The invention relates to a method of raising a loss of frame alarm in an optical network. Background

In optical transmission networks, such as an Optical Transport Network (OTN), a network of

Synchronous Digital Hierarchy (SDH) or a Synchronous Optical Network (SONET), data is transmitted using framed optical data signals. An optical network element receiving such a framed signal needs to detect the frame position for proper data reception. Otherwise, the receiving network element will not be able to properly de-map data from the one or more received frames. In other words, a framer of a receiving network element has to lock to the start position, also called frame position, of received frames.

According to different telecommunication standards, there is a demand that a framer has to be able to lock to a start position of received frames even if the bit error rate of the received data representing the one or more frames rises up to a threshold of 10e-3. If the framer is able to lock to the start position, then this is called an in-frame (IF) state.

If the bit error rate of the received data rises above the threshold value up to for example 10e-2, then it may occur, that the framer does not lock to the start position of one or more received frames, thus being in an out-of-frame (OOF) state.

According to the standard ITU-T G.783, the presence of the detected OOF state is detected, by filtering the OOF state in time, using preferably a persistency filter with a time window of 3 milliseconds (ms). A loss of frame defect (dLOF) is detected in the case that the OOF state persists for at least these 3 ms. The dLOF is then correlated with one or more other defects, using Boolean AND concatenation, to yield a defect correlation (cLOF).

Such defect correlation cLOF may then be filtered by a further persistency filter having preferably a time window of 2.5 seconds (sec). A frame failure fLOF is detected in the case that the cLOF persists for at least these 2.5 sec. In the case of a frame failure fLOF an alarm such as an Alarm Indication Signal (AIS) is raised by the optical network element. Another device or a network operator receiving this alarm AIS may then derive from this alarm that the network element that raised the alarm AIS is not able to lock its framer to the start position of one or more frames, possibly due to degraded optical signal transmission towards this network element.

It is an object of the invention to improve the known methods for detecting degraded optical signal transmission causing a problem for a framer of an optical network element. Summary

Proposed is a method of raising a loss of frame alarm in an optical transmission network at an optical network element. The method comprises the steps of detecting an out-of-frame (OOF) state, detecting a presence of the OOF state, by filtering the OOF state in time using at least one time window, detecting a loss of frame defect , in case the presence of the OOF state is detected, determining a defect correlation, by correlating the loss of frame defect with one or more further defects, and finally, in case of a presence of the defect correlation, instantaneously detecting a frame failure and raising a loss of frame alarm.

In order to grasp the advantages of the proposed method, the following aspects have to be taken into consideration.

When detecting a presence of an OOF state, such presence is usually detected by a persistency filter filtering the OOF state in time with a time window of preferably 3 ms, although different time filtering is possible. A loss of frame defect, preferably a dLOF, is detected due to the detected presence of the OOF and then furthermore a defect correlation, preferably a cLOF, is determined in dependence of the loss of frame defect, dLOF. Usually, according to known solutions, such a defect correlation, cLOF, is then filtered by a persistency filter with a time window of preferably 2.5 sec before detecting the failure and raising the loss of frame alarm. Such a solution may be detrimental in a situation, in which the framer may toggle between an OOF state and an IF state.

For example, it may be the case that the framer stays in the OOF state long enough, such that the presence of the OOF state is detected even after filtering the OOF state by the time window of preferably 3 ms duration. But the toggling of the framer between the OOF state and the IF state may be at a frequency, such that the subsequent filtering of the defect correlation, cLOF, by the persistency filter with a time window of preferably 2.5 sec causes the loss of frame alarm never to be raised, since the defect correlation, cLOF, may persist only for a time duration that is less than the time window of the persistency filter applied to the defect correlation, cLOF. Thus, although the framer is repeatedly in an OOF state, the frame failure is not detected and the loss of frame alarm is not raised, which leads to the fact that the operator is not informed about the degraded data reception at the network element. In other words, such failure detection leads to the paradox effect that the data transmission may be degraded, but the network system is alarm free, i.e. from looking at the raised alarms the operator cannot determine that there is a transmission problem.

