Method of estimating a propagation delay difference of an optical link and apparatus for same

Abstract

An optical link for a communication network, the optical link having an optical fibre link, a downstream transmitter, a downstream receiver, an upstream transmitter and an upstream receiver. The upstream and downstream transmitters are configured to transmit a respective pilot tone on a respective optical carrier and are configured to tune a frequency of the pilot tone within a preselected frequency range. The upstream and downstream receivers are configured respectively to determine an upstream notch frequency, f.sub.notch-US, and a downstream notch frequency, f.sub.notch-DS, of respective detected photocurrents from at least one respective pilot tone frequency at which the respective detected photocurrent is equal to or lower than a photocurrent threshold. The optical link also includes processing circuitry configured to receive the upstream and downstream notch frequencies and configured to estimate a propagation delay difference of the optical link depending on the upstream and downstream notch frequencies.

Claims

1. An optical link for a communication network, the optical link comprising: an optical fibre link; a downstream transmitter, a downstream receiver, an upstream transmitter and an upstream receiver, the upstream and downstream transmitters being configured to transmit a respective pilot tone on a respective optical carrier and are configured to tune a frequency of the pilot tone within a preselected frequency range, and the upstream and downstream receivers being configured respectively to determine an upstream notch frequency, f.sub.notch-US, and a downstream notch frequency, f.sub.notch-DS, of respective detected photocurrents from at least one respective pilot tone frequency at which the respective detected photocurrent is equal to or less than a photocurrent threshold; and processing circuitry configured to receive the upstream and downstream notch frequencies and configured to estimate a propagation delay difference of the optical link depending on the upstream and downstream notch frequencies.

2. The optical link according to claim 1, wherein the upstream and downstream receivers are configured to determine the respective notch frequency from at least first and second pilot tone frequencies at which the respective detected photocurrents equal the photocurrent threshold.

3. The optical link according to claim 2, wherein the upstream and downstream receivers are configured to calculate the respective notch frequency as an average of first and second pilot tone frequencies at which the respective detected photocurrents equal the photocurrent threshold.

4. The optical link according to claim 2, wherein the upstream and downstream receivers are configured to calculate the respective notch frequency by linear interpolation of four pilot tone frequencies, two pilot tone frequencies at which the respective detected photocurrents equal the photocurrent threshold and two further pilot tone frequencies at which the respective detected photocurrents equal a second, different, photocurrent threshold.

5. The optical link according to claim 1, wherein the processing circuitry is configured to estimate the propagation delay difference depending inversely on a difference of the upstream and downstream notch frequencies.

6. The optical link according to claim 5, wherein the respective optical carriers respectively have an upstream wavelength, λ.sub.US, and a downstream wavelength, λ.sub.DS, and wherein the processing circuitry is configured to estimate the propagation delay difference, ΔT, as $\Delta T=\frac{c}{4}\left(\frac{1}{{\lambda }_{\mathrm{DS}}{f}_{\mathrm{notch}-\mathrm{DS}}^2}-\frac{1}{{\lambda }_{\mathrm{US}}{f}_{\mathrm{notch}-\mathrm{US}}^2}\right).$

7. The optical link according to claim 1, wherein the optical fibre link further comprises a first optical fibre for downstream transmission and a second optical fibre for upstream transmission, and wherein the processing circuitry is additionally configured to estimate respective lengths of the optical fibres depending on respective dispersion coefficients of the optical fibres at preselected respective wavelengths and respective notch frequencies of respective pilot tones transmitted on respective optical carriers at the respective wavelengths.

8. A method of estimating a propagation delay difference of an optical link of a communication network, the method comprising steps of: a. transmitting respective pilot tones on upstream and downstream optical carriers, frequencies of the respective pilot tones being tuned within a preselected frequency range; b. determining an upstream notch frequency, f.sub.notch-US, and a downstream notch frequency, f.sub.notch-DS, of respective detected photocurrents from at least one respective pilot tone frequency at which the respective detected photocurrent is equal to or less than a photocurrent threshold; and c. estimating a propagation delay difference of the optical link depending on the upstream and downstream notch frequencies.

9. The method according to claim 8, wherein the respective notch frequency is determined from at least first and second pilot tone frequencies at which the respective detected photocurrents equal the photocurrent threshold.

