Controlling optical signal power levelling in an optical communication network

Abstract

A method (10) of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation, α, to a pass-through optical signal. The method comprises: a. performing the following steps i. to iii. until an attenuation variation value, Δα, is greater than a preselected attenuation variation threshold value (18), Δα.sub.TH: i. measuring (12) an optical signal power of an optical signal; ii. calculating (14) a difference, ΔP, between the measured optical signal power and a target optical signal power; iii. calculating (16) a value for the attenuation variation, Δα, to be applied to the optical attenuation taking account of ΔP; b. obtaining (20) a current value of the optical attenuation, α.sub.n, and obtaining (22) a new optical attenuation value, α.sub.n+1, in dependence on the current value of the optical attenuation, a current value of the attenuation variation, Δα.sub.n, and at least an earlier value of the attenuation variation, Δ.sub.αn−1; and c. generating (24) a control signal arranged to configure the node to apply the new optical attenuation value, α.sub.n+1.

Claims

1. A method of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation, α, to a pass-through optical signal, the method comprising: a. performing the following steps i. to iii. until an attenuation variation value, Δα, is greater than a preselected attenuation variation threshold value, Δα.sub.TH: i. measuring an optical signal power of an optical signal; ii. calculating a difference, ΔP, between the measured optical signal power and a target optical signal power; iii. calculating a value for the attenuation variation, Δα, to be applied to the optical attenuation taking account of ΔP; b. obtaining a current value of the optical attenuation, α.sub.n, and obtaining a new optical attenuation value, α.sub.n+1, in dependence on the current value of the optical attenuation, a current value of the attenuation variation, Δα.sub.n, and at least an earlier value of the attenuation variation, Δα.sub.n−1; and c. generating a control signal arranged to configure the node to apply the new optical attenuation value, α.sub.n+1.

2. A method as claimed in claim 1, wherein step a. comprises performing steps i. to iii. and cumulatively summing each calculated value of the attenuation variation, Δα.sub.n, until a current value of the cumulated sum, Δα(sum).sub.n, is greater than the preselected attenuation variation threshold value, Δα.sub.TH, and storing each value of the cumulated sum, and in step b. the new optical attenuation value, α.sub.n+1, is obtained in dependence on the current value of the cumulated sum, Δα(sum).sub.n, and at least an earlier stored value of the cumulated sum, Δα(sum).sub.n−1.

3. A method as claimed in claim 2, wherein in step b., the new optical attenuation value, α.sub.n+1, is obtained if there is at least one earlier stored value of the cumulated sum, Δα(sum).sub.n−1, which is greater than the threshold value, Δα.sub.TH, the new optical attenuation value, α.sub.n+1, being obtained by summing the current value of the optical attenuation, α.sub.n, with the current value of the cumulated sum, Δα(sum).sub.n, and if there is not at least one earlier stored value of the cumulated sum which is greater than the threshold value, the method comprises clearing the cumulated sum and recommencing at step a.

4. A method as claimed in claim 3, wherein in step b., the new optical attenuation value, α.sub.n+1, is obtained if there are two consecutive stored values of the cumulated sum, Δα(sum).sub.n, Δα(sum).sub.n−1, which are each greater than the threshold value, Δα.sub.TH, and if there are not two consecutive stored values of the cumulated sum which are greater than the threshold value, the method comprises clearing the cumulated sum and recommencing at step a.

5. A method as claimed in claim 2, wherein in step a.iii, the value for the attenuation variation, Δα.sub.n, is calculated as Δα.sub.n=ΔP*K, where K is a preselected first smoothing factor, and wherein each value of the cumulated sum, Δα(sum).sub.n, is stored as a sample in a register configured to store N samples, where N is proportional to the inverse of the first smoothing factor, K.

6. A method as claimed in claim 5, wherein N is at least (1/K)+1.

7. A method as claimed in claim 5, wherein in step b. the method comprises multiplying the current value of the cumulated sum, Δα(sum).sub.n, with a second smoothing factor, having a value of less than 1, before summing with the current value of the optical attenuation, α.sub.n.

8. A method as claimed in claim 5, the new optical attenuation value, α.sub.n+1, is obtained if there are at least M consecutive stored values of the cumulated sum, Δα(sum), which are each greater than the threshold value, Δα.sub.TH, where M is a positive integer calculated as a function of a number of optical communication network nodes configured to apply an optical attenuation to a pass-through optical signal which precede the said optical communication network node in a path of the optical signal.

9. A method as claimed in claim 8, wherein M is a positive integer calculated as a monotonically increasing function of said number of optical communication network nodes.

