SUBCARRIER DIVERSITY IN OPTICAL COMMUNICATION SYSTEMS

Abstract

Disclosed herein is a method of transmitting a data stream from a first location to a second location through an optical network, as well as a corresponding performance monitoring unit, a transmitting arrangement and a receiving arrangement. The method comprises the steps of transmitting said data stream from said first location to said second location along a working path, wherein said data stream is transmitted in the form of a super channel comprising a number of n wavelengths within a predefined reserved wavelength range, monitoring the performance of the transmission based on the super channel signal received at said second location, and in case the performance is observed to drop below a predetermined performance threshold, unburdening the super channel from a part of said data stream while maintaining the reserved wavelength range for said super channel, by redirecting said part of the data stream for transmittal along at least one restoration path connecting said first and second locations, and transmitting the remainder of said data stream within the super channel on a number of wavelengths that is less than n, and/or with a reduced transmission rate for at least some of the wavelengths in said super channel.

Claims

1. A method of transmitting a data stream from a first location to a second location through an optical network, comprising the following steps: transmitting said data stream from said first location to said second location along a working path established in said optical network, wherein said data stream is transmitted in the form of a super channel comprising a number of n wavelengths within a predefined reserved wavelength range, monitoring performance of the transmission based on the super channel signal received at said second location, and in case the performance is observed to drop below a predetermined performance threshold, unburdening the super channel from a part of said data stream while maintaining the reserved wavelength range for said super channel, by redirecting said part of the data stream for transmittal along at least one restoration path connecting said first and second locations, said at least one restoration path being different from said working path, and transmitting the remainder of said data stream within the super channel on a number of wavelengths that is less than n, and/or with a reduced transmission rate for at least some of the wavelengths in said super channel.

2. The method of claim 1, wherein said part of the redirected data stream is chosen such that the performance of the transmission of the remainder of said data stream within the super channel exceeds said predetermined performance threshold.

3. The method of claim 1, further comprising a step of additionally monitoring the performance of the transmission of the redirected part of the data stream via said at least one restoration path, and dynamically adjusting the share of the redirected data stream and of the remainder of the data stream based on the two monitored performances.

4. The method of claim 1, wherein said redirected part of the data stream is transmitted along said at least one restoration path with a reduced transmission rate per wavelength as compared to the transmission rate of the working path.

5. The method of claim 1, wherein the remainder of the datastream is transmitted within the super channel on the working path on a sub-set of the original n wavelengths, wherein in said subset, one or both of the largest and smallest among the n original wavelengths is missing and/or wavelengths are missing which were mutually non-adjacent in the original set of n wavelengths.

6. The method of claim 1, wherein said redirected part of the data stream is transmitted along said at least one restoration path in the form of a super channel as well.

7. The method of claim 1, wherein said performance is monitored by analyzing one or more of the following performance indicators, high error rate contributors (HERC), signal-to-noise-ratio (SNR), power spectral density (PSD), pre-forward-error-correction (FEC) bit error rate, post-FEC bit error rate, rate of corrected symbols, peak-to-average ratio, or error vector magnitude (EVM).

8. The method of claim 1, wherein said predetermined performance threshold is chosen according to a service level agreement.

9. The method of claim 1, wherein said data stream is a predefined amount of data traffic under a service level agreement.

10. The method of claim 1, wherein said redirecting of said part of said data stream is carried out by using wavelength selective switches.

11. The method of claim 1, wherein said redirecting of said part of said data stream is carried out by means of a cyclic filter in combination with a suitable control of the transmitted wavelengths.

12. The method of claim 1, further comprising a step of buffering one of said redirected part of the data stream and said remainder of the data stream, such as to compensate for a difference in optical path lengths of said working path and said redirection path.

13. The method of claim 1, wherein said step of reducing the transmission rate for a given wavelength in said super channel comprises changing the modulation format and/or the symbol rate of the corresponding optical signal.

14. The method of claim 1, further comprising a step of establishing said at least one restoration path in response to the monitored performance being observed to approach said predetermined performance threshold.