10. The method according to claim 9, wherein the respective notch frequency is calculated as an average of first and second pilot tone frequencies at which the respective detected photocurrents equal the photocurrent threshold.

11. The method according to claim 9, wherein the respective notch frequency is calculated by linear interpolation of four pilot tone frequencies, two pilot tone frequencies at which the respective detected photocurrents equal the photocurrent threshold and two further pilot tone frequencies at which the respective detected photocurrents equal a second, different, photocurrent threshold.

12. The method according to claim 8, wherein the estimate of the propagation delay difference depends inversely on a difference of the upstream and downstream notch frequencies.

13. The method according to claim 8, wherein the upstream and downstream optical carriers respectively have an upstream wavelength, λ.sub.US, and a downstream wavelength, λ.sub.DS, and wherein the propagation delay difference, ΔT, is estimated as $\Delta T=\frac{c}{4}\left(\frac{1}{{\lambda }_{\mathrm{DS}}{f}_{\mathrm{notch}-\mathrm{DS}}^2}-\frac{1}{{\lambda }_{\mathrm{US}}{f}_{\mathrm{notch}-\mathrm{US}}^2}\right).$

14. The method according to claim 8, wherein the optical link comprises a first optical fibre for downstream transmission and a second optical fibre for upstream transmission, and wherein the method additionally comprises estimating respective lengths of the optical fibres depending on respective dispersion coefficients of the optical fibres at preselected respective wavelengths and respective notch frequencies of respective pilot tones transmitted on respective optical carriers at the respective wavelengths.

15. A node for a communication network, the node comprising: a transmitter configured to transmit a downstream pilot tone on a downstream optical carrier and configured to tune a frequency of the downstream pilot tone within a preselected frequency range; and a receiver configured to receive an upstream pilot tone on an upstream optical carrier from a second node, a frequency of the upstream pilot tone varying within a preselected frequency range, and configured to determine an upstream notch frequency from at least one upstream pilot tone frequency at which a respective detected photocurrent is equal to or less than a photocurrent threshold; and the node being configured to provide an indication of the upstream notch frequency to processing circuitry configured to estimate a propagation delay difference of an optical link comprising the node and the second node.

16. A method at a node for a communication network, the method comprising: a. transmitting a downstream pilot tone on a downstream optical carrier, a frequency of the downstream pilot tone being tuned within a preselected frequency range; b. receiving an upstream pilot tone on an upstream optical carrier from a second node, a frequency of the upstream pilot tone varying within a preselected frequency range; c. determining an upstream notch frequency from at least one upstream pilot tone frequency at which a respective detected photocurrent is equal to or less than a photocurrent threshold; and d. providing an indication of the upstream notch frequency to processing circuitry configured to estimate a propagation delay difference of an optical link comprising the node and the second node.

17. A control system comprising processing circuitry configured to: receive an upstream notch frequency and a downstream notch frequency of an optical link, the upstream notch frequency and the downstream notch frequency corresponding to respective detected photocurrents and being determined from at least one respective tone frequency at which the respective detected photocurrent is equal to or less than a photocurrent threshold; and estimate a propagation delay difference of the optical link depending inversely on a difference of the upstream and downstream notch frequencies.

18. The optical link according to claim 2, wherein the processing circuitry is configured to estimate the propagation delay difference depending inversely on a difference of the upstream and downstream notch frequencies.

19. The optical link according to claim 2, wherein the optical fibre link further comprises a first optical fibre for downstream transmission and a second optical fibre for upstream transmission, and wherein the processing circuitry is additionally configured to estimate respective lengths of the optical fibres depending on respective dispersion coefficients of the optical fibres at preselected respective wavelengths and respective notch frequencies of respective pilot tones transmitted on respective optical carriers at the respective wavelengths.

20. The method according to claim 9, wherein the estimate of the propagation delay difference depends inversely on a difference of the upstream and downstream notch frequencies.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1, 2 and 5 illustrate optical links for a communication network according to embodiments of the invention;

(2) FIGS. 3 and 4 illustrate estimation of a notch frequency according to embodiments of the invention;

(3) FIG. 6 illustrates notch frequency as a function of fibre length of the optical link;

(4) FIG. 7 illustrates required accuracy of the pilot tone frequency;

(5) FIGS. 8 to 12 illustrate steps of methods of estimating a propagation delay difference of an optical link of a communication network according to embodiments of the invention;

(6) FIGS. 13 and 14 illustrate nodes for a communication network according to embodiments of the invention; and

(7) FIG. 15 illustrates steps of a method at a node for a communication network according to an embodiment of the invention.