But, according to the proposed method, since the frame alarm is detected and the loss of frame alarm is raised instantaneously in case of a presence of the defect correlation, the loss of frame alarm is also raised in the case that the frame is in the OOF state for much shorter time periods, only defined by the time window of the filter used for detecting the presence of the OOF. Thus, the operator is informed about the degraded data reception also in the case that the framer toggles between the OOF state and the IF state.

Detecting the frame failure and raising the alarm instantaneously is in other words detecting the frame failure and raising the alarm directly at the presence of the defect correlation. In other words, detecting the frame failure and raising the alarm is carried out without filtering the defect correlation in time.

For time-division-multiplex (TDM) systems, which are operated without wavelength-division- multiplex (WDM) systems, the above described problem may be only a theoretical problem, as the vendors carefully specify the margins for their optics, and if the systems are operated within the specified margins this problem may not occur, as the margins are defined such that an error rate of 10e-3 will not be reached. The situation may change with the introduction of WDM systems, especially with WDM systems with protection and/or restoration: In WDM transmission it may happen that single wavelengths degrade to a degree that a bit error rate of 10e-3 occurs, which then leads to the above described problem. The root cause can be manifold, such as degraded optical components, optical cross talk, or too much dispersion, respectively too much attenuation after a protection/restoration in the optical domain.

Instead of using the proposed method, performance monitoring may be used to detect the degradation of the signal. However, many operators are not willing to use performance monitoring. Furthermore, many operators trigger repair actions based on alarms. As long as there is no alarm, the operators perform no corrective actions.

A further different solution may be, that the optical analogue parameters of the network are monitored by the WDM systems. This however is unpractical, as it requires detailed knowledge about the characteristics of the optical components used in the transmission path.

Thus, the proposed method provides a solution for detecting a degraded data reception with regard to a framer reaching one or more OOF states without having to implement performance monitoring and/or without having to monitor optical analogue parameters of the network, but instead only relying on raised alarms. In other words, the operator is alarmed, in case the optical signal has degraded to a level where the framer starts missing frames, i.e. to a level where transmission is severely impacted. This allows the operator to realize that he has a transmission problem and to perform immediate repair actions.

Preferably, the OOF states are detected according to ITU-T G.798 8.2.1

Preferably, the step of filtering the OOF state is performed by applying a persistency filter with a time window and the presence of the OOF state is detected in case the OOF state persists for the whole of the time window. Preferably, this time window is of 3 ms length.

Preferably, the step of filtering the OOF state is performed by applying a filter with a time window and the presence of the OOF state is detected in case the OOF state is present within at least two consecutive instances of the time window. Preferably, this time window is of 2.5 sec length.

Preferably, the loss of frame defect is correlated with the one or more further defects using one or more Boolean AND functions.

Preferably, the loss of frame alarm is an Alarm Indication Signal. Preferably, the loss of frame defect is a dLOF.

Preferably, the loss of frame defect is a defect indicating an unstable defect. Preferably, the loss of frame defect is a dULOF. Preferably, the defect correlation is a cLOF. Preferably, the failure is a fLOF.

Brief description of the Figures

Figure 1 a shows steps of detecting a loss of frame defect. Figure 1 b shows steps of detecting an IF state or an OOF state.

Figure 1 c shows steps of detecting a frame failure according to a preferred embodiment. Figure 2 shows steps of detecting a loss of frame defect according to a preferred embodiment.

Description of embodiments

Figure 1 b shows steps of detecting an IF state or an OOF state. It shall be assumed, that initially the framer is in a state IFS, in which the framer has the in-frame (IF) state and is locked to the starting position of received frames. In the case, that according to the condition described in ITU-T G.798 (12/2012), clause 8.2.1, the framing is lost, a transition from the IF state IFS to the OOF state OFS is performed. Furthermore, in the case that according to the condition described in ITU-T G.798 (12/2012), clause 8.2.1, the framing is found, a transition from the OOF state OFS back to the IF state IFS is performed.