10. A method as claimed in claim 8, wherein N is (1/K)*M+1.

11. A method as claimed in claim 1, wherein the optical communication network node is further configured to apply a further optical attenuation, α(add), to an add optical signal and the method further comprises, if the optical signal is an add optical signal, performing the following steps d. and e. in place of steps b. and c.: d. obtaining a current value of the further optical attenuation, α(add).sub.n, and summing said value, α(add).sub.n, with the current attenuation variation value, Δα.sub.n, to form a new further optical attenuation value, α(add).sub.n+1; and e. generating a control signal arranged to configure the node to apply the new further optical attenuation value, α(add).sub.n+1.

12. A method of controlling optical signal power levelling in an optical communication network path comprising a plurality of optical communication network nodes each configured to apply a respective optical attenuation, α, to a pass-through optical signal propagating along the path, the method comprising configuring each optical communication network node to perform the method of controlling optical signal power levelling as claimed in claim 1.

13. A method as claimed in claim 12, wherein the method comprises: generating a path message arranged to configure the path and transmitting the path message to each node serially along the path, the path message comprising an indication of a number of nodes crossed by the path message and the indication of the number of nodes being respectively updated at each node.

14. A method as claimed in claim 13, wherein the path message is one of a strict explicit route object RSVP-TE protocol message and a loose explicit route object RSVP-TE protocol message.

15. A method as claimed in claim 13, wherein the indication of a number of nodes crossed by the path message is one of a node count field arranged to store the number of nodes crossed by the path message and a node list comprising an identification of each node crossed by the path message.

16. A non-transient computer readable medium having computer readable instructions embodied therein, the computer readable instructions being for providing access to resources available on a processor and the computer readable instructions comprising instructions to cause the processor to perform the steps of claim 1.

17. A non-transient computer readable medium having computer readable instructions embodied therein, the computer readable instructions being for providing access to resources available on a processor and the computer readable instructions comprising instructions to cause the processor to perform the steps of claim 12.

18. An optical communication network node comprising: an input arranged to receive an optical signal; optical attenuation apparatus configured to apply an optical attenuation, α, to the optical signal; optical signal power measurement apparatus arranged to measure an optical signal power of the optical signal; and a controller arranged to: a. perform the following steps i. to iii. until an attenuation variation value, Δα, is greater than a preselected attenuation variation threshold value, Δα.sub.TH: i. receive a measured optical signal power from the optical signal power measurement apparatus; ii. calculate a difference, ΔP, between the measured optical signal power and a target optical signal power; iii. calculate a value for the attenuation variation, Δα, to be applied to the optical attenuation taking account of ΔP; b. obtain a new optical attenuation value, α.sub.n+1, in dependence on the current value of the optical attenuation, the current attenuation variation value, Δα.sub.n, and at least an earlier attenuation variation value Δα.sub.n−1; and c. generate a control signal arranged to configure the optical attenuation apparatus to apply the new optical attenuation value, α.sub.n+1.

19. The optical communication network node of claim 18, wherein the optical attenuation apparatus comprises a wavelength selective switch.

20. The optical communication network node of claim 18, wherein the optical signal power measurement apparatus comprises an optical channel monitor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the steps of a method according to a first embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(2) FIG. 2 shows the steps of a method according to a second embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(3) FIG. 3 shows the steps of a method according to a third embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(4) FIG. 4 shows the steps of a method according to a fourth embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(5) FIG. 5 shows the steps of a method according to a fifth embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(6) FIG. 6 shows the steps of a method according to a sixth embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(7) FIG. 7 shows the steps of a method according to a seventh embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(8) FIG. 8 shows the steps of a method according to an eighth embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(9) FIG. 9 shows the steps of a method according to an eleventh embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(10) FIG. 10 shows the steps of a method according to a twelfth embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(11) FIG. 11 shows the steps of a method according to a thirteenth embodiment of the invention of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal;

(12) FIG. 12 shows simulated optical signal power readings at the 1.sup.st, 5.sup.th, 10.sup.th and 20.sup.th nodes of an optical communication network path comprising 20 nodes as a function of cycles of the method of FIG. 10, excluding steps 120, 124, 126, 128 (step 122 proceeds directly to step 132), where K=0.1, Δα.sub.TH=0.2 dB;

(13) FIG. 13 shows simulated optical signal power readings at the 1.sup.st, 5.sup.th, 10.sup.th and 20.sup.th nodes of an optical communication network path comprising 20 nodes as a function of cycles of the method of FIG. 10, where N=11, K=0.1, Δα.sub.TH=0.2 dB, and no second smoothing factor is applied (K′=0);

(14) FIG. 14 shows simulated optical signal power readings each of the 20 nodes of an optical communication network path as a function of cycles of the method of FIG. 10, where N=11, K=0.1, Δα.sub.TH=0.2 dB, and K′=0.2;

