Method and system of monitoring a data transmission link, particularly an optical, bidirectional data transmission link

Systems and methods for monitoring a data transmission link, especially an optical, bidirectional data transmission link, in which a digital transmit signal is transmitted on a first transmission path from a local end of the data transmission link toward a remote end of the data transmission link. A portion of the power of the transmit signal sent at the local end is transmitted, delayed by a non-zero delay time on a second transmission path as a control signal toward the remote end of the data transmission link. Both signals are received at the remote end and are tested for the presence of events of a predetermined type. A conclusion can be reached on the quality of the transmission link as a function of a time correlation and frequency of the appearance of events in the received transmit signal and in the received control signal.

CROSS REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. §119, this application claims the benefit of prior German Patent Application No. 10 2007 015 628.8, filed Mar. 29, 2007. The prior application is hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

The invention relates to a method for monitoring a data transmission link, especially an optical data transmission link, with the goal of reaching a conclusion on the performance of the overall data transmission or on an individual service as part of the overall data transmission to a certain endpoint of the transmission link, a so-called demarcation line.

BACKGROUND

For the commercial preparation of Ethernet-based connection services, so-called Service Level Agreements are usually a central component of the contract between the network operator or service provider and the customer. Such Service Level Agreements describe in detail a minimum continuous performance of data transmission or service to be guaranteed by the network operator or service provider to the fixed demarcation line, that is, typically to the point at which the data or the service is transferred to the application device of the customer or is fed from this device onto the transmission link. To guarantee a certain performance of such a service or a certain quality of the data transmission, the continuous digital monitoring of the data transmission or the protocol at the demarcation line is necessary. This is realized according to known methods usually through the use of active transmission technology at the demarcation line, that is, at the customer site, and through the measurement and evaluation of digital performance data, such as, e.g., “loss of Ethernet frame” or the like. The performance data is then typically transmitted via an additional transmission channel from the customer site to the location of the network operator.

This type of active performance monitoring allows the verification and guaranteeing of compliance with agreements set down in Service Level Agreements. In addition, in this way a detailed error analysis is possible in the case of a fault.

However, the use of active transmission technology for determining performance data at a demarcation line, especially for the purpose of monitoring special services, leads to an increase in the hardware and software costs, as well as to an increase of the operating costs due to the necessary maintenance and commissioning of the active transmission technology at the customer site. In addition, network availability decreases due to the ever-present failure probability of active transmission technology.

SUMMARY OF THE INVENTION

The present invention is devised to provide a method for monitoring a data transmission link, especially an optical, bidirectional data transmission link, with which in a simple and economical way performance data of the overall data transmission via the transmission link or individual services can be determined. In addition, the invention presents devices for implementing such methods.

The invention starts from the idea that expensive installation and maintenance work at the demarcation line or at the customer site can be avoided if only passive transmission-related means have to be used at the customer site, in order to carry out the monitoring of the data transmission link in terms of its performance at the demarcation line from a remote end, for example, from the location of the network operator or service provider.

According to the invention, at the demarcation line or the local end of the transmission link, only a splitting-and-delay unit is provided, by means of which the transmit signal to be sent at the demarcation line, that is, the local end of the transmission link, is fed not only to a first data transmission path, but also, after a predetermined non-zero delay time, to a second transmission path of the transmission link. The second transmission path is already present in bidirectional transmission links, which are realized by means of two separate data transmission paths, for example, by means of two optical waveguides. The two transmission paths, however, can also be realized by means of a single transmission medium, for example, a single optical waveguide. In order to avoid undesired superpositioning of signals with the same transmission direction, for this purpose, different wavelengths or signals with orthogonal polarization directions could be used.

At the remote end of the data transmission link, that is, for example, at the location of the network operator or service provider, according to the invention, only a receiving and evaluation unit must be provided, to which is fed the transmit signal received at the remote end on the first and second transmission path. The receiving and evaluation unit can then check the received transmit signals for the appearance of events of a predetermined type and can arrive at a conclusion on the quality of the transmission link as a function of a time correlation and as a function of the frequency of the appearance of the events in the transmit signal transmitted and received via the first transmission path and in the control signal transmitted via the second transmission path.

