only mode to estimate the instantaneous phase offset based on the state vector derived from the composite timing messages from the previous time window;
generating the composite timing messages for the current time window; and
at the end of each time window, updating the Kalman filter with the values from the composite timing messages from the time window just ended,

wherein the Kalman filter operates with a measurement equation:

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and a state equation:

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Assignees

• Khalifa Univ Of Science Technology And Research
• British Telecomm
• Emirates Telecommunications Corp

Inventors

• Chaloupka Zdenek
• Boyd Ivan

Classifications

• H04L43/0852 Primary

  Delays · CPC title

• H04L67/42

  Electricity · mapped topic

• H04L7/0079

  Receiver details · CPC title

• H04L27/2657 Primary

  Carrier synchronisation · CPC title

• H04L67/01

  Protocols · CPC title