(15) FIG. 15 shows simulated optical signal powers each of the 20 nodes of an optical communication network path as a function of cycles of the method of FIG. 10, where N=(10*M)+1, K=0.1, Δα.sub.TH=0.2 dB, K′=0.2, and M=sqrt (no. nodes);

(16) FIG. 16 shows simulated optical signal power readings at the 1.sup.st, 5.sup.th, 10.sup.th and 20.sup.th nodes of an optical communication network path comprising 20 nodes in response to a ΔP of 1 dB at the 1.sup.st node of the path, applying the method of FIG. 10 excluding steps 120, 124, 126, 128 (step 122 proceeds directly to step 132), where K=0.1, Δα.sub.TH=0.2 dB;

(17) FIG. 17 shows simulated optical signal power readings at the 1.sup.st, 5.sup.th, 10.sup.th and 20.sup.th nodes of an optical communication network path comprising 20 nodes in response to a ΔP of 1 dB at the 1.sup.st node of the path, applying the method of FIG. 10, where N is 11, K=0.1, Δα.sub.TH=0.2 dB, and K′=0.2;

(18) FIG. 18 shows simulated optical signal power readings at the 1.sup.st, 5.sup.th, 10.sup.th and 20.sup.th nodes of an optical communication network path comprising 20 nodes in response to a ΔP of 1 dB at the 1.sup.st node of the path, applying the method of FIG. 10, where N=(10*M)+1, K=0.1, Δα.sub.TH=0.2 dB, K′=0.2, and M=sqrt (no. nodes);

(19) FIG. 19 shows the steps of a method according to a fifteenth embodiment of the invention of controlling optical signal power levelling in an optical communication network path;

(20) FIG. 20 is a schematic representation of an optical communication network node according to a sixteenth embodiment of the invention;

(21) FIG. 21 is a schematic representation of an optical communication network node according to a seventeenth embodiment of the invention;

(22) FIG. 22 shows the steps of a method according to an eighteenth embodiment of the invention of configuring a path across an optical communication network; and

(23) FIG. 23 is a schematic representation of an optical communication network path message according to a nineteenth embodiment of the invention.

DETAILED DESCRIPTION

(24) Referring to FIG. 1 a first embodiment of the invention provides a method 10 of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to a pass-through optical signal.

(25) The method comprises: a. performing the following steps i. to iii. until an attenuation variation value, Δα, is greater than a preselected attenuation variation threshold value 18, Δα.sub.TH: i. measuring 12 an optical signal power of an optical signal; ii. calculating 14 a difference, ΔP, between the measured optical signal power and a target optical signal power; iii. calculating 16 a value for the attenuation variation, Δα, to be applied to the optical attenuation taking account of ΔP; b. obtaining 20 a current value of the optical attenuation, α.sub.n, and obtaining 22 a new optical attenuation value, α.sub.n+1, in dependence on the current value of the optical attenuation, a current value of the attenuation variation, Δα.sub.n, and at least an earlier value of the attenuation variation, Δα.sub.n−1; and c. generating 24 a control signal arranged to configure the node to apply the new optical attenuation value, α.sub.n+1.

(26) Referring to FIG. 2 a second embodiment of the invention provides a method 30 of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to a pass-through optical signal.

(27) The method 30 of this embodiment is similar to the method 10 of FIG. 1, with the following modifications. The same reference numbers are retained for corresponding steps.

(28) In this embodiment, step a. comprises performing steps i. to iii. and cumulatively summing 32 each calculated value of the attenuation variation, Δα.sub.n. Step a. further comprises storing each calculated value of the cumulated sum 34. Step a. repeats until a current value of the cumulated sum, Δα(sum).sub.n, is greater than the preselected attenuation variation threshold value 36, Δα.sub.TH.

(29) In step b., the new optical attenuation value, α.sub.n+1, is obtained 38 in dependence on the current value of the cumulated sum, Δα(sum).sub.n, and at least an earlier stored value of the cumulated sum, Δα(sum).sub.n−1.

(30) Referring to FIG. 3 a third embodiment of the invention provides a method 40 of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to a pass-through optical signal.

(31) The method 40 of this embodiment is similar to the method 30 of FIG. 2, with the following modifications. The same reference numbers are retained for corresponding steps.

(32) In this embodiment, in step a.iii, the value for the attenuation variation, Δα.sub.n, is calculated 42 as Δα.sub.n=ΔP*K, where K is a preselected first smoothing factor.

(33) In this embodiment, in step b., the new optical attenuation value, α.sub.n+1, is obtained if 44 there is at least one earlier stored value of the cumulated sum, Δα(sum).sub.n+1, which is greater than the threshold value, Δα.sub.TH. The new optical attenuation value, α.sub.n+1, is obtained 46 by summing the current value of the optical attenuation, α.sub.n, with the current value of the cumulated sum, Δα(sum).sub.n. If there is not at least one earlier stored value of the cumulated sum which is greater than the threshold value, the method comprises clearing the cumulated sum 46 and recommencing at step a.