According to one implementation of the invention, the transmission link can be bidirectional; that is, on the second transmission path, a receive signal can be transmitted from the remote end in the direction toward the local end of the data transmission link and can be received there, that is, at the demarcation line. The opposite-direction signals on the second data transmission path, that is, the control signal, on the one hand, and the receive signal transmitted from the remote end to the local end, on the other, do not influence the selection of a suitable transmission technology and a suitable transmission medium for forming the second transmission path. For example, for an optical transmission link, an optical waveguide can guide the opposite-direction signals, wherein these can also feature different wavelengths.

According to one embodiment of the invention, for the appearance of an event in the received transmit signal and in the received control signal delayed by an amount less than the amount of the difference between the signal propagation times via the first and second transmission paths, the decision is made that the cause of the event is impairment of both transmission paths. This is because, in this case, the impairment of the two signals must take place at a point of the transmission link that, viewed in the transmission direction, lies after the splitting of the transmit signal and the feeding of the control signal onto the second transmission path. In addition, an impairment that affects the two transmission paths in the same way would have to exist or have existed in the transmission link.

According to another implementation of the invention, the decision is made that the cause of the events, which feature both signals, but delayed by the delay time plus the difference between the signal propagation times via the first and second transmission paths, is not impairment of one of the two transmission paths, but rather that the event in question was already contained in the transmit signal before the splitting of the transmit signal at the demarcation line. Such events can thus remain outside of consideration for reaching a conclusion on the quality of the transmission link.

According to another embodiment of the invention, for the appearance of the event only in either the received transmit signal or the received control signal, the decision is made that the cause of this event was an impairment in either the first or the second transmission path.

According to the preferred embodiment of the invention, for making the decisions explained above, the cross-correlation function is used for the transmit signal received at the remote end of the data transmission link and the control signal received there or for corresponding signals derived from the received transmit signal and the received control signal. Preferably, the normalized cross-correlation function is used for evaluating the signal to be examined. The cross-correlation function can be determined continuously (quasi-continuously) for each time section of predetermined length of the two signals.

Instead of a cross-correlation of the received signals, digital event signals derived from these signals can also be used, in which the appearance of an event is indicated, for example, by a pulse. In practice, for this purpose an event counter, for example, a “bad frame counter” can be used.

Preferably, however, the correlation function is calculated only when an event is contained in the relevant time section. The time length of the section is selected to be at least as large as the sum of the amount of the propagation time difference via the two transmission paths and the predetermined delay time. In addition, it is sufficient to calculate the cross-correlation function at selected points.

If the cross-correlation of the two signals has the value one for a time shift τ less than or equal to the different between the signal propagation times, then it is assumed that the two signals containing the event are identical and were merely received at the remote end delayed by an amount less than the difference between the signal propagation times via the two transmission paths. In this case, the relevant event is traced back to an impairment in the transmission link and therefore must be considered for determining the performance or quality of the transmission link.

This conclusion can also be reached by evaluating the cross-correlation function at the position τ equal to the sum of the delay time and the difference between the signal propagation times. If a value of zero is determined at this position, even though an event is present in both signals, then the event must have originated after the splitting of the transmit signal in this signal or in the control signal. The event therefore must be considered. This procedure has the advantage that the cross-correlation function must be calculated merely for a certain τ.

For the cross-correlation function, if a value of one is determined for a time shift equal to the delay time plus the difference between the signal propagation times via the two transmission paths, then it can be definitively concluded that the relevant impairment came about not in the course of the transmission link to be monitored, but rather before the point of the splitting of the transmit signal at the local end of the transmission link.

According to one embodiment of the invention, the total delay time of the two remote-side receive signals can be determined as the sum of the delay time and the difference between the signal propagation times in the first and second transmission paths determined in an initialization process. For this purpose, at the remote end of the transmission link, the transmit signal received on the first transmission path can be compared with the control signal received on the second transmission path with reference to a certain signal pattern. The time shift between two identical patterns in the two signals then can be determined similarly by evaluating the cross-correlation function. Optionally, such a measurement can be performed with or without the delay element at the local end of the transmission link. According to another embodiment of the invention, for a bidirectional data transmission, a portion of the power of the receive signal received on the second transmission path at the local end of the data transmission link can be coupled onto the first transmission path in the direction toward the remote end and received at the remote end. The receive signal, which is received at the remote end and which obviously may be superimposed in a non-separable way with the transmit signal already transmitted on the first transmission path, is checked for the appearance of events of the same type or a predetermined type, wherein the number of events appearing per unit time is used as a measure for reaching a conclusion on the quality of the second transmission path of the transmission link. Here, it involves a worst-case scenario, because the number of impairments possibly occurring in sequence on the transmission link in both the first and also second transmission paths obviously represents an upper limit for the impairments that occurred in just the second transmission path.

The preferred embodiments of the invention are explained in more detail below with reference to an embodiment shown in the drawing.

DETAILED DESCRIPTION

The optical transmission link10shown inFIG. 1includes two optical waveguides12,14, of which the optical waveguide12transmits an optical receive signal SRat the wavelength λ1from a remote end16to a local end18and an optical waveguide14transmits an optical transmit signal STat the wavelength λ2from the local end18to the remote end16. At the remote end16of the transmission link10there is an electro-optical converter unit20, which performs an electro-optical conversion of the electrical receive signal SR,einto the optical receive signal SRand an opto-electrical converter unit22, which converts the remote-side, received optical transmit signal STinto an electrical transmit signal ST,e. In practice, the electro-optical converter unit20and the opto-electrical converter unit22can be contained, for example, in one channel card.

At the local end18of the transmission link10there is a splitting-and-delay unit24, which includes a first coupler26, a second coupler28, and a delay unit30. The first coupler26decouples a predetermined part of the optical power of the optical transmit signal STfrom the optical waveguide14and feeds this signal to the delay unit30, which delays the signal by a predetermined non-zero delay time. The delay unit can consist, for example, of an optical waveguide of predetermined length, wherein the signal group velocity multiplied with the length of the optical waveguide gives the delay time. The delayed signal is coupled into the optical waveguide12via the second coupler28as the control signal SC.

At the remote end16, the control signal SCis decoupled from the optical waveguide12by means of a third coupler32and is fed to a receiving and evaluation unit34. At this point, it should be mentioned that the couplers26,28and32involve typical, also wavelength-dependent couplers.

In addition, the transmit signal ST,ealready opto-electrically converted by the opto-electrical converter unit22is also fed to the receiving and evaluation unit34.

The receiving and evaluation unit34first converts the control signal SCfed to it opto-electrically by means of an opto-electrical converter unit36contained in the receiving and evaluation unit and feeds the converted signal to a second event detector40. This detector evaluates the opto-electrically converted control signal SC,ewith respect to the appearance of events of a certain type, such as, for example, “loss of frame” in an Ethernet signal. Such methods for Ethernet-frame analysis are known. The output signal of the event detector40, the event signal EC, in which, for example, each “bad frame” is represented by a pulse, then can be further processed or evaluated at less expense than the high-bit rate transmit signal ST,e.

In the same way, a first event detector38contained in the receiving and evaluation unit34evaluates the already opto-electrically converted transmit signal ST,efed to it with respect to the appearance of events of the predetermined type and generates a corresponding event signal ET.

The receiving and evaluation unit34then evaluates the event signals ECand ETaccording to the following method, wherein it is noted that this method, which concerns the correlation analysis, could also be performed with the high-bit rate signals SC,eand ST,edirectly.

For performing this evaluation method, the total propagation time difference Ttotalbetween the signals SCand STmust be known to the receiving and evaluation unit34. This value either can be reported to the receiving and evaluation unit34or it can be determined by it in an initialization process. For this purpose, the receiving and evaluation unit34can calculate the cross-correlation function, for example, for two time segments of the signals SCand STand can determine the maximum of the cross-correlation function. If the signals were not interfered with in the transmission from the local end18to the remote end16and are therefore identical, then this produces the maximum value of the cross-correlation function for a time shift τ equal to the time shift of the received signals. This value then can be stored, if necessary, by the receiving and evaluation unit34. Obviously, the accuracy can be improved through several repetitions of this procedure and by averaging the resulting values.

The propagation time difference of the signals via the two optical waveguides12,14can be determined in an analogous manner, wherein, for this purpose, the delay unit30must be replaced with a corresponding coupler unit without a delay time. Alternatively, a delay unit can be used, which allows a continuous or stepped adjustment of the delay time, including the delay time TD=0.

At this point, it should be noted that the delay time must be meaningfully selected, preferably significantly greater than the amount of the propagation time difference via the two transmission paths. Here, the propagation time difference can be determined in the scope of the initialization process, and then the delay time TDcan be selected in a suitable way.