15. A performance monitoring unit for monitoring and controlling the transmission of a data stream in the form of a super channel from a first location to a second location through an optical network, wherein the super channel comprises a number of n wavelengths within a predefined reserved wavelength range, and wherein the performance monitoring unit is configured for: monitoring the performance of a transmission of the data stream from said first location to said second location along a working path established in said optical network based on the super channel signal received at said second location; and in case the performance is observed to drop below a predetermined performance threshold, unburdening the super channel from a part of said data stream while maintaining the reserved wavelength range for said super channel, by controlling components within said optical network for: redirecting said part of the data stream for transmittal along at least one restoration path connecting said first and second locations, said at least one restoration path being different from said working path, and transmitting the remainder of said data stream within the super channel on a number of wavelengths that is less than n, and/or with a reduced transmission rate for at least some of the wavelengths in said super channel.

16. The performance monitoring unit of claim 15, further configured for controlling components within said optical network for redirecting said part of the redirected data stream such that the performance of the transmission of the remainder of said data stream within the super channel exceeds said predetermined threshold.

17. The performance monitoring unit of claim 15, further configured for additionally monitoring the performance of the transmission of the remainder of the redirected part of the data stream via said at least one restoration path, and for controlling components within said optical network for dynamically adjusting the share of the redirected data stream and of the remainder of the data stream based on the two monitored performances.

18. The performance monitoring unit of claim 15, further configured for controlling components within said optical network for transmitting said redirected part of the data stream along said at least one restoration path with a reduced transmission rate per wavelength as compared to the transmission rate of the working path.

19. The performance monitoring unit of claim 15, further configured for controlling components within said optical network for transmitting said redirected part of the data stream along said at least one restoration path in the form of a super channel as well.

20. The performance monitoring unit of claim 15, further configured for monitoring the performance by analyzing of the following one or more performance indicators, of high error rate contributors (HERC), signal-to-noise-ratio (SNR), power spectral density (PSD), pre-forward-error-correction (FEC) bit error rate, post-FEC bit error rate, rate of corrected symbols, peak-to-average ratio, or error vector magnitude (EVM).

21. The performance monitoring unit of claim 15, further configured for establishing said at least one restoration path in response to the performance being observed to approach said predetermined performance threshold.

22. The performance monitoring unit of claim 15, further configured for calculating a prediction for the performance or one or more performance indicators expected at a future time, distant from a present time by at least the time required to carry out said step of unburdening the super channel.

23. A transmitting arrangement for transmitting a data stream in the form of a super channel from a first location to a second location through an optical network along a working path, and optionally along at least one restoration path, wherein the working path and the at least one restoration path connect said first and second locations, the at least one restoration path being different from the working path, wherein the super channel comprises a number of n wavelengths within a predefined reserved wavelength range, and wherein the transmitting arrangement comprises: at least n light sources configured to emit the n wavelengths constituting the carriers of said super channel; one or more modulators for modulating said carrier with data such as to generate optical data signals; and a redirecting device configured for: redirecting part of said optical data signal for transmittal along the at least one restoration path, and transmitting the remainder of the optical data signal along the working path, wherein said transmitting arrangement is further configured for transmitting said remainder of the optical data signal within the super channel on a number of wavelength that is less than n, and/or with a reduced transmission rate for at least some of the wavelengths in said super channel.

24. The transmitting arrangement of claim 23, wherein the redirecting device comprises a cyclic filter.

25. The transmitting arrangement claim 23, wherein the redirecting device comprises a wavelength selective switch (WSS).

26. The transmitting arrangement of claim 23, wherein each of the at least n light sources can emit light at each of the n wavelengths of the super channel.

27. The transmitting arrangement of claim 23, further comprising one or more additional light sources, wherein each additional light source can emit light at each of the n wavelengths of the super channel.

28. The transmitting arrangement of claim 23, wherein the predefined reserved wavelength range has a width of between 0.8 and 5.0 nm.

29. The transmitting arrangement of claim 23, wherein the n wavelengths are spaced apart by a frequency difference of between 20 GHz and 100 GHz.

30. The transmitting arrangement of claim 23, wherein each of the n wavelengths carry a portion of said data stream with a transmission rate between 40 Gbit/s and 1 Tbit/s.