(34) Optical signal power variations are not expected on pass-through optical signals, therefore if a ΔP is detected the method initially assumes that the ΔP is noise and discards the first value of the cumulated sum which is greater than the threshold value. If there is a second occurrence of the cumulated sum being greater than the threshold value then the method assumes that the ΔP is not noise but rather is an optical signal power different which needs to be corrected.

(35) Referring to FIG. 4 a fourth embodiment of the invention provides a method 50 of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to a pass-through optical signal.

(36) The method 50 of this embodiment is similar to the method 40 of FIG. 3, with the following modifications. The same reference numbers are retained for corresponding steps.

(37) In this embodiment, in step b., the new optical attenuation value, α.sub.n+1, is obtained 52 if there are two consecutive stored values of the cumulated sum, Δα(sum).sub.n, Δα(sum).sub.n−1, which are each greater than the threshold value, Δα.sub.TH. If there are not two consecutive stored values of the cumulated sum which are greater than the threshold value, the method comprises clearing the cumulated sum 46 and recommencing at step a.

(38) Referring to FIG. 5 a fifth embodiment of the invention provides a method 60 of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to a pass-through optical signal.

(39) The method 60 of this embodiment is similar to the method 50 of FIG. 4, with the following modifications. The same reference numbers are retained for corresponding steps.

(40) The method further comprises storing each value of the cumulated sum, Δα(sum).sub.n, as a sample in a register configured to store N samples 64. N is proportional to the inverse of the first smoothing factor, K. N is the number of values of the cumulated sum that need to be stored in the register to enable the method 60 is able to check that there are two consecutive stored values of the cumulated sum which are greater than the threshold value. Storing each value of the cumulated sum, Δα(sum).sub.n, as a sample in a register configured to store N samples, where N is proportional to 1/K, enables the method to check that there are two consecutive stored cumulated sum values which are each greater than the threshold value, Δα.sub.TH, by simply checking whether two of the values stored as samples in the register are greater than the threshold value.

(41) Referring to FIG. 6, a sixth embodiment of the invention provides a method 70 of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to a pass-through optical signal.

(42) The method 70 of this embodiment is similar to the method 60 of FIG. 5, with the following modifications. The same reference numbers are retained for corresponding steps.

(43) The register is configured to store at least (1/K)+1 samples, N (1/K)+1, and in this embodiment the register is configured to store N=(1/K)+1 samples. By storing N samples the method is able to check whether there are two consecutive stored cumulated sum values which are each greater than the threshold value, Δα.sub.TH, by simply checking whether two of the values stored as samples in the register are greater than the threshold value.

(44) Referring to FIG. 7, a seventh embodiment of the invention provides a method 80 of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to a pass-through optical signal.

(45) The method 80 of this embodiment is similar to the method 70 of FIG. 6, with the following modifications. The same reference numbers are retained for corresponding steps.

(46) In this embodiment, in step b. the cumulated sum, Δα(sum).sub.n, is multiplied 82 with a second smoothing factor, K′, before being summed 84 with the current value of the optical attenuation, α.sub.n, to obtain a new optical attenuation value, Δα.sub.n+1:
α.sub.n+1=α.sub.n+K′[Δα(sum).sub.n]

(47) The second smoothing factor has a value of less than 1. K′ may have a value in the range 0.05 to 0.5.

(48) Optical signal power variations are not expected on pass-through optical signals, therefore if a ΔP is detected the method initially assumes that the ΔP is noise and discards the first value of the cumulated sum which is greater than the threshold value. If there is a second occurrence of the cumulated sum being greater than the threshold value then the method assumes that the ΔP is not noise but rather is an optical signal power different which needs to be corrected. There is however still a possibility that the ΔP is noise, and applying the attenuation variation may therefore result in oscillations being created. Applying the second smoothing factor, K′, to the current value of the cumulated sum, Δα(sum).sub.n, before summing it with the current value of the attenuation means that only part of the power difference, ΔP, is corrected, in case it is noise. The second smoothing factor therefore slows the correction of an optical signal power difference, ΔP, between the detected optical signal power and the target optical signal power, which prevents the occurrence of optical signal power oscillations on the optical signal.

(49) Referring to FIG. 8, an eighth embodiment of the invention provides a method 90 of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to a pass-through optical signal.

(50) The method 90 of this embodiment is similar to the method 70 of FIG. 6, with the following modifications. The same reference numbers are retained for corresponding steps.

