Timing recovery over packet networks
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.
1. A method of recovering timing information over packet networks, comprising:
measuring raw network time delays from timing packets of a single stream sent between a transmitter and a receiver over a packet network;
predicting a current delay estimate using a minimum statistics adaptive filter to track local minima of the measured raw network time delays over a series of successive smoothing windows of L received timing packets of the single stream, by:
maintaining a first variable tracking a local minimum measured raw network time delay within a smoothing window,
maintaining a second variable representing the current delay estimate, said second variable being regularly updated within each smoothing window and being set equal to a minimum between the current value thereof and the measured raw network time delay for the current timing packet,
after L received timing packets at the start of a new smoothing window setting said second variable equal to the current value of said first variable and subsequently updating the value of said first variable, and
adaptively varying the value of L based on estimated delay variations between the transmitter and the receiver to change the tracking characteristics of the minimum statistics adaptive filter;
selecting those incoming timing packets of the single stream within the smoothing window which meet a particular criterion relative to the current delay estimate; and
making frequency adjustments determined from the measured raw network time delays of the selected timing packets of the single stream 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 raw network time delay and the current 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 z 1 _min(k) and the second variable is z_min(k), and z(k) is raw delay estimate for timing packet k, and the following loop is implemented:
set z_min(k) = minimum (z_min(k−1),z(k))
set z1_min(k) = minimum(z1_min(k−1),z(k))
IF win_cnt = L
increment win_cntr
ELSE
set z_min(k) = Z1_min)k)
z1_min(k) = z(k)
win_cntr = 0
where win_cntr is a counter value and k is an integer.
8. A method as claimed in claim 1 , wherein said first variable is updated to the value of the raw measured timing delay of the current packet at the start of said new smoothing window.
9. A timing recovery unit for recovering timing information over packet networks, comprising:
a block for measuring raw network time delays from timing packets of a single stream sent between a transmitter and a receiver over a packet network;
a minimum statistics adaptive filter to track local minima of the measured raw network time delays over a smoothing window to predict a current delay estimate over a series of successive smoothing windows of L received timing packets of the single stream, and wherein the minimum statistics filter is configured to:
maintain a first variable tracking a local minimum raw network time delay within a smoothing window,
maintain a second variable representing the current delay estimate, said second variable being regularly updated within each smoothing window and being set equal to a minimum between the current value thereof and the measured raw network time delay for the current timing packet,
after L received timing packets at the start of a new smoothing window, to set the second variable equal to the current value of the first variable and subsequently update the value of said first variable, and
to adaptively vary the size of smoothing window based on estimated delay variations between the transmitter and the receiver to change the tracking characteristics of the minimum statistics adaptive filter;
a selection block for selecting those incoming timing packets of the single stream within the smoothing window which meet a particular criterion relative to the current delay estimate; and
a phase locked loop for outputting a local clock at the receiver using frequency adjustments determined from the measured raw network time delays of the selected timing packets of the single stream.
10. A timing recovery unit as claimed in claim 9 , wherein the selection block determines the difference between the measured raw network 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.
11. A timing recovery unit as claimed in claim 9 , wherein the second variable is updated for each incoming timing packet within the smoothing window.
12. A timing recovery unit as claimed claim 9 , further comprising a resampling unit for resampling the output of the minimum statistics adaptive delay filter.
13. A timing recovery unit as claimed in claim 9 , wherein the phase locked loop includes a loop filter that filters the frequency adjustments.
14. A timing recovery unit as claimed in claim 9 , 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.
15. A timing recovery unit as claimed in claim 9 , wherein the minimum statistics adaptive filter implements the following loop:
set z_min(k) = minimum (z_min(k−1),z(k))
set z1_min(k) = minimum(z1_min(k−1),z(k))
IF win_cnt = L
increment win_cntr
ELSE
set z_min(k) = z1_min(k)
z1_min(k) = z(k)
win_cntr = 0
wherein the first variable is z 1 _min(k), the second variable is z_min(k), win_cntr is a counter value, and k is an integer, and z(k) is raw delay estimate for timing packet k.
16. A timing recovery unit as claimed in claim 9 , wherein the minimum statistics adaptive filter is configured to update said first variable to the value of the raw measured timing delay of the current packet at the start of said new smoothing window.