DETAILED DESCRIPTION

(8) The same reference numbers will used for corresponding features in different embodiments.

(9) Referring to FIG. 1, an embodiment of the invention provides an optical link 100 for a communication network, comprising an optical fibre link 102, a downstream transmitter 112, a downstream receiver 114, an upstream transmitter 122 and an upstream receiver 124 and processing circuitry 130.

(10) The downstream transmitter 112, upstream receiver and processing circuitry 130 are provided at node A 110. The upstream transmitter 122 and downstream receiver 124 are provided at node B 120. A bandsplitter 116, 126 is provided at each node for routing upstream and downstream optical signals to and from the optical fibre link 102, which is configured for bidirectional propagation.

(11) The downstream transmitter 112 is configured to transmit a downstream pilot tone on a downstream optical carrier and is configured to tune a frequency of the downstream pilot tone within a preselected frequency range. The upstream transmitter 122 is configured to transmit an upstream pilot tone on an upstream optical carrier and is configured to tune a frequency of the upstream pilot tone within the preselected frequency range.

(12) The upstream receiver 114 is configured to determine an upstream notch frequency, f.sub.notch-US, of detected photocurrents from at least one upstream pilot tone frequency at which the respective detected photocurrent is equal to or lower than a photocurrent threshold. The downstream receiver 124 is similarly configured to determine a downstream notch frequency, f.sub.notch-DS, from at least one downstream pilot tone frequency at which the respective detected photocurrent is equal to or lower than the photocurrent threshold.

(13) The processing circuitry 130 is configured to receive the upstream and downstream notch frequencies and is configured to estimate a propagation delay difference of the optical link depending on the upstream and downstream notch frequencies.

(14) An optical link 200 according to another embodiment of the invention is illustrated in FIG. 2. In this embodiment, the processing circuitry is remotely located in a control system 230 such as a network control system or a network management system.

(15) Referring to FIG. 3, the receivers 114, 124 are configured to calculate the respective notch frequency as an average of a first pilot tone frequency, f.sub.1, and a second pilot tone frequency, f.sub.2, at which respective detected photocurrents 140 equal the photocurrent threshold, 142. The first pilot tone frequency is below the notch frequency, f.sub.notch, and the second pilot tone above the notch frequency.

(16) When a pure tone, e.g. a sinusoidal signal, is sent into an optical fibre system, the detected photocurrent is proportional to:

(17) $\begin{array}{cc}\cos \left(\frac{\pi f_{\mathrm{sc}}^2{\lambda }^2\mathrm{DL}}{c}\right)& \mathrm{Equation}1\end{array}$

(18) where f.sub.SC is the tone frequency, λ is the optical carrier wavelength, D is the chromatic dispersion coefficient in ps/(nm.Math.km), L is the fibre length, and c is the speed of light in vacuum.

(19) The photocurrent presents a notch at the tone frequency:

(20) $\begin{array}{cc}{f}_{\mathrm{notch}}=\sqrt{\frac{c}{2{\lambda }^2\mathrm{DL}}}& \mathrm{Equation}1\end{array}$

(21) f.sub.notch is the frequency at which, in theory, no power is detected at the tone frequency regardless of its transmitted power.

(22) In practice, due to the noise floor of the transmission system, a small power will still be detected at the notch frequency. A reasonable accuracy in determining the notch frequency can be achieved by setting a photocurrent threshold slightly higher than the noise floor. For example, the threshold may be 50-60 dB lower than the photocurrent detected at the tone frequency in a system where the optical fibre is replaced by an attenuator having an equivalent attenuation to the fibre.

(23) If the noise floor is significant, a higher threshold can be used, continuing to move the detected tone frequency beyond the notch frequency. The notch frequency can be estimated as the average of two tone frequencies, one lower than the notch frequency, the other higher, for which the fibre response equals the photocurrent threshold.