(51) In this embodiment, the new optical attenuation value, α.sub.n+1, is obtained if 92 there are at least M consecutive stored values of the cumulated sum, Δα(sum), which are each greater than the threshold value, Δα.sub.TH. M is a positive integer calculated as a function of a number of optical communication network nodes configured to apply an optical attenuation to a pass-through optical signal which precede the said optical communication network node in a path of the optical signal.

(52) In a ninth embodiment of the invention, having the same steps as shown in FIG. 8, M is a positive integer calculated as a monotonically increasing function of the said number of optical communication network nodes configured to apply an optical attenuation to a pass-through optical signal which precede the said optical communication network node. M may, for example, be one of the following: the said number of preceding nodes; the said number of preceding nodes multiplied by a constant, M=c*N; or the square root of the said number of preceding nodes.

(53) A tenth embodiment of the invention provides a method of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to a pass-through optical signal. The method of this embodiment is similar to the method 10 of FIG. 1, with the following modifications.

(54) In this embodiment, the optical communication network node is further configured to apply a further optical attenuation, α(add), to add optical signals.

(55) The method further comprises, if the optical signal is an add optical signal, performing the following steps d. and e. in place of steps b. and c.: d. obtaining a current value of the further optical attenuation, α(add).sub.n, and summing said value, α(add).sub.n, with the current attenuation variation value, Δα.sub.n, to form a new further optical attenuation value, α(add).sub.n+1; and e. generating a control signal arranged to configure the node to apply the new further optical attenuation value, α(add).sub.n+1.

(56) Referring to FIG. 9, an eleventh embodiment of the invention provides a method 100 of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to a pass-through optical signal.

(57) The method 100 of this embodiment is similar to the method 30 of FIG. 2, with the following modifications. The same reference numbers are retained for corresponding steps.

(58) In this embodiment, the optical communication network node is further configured to apply a further optical attenuation, α(add), to add optical signals.

(59) The method further comprises, if the optical signal is an add optical signal 102 performing the following steps d. and e. in place of steps b. and c.: d. obtaining 104 a current value of the further optical attenuation, α(add).sub.n, and summing 106 said value, α(add).sub.n, with the cumulated sum, Δα(sum).sub.n, to form a new further optical attenuation value, α(add).sub.n+1; and e. generating 108 a control signal arranged to configure the node to apply the new further optical attenuation value, α(add).sub.n+1.

(60) As in FIG. 3, the value for the attenuation variation, Δα, is calculated 42 as Δαn=ΔP*K, where K is a preselected first smoothing factor.

(61) It will be appreciated that the steps of this embodiment may be added to any of the preceding embodiments, in order that the methods described in those embodiments may handle both pass-through and add optical signals.

(62) FIG. 10 shows the steps of a method 110 according to a twelfth embodiment of the invention of controlling optical signal power levelling in an optical communication network node. The node is configured to apply an optical attenuation, α, to an optical signal, which may be pass-through or add.

(63) The node comprises an optical channel monitor, OCM, and a wavelength selective switch, WSS. The WSS is arranged to apply the optical attenuation, α, to the optical signal. The OCM is arranged to measure an optical signal power of an optical signal.

(64) The method 110 comprises step a. of performing the following steps i. to iii. and cumulatively summing 118 each calculated value of an attenuation variation, Δα.sub.n, until the cumulated sum, Δα(sum).sub.n, is greater than a preselected attenuation variation threshold value 122, Δα.sub.TH: i. measuring an optical signal power of an optical signal 112 using the OCM; ii. calculating a difference, ΔP, between the measured optical signal power and a target optical signal power 114; iii. calculating 116 a value for the attenuation variation, Δα.sub.n, to be applied to the optical attenuation, applying a first smoothing factor, K, as Δα.sub.n=ΔP*K.

(65) The attenuation variation threshold value, Δα.sub.TH, is the dead-zone threshold setting of the WSS, being the threshold below which no attenuation variation is applied to the WSS.

(66) The current value of the cumulated sum, Δα(sum).sub.n, is stored 120 in a register arranged to store the last N values of Δα(sum).sub.n.

(67) The method 110 comprises a step 124 of checking whether the optical signal which has been measured is a pass-through optical signal. If the optical signal is not a pass-through optical signal, i.e. it is an add signal, then the current cumulated sum value, Δα(sum).sub.n, is applied to the WSS 132, as follows. A current value of the WSS optical attenuation is obtained, α(add).sub.n, and summed with the current cumulated sum value, Δα(sum).sub.n, to form a new optical attenuation value, α(add).sub.n+1. A control signal arranged to configure the WSS to apply the new optical attenuation value, α(add).sub.+1. The cumulated sum value, Δα(sum).sub.n, is then cleared 130, so that the method can restart.