31. A receiving arrangement configured for receiving a data stream from a transmitting arrangement, wherein said receiving arrangement is configured to selectively receive a datastream, from said transmitting arrangement, in the form of a super channel comprising a number of n wavelengths within a predefined reserved wavelength range via a working path connecting first and second locations, or a redirected part of said data stream via at least one restoration path, and a remainder of said data stream within the super channel via said working path, said remainder of said data stream being transmitted on a number of wavelengths that is less than n and/or with a reduced transmission rate for at least some of the wavelengths in said super channel. wherein the receiving arrangement further comprises at least one buffer configured for buffering said redirected part of said data stream and said remainder of said data stream, such as to compensate for a difference in optical path lengths of said working path and said restoration path connecting said first and second locations and being different front the working path.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] FIG. 1 shows a schematic view of a super channel.

[0058] FIG. 2 shows a schematic representation of an optical network in which a working path and a restoration path are established.

[0059] FIG. 3 shows a redirection of individual carriers of a super channel according to an embodiment of the invention.

[0060] FIG. 4 shows a redirection of individual carriers of a super channel and change of transmission rate according to an embodiment of the invention.

[0061] FIG. 5 shows various examples of a redistribution of the carriers of a super channel according to an embodiments of the invention.

[0062] FIG. 6 shows a transponder for use in the present invention.

[0063] FIG. 7 shows an optical network according to an embodiment of the invention.

[0064] FIG. 8 shows a performance-time diagram for an edge carrier and an inner carrier of a super-channel and a performance extrapolation based thereon.

[0065] FIG. 9 shows an optical network comprising a performance monitoring unit according to an embodiment of the invention.

[0066] FIG. 10 shows an optical network comprising a performance monitoring unit according to an embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0067] For the purposes of promoting an understanding of the principles of the invention, reference will now be made to a preferred embodiment illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated apparatus and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now or in the future to one skilled in the art to which the invention relates.

[0068] FIG. 2 schematically shows an optical network comprising six nodes 1 to 6 connected by optical links. According to an embodiment of the method of the invention, a data stream is transmitted from a first location at node 1 to a second location at node 6 along a working path via node 2, wherein the data stream is transmitted in the form of a super channel like that shown in FIG. 1. The performance of the transmission of the data stream between node 1 and node 6 is monitored based on the super channel signal received at node 6. In the case shown, there is some performance degradation along the working path between node 1 and node 6 (more precisely between node 1 and intermediate node 2) which leads said performance to drop below a predetermined performance threshold.

[0069] When this happens, the super channel is unburdened from a part of the data stream while maintaining the reserved wavelength range (i.e. the wavelength range L shown in FIG. 1) for said super channel, by redirecting a part of the data stream for transmittal along a restoration path connecting node 1 and node 6 via nodes 4 and 5. The remainder of the data stream continues to be transmitted along the working path between node 1 and node 6 via node 2 within the super channel. However, since now only a fraction of the original data stream (the remainder as referred to herein) needs to be transmitted via the super channel, this remainder can be transmitted on a number of wavelengths that is less than the number of wavelengths originally comprised in the super channel. In other words, depending on the fraction of the data stream or traffic redirected along the restoration path, the remainder can for example be transmitted on two or three wavelengths within the super channel only. Thus, performance is already improved thanks to reduced nonlinear interaction. Since this furthermore allows to increase the size of the guard bands and/or the spectral distance between neighboring carriers, this may lead to an additional improvement of the performance of the transmission along the working path. In addition or alternatively, the reduced payload on the working path would also allow for reducing the transmission rate for at least some of the wavelengths in the super channel, which likewise allows for improving the transmission performance.

[0070] Note that conventionally, detecting insufficient transmission performance would have typically caused a network management system to redirect the entire data stream along a restoration path or, if present, a protection path. According to the present invention, however, the super channel as such is maintained on the working path, but it is only unburdened from some of its traffic to thereby facilitate maintaining the required performance on the super channel. This removal of traffic from the super channel can be carried out dynamically and precisely to the extent that the desired performance is reached, thereby exploiting the current physical capability of the working path as much as possible, i.e. with optimum efficiency. Also, since the super channel is maintained, no reconfiguration of the working path is required.

[0071] The restoration channel can be calculated in advance, or can be calculated when the monitoring of the performance of the transmission based on the super channel along the working path indicates that due to gradual degradation or the like, the performance is expected to drop below a predetermined performance threshold in the near future. Since the restoration channel only needs to transmit a fraction of the original data stream, it is comparatively easy to find a suitable restoration path with sufficient resources for the redirected part of the data stream.