If the receiving and evaluation unit34knows the sum from the delay time TDand the propagation time difference of the signals via the optical waveguide14(first transmission path) and the optical waveguide12(second transmission path), then this unit performs the evaluation of the event signals ETand ECas follows.

If the receiving and evaluation unit34determines the appearance of an event in at least one of the event signals ETand EC, then it calculates the cross-correlation function of these signals for a sufficiently large time section, in which lies the one or more detected events. The time section is selected using information on the total propagation time difference Ttotal, with the time section being selected at least as large as the propagation time difference Ttotal.

For the cross-correlation function, if a value of one is determined for a time shift τ equal to Ttotal, then this means that an event was contained in both signals and these were received at the remote end delayed by Ttotalexactly. However, this is only possible if the relevant event was contained in the transmit signal STbefore the splitting and delay unit. Thus, the appearance of this event cannot be assigned to the transmission link.

If a conclusion on the performance of the transmission link formed by the optical waveguide14is reached by counting events within a certain time unit, then the event can be used for the cross-correlation function for τ equal to Ttotal, in order to evaluate the signal ET. If the value of the cross-correlation function is equal to one, then the relevant event is not counted. If the value of the cross-correlation function is equal to zero for τ equal to Ttotal, then the event is counted.

This procedure is described again briefly in the following table, where K(τ) designates the normalized cross-correlation function of the two signals STand SCto be correlated or the corresponding time sections of these signals:

In other words, an event contained in the signal ETcan remain outside of consideration if an event delayed by the delay time TDplus the difference between the signal propagation times in the optical waveguides12and14is contained in the signal EC. This is because, in this case, the relevant event must have already been contained in the signal ST, which was fed to the local end18. In contrast, if the delay of the event contained in the signal ECis less than or equal to the pure propagation time difference between the signals via the optical waveguides12,14, then the error must be a result of impairment of both optical waveguides.

Thus, according to this method for reaching a conclusion on the performance of the overall data transmission or an individual service via the transmission link, which is made available through the optical waveguide14to a subscriber at the demarcation line or the local end18, only the passive splitting-and-delay unit24is necessary. The evaluation can be performed remotely.

With this method, the receiving and evaluation unit34reaches a conclusion on the performance on the local-side demarcation line and can deliver this information by means of a performance signal SPto arbitrary locations or units.

FIG. 2shows a refinement of the transmission link10inFIG. 1, with another coupling unit42being provided on the local end and another coupler48being provided on the remote end.

The coupling unit42can be formed, as shown inFIG. 2, from two separate couplers44,46. However, it can also be constructed together with the coupling unit24. Through the use of the additional coupling unit42, a portion of the power of the signal SRis decoupled from the optical waveguide12by means of the coupler44and is coupled into the optical waveguide14via the coupler46in the direction toward the remote end. This signal RR,back, which is fed back in the direction toward the remote end and received there and which has a wavelength λ1that is different from the wavelength λ2of the transmit signal ST.

The signal SR,backis decoupled at the remote end by means of the preferably wavelength-selective coupler48and fed to the receiving and evaluation unit34. This unit converts the signal SR,backby means of an opto-electrical converter unit50contained by this receiving and evaluation unit into a corresponding electrical signal and feeds it to another event detector50also contained therein. The event detector50, which can be constructed in the same way as the event detectors38,40, can check the signal fed to it and opto-electrically converted, SR,back, for the presence of the same type of events as the event detectors34,40or also for a different type, as a function of how the performance of the transmission link formed by the optical waveguide12is defined at the local-side demarcation line.

For example, the performance conclusion can be redefined by means of the events of a certain type appearing per unit time, with these events being detected by an event detector50. The receiving and evaluation unit34can also store this information in the performance signal SPand can transmit it to any other locations or units.

However, because the signal SR,backreceived at the remote end also includes influences of the transmission path from the local end to the remote end, i.e., in particular, influences of the optical waveguide14, the performance conclusion can be reached only in the way that it is assumed as a worst-case scenario that the agreed performance is fulfilled in each case at the local end, if this is determined not on the basis of a signal received at the local end, but rather on the basis of an additional signal transmitted back from the local end to the remote end.