(68) If the optical signal is a pass-through optical signal, the method proceeds by considering the last N cumulated sum values stored in the register 126, to check whether there are two stored values which are greater than the attenuation variation threshold. If the current value is the only value of the cumulated sum in the register which is greater than the attenuation variation threshold, then it is the first time that the cumulated sum, Δα(sum), has exceeded than the attenuation variation threshold and it will be disregarded. The method then clears the cumulated attenuation 130, to allow steps i. to iii. to be repeated. If instead there are two stored cumulated sum values in the register which are greater than the attenuation variation threshold, this means that last cumulated sum value is not the first to exceed the attenuation variation threshold value. The method then proceeds to apply the current cumulated sum value, Δα(sum).sub.n, to the WSS 128, as follows. The current value of the optical attenuation, α.sub.n, of the WSS is obtained and a new optical attenuation value, α.sub.n+1, is obtained by summing it with the current cumulated sum value, Δα(sum).sub.n. A control signal arranged to configure the WSS to apply the new optical attenuation value, α.sub.n+1, is then generated.

(69) The number of samples in the register, N, sets the time, expressed in loop cycles, over which the method checks whether there are two stored cumulated sum values, Δα(sum), which are greater than attenuation variation threshold, Δα.sub.TH. This time period is proportional to 1/K to enable the node to compensate for an optical signal power different, ΔP, effectively related to that node, and to keep the level of accuracy determined by the attenuation variation threshold, being the dead zone threshold setting of the WSS. The number of register samples, N, is given as: N≧(1/K)+1. This enables the method to check whether there are two consecutive stored cumulated sum values which are each greater than the threshold value, Δα.sub.TH, by simply checking whether two of the N stored samples are greater than the threshold value.

(70) The method may also multiply the current cumulated sum value, Δα(sum).sub.n, with a second smoothing factor, K′, before summing it with the current value of the optical attenuation, α.sub.n. This may enable better optical signal power oscillation suppression.

(71) The second smoothing factor further reduces the occurrence of optical signal power oscillations. If the ΔP that is detected is in fact noise then trying to correct for this ΔP may make the noise worse, which may lead to oscillations. Noise on the optical signal power reading in the node may result in more than two stored values of the cumulated sum being greater than the attenuation variation threshold, Δα.sub.TH, leading to the attenuation variation being incorrectly applied. By applying the second smoothing factor the attenuation variation which is applied is reduced, therefore the correction to the ΔP that is applied is reduced, so if ΔP is noise the applied correction will not result in optical signal power oscillations. The second smoothing factor, K′, is in effect slowing down the response speed of the node compared to the speed at which the optical signal power of a pass-through channel may be changed by a network operator, but this is not an issue since the optical signal power control to respond to a fast event, like a line failure, which causes a variation in the aggregated input power of a WDM optical signal is managed with transient management of amplifiers and/or other control loops to maintain the correct power level for the aggregate WDM signal, which may comprise up to 96 optical signals/channels.

(72) FIG. 11 shows the steps of a method 140 according to a thirteenth embodiment of the invention of controlling optical signal power levelling in an optical communication network node. The method 140 is similar to the method 110 of FIG. 10, with the following modifications. The same reference numbers are retained for corresponding steps.

(73) In this embodiment, the method checks whether there are M stored cumulated sum values which are each greater than the threshold value, Δα.sub.TH, by checking whether M of the N stored samples are greater than the threshold value.

(74) Simulations of the method were performed for an optical signal propagating in an optical communication network path comprising 20 nodes, the nodes comprising ROADMs. The optical signal is added at the first node and then propagates as a pass-through optical signal through node 2 to 20. A random error of 0.3 dB (+/−0.15) was added to the simulated optical signal power reading at each node to simulate random optical noise in the optical signal.

(75) FIG. 12 shows the simulated optical signal power readings at the 1.sup.st, 5.sup.th, 10.sup.th and 20.sup.th nodes as a function of cycles of steps 112 to 118, 122, 132 and 130 of the method of FIG. 10. It will be appreciated that in this scenario step 122 proceeds directly to step 132. These steps comprise a prior art method of optical signal power levelling applied by ROADMs to both pass-through and add/optical signals (channels), that is to say without differentiating between pass-through and add signals. In this simulation K=0.1, and Δα.sub.TH=0.2 dB. The simulations show that, the optical signal power oscillations are above 1 dB after 10 nodes and above 2 dB after node 20.

(76) FIG. 13 shows the simulated optical signal power readings at the 1.sup.st, 5.sup.th, 10.sup.th and 20.sup.th nodes of the same path as used for the simulations of FIG. 12. In this simulation the method shown in FIG. 10 is implemented. That is to say, pass-through and add optical signals are treated differently. Only steps 112 to 118, 122, 132 and 130 are applied at the 1.sup.st node, since the optical signal is an add signal at this node. All of the steps of FIG. 10 are relevant for the 2.sup.nd to 20.sup.th nodes.

