Method and apparatus for time and frequency transfer in communication networks

Abstract

A timing system for time synchronization between a time server and a time client. The timing system includes a time server for generating current timestamp information and a time client having a phase-locked loop driven client clock counter. The time client periodically exchanges time transfer protocol messages with the time server over a packet network, and calculates an estimated client time based on the timestamp information. The phase-locked loop in the time client receives periodic signals representing the estimated server time as its input and calculates a signal which represents the error difference between the estimated server time and the time indicated by the time client clock counter. The error difference eventually converges to zero or a given error range indicating the time presented by the client clock counter, which is driven by the phase-locked loop having locked onto the time of the time server.

What is claimed is: 
     
         1 . A method of time synchronization over a packet network between a time server having a server clock and a time client having a client clock, the method comprising, at the time client:
 sending time transfer protocol messages from the time client to the time server over a packet network, the time transfer protocol messages comprising time stamp information;   receiving time transfer protocol messages from the time server over the packet network, the time transfer protocol messages comprising time stamp information;   identifying time transfer protocol messages within a monitoring interval having lowest one-way transmission delay based on the time stamp information;   periodically calculating an estimated server time using time stamp information in the time transfer protocol messages identified as having lowest one-way transmission delay within the monitoring interval; and   calculating an estimated client time based on the estimated server time.   
     
     
         2 . The method of  claim 1 , wherein the time stamp information in each time transfer protocol message sent from the time client to the time server comprises a respective first time stamp indicating when that time transfer protocol message was sent from the time client. 
     
     
         3 . The method of  claim 2 , wherein the time stamp information in each time transfer protocol message received at the time client from the time server comprises a respective first time stamp indicating when a respective associated time transfer protocol message was sent from the time client, a respective second time stamp indicating when the respective associated time transfer protocol message was received at the time server and a respective third time stamp indicating when the time transfer protocol message was sent from the time server. 
     
     
         4 . The method of  claim 3 , comprising capturing a respective fourth time stamp for each time transfer protocol message received at the time client, the respective fourth time stamp indicating when the time transfer protocol message was received by the time client. 
     
     
         5 . The method of  claim 4 , wherein periodically calculating an estimated server time using the time stamp information in time transfer protocol messages identified as having relatively low one-way transmission delay comprises calculating the estimated server time using:
 first and second time stamps associated with time transfer protocol messages identified as having lowest one-way transmission delay within the monitoring interval for transmission from the time client to the time server; and   third and fourth time stamps associated with time transfer protocol messages identified as having lowest one-way transmission delay within the monitoring window for transmission from the time server to the time client.   
     
     
         6 . The method of  claim 1 , wherein identifying time transfer protocol messages having lowest one-way transmission delay within the monitoring interval comprises:
 capturing time stamp information generated in a moving time window; and   identifying time transfer protocol messages having lowest one-way transmission delay in the moving time window based on the captured time stamp information.   
     
     
         7 . The method of  claim 6 , wherein the moving time window has a duration long enough to capture at least eight time transfer protocol message exchanges. 
     
     
         8 . The method of  claim 1 , comprising computing an error signal, the error signal representing a difference between the estimated server time and the estimated client time. 
     
     
         9 . The method of  claim 8 , wherein the time client comprises a phase-locked loop driven client clock counter and the estimated client time is a time indicated by the client clock counter. 
     
     
         10 . The method of  claim 9 , further comprising transforming the error signal into a control signal for driving an oscillator, the oscillator providing a clock frequency for the client clock counter. 
     
     
         11 . The method of  claim 8 , further comprising, when the error signal is in a predetermined error range, deeming the estimated client time to be synchronized with the estimated server time. 
     
     
         12 . The method of  claim 1 , wherein the estimated server time is recalculated after each exchange of time transfer protocol messages between the time client and the time server. 
     
     
         13 . The method of  claim 1 , further comprising:
 receiving time transfer protocol messages sent by the time client at the time server; and   responsive to receipt of each time transfer protocol message sent by the time client at the time server, sending a respective associated time transfer protocol message from the time server to the time client.   
     
     
         14 . The method of  claim 13 , further comprising:
 receiving time transfer protocol messages sent by plural time clients at the time server; and   responsive to receipt of each time transfer protocol message sent by each time client, sending a respective associated time transfer protocol message from the time server to that respective time client.   
     
     
         15 . The method of  claim 14 , further comprising, at each respective time client:
 periodically calculating an estimated server time using time stamp information in time transfer protocol messages identified as having lowest one-way transmission delay within a respective monitoring interval; and   calculating a respective estimated client time based on the estimated server time.   
     
     
         16 . The method of  claim 16 , wherein the time stamp information in each time transfer protocol message sent from each time client to the time server comprises a respective first time stamp indicating when that time transfer protocol message was sent from the time client. 
     
     
         17 . The method of  claim 16 , wherein the time stamp information in each time transfer protocol message received at each time client from the time server comprises a respective first time stamp indicating when a respective associated time transfer protocol message was sent from a respective time client, a respective second time stamp indicating when the respective associated time transfer protocol message was received at the time server and a respective third time stamp indicating when the time transfer protocol message was sent from the time server. 
     
     
         18 . The method of  claim 17 , further comprising, at each time client, capturing a respective fourth time stamp for each received time transfer protocol message indicating when that time transfer protocol message was received by the time client. 
     
     
         19 . The method of  claim 18 , wherein periodically calculating an estimated server time using the time stamp information in time transfer protocol messages identified as having lowest one-way transmission delay within a monitoring interval comprises calculating the estimated server time using:
 first and second time stamps associated with time transfer protocol messages identified as having lowest one-way transmission delay within the monitoring interval for transmission from the time client to the time server; and   third and fourth time stamps associated with time transfer protocol messages identified as having lowest one-way transmission delay within the monitoring interval for transmission from the time server to the time client.   
     
     
         20 . The method of  claim 14 , wherein identifying time transfer protocol messages having lowest one-way transmission delay within the monitoring interval comprises, at each time client:
 capturing time stamp information generated in a moving time window; and   identifying time transfer protocol messages having lowest one-way transmission delay within the moving time window based on the captured time stamp information.