[0072] Importantly, the share of the redirected datastream and the remainder of the data stream can be dynamically adjusted, to give an optimum overall result. For this purpose, in preferred embodiments of the invention, the performance of both, the transmission of the redirected part and of the remainder of the data stream received along the restoration and working paths, respectively, is monitored, and the shares of these parts are adjusted based on the two monitored performances. This would in particular allow for increasing the traffic on the working path again, if the working path has recovered from transient complications or due to maintenance work.

[0073] Note further that according to the wording used herein, every data stream currently transmitted on the working path can be regarded as the original data stream, i.e. irrespectively of whether an even larger data stream has previously been transmitted on the working path. This is because the redirection of part of the current data stream is a dynamical process, but at any point in time, there is a possibility to redirect part of the current data stream transmitted along the working path in the form of said super channel.

[0074] With reference to FIG. 3, an embodiment of the method and apparatus of the invention is described. FIG. 3 shows a transmitting arrangement 10, comprising first to fourth light sources 12a to 12d and a redirecting device 14 which in the embodiment shown is formed by a cyclic filter. The cyclic filter 14 has on its left side first to fourth input ports, which are connected with the first to fourth light sources 12a to 12d, respectively. The cyclic filter 14 further has on its right side first to fourth output ports, of which the first output port is connected with a working path 16 and the fourth output port is connected with a restoration path 18. The second and third output ports of the cyclic filter 14 are not occupied and hence not shown in the Figure. However, the protection path could also be connected to any of these ports. Furthermore, unused ports might be termined inside of housing of the cyclic filter and thus might not be visible from outside.

[0075] Further shown in FIG. 3 is a receiving arrangement 20 which includes first to fourth receiving units 22a to 22d and a redirecting device, which is formed by a further cyclic filter 24. The cyclic filter 24 has four input ports on its left side, of which the first is connected with the working path 16 and the fourth is connected with the restoration path 18, while the second and third input ports are not occupied. On its right side, the cyclic filter 24 has first to fourth output ports connected to the first to fourth receiver units 22a to 22d, respectively.

[0076] Shown in the table below is the connectivity of the cyclic filters 14, 16 depending on the wavelength.

TABLE-US-00001 Output Output Output Output port 1 port 2 port 3 port 4 Input port 1 .sub.1 .sub.2 .sub.3 .sub.4 Input port 2 .sub.2 .sub.3 .sub.4 .sub.1 Input port 3 .sub.3 .sub.4 .sub.1 .sub.2 Input port 4 .sub.4 .sub.1 .sub.2 .sub.3

[0077] In the situation of normal operation, shown in the upper half of FIG. 3, the first to fourth light sources 12a to 12d emit wavelengths .sub.1 to .sub.4, respectively, which according to the connectivity table above means that all four wavelengths are sent via the first output port to the working path 16, where they form the super channel. At the receiving arrangement 20, the performance of the super channel transmission is monitored. In particular, the monitoring comprises analyzing one or more performance indicators, such as high error rate contributors (HERC), signal-to-noise ratio (SNR), power spectral density (PSD), pre-forward-error-correction (FEC) bit error rate, post-FEC bit error rate, rate of corrected symbols, peak-to-average ratio, and error vector magnitude (EVM). When it is determined that the performance as indicated by the performance indicator drops below a predetermined performance threshold, the transmitting arrangement 10 is controlled by a performance monitoring unit (not shown) to change the wavelength of the second light source 12b from .sub.2 to .sub.1 and the wavelength of the fourth light source 12d from .sub.4 to .sub.3. Consequently, the carriers generated by the second and fourth light sources 12b and 12d, respectively, are redirected to the fourth output port of the cyclic filter 14, and hence transmitted via the restoration path 18, as shown in the lower half of FIG. 3.

[0078] At the receiving arrangement 20, the cyclic filter 24 redirects the light received at its fourth input port from the second and fourth light sources 12b and 12d, to the second and fourth receiving units 22b, 22d, respectively, as before. Accordingly, from the receiving arrangement's point of view, nothing has changed, except that the second and third receiving units 22b, 22d receive different wavelengths. Note in this regard that the cyclic filter 14, 24 is symmetrical or reciprocal with regard to the input and output ports, or in other words, that the table above is mirror symmetric with regard to its diagonal.