(77) In this simulation, K=0.1, N=(1/K+1)=11, and Δα.sub.TH=0.2 dB. No second smoothing factor was applied for this simulation (K′=0). It can be seen that the optical signal power oscillations are noticeably reduced as compared to those shown in FIG. 12. The oscillations are very small also after 5 nodes, but after 10 nodes the sum of the optical signal power variations caused by the optical attenuation variations applied by each of the 10 nodes may be enough to trigger oscillations. The optical signal power oscillations are above 1 dB after 10 nodes and reach about 2 dB at node 20.

(78) FIG. 14 shows the simulated optical signal power readings at each of the 20 nodes of the same path as used for the simulations of FIGS. 12 and 13, as a function of cycles of the method of FIG. 10. As for FIG. 12, N=11, K=0.1, Δα.sub.TH=0.2 dB. In this embodiment a second smoothing factor, K′, is also applied, 128 in FIG. 10. K′=0.2. As can be seen, applying the second smoothing factor produces a significant further reduction in optical signal power oscillations in the simulated optical signal power readings at each of the nodes. The oscillations are no longer amplified as they are propagated along the path; only the optical signal power variation caused by the optical attenuation variation applied at the first node is significant, and is generally maintained by the following nodes, which each add only very small optical signal power variation contributions. This is because the second smoothing factor K′ reduces the optical signal power difference, ΔP, that is able to trigger the oscillations shown in FIG. 13 The optical signal power oscillations are below 0.5 dB (0.35 dB for this specific simulation) after 20 nodes, and the net contribution of the 19 nodes after the first is below 0.2 dB.

(79) FIG. 15 shows the simulated optical signal power readings at each of the 20 nodes of the same path as used for the simulations of FIGS. 12 to 14, as a function of cycles of the method of FIG. 11. As for FIG. 14, K=0.1, Δα.sub.TH=0.2 dB, and K′=0.2. In this simulation the number of samples, N, in the register is calculated as N=(10*M)+1, where M=sqrt (preceding no. nodes) and therefore changes for each node along the path. The value of M is truncated to an integer. As can be seen, compared to FIGS. 13 to 14, there is no change for nodes 2 and 3, since we have the same settings: M=1 and N=11. For nodes 4 to 8 M=2 and N=21, and so on. In this simulation there is no further, or negligible, contribution from all the nodes of the path, as indicated by the fact that the simulated optical signal powers shown for nodes 3 to 20 are almost overlapped.

(80) The method of FIG. 11 does not prevent or slow down the capability of the nodes in the path to follow a real change of optical signal power introduced at the 1.sup.st node, where the signal is added, which needs to be compensated for but actually also helps to achieve this. FIGS. 16 to 18 illustrate how the nodes of the same path as used for the simulations of FIGS. 12 to 15 react to an optical signal power variation which needs to be compensated for. The simulated optical signal power readings at nodes 1, 5, 10 and 20 are shown for an optical signal power difference, ΔP=1 dB at the input of node 1. No random error has been used in these simulations so that the effect on compensating a real optical signal power variation of applying the method of FIG. 10 can be clearly seen.

(81) FIG. 16 shows the simulated optical signal power readings at the 1.sup.st, 5.sup.th, 10.sup.th and 20.sup.th nodes of the same path as used for the simulations of FIGS. 12 to 15 in response to a ΔP of 1 dB at the 1.sup.st node of the path. As for the simulation of FIG. 12, only steps 112 to 118, 122, 132 and 130 of the method of FIG. 10 are implemented. In this simulation K=0.1, and Δα.sub.TH=0.2 dB.

(82) FIG. 17 shows the simulated optical signal power readings at the 1.sup.st, 5.sup.th, 10.sup.th and 20.sup.th nodes of the same path as used for the simulations of FIGS. 12 to 16 in response to a ΔP of 1 dB at the 1.sup.st node of the path, applying the method of FIG. 10. Similarly to FIG. 13, only steps 112 to 118, 122, 132 and 130 are applied at the 1.sup.st node, since the optical signal is an add signal at this node. All of the steps of FIG. 10 are relevant for the 2.sup.nd to 20.sup.th nodes. N is 11, K=0.1, Δα.sub.TH=0.2 dB, and K′=0.2.

(83) FIG. 18 shows simulated optical signal power readings at the 1.sup.st, 5.sup.th, 10.sup.th and 20.sup.th nodes of the same path as used for the simulations of FIGS. 12 to 17 in response to a ΔP of 1 dB at the 1.sup.st node of the path, applying the method of FIG. 11, where N=(10*M)+1, K=0.1, Δα.sub.TH=0.2 dB, K′=0.2, and M=sqrt (no. nodes).

