Method of transferring timing information over packet network
Timing information is transmitted over a network by sending time-stamped packets between a transmitter and receiver. The time-stamped packets are used to compute an estimate of the frequency deviation between the transmitter clock and the receiver clock. The local receiver clock is periodically updated to match the transmitter clock based on the estimate, which is preferably a least squares estimate. A multiple window approach is employed to increase update speed.
1. A method of transferring timing information over a network between a transmitter and receiver, comprising:
generating a clock signals with a first digital controlled oscillator having a set center frequency at the transmitter based on a local oscillator at the transmitter;
generating clock signals at the receiver with a digital controlled oscillator having a set center frequency based on a local oscillator at the receiver;
sending time-stamped packets derived from the local transmitter clock to the receiver;
creating time-stamped packets at the receiver containing timing information derived from the local receiver clock;
processing said time-stamped packets to compute an estimate of the frequency deviation between the transmitter clock and the receiver clock;
adjusting the center frequency of the digital controlled oscillator at the receiver so that it is synchronized with the digital controlled oscillator at the receiver, and
wherein an overlapping multi-window approach is adopted wherein data adjustment blocks compute frequency adjustments from timing data accumulating during overlapping time intervals, and the data adjustment blocks take turns to output frequency adjustment data for the digital controlled oscillator at the receiver every T/M time intervals, where T is the time required to collect enough data to make an adjustment with a single adjustment block, and M is the number of adjustment blocks, and
wherein an adjustment block is reset after outputting a frequency adjustment to update the local receiver clock, and variables stored in remaining adjustment blocks are adjusted to compensate for the updating of the local receiver clock, and wherein the compensation adjustments for variables S y and S xy stored in an adjustment block are given by the expressions
S y =S y +(α 1 −α)( S x −T 1 )
S xy =S xy +(α 1 −α)( S 2x −T 1 ·S x )
where T 1 is the time elapsed during the data collection of S y , S xy , S x and S 2x , and S x =Σ(x k ), S xy =Σ(x k y k ), S 2x =Σ(x k 2 ), and S y =Σ(y k ).
2. A method as claimed in claim 1 , wherein the transit time over said network is computed from the difference between the transmitter time-stamped packets and the receiver time-stamped packets, and said estimate is computed from transit time.
3. A method as claimed in claim 1 , wherein said estimate is a computed least squares estimate.
4. A method as claimed in claim 3 , wherein said estimate is a computed recursive least square estimate.
5. A method as claimed in claim 3 , wherein the least squares estimate a is computed in accordance with the formula:
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where x k =k·T·f tx , y k =α·x k +v k , where k is an index, T is the time interval between consecutive timing packets, and v k represents the network delay variation plus a phase offset.
6. An apparatus for updating the set frequency of a digital controlled oscillator at a receiver for timing recovery in an asynchronous packet network, comprising:
a plurality of data accumulation blocks for accumulating timing data from time-stamped packets transmitted through a packet network from a transmitter;
a plurality of adjustment blocks for computing an estimate of the frequency deviation from said accumulated timing data; and
an updater for periodically updating said set frequency based on said computed estimate to synchronize said digital controlled oscillator at the receiver with a digital controlled oscillator at the transmitter; and
wherein the data adjustment blocks are configured to compute frequency adjustments from timing data accumulating during overlapping time intervals, and the data adjustment blocks are configured to take turns to output frequency adjustment data for the digital controlled oscillator at the receiver every T/M time intervals, where T is the time required to collect enough data to make an adjustment with a single adjustment block, and M is the number of adjustment blocks, and
wherein the data accumulation blocks are reset after their output has been used to update the local oscillator, and
wherein the apparatus further comprises a unit for changing at least some variables in the remaining data accumulation block(s) to compensate for the different frequency deviation, and
wherein the compensation adjustments for variables S y and S xy stored in an adjustment block are given by the expressions
S y =S y +(α 1 −α)( S x −T 1 )
S xy =S xy +(α 1 −α)( S 2x −T 1 ·S x )
where T 1 is the time elapsed during the data collection of S y , S xy , S x and S 2x , and S x =Σ(x k ), S xy =Σ(x k y k ), S 2x =Σ(x k 2 ), and S y =Σ(y k ).
7. An apparatus as claimed in claim 6 , further comprising a main counter for counting a predetermined number of time-stamped packets and outputting data from an adjustment block when the counter reaches a preset value representing the number of time-stamped packets required to provide a valid update for the local oscillator.
8. An apparatus as claimed in claim 7 , further comprising a selection unit for switching between said data adjustment blocks.
9. An apparatus as claimed in claim 8 , comprising two said data accumulation blocks and two said adjustment blocks, and wherein said selection includes a 1-bit counter.
10. An apparatus as claimed in claim 6 , wherein M is a number greater than 2.
11. An apparatus as claimed in claim 10 , wherein said selection unit includes a secondary counter that counts up to M.
12. A method of recovering timing information over a network between a transmitter and receiver, comprising:
generating clock signals at the receiver with a digital controlled oscillator having a set center frequency based on a local oscillator at the receiver;
receiving time-stamped packets derived from a local clock at the transmitter;
creating time-stamped packets at the receiver containing timing information derived from the local receiver clock;
processing said time-stamped packets to compute an estimate of the frequency deviation between the transmitter clock and the receiver clock;
adjusting the center frequency of the digital controlled oscillator at the receiver so that it is synchronized with the digital controlled oscillator at the receiver, and
wherein an overlapping multi-window approach is adopted wherein data adjustment blocks compute frequency adjustments from timing data accumulating during overlapping time intervals, and the data adjustment blocks take turns to output frequency adjustment data for the digital controlled oscillator at the receiver every T/M time intervals, where T is the time required to collect enough data to make an adjustment with a single adjustment block, and M is the number of adjustment blocks, and
wherein an adjustment block is reset after outputting a frequency adjustment to update the local receiver clock, and variables stored in remaining adjustment blocks are adjusted to compensate for the updating of the local receiver clock, and wherein the compensation adjustments for variables S y and S xy stored in an adjustment block are given by the expressions
S y =S y +(α 1 −α)( S x −T 1 )
S xy =S xy +(α 1 −α)( S 2x −T 1 ·S x )
where T 1 is the time elapsed during the data collection of S y , S xy , S x and S 2x , and S x =Σ(x k ), S xy =Σ(x k y k ), S 2x =Σ(x k 2 ), and S y =Σ(y k ).
13. A method as claimed in claim 12 , wherein the transit time over said network is computed from the difference between the transmitter time-stamped packets and the receiver time-stamped packets, and said estimate is computed from transit time.
14. A method as claimed in claim 12 , wherein said estimate is a computed least squares estimate.
15. A method as claimed in claim 14 , wherein the least squares estimate {circumflex over (α)} is computed in accordance with the formula:
α
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=
∑
x
k
y
k
-
1
N
∑
x
k
∑
y
k
∑
x
k
2
-
1
N
(
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x
k
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2
where x k =k·T·f tx , y k =α·x k +v k , where k is an index, T is the time interval between consecutive timing packets, and v k represents the network delay variation plus a phase offset.