[0079] As is seen from the above embodiment, by using cyclic filters 14, 24, the redirection of individual carriers of the super channel can be carried out simply by a suitable control of the wavelengths at the transmitting arrangement 10. For this purpose, the light sources 12a to 12d must be wavelength adjustable, in the sense that every light source 12a to 12d has to be capable of producing light with the wavelength of each of the carriers of the super channel.

[0080] Since the super channel now only includes two carriers instead of four, the spectral distance between the two remaining wavelengths (.sub.1 and .sub.3) has been increased, thereby reducing performance losses due to nonlinear effects occurring when the carriers are located spectrally closely together. Moreover, the super channel no longer has to carry the edge wavelength .sub.4, which is likely to suffer from filtering effects upon passing the filters (not shown) provided on the working path 16. Accordingly, it can be expected that the performance of the received super channel is increased to above the predetermined performance threshold.

[0081] FIG. 4 discloses a further embodiment, which is similar to that of FIG. 3, except that the transmitting arrangement 10 comprises an additional fifth light source 12e connected to a fifth input port of the redirecting device 14 and the receiving arrangement 20 comprises a fifth receiver unit 22e connected to a fifth output port of the redirecting device 24. The connectivity of the redirecting devices 14 and 24 is shown in the table below.

TABLE-US-00002 Output Output Output Output Output port 1 port 2 port 3 port 4 port 5 Input port 1 .sub.1 .sub.2 .sub.3 .sub.4 .sub.5 Input port 2 .sub.2 .sub.3 .sub.4 .sub.5 .sub.1 Input port 3 .sub.3 .sub.4 .sub.5 .sub.1 .sub.2 Input port 4 .sub.4 .sub.5 .sub.1 .sub.2 .sub.3 Input port 5 .sub.5 .sub.1 .sub.2 .sub.3 .sub.4

[0082] Under normal operation, the situation is the same as in FIG. 3, i.e. optical signals generated by the first to fourth light sources 12a to 12d are jointly transmitted as a super channel on the working path 16, where in this embodiment each wavelength carries data at a data rate of 100 Gbit/s. The total data stream is therefor transmitted at a rate of 400 Gbit/s. When the performance of the received super channel decreases below the predetermined performance threshold, in this embodiment the wavelengths of the first to third light sources 12a to 12d remain unchanged, and hence continue to be part of the super channel. However, the transmission rate of the lowest wavelength .sub.1, i.e. the edge channel that is particularly prone to performance loss, is reduced from 100 Gbit/s to 50 Gbit/s only. This can e.g. be achieved by changing the modulation format to a modulation format with less constellation points per symbol, or decreasing the symbol rate of the optical signal at wavelength .sub.1. Although not shown in the figures for simplicity, the skilled person will appreciate that the transmitting arrangement 10 further comprises suitable modulators that allow for adjusting the modulation format and the symbol rate.

[0083] In addition, the wavelength of the fourth light source 12d is changed to .sub.3 such that it is directed to the restoration path 18. Since the first wavelength .sub.1 only carries data at a data rate of 50 Gbit/s, to account for the full data stream with a rate of 400 Gbit/s, another 50 Gbit/s are missing. These missing 50 Gbit/s are provided by the fifth light source 12e (and corresponding modulator, not shown) and are transmitted via the restoration path 18 as well. In this case too, the super channel on the working path is maintained, but it is unburdened from its one wavelength .sub.4, Moreover, while the other edge wavelength .sub.1 is still present, the super channel is unburdened with regard to this wavelength as well due to the reduced transmission rate. Accordingly, it is expected that the performance of the transmission of the remaining part of the datastream in the super channel is increased.

[0084] FIG. 5 schematically shows favorable transmission rate and wavelength assignments during normal operation and when part of the datastream is redirected for transmittal along the restoration path 18. In FIG. 5, if a carrier of the super channel is shown to be narrow, this shall mean that this wavelength is utilized at a reduced transmission rate.

[0085] The diagram on the top left shows for comparison the complete rerouting of the four carrier super channel from the working path to a protection path, which would resemble the ordinary practice known from prior art. The diagram on the top right shows a situation where the first and third carriers in wavelength ascending order remain on the super channel on the working path, while the second and fourth carriers are redirected to the restoration path This way, the spectral distance between the carriers remaining in the super channel is increased, thereby decreasing performance losses due to nonlinear interference occurring for optical signals that are spectrally close to each other. This is an example of the general concept stated in the summary of the invention that the remainder of the data stream is transmitted within the super channel on the working path on a subset of the original n wavelengths, wherein in the subset wavelengths are missing which were mutually non-adjacent in the original set of n wavelengths. In other words, the remaining carriers and the carriers to be redirected are interleaved with each other.