(84) A fourteenth embodiment of the invention provides a method of controlling optical signal power levelling in an optical communication network path. The network path comprises a plurality of optical communication network nodes each configured to apply a respective optical attenuation, α, to a pass-through optical signal propagating along the path.

(85) The method comprises configuring each optical communication network node to perform the method of controlling optical signal power levelling as described in any of the previous embodiments.

(86) The steps of a method 150 according to a fifteenth embodiment of the invention of controlling optical signal power levelling in an optical communication network path are shown in FIG. 19. The network path comprises a plurality of optical communication network nodes each configured to apply a respective optical attenuation, α, to a pass-through optical signal propagating along the path.

(87) The method comprises:

(88) configuring each optical communication network node to perform the method of controlling optical signal power levelling as described in any of the previous embodiments 152;

(89) generating a path message arranged to configure the path, the path message comprising an indication of a number of nodes crossed by the path message 154;

(90) transmitting the path message to each node serially along the path 156; and

(91) updating the indication of the number of nodes respectively at each node 158.

(92) The path message is one of a strict explicit route object RSVP-TE protocol message and a loose explicit route object RSVP-TE protocol message.

(93) The indication of the number of nodes crossed by the path message is one of a node count field arranged to store the number of nodes crossed by the path message and a node list comprising an identification of each node crossed by the path message.

(94) Referring to FIG. 20, a sixteenth embodiment of the invention provides an optical communication network node 160 comprising an input 162, optical attenuation apparatus 164, optical signal power measurement apparatus 166, and a controller 168.

(95) The input 162 is arranged to receive an optical signal. The optical attenuation apparatus 164 is configured to apply an optical attenuation, α, to the optical signal. The optical signal power measurement apparatus 166 is arranged to measure an optical signal power of the optical signal.

(96) The controller 168 is arranged to: a. perform the following steps i. to iii. until the an attenuation variation value, Δα, is greater than a preselected attenuation variation threshold value, Δα.sub.TH: i. receive a measured optical signal power from the optical signal power measurement apparatus 166; ii. calculate a difference, ΔP, between the measured optical signal power and a target optical signal power; iii. calculate a value for the attenuation variation, Δα.sub.n, to be applied to the optical attenuation taking account of ΔP; b. a new optical attenuation value, α.sub.n+1, in dependence on the current value of the optical attenuation, the current attenuation variation value, Δα.sub.n, and at least an earlier attenuation variation value Δα.sub.n−1; and c. generate a control signal arranged 170 to configure the optical attenuation apparatus 164 to apply the new optical attenuation value, α.sub.n+1.

(97) Referring to FIG. 21, a seventeenth embodiment of the invention provides an optical communication network node 180. The node 180 of this embodiment is similar to the node 160 of the previous embodiment, with the following modifications. The same reference numbers are retained for corresponding features.

(98) In this embodiment, the optical attenuation apparatus is a wavelength selective switch, WSS, 182. The optical signal power measurement apparatus is an optical channel monitor, OCM, 184.

(99) The controller 168 is arranged to receive a measured optical signal power from the OCM. The controller is arranged to generate a control signal 170 arranged to configure the WSS 182 to apply the new optical attenuation value, α.sub.n+1.

(100) The steps of a method 190 according to an eighteenth embodiment of the invention of configuring a path across an optical communication network are shown in FIG. 22.

(101) The method comprises:

(102) generating a path message arranged to configure the path 192, the path message comprising an indication of a number of nodes crossed by the path message;

(103) transmitting the path message to each node serially along the path 194; and

(104) at each node, updating the indication of the number of nodes crossed by the path message 196.

(105) Referring to FIG. 23, a nineteenth embodiment of the invention provides an optical communication network path message 200 comprising instructions 202 for configuring a path across an optical communication network and an indication of a number of nodes crossed by the path message 204.

(106) A twentieth embodiment of the invention provides a data carrier having computer readable instructions embodied therein. The computer readable instructions are for providing access to resources available on a processor and comprise instructions to cause the processor to perform any of the steps of the method of controlling optical signal power levelling in an optical communication network node configured to apply an optical attenuation to an optical signal as described in any of the above embodiments.

(107) The data carrier may be a non-transient data carrier.

(108) A twenty-first embodiment of the invention provides a data carrier having computer readable instructions embodied therein. The computer readable instructions are for providing access to resources available on a processor and comprise instructions to cause the processor to perform any of the steps of the method of controlling optical signal power levelling in an optical communication network path as described in the thirteenth or fourteenth embodiments.

(109) A twenty-second embodiment of the invention provides a data carrier having computer readable instructions embodied therein. The computer readable instructions are for providing access to resources available on a processor and comprise instructions to cause the processor to perform any of the steps of the method of configuring a path across an optical communication network according to the seventeenth embodiment.