Figure 1 a shows steps of detecting a loss of frame defect. The loss of frame defect is preferably a dLOF. It shall be assumed, that initially the system is in the state Dl, in which no loss of frame defect is detected. In a state D2, the status of the framer as detected according to Figure 1 b is read. This status is filtered in time using a filter function with a time window. In this example, the status is filtered using a persistency filter with a time window of 3 ms. If the OOF state persists for at least 3ms, then a transition to the state D3 is carried out, in which the loss of frame defect, dLOF, is detected. In the case, that the IF state persists for at least 3 ms, then a transition from the state D3 back to the state Dl is carried out, in which no loss of frame defect, dLOF, is detected, or is in other words cleared.

A detected loss of frame defect, dLOF, is then correlated with one or more further defects using Boolean AND-operations for determining a defect correlation, which is preferably a cLOF. This correlation is preferably carried out as described in ITU-T G.798 (12/2012), page 119, as cLOF <- dLOF and (not dAIS) and (not AI_TSF-P) .

Such correlation is not shown in detail in Figure la.

Figure 1 c shows steps of detecting a frame failure according to a preferred embodiment. The system shall be assumed to be initially in a state D10, in which not frame failure is detected.

In a state Dll, the status of the defect correlation, cLOF, is read.

In the case, that a defect correlation, cLOF, is present, an immediate and/or direct transition to a state D12 is carried out, in which a frame failure, preferably a fLOF, is detected. In case of detecting a frame failure, a loss of frame alarm is raised. The alarm is preferably an AIS.

In the case, that a frame failure was detected, and that the defect correlation, cLOF, read out in the state D13, is absent for a predefined number of consecutive seconds, preferably 10 seconds, the system carries out a transition from the state D12 to the state D10.

In other words, a persistency filter is applied to the OOF anomaly to detect a presence of a loss of frame defect. This immediately leads to consequent actions such as inserting AIS - presenting a forward defect indication so subsequent systems - and potentially also triggering, after a individually configured hold-off time, a transmission protection switch. The proposed method has the advantage, as already described above in detail , that the alarm is raised even in the case that the framer toggles between the IF state and the OOF state.

Figure 2 shows steps of detecting a loss of frame defect according to a preferred embodiment. Instead of using as the loss of frame defect the dLOF as mentioned with regard to Figure la, a different loss of frame defect may be used. This is a loss of frame defect indicating an unstable state of the framer. Preferably, this loss of frame defect is a dULOF.

It shall be assumed, that the system is initially in a state D20, in which no loss of frame defect is detected. In a state D21, the status of the framer as detected according to Figure 1 b is read. This status is filtered in time using a filter function with a time window. In this example, the filtering is performed by applying a filter with a time window of 2.5 sec length. The presence of the OOF state is detected in case that the OOF state is present at least once within each of at least two consecutive time windows. If the presence of the OOF state is detected, then a transition to the state D22 is carried out, in which the loss of frame defect, dULOF, is detected. The loss of frame defect, dULOF, is a loss of frame defect indicating an unstable state of the framer.

The loss of frame defect dULOF may be additional to the regular standard loss of frame defect dLOF. The loss of frame defect dULOF needs additional correlation and also contributes to consequent actions.

The defect correlation cULOF is determined, by correlating the loss of frame defect dULOF with one or more further defects. Preferably, the defect correlation cULOF is determined as cULOF - dULOF and (not dLOF) and (not dAIS) and (not AI_TSF<server>)

Preferably, the loss of frame defect dULOF contributes to the consequent action aSSF via aSSF - dAIS or dLOF or dULOF or dLOM or AI_TSF<server>

or (not MI_Active < Adaptation in question>) .

In the case, that a loss of frame defect dULOF was detected, as in state D22, and that the OOF state read out in the state D23 is absent for two consecutive time windows of preferably 2.5sec length, the system carries out a transition back to the state D20.

Proposed is furthermore an optical network element, which contains an optical interface operable to receive a framed optical data signal and that contains furthermore one or more processors operable to carry out the steps of the proposed method. The network element contains the optical interface for receiving the framed optical data signal as well as a receiver for deriving from the signal transmission data. The network element contains the processor that is coupled to the receiver. The processor is operable to carry out the steps of the proposed method. The functions of the processor may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term 'processor' should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage. Other hardware, conventional and/or custom, may also be included.