[0086] The diagram on the bottom left shows a situation, where the data rate of one of the edge carriers (in the embodiment shown .sub.1) is reduced, indicated by a narrow representation of the carrier in the spectrum, while the missing data rate is compensated for by transmission on the restoration path using an additional light source such as light source 12e shown in FIG. 4. The example on the bottom right shows a combination of the two previous scenarios.

[0087] While in FIGS. 3 and 4 separate transmitting and receiving arrangements 10, 20 are shown, in various embodiments the invention will employ transponders, such as a super channel transponder as shown under reference sign 28 in FIG. 6. In the schematic representation of the transponder 28, the boundary between optical signals and electrical signals is indicated which runs right through the light sources 12a to 12d and receiver units 22a to 22d. Further shown in FIG. 6 is an optical demultiplexer 30, an optical multiplexer 32, an electrical multiplexer 34 and an electrical demultiplexer 36. The transponder 28 comprises buffers 38 for compensating for a difference in optical path lengths between the working path 16 and the restoration path 18. In the embodiment shown in FIG. 6, each buffer 38 has a size of Mbit. Assuming that a carrier originally transmits 100 Gbit/s and is then split into two 50 Gbit/s signals, then at a propagation time per kilometer of about 5 s, the buffer must be able to store 250 kbit for each kilometer of path length difference. Accordingly, with the buffer size of 12.5 Mbit, a typical path length difference of 50 km can be compensated for. The buffers embedded in the electrical domain can be used for re-synchronizing data streams in order to form a single output signal. In the transponder 28 shown in FIG. 6, buffers 38 are provided both on the transmitter side and on the receive side. However, in simple, embodiments, it would be possible to provide the buffers 38 on only one of the transmit and receive side only.

[0088] While in the embodiment shown in FIGS. 3 and 4 the transmitting arrangement 10 and receiving arrangement 20 contain the corresponding redirecting device 14, 24, such as a corresponding cyclic filter, this is not mandatory for practicing the invention. Namely, instead of providing such redirecting devices 14, 24 in the transmitting and receiving arrangements 10, 20, or on a corresponding transponder card, it is possible to carry out the combination of carriers for forming the super channels and redirecting of part of the data stream outside the transmitting and receiving arrangements 10, 20, using e.g. wavelength selective switches (WSS) 40, as schematically shown in FIG. 7.

[0089] The term wavelength selective switch is used in a broad sense here and comprises all kind of configurable routing devices that are designed for directing an incoming lightwave to one out of at least two output ports independent of other lightwaves entering the device at the same port but having different wavelength. Thus, the term is not limiting with respect to the used technology.

[0090] Modern transponders make use of coherent detection. Thus, the sub-channel detected by the receiver can be selected from the entire super-channel signal by tuning the local oscillator. This features is known in the art under the term of coherent channel selection and allows to use a simple splitter instead of the cyclic filter 24 in the receiving arrangement 20.

[0091] Finally, FIG. 8 shows a performance-time diagram for illustrating a method for proactive reconfiguration of the transmission according to an embodiment of the invention. In FIG. 8, two carriers or sub-channels of the super channel, namely an edge carrier and an inner carrier, are considered. Performance of the carriers is monitored continuously, wherein the measurement data are represented by black and white dots. On the left side of the time axis, the performance of both carriers degrades over time due to changes on the working path. As is seen in the diagram, the performance of the edge carrier is always worse than that of the inner carrier, because it is more exposed to filtering effects. Furthermore, the expected (extrapolated) performance at a time instant later by a time T required for communication and rerouting to the restoration path is calculated continuously. At time T.sub.0, the expected performance of the edge carrier at time T.sub.0+T drops below a threshold indicating the required performance. At this time, the edge carrier is switched to the restoration path, whereas the inner carrier having a higher performance is still transmitted as part of the super channel on the working path and is further monitored. The further evolution of the performance of the inner carrier represented by white dots is used to obtain an estimate of the theoretical performance of the edge carrier if it were transmitted on the working path. The required information on the correlation of the performance of the two carriers is derived from the behaviour observed before time T.sub.0.

[0092] As is seen in FIG. 8, at some time after T.sub.0, the performance observed for the inner carrier of the super channel on the working path is improving again, for example because some transient disturbance seizes to exist, or due to completion of maintenance work. At time T.sub.1, the performance predicted for the edge carrier indicates that error-free data transmission of the edge carrier over the working path is possible, and the edge channel is switched back to the working path.

[0093] The function of a performance monitoring unit 42, 42 is detailed in the following by referring to the optical network shown FIG. 9. The embodiment shows a typical configuration of optical networks wherein data is transmitted bidirectionally by using fiber pairs. Via the working path 16, data is transmitted from a transponder 10 to a transponder 10 and vice versa. In the example shown, two separate fibers are used. However, using a single fiber for transmitting lightwaves in both directions would also be possible. Furthermore, different cores of a multi-core fiber or different modes of a few-mode fiber could be used. Both transponders comprise a transmitting part comprising transmitters 12a to 12d, 12a to 12d and a receiving part comprising receivers 22a to 22d, 22a to 22d. Furthermore, cyclic filters 14, 24, 14 and 24 are embedded in the respective transponder card for switching the data signals from the working path 16 to the restoration path 18. The function of these elements has already been described with reference to the previous figures and shall not be repeated again. Furthermore, the receivers 22a to 22d are connected to the performance monitoring unit 42 receiving performance related information from the receivers 22a to 22d and analysing them. Based on the result of this analysis, the performance monitoring unit 42 may initiate rerouting as for example described with reference to FIG. 5. A similar monitoring unit 42 is provided in the transponder 10.

[0094] In case performance degradation is detected and there is a need for switching at least part of the traffic from the working path to the protection path, the transmitters on the opposite side of the optical link need to be instructed to change wavelength. Different solutions are known for this kind of information exchange. For example, if the performance monitoring unit 42 detects some performance degradation for the data transmission from transponder 10 to transponder 10, this information can be transmitted to the transmitters 12a to 12d via an in-band communication channel such as the general communication channel GCCo of a sub-channel transmitted from transponder 10 to transponders 10. Most optical communication systems also provide an optical supervisory channel (OSC) that can be used for exchanging information between network elements and their cards. In addition, both transponders are connected to a telecommunications network management system (TNMS) 44. Since timing requirements are less strict when switching from a working path to a restoration path due to performance degration as compared to a complete traffic interruption, configuration data can also be exchanged via the TNMS 44. Furthermore, the connection to the TNMS 44 can also be used to make the restoration path available, for example by configuring some intermediate nodes such as photonic crossconnects comprising WSSes. Eventually, the changed wavelength information also needs to be communicated to the receiver. For example, when using coherent detection the wavelength of the local oscillator that is mixed with the received signal in order to generate an electrical signal carrying the transmitted information needs to be adapted.

[0095] Monitoring the performance of the data signals received by the receivers 22a to 22d is the key task of the performance monitoring unit 42. However, this unit can assume further tasks such as taking the decision to switch channels (at least partly) from the working path 16 to the restoration path 18. However, in alternative embodiments, these additional tasks can also be assumed by the controller of the respective network element or the TNMS 44. Furthermore, the performance monitoring unit does not need to be integrated in the transponder card. It is also possible to provide this functionality by a separate card.

[0096] In the embodiment described above, the transponder determines the used optical path (working path or protection path) by setting the wavelengths of the transponders. However, the optical path might also be selected by means of WSSes, as already explained with reference to FIG. 7. FIG. 10 shows a related setup comprising the performance monitoring units 42 and 42 as well as the TNMS 44. In this example, the TNMS 44 receives performance data from the performance monitoring units 42 and 42 and instructs the WSSes accordingly. If the transponder forms part of the same optical network element as the WSS determining the optical path (transponder 10 and WSSes 40, transponder 10 and WSSes 10), it is also possible to provide communication between the transponders and the WSSes via communication channels that are internal to the optical network elements.

[0097] Although preferred exemplary embodiments are shown and specified in detail in the drawings and the preceding specification, these should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiments are shown and specified, and all variations and modifications should be protected that presently or in the future lie within the scope of protection of the invention as defined in the claims.