(24) Referring to FIG. 4, the receivers 114, 124 are configured to calculate the respective notch frequency, f.sub.notch, by linear interpolation of four pilot tone frequencies; two pilot tone frequencies, f.sub.2 and f.sub.3, at which respective detected photocurrents equal a first photocurrent threshold 140; and two pilot tone frequencies, f.sub.1 and f.sub.4, at which respective detected photocurrents equal a second, higher, photocurrent threshold 152. Two of the pilot tone frequencies, f.sub.3 and f.sub.4, are above the notch frequency and two of the pilot tone frequencies, f.sub.1 and f.sub.2, are below the notch frequency.

(25) Using two thresholds, enables the accuracy of determining the notch frequency to be improved by considering that the slope of the photocurrent is slightly different below and above the notch frequency. The notch frequency may be calculated as:

(26) $f_{\mathrm{notch}}=\frac{f_4{f}_1-f_2{f}_3}{f_1-f_2-f_3+f_4}$

(27) In an embodiment, the receivers 114, 124 are direct detection receivers, such as small form-factor pluggable, SFP, transceivers.

(28) In an embodiment, the processing circuitry 130, 230 is configured to estimate the propagation delay difference depending inversely on a difference of the upstream and downstream notch frequencies.

(29) In an embodiment, the upstream optical carrier has an upstream wavelength, λ.sub.US, and the downstream optical carrier has a downstream wavelength, λ.sub.DS. The processing circuitry 130, 230, 330 is configured to estimate the propagation delay difference, ΔT, as:

(30) $\Delta T=\frac{c}{4}\left(\frac{1}{{\lambda }_{\mathrm{DS}}{f}_{\mathrm{notch}-\mathrm{DS}}^2}-\frac{1}{{\lambda }_{\mathrm{US}}{f}_{\mathrm{notch}-\mathrm{US}}^2}\right)$

(31) It can be demonstrated, as below, that propagation delay, T, depends on the product λDL as:

(32) $\begin{array}{cc}T=\mathrm{AL}+\frac{\lambda \mathrm{DL}}{2}& \mathrm{Equation}2\end{array}$

(33) where A is an unknown constant.

(34) According to ITU-T G.652 Recommendation, for a standard single mode optical fibre, the fibre chromatic coefficient depends on wavelength as:

(35) 0$\begin{array}{cc}D\left(\lambda \right)=\frac{\lambda S_0}{4}\left[1-{\left(\frac{{\lambda }_0}{\lambda }\right)}^4\right]& \mathrm{Equation}A1\end{array}$

(36) where D is the chromatic dispersion coefficient in ps/nm/km, λ is the wavelength in nm, λ.sub.0 is the zero dispersion wavelength (1300 nm for a standard single mode fibre), and S.sub.0 is the slope at λ.sub.0 (0.092 ps/nm.sup.2×km for a standard single mode fibre).

(37) A delay coefficient in ps/km can be calculated by integrating equation A1 with respect to wavelength

(38) $\begin{array}{cc}\frac{T\left(\lambda \right)}{L}=A+\frac{{\lambda }^2{S}_0}{8}\left[1-{\left(\frac{{\lambda }_0}{\lambda }\right)}^4\right]& \mathrm{Equation}\mathrm{A2}\end{array}$

(39) where L is the fibre length in km.

(40) From Equation 1 and Equation 2, we obtain:

(41) $\begin{array}{cc}T=\mathrm{AL}+\frac{c}{4\lambda f_{\mathrm{notch}}^2}& \mathrm{Equation}3\end{array}$

(42) The difference between downstream and upstream delay, in a bidirectional system, where downstream and upstream fibre lengths are equal, can be calculated as:

(43) $\begin{array}{cc}\Delta T=\frac{c}{4}\left(\frac{1}{{\lambda }_{\mathrm{DS}}{f}_{\mathrm{notch}-\mathrm{DS}}^2}-\frac{1}{{\lambda }_{\mathrm{US}}{f}_{\mathrm{notch}-\mathrm{US}}^2}\right)& \mathrm{Equation}4\end{array}$

(44) An optical link 300 according to a further embodiment of the invention is illustrated in FIG. 3. In this embodiment, the optical fibre link comprises a first optical fibre 302 for downstream transmission and a second optical fibre 304 for upstream transmission. Each optical fibre 302, 304 is configured for uni-directional propagation.

(45) The processing circuitry 300 is additionally configured to estimate a respective length each of the optical fibres 302, 304, depending on respective dispersion coefficients of the optical fibres at preselected respective wavelengths and respective notch frequencies of respective pilot tones transmitted on respective optical carriers at the respective wavelengths.

(46) Unidirectional fibre systems use two fibres for downstream and upstream transmission so the same fibre length cannot be assumed for the two directions. Since the fibres are different, the chromatic dispersion coefficients also cannot be assumed to be equal. The fibre length needs to be estimated in order to use Equation 3. This can be done assuming that: the constant A (the inverse of fibre group velocity or, equivalently, the ratio between the light speed in vacuum and the effective refractive index) is known, e.g. from the fibre data sheet; and the chromatic dispersion coefficient at one wavelength, λ.sub.1, is known (e.g. from the fibre data sheet).

(47) Measuring the notch frequency at λ.sub.1 the fibre length L can be estimated using Equation 1:

(48) $L=\frac{c}{2{\lambda }^{2}D\left({\lambda }_1\right)f_{\mathrm{notch}}^2\left({\lambda }_1\right)}$

(49) Using Equation 2 again, we obtain the chromatic dispersion coefficient at any wavelength:

(50) $\begin{array}{cc}D\left(\lambda \right)=\frac{{\lambda }_1^2{f}_{\mathrm{notch}}^2\left({\lambda }_1\right)}{{\lambda }^2{f}_{\mathrm{notch}}^2\left(\lambda \right)}D\left({\lambda }_1\right)& \mathrm{Equation}5\end{array}$

(51) In an embodiment, the upstream and downstream transmitters 112, 122 are configured to tune a frequency of the pilot tone within the preselected frequency range, starting at an initial frequency which is not equal to a transfer function notch frequency of the optical fibre link 102, 302, 304.

(52) In an embodiment, referring to FIGS. 1, 2 and 5, the receivers 114, 124 are configured to send an acknowledgement message to the respective transmitter 122, 112 to indicate receipt of the pilot tone at a currently set frequency. The transmitters are configured to vary the frequency of the pilot tone by a predetermined amount in response to receiving a said upstream acknowledgement message. The transmitters may therefore be instructed to stop transmission of the pilot tone once the notch frequency has been determined.

(53) A pilot tone is sent downstream, together with the signal data, into an optical fibre link 102, 302. When the receiver 124 detects the tone, it acknowledges the transmitter by means of an upstream message.

(54) The pilot tone frequency is gradually tuned until the receiver communicates through the upstream message that it is longer able to detect the tone.

(55) When this happens, it means that the pilot tone frequency equals the frequency of the notch of the fibre response, which depends on fibre chromatic dispersion and length; that is, ultimately, on the propagation delay in the downstream direction.

(56) Performing the same procedure in the upstream direction, the upstream propagation delay is estimated so that the delay asymmetry can be calculated by difference.

(57) The optical link offers various advantages, as follows. It is applicable to cost effective direct detection systems, and does not require coherent transmission. It requires a limited number of additional blocks, all of which are easy to integrate in a cost effective and small form factor transceiver, e.g. in a SFP transceiver. It enables accuracy of propagation delay difference in the ns range to be determined, which covers all currently relevant applications, such as those in the NG-PON2 and G.metro standards. It allows calculation of propagation delay asymmetry also when traffic data are transmitted; that is to say, in service operation is possible.

(58) In an embodiment, the transmitters 112, 122 are configured to tune the frequency of the pilot tone in steps of a predetermined step size starting at an initial frequency. The receivers 114, 124 are configured to send a reporting message to the respective transmitter when an expected pilot tone is not received. The notch frequency may therefore be determined without requiring signalling between the receiver and the respective transmitter.

(59) In an embodiment, the receivers 114, 124 are configured to extract the respective pilot tone from the respective traffic signal. The receivers may comprise a narrowband phase locked loop to extract a sinusoidal pilot tone from traffic signal. Alternatively, the pilot tone may comprise a codeword that the receivers are configured to extract from the traffic signal.

(60) In an embodiment, the pilot tones are transmitted in an available field of the respective traffic signal's data frame, such as a reference frame or an optical transport network, OTN, frame.

(61) In an embodiment, the transmitters 112, 122 are configured to generate and transmit the pilot tones as an overmodulated channel.

(62) In an embodiment, the transmitters 112, 122 each comprise an oscillator configured to generate the respective pilot tone with a stability of a few parts per million, ppm. A stability of a few ppm is sufficient to estimate the propagation delay difference to an accuracy in the order of nanoseconds, ns, which meets currently envisaged fronthaul network applications in the NG-PON2 and G.metro standards. Low cost, free to run oscillators may therefore be used.

(63) To demonstrate that the notch frequency can be determined using low cost oscillators that can be integrated in a small form factor module, for example a SFP, notch frequency, GHz, as a function of fibre length, Km, at a wavelength of 1550 nm and fibre dispersion coefficient of 17 ps/nm/km was plotted, as shown in FIG. 6.

(64) It can be seen in FIG. 6 that for any practical distance between 5 km and 100 km, the notch frequency is in the order of 10 GHz, which is feasible with off the shelf oscillators. The frequency steeply increases for very fibre lengths but in this range fibre delay is less critical.

(65) For long fibre lengths, the curve becomes flat, indicating that big variations in length (and delay) correspond to small variations in the notch frequency. This may give rise to potential accuracy issues if the oscillator is not stable enough. To verify the absence of accuracy issues, the notch frequency accuracy, in ppm, required to achieve 1 m of length resolution, was plotted as a function of fibre length, in Km, as shown in FIG. 7.

(66) The notch frequency accuracy is calculated as:

(67) $\begin{array}{cc}\mathrm{Frequency}\mathrm{accuracy}=\frac{f_{\mathrm{notch}}\left(L\right)-f_{\mathrm{notch}}\left(L+1m\right)}{f_{\mathrm{notch}}\left(L\right)}& \mathrm{Equation}6\end{array}$

(68) With a typical effective refractive index value of 1.5, 1 m of optical fibre corresponds to 5 ns of delay. FIG. 7 shows that cheap, free run oscillators with a stability of a few ppm are sufficient to meet the accuracy requirements of a fronthaul system.

(69) In an embodiment, the transmitters 112, 122 are configured to generate and transmit pilot tones in a frequency range in which a spectrum of the respective traffic signal is negligible.

(70) It is desirable, even if not necessary, that the pilot tone frequency is within a frequency range where the traffic signal spectrum is negligible. Considering a fibre length of up to 30 km, which is the case for time-sensitive fronthaul, and looking at the vertical axis of FIG. 6, this is the case for data rates up to 10 Gbit/s, which typically have significant spectral content below 7 GHz. For 25 Gbit/s data rates, the pilot tone needs to be extracted from the data signal. This can be done by sending a codeword instead of a pure tone or, the tunable pilot tone can be synthetized and discriminated by means of a phase-locked loop, PLL, that can be easily integrated in a small form factor transceiver (e.g. a SFP) with acceptable additional cost.

(71) For an optical carrier generated using a direct modulation laser, the pilot tone is generated by modulating the laser bias current. Where the optical carrier is externally modulated, the pilot tone may be summed to the voltage or current carrying the traffic data and then applied to the electrical input of the modulator.

(72) Corresponding embodiments apply to the method of estimating a propagation delay difference of an optical link, the node for a communication network and the control system described below.

(73) Referring to FIG. 8, an embodiment of the invention provides method 400 of estimating a propagation delay difference of an optical link of a communication network.

(74) The method comprises steps of:

(75) transmitting 402 respective pilot tones on upstream and downstream optical carriers, the frequencies of the pilot tones being tuned within a preselected frequency range;

(76) determining 404 an upstream notch frequency, f.sub.notch-US, and a downstream notch frequency, f.sub.notch-DS, of respective detected photocurrents from at least one respective pilot tone frequency at which the respective detected photocurrent is equal to or lower than a photocurrent threshold; and

(77) estimating 406 a propagation delay difference of the optical link depending on the upstream and downstream notch frequencies.

(78) Referring to the embodiment illustrated in FIG. 9, the respective notch frequency is determined 502 from at least first and second pilot tone frequencies at which the respective detected photocurrents equal the photocurrent threshold.

(79) As described above with reference to FIG. 3, the notch frequency is calculated as an average of a first pilot tone frequency, f.sub.1, and a second pilot tone frequency, f.sub.2, at which respective detected photocurrents 140 equal the photocurrent threshold, 142. The first pilot tone frequency is below the notch frequency, f.sub.notch, and the second pilot tone above the notch frequency.

(80) As described above, if the noise floor is significant, a higher threshold can be used, continuing to move the detected tone frequency beyond the notch frequency.

(81) As described above with reference to FIG. 4, the notch frequency, f.sub.notch, may alternatively be calculated by linear interpolation of four pilot tone frequencies; two pilot tone frequencies, f.sub.2 and f.sub.3, at which respective detected photocurrents equal a first photocurrent threshold 140; and two pilot tone frequencies, f.sub.1 and f.sub.4, at which respective detected photocurrents equal a second, higher, photocurrent threshold 152. Two of the pilot tone frequencies, f.sub.3 and f.sub.4, are above the notch frequency and two of the pilot tone frequencies, and f.sub.2, are below the notch frequency. The notch frequency may be calculated as:

(82) $f_{\mathrm{notch}}=\frac{f_4{f}_1-f_2{f}_3}{f_1-f_2-f_3+f_4}$

(83) The pilot tone frequency is tuned with a step size small enough to allow detection of the decreasing slope of the first lobe of the fibre link frequency response, as illustrated in FIG. 4.

(84) In an embodiment, the estimate of the propagation delay difference depends inversely on a difference of the upstream and downstream notch frequencies.

(85) Referring to FIG. 10, in an embodiment the upstream pilot tone is transmitted 602 on an upstream optical carrier having an upstream wavelength, λ.sub.US, and the downstream pilot tone is transmitted 602 on a downstream optical carrier having a downstream wavelength, λ.sub.DS. The propagation delay difference, ΔT, is estimated 604 as:

(86) $\Delta T=\frac{c}{4}\left(\frac{1}{{\lambda }_{\mathrm{DS}}{f}_{\mathrm{notch}-\mathrm{DS}}^2}-\frac{1}{{\lambda }_{\mathrm{US}}{f}_{\mathrm{notch}-\mathrm{US}}^2}\right).$

(87) In an embodiment the optical link comprises a first optical fibre for downstream transmission and a second optical fibre for upstream transmission, as illustrated in FIG. 5. As illustrated in FIG. 11, the method additionally comprises estimating 702 respective lengths of the optical fibres 302, 304 depending on respective dispersion coefficients of the optical fibres at preselected respective wavelengths and respective notch frequencies of the pilot tones transmitted on respective optical carriers at the respective wavelengths.

(88) In an embodiment, the frequency of the pilot tone is tuned within the preselected frequency range, starting at an initial frequency which is not equal to a transfer function notch frequency of the optical fibre link 102, 302, 304.

(89) Referring to FIG. 12, in an embodiment a pilot tone is sent 802 downstream, together with the signal data, into an optical fibre link. An upstream acknowledgement message is sent 804 when a detected photocurrent is greater than the photocurrent threshold.

(90) The pilot tone frequency is gradually tuned 806 until the receiver communicates through the upstream message that it is longer able to detect the tone.

(91) When this happens, it means that the pilot tone frequency equals the frequency of the notch of the fibre response, which depends on fibre chromatic dispersion and length; that is, ultimately, on the propagation delay in the downstream direction.

(92) Performing the same procedure in the upstream direction, the upstream propagation delay is estimated so that the delay asymmetry can be calculated by difference.

(93) A fronthaul system can tolerate differences of propagation delay in the two propagation directions in the order of 10 ns. As discussed above, such small values of resolution can be estimated by transmitting a tunable pilot tone, and varying its frequency until detecting a notch. This requires a bidirectional channel where: 1) The pilot tone frequency is communicated downstream from the transmitter to receiver. To this purpose, an available field in the data frame can be used, as well as a message channel, e.g. generated by superimposing a small modulation to the data modulation, as in the system which is being standardized in the new G. Metro Recommendation. 2) The pilot tone is transmitted at an arbitrary initial frequency. It is a good choice to select a frequency high enough to be sure that it does not equal the notch corresponding to length and chromatic dispersion of the optical fibre link. 3) When the tone is detected, the receiver acknowledges the transmitter via an upstream message. Either a data field or a dedicated channel can be used. 4) The transmitter sets a higher frequency the pilot tone. The incremental step is a design choice depending on the desired frequency accuracy. According to the curve of FIG. 6., the shorter the fibre, the higher the step. In any case, the step should be small enough to allow to detect the decreasing slope of the first lobe of the frequency response in FIG. 4. and FIG. 5. 5) Steps 3 and 4 are repeated until the receiver detects a notch. This condition is assumed to be met when the tone amplitude is below a given threshold. Alternatively, the notch frequency can be estimated by averaging frequencies at which the tone amplitude has equal values below and above the notch frequency (see FIG. 3 and FIG 4. 6) The receiver tells the transmitter to stop sending the pilot tone since the notch frequency has been reached.

(94) Then, steps 1 to 5 are repeated in the upstream direction, by an upstream transmitter and receiver.

(95) Finally, the upstream and downstream notch frequencies are used to estimate the propagation delay difference between the two directions using Equation 5.

(96) FIG. 13 illustrates a node 900 for a communication network, according to an embodiment of the invention. The node 900 comprises a transmitter 902 and a receiver 904. A bandsplitter 906 is provided for routing upstream and downstream optical signals to and from an optical fibre link 908, configured for bidirectional propagation.

(97) The transmitter 902 configured to transmit a downstream pilot tone on a downstream optical carrier and is configured to tune a frequency of the downstream pilot tone within a preselected frequency range. The receiver 904 is configured to receive an upstream pilot tone on an upstream optical carrier from a second node; a frequency of the received upstream pilot tone varies within a preselected frequency range. The receiver is configured to determine an upstream notch frequency, f.sub.notch-US, from at least one upstream pilot tone frequency at which a respective detected photocurrent is equal to or lower than a photocurrent threshold.

(98) The node 900 is configured to generate an output signal 910 comprising an indication of the upstream notch frequency to processing circuitry configured to estimate a propagation delay difference of an optical link comprising the node 900 and the second node.

(99) The processing circuitry in this embodiment is remotely located, in for example a network management system or network control system.

(100) A further embodiment of the invention provides a control system comprising processing circuitry. The processing circuitry is configured to receive an upstream notch frequency and a downstream notch frequency of an optical link, and is the processing circuitry is configured to estimate a propagation delay difference of the optical link depending on the upstream and downstream notch frequencies.

(101) FIG. 14 illustrates a node 1000 for a communication network, according to another embodiment of the invention. In this embodiment, the node additionally comprises processing circuitry 1010 configured to determine an upstream notch frequency, f.sub.notch-US, from at least one upstream pilot tone frequency at which a respective detected photocurrent is equal to or lower than a photocurrent threshold. The receiver 904 is configured to provide an indication of the upstream notch frequency to the processing circuitry.

(102) FIG. 15 illustrates steps of a method 1100 at a node for a communication network according to an embodiment of the invention. The method comprises steps of:

(103) a. transmitting 1002 a downstream pilot tone on a downstream optical carrier, the frequency of the downstream pilot tone being tuned within a preselected frequency range;

(104) b. receiving 1004 an upstream pilot tone on an upstream optical carrier from a second node, a frequency of the upstream pilot tone varying within a preselected frequency range;

(105) c. determining 1006 an upstream notch frequency from at least one upstream pilot tone frequency at which a respective detected photocurrent is equal to or lower than a photocurrent threshold; and

(106) d. providing 1008 an indication of the upstream notch frequency to processing circuitry configured to estimate a propagation delay difference of an optical link comprising the node and the second node.

(107) In an embodiment, the method comprises generating an output signal comprising an indication of the upstream notch frequency and sending the output signal to remotely located processing circuitry.

(108) An embodiment of the invention provides a computer program, comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out any of the steps of the methods of the above embodiments.

(109) An embodiment of the invention provides a carrier containing a computer program, comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out any of the steps of the methods of the above embodiments.

(110) The carrier is one of an electronic signal, optical signal, radio signal, or computer readable storage medium.