As already mentioned above, the first and the second transmission paths can be formed by means of a single transmission medium, for example, by means of a single optical waveguide. Such an embodiment, whose function incidentally corresponds to the embodiment inFIG. 2, is shown inFIG. 3. Instead of two optical waveguides for the first or second transmission path, a single optical waveguide13is provided, which guides all of the optical signals. To allow optical separation of the optical signals guided in a direction, each is provided with polarization directions that are mutually orthogonal. Because the electrical signal processing after the opto-electrical or electro-optical conversion of the signals is identical to the embodiment inFIG. 2, inFIG. 3only the purely optical transmission path between the remote end16and the local end18of the optical transmission link is shown.

The optical transmit signal STat a wavelength λ2is fed with a first polarization direction P1to the transmission link at the local end18. By means of a first, preferably wavelength-selective coupler60, a portion of the optical power of the signal STis decoupled and fed to a polarization rotation unit62. This sub-signal represents the optical control signal and is therefore influenced in its polarization direction, so that it is orthogonal to the polarization direction P1of the transmit signal ST. This optical signal with the polarization direction P2is then delayed by the predetermined delay time TDrelative to the transmit signal STby means of the delay unit24and is fed by means of another, wavelength-selective coupler64as optical control signal SCback to the optical waveguide13. The couplers60and64are preferably created so that signals with the optical wavelength λ2are merely coupled to or decoupled from the optical waveguide13.

In the region of the remote end16, a wavelength-selective coupler66is provided, which separates the optical waveguide13of the signals with the wavelengths λ2or λ1into separate signal paths or combines these signals. In each separate signal path there is a splitting unit68, which feeds its input optical signals with the orthogonal polarization devices P1and P2to separate signal paths.

Thus, the transmit signal STand the optical control signal SCwith the wavelength λ2are fed through the coupler66to the splitting unit68provided in the lower signal path inFIG. 3. This splitting unit separates the signals STand SC, so that these can be electro-optically converted and further processed at the remote end in the way explained in connection withFIG. 2.

In an analogous way, the receive signal SRis fed to the splitting unit68arranged in the remote-side signal path for the signals with the wavelength λ1at the port for signals of the polarization direction P1. If this signal already exists as a signal with the polarization direction P1, then a corresponding polarization filter can be eliminated in the splitting unit68, or the splitting unit for feeding the signal SRwith the wavelength λ1to the coupler66must not have a corresponding polarizing property. The signal SRis then fed to the optical waveguide13via the wavelength-selective coupler66.

In the region of the local end, another, preferably wavelength-selective coupler70is provided, which decouples a portion of the power of the signal SRand feeds it to a polarization rotation unit72. This unit rotates the polarization direction P1of the signal SRin the orthogonal polarization direction P2and feeds this signal SR,backto the other wavelength-selective coupler74, which feeds the signal SR,backback to the optical waveguide13in the direction toward the remote end. In order not to allow also a signal path from the coupler74via the coupler70, in the splitting unit92an optical isolator (not shown) can be provided, which absorbs the signal fed via the coupler74on the output side.

At the remote end, the signal SR,backis fed via the coupler66to the upper signal path inFIG. 3and by means of the splitting unit to the relevant port for the signals of the polarization direction P2. Thus, after the electro-optical conversion of the signal SR,back, the signals SRand SR,backcan be further processed at the remote end in the way (electrically) explained in connection withFIG. 2.

In the variant shown inFIG. 3, the couplers60and64as well as the delay unit24and the polarization rotation unit62can be combined to form a splitting-and-delay unit30′. This can be easily installed at the local end18, for the purpose of which the splitting-and-delay unit30′ can be connected with its input and output ports to the actual optical waveguide transmission link by means of detachable or non-detachable connection devices, for example, by means of plugs and/or sockets. The same also applies for the couplers70and74, as well as the polarization rotation unit72, which likewise can be combined to form a coupling unit42′. Likewise, the splitting units68and the wavelength-selective coupler66can be combined to form a unit76, which assigns signals of the wavelengths λ1and λ2and with the orthogonal polarization directions P1and P2each to different ports. Obviously, the units30′,42′, and76, can also be realized like the units24and30inFIG. 1orFIG. 2by means of other components, as long as the functions described above with respect to input ports and output ports of these units are realized.

The above described exemplary embodiments are intended to illustrate the principles of the invention, but not to limit the scope of the invention. Various other embodiments and modifications to these example embodiments may be made by those skilled in the art without departing from the scope of the present invention.