EP4165800A1

EP4165800A1 - Fault location in an optical ring network

Info

Publication number: EP4165800A1
Application number: EP20733234.7A
Authority: EP
Inventors: Riccardo Ceccatelli; Giacomo AGOSTINI; Roberto Magri; Stefano Parodi; Luca Risso; Marina VALERI
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2020-06-10
Filing date: 2020-06-10
Publication date: 2023-04-19
Also published as: WO2021249639A1

Abstract

Embodiments described herein relate to methods and apparatus for determining a fault location in an optical ring network. There is provided a method of determining a fault location in an optical ring network (100) having a number of nodes (110A, 110B, 110C). The method comprises monitoring a parameter (205) of a traffic signal (170D, 170U) associated with each node of the optical ring network (305), and in response to receiving an indication of a fault event (215, 220, 310), comparing a value of the parameter of each node before and after the fault event to determine the fault location (315, 325, 335, 340).

Description

FAULT LOCATION IN AN OPTICAL RING NETWORK

Technical Field

Examples of the present disclosure relate to methods and apparatus for locating a fault in an optical ring network.

Background

Fronthaul networks are typically based on a single optical fiber infrastructure from a main node (Central Office) to remote nodes (radio antenna sites) using WDM technology for multiplexing multiple optical channels over the same optical fiber. In Single Fiber Working, SFW, the optical channels are transported using different wavelengths for uplink and downlink transmission on the same optical fiber. In the Fronthaul network segment, WDM pluggable modules are fitted in the radio access network, RAN, nodes (passive mode) or in transponders (active mode) and connected to passive optical filters.

Fronthaul network infrastructure typically reuses existing deployed fiber and can be based on point to point, P2P, topologies, point to multi-point, P2MP, topologies, such as star topologies with a remote feeder node, or a ring topology with a hub node communicating to all other nodes. A ring network topology allows a main node to be connected to many remote nodes by means of a single optical fiber, offering traffic protection and limiting latency. In many cases optical fiber is a valuable resource and network operators tend to minimize the number of fibers deployed in the fronthaul and backhaul network segments. Each node may be allocated a different pair of wavelengths for downlink and uplink transmissions.

A fault in a ring network may result in some wavelengths becoming unavailable to their respective nodes. Protection schemes may switch the direction of some or all wavelengths to avoid the fault by using a part of the ring network that is still intact between the hub and node.

In order to minimize the risk of failure of some nodes due to another fault, to increase serviceability and to minimize costs and time to repair, it is desirable to locate the fault within the ring network. This can be achieved using an Optical Time Domain Reflectometer (OTDR) where pulses are sent into the optical fiber and the round trip time of reflected signals measured to localize fiber damage. However, the accuracy and range of this approach can be limited. W02007/044939 describes a method of automatically determining a fault location by monitoring photodiodes at each node to detect a probe signal and to communicate this using an Optical Supervisory Channel (OSC). By analyzing which nodes can and cannot detect the probe signal, the location of the fault can be inferred. However, this approach requires additional hardware and complexity and can only be applied to active nodes.

US2009/0074403 describes another method of automatically determining a fault location using the MAC layer to detect and control switching at each node. However, again this requires additional equipment and is protocol dependent.

Summary

In one aspect there is provided a method of determining a fault location in an optical ring network having a number of nodes. The method comprises monitoring a parameter of a traffic signal associated with each node of the optical ring network. In response to receiving an indication of a fault event, the method compares a value of the parameter of each node before and after the fault event to determine the fault location.

Parameters at different layers may be used, for example received power, round trip delay and traffic signal quality parameters such as bit error rate (BER).

By analyzing the value of one or more parameters of a traffic signal for each node before and after a fault event, the effect of the fault on each node can be determined and from this the location of the fault can be automatically determined without the need for additional equipment at the nodes. This simple approach is applicable to passive as well as active or semi-active nodes and does not require any additional hardware but can be implemented using only new software. A variety of different types of parameter may be monitored providing implementation flexibility depending on factors such as availability of monitoring of parameters.

In another aspect there is provided apparