Asymmetry correction for precise clock synchronization over optical fiber
View Patent ↗A clock at a first network element that is connected to a second network element over an optical fiber link is aligned in time/phase using packet protocols such as PTP. The invention discloses how to correct the asymmetry error inherent in traditional packet-based time-transfer methods.
1. In an optical network including a first network element that is connected to a second network element over first and second optical links that are physically distinct from each other, a method for estimating an asymmetry of transit delay in the first and second optical links, the method comprising:
aligning a first clock at the first network element and a second clock at the second network element to a nominal frequency; and
subsequent to aligning the first clock and the second clock to the nominal frequency:
measuring a first phase shift at the second clock of a clock signal that is recovered when a first physical layer clock is operated, according to a first control signal from the first clock, at a frequency greater than the nominal frequency,
measuring a second phase shift at the second clock of the clock signal that is recovered when the first physical layer clock is operated, according to a second control signal from the first clock, at a frequency less than the nominal frequency, and
estimating the asymmetry of transit delay in the first and second optical links based on the measured first and second phase shifts.
2. The method according to claim 1 , wherein the first phase shift is determined relative to a first phase reference corresponding to the nominal frequency, and wherein the second phase shift is determined relative to a second phase reference corresponding to the frequency greater than the nominal frequency.
3. The method according to claim 2 , wherein, before measuring the second phase shift, the second clock is permitted to align to the frequency greater than the nominal frequency.
4. The method according to claim 1 , further comprising repeating the steps of aligning the first and second clocks, measuring the first phase shift, measuring the second phase shift, and estimating the asymmetry based on the measured first and second phase shifts.
5. The method according to claim 1 , wherein estimating the asymmetry includes estimating a path length x 1 of the first optical link using the formula
x
1
=
v
·
(
ϕ
S
(
Δ
)
2
ω
0
Δ
)
,
where v is a velocity of propagation, ω 0 is the nominal frequency, Δ is an absolute value of an offset from the nominal frequency of the frequencies greater and less than the nominal frequency, and φ S (Δ) is a phase difference between the first and second phases.
6. The method according to claim 1 , further comprising correcting the alignment of the first and second clocks based on the estimated asymmetry.
7. The method according to claim 1 , wherein correcting the alignment includes calibrating at least one of the first and second optical links based on the estimated asymmetry.
8. In an optical network including a first network element that is connected to a second network element over first and second optical links that are physically distinct from each other, a method for estimating a path length of the first optical link, the method comprising:
aligning a first clock at the first network element and a second clock at the second network element to a nominal frequency; and
subsequent to aligning the first clock and the second clock to the nominal frequency:
measuring a first phase shift at the second clock of a clock signal that is recovered when a first physical layer clock is operated, according to a first control signal from the first clock, at a frequency greater than the nominal frequency,
measuring a second phase shift at the second clock of the clock signal that is recovered when the first physical layer clock is operated, according to a second control signal from the first clock, at a frequency less than the nominal frequency, and
estimating the path length of the first optical link based on the measured first and second phase shifts.
9. The method according to claim 8 , wherein the first phase shift is determined relative to a first phase reference corresponding to the nominal frequency, and wherein the second phase shift is determined relative to a second phase reference corresponding to the frequency greater than the nominal frequency.
10. The method according to claim 9 , wherein the path length of the first optical link is estimated using the formula
x
1
=
v
·
(
ϕ
S
(
Δ
)
2
ω
0
Δ
)
,
where x 1 is the path length of the first optical link, v is a velocity of propagation, ω 0 is the nominal frequency, Δ is an absolute value of an offset from the nominal frequency of the frequencies greater and less than the nominal frequency, and φ S (Δ) is a phase difference between the first and second phases.
11. In an optical network including a first network element that is connected to a second network element over first and second optical links that are physically distinct from each other, a method for estimating an asymmetry of transit delay in the first and second optical links, the method comprising:
aligning a first clock at the first network element and a second clock at the second network element to a nominal frequency; and
subsequent to aligning the first clock and the second clock to the nominal frequency:
measuring a first relative phase progression between a transmitted and a received wave at the first network element, wherein the first relative phase progression is measured with reference to a local time at the first network element,
measuring a second relative phase progression between a transmitted and a received wave at the second network element, wherein the second relative phase progression is measured with reference to a local time at the second network element, and
estimating the asymmetry of transit delay in the first and second optical links based on the first and second relative phase progressions.
12. The method according to claim 11 , further comprising managing phase cycles while measuring the first and second relative phase progressions.
13. The method according to claim 12 , wherein the phase cycles are managed by numbering edges, and wherein phase differences are measured between corresponding edges based on the numbering.
14. The method according to claim 11 , wherein the asymmetry is estimated using the formula
(
x
1
-
x
2
v
)
=
1
2
ω
0
·
(
φ
S
-
φ
M
)
,
where v is a velocity of propagation, ω 0 is the nominal frequency, x 1 is a path length of the first optical link, x 2 is a path length of the second optical link, φ S is the first relative phase progression, and φ M is the second relative phase progression.
15. The method according to claim 11 , further comprising correcting the alignment of the first and second clocks based on the estimated asymmetry.
16. The method according to claim 15 , wherein correcting the alignment includes calibrating at least one of the first and second optical links based on the estimated asymmetry.
17. In an optical network including a first network element that is connected to a second network element over first and second optical links that are physically distinct from each other, a method for estimating a path length of the first optical link, the method comprising:
aligning a first clock at the first network element and a second clock at the second network element to a nominal frequency; and
subsequent to aligning the first clock and the second clock to the nominal frequency:
measuring a first relative phase progression between a transmitted and a received wave at the first network element, wherein the first relative phase progression is measured with reference to a local time at the first network element;
measuring a second relative phase progression between a transmitted and a received wave at the second network element, wherein the second relative phase progression is measured with reference to a local time at the second network element; and
estimating the path length of the first optical link based on the first and second relative phase progressions.
18. The method according to claim 17 , further comprising managing phase cycles while measuring the first and second relative phase progressions.
19. The method according to claim 18 , wherein the phase cycles are managed by numbering edges, and wherein phase differences are measured between corresponding edges based on the numbering.
20. The method according to claim 17 , wherein the path length of the first optical link is estimated using one of the formulas
x
=
(
3
φ
S
-
φ
M
)
4
κ
and
x
=
(
φ
S
-
3
φ
M
)
4
κ
,
where x is the path length of the first optical link, φ S is the first relative phase progression, φ M is the second relative phase progression, and κ is equal to a nominal frequency divided by a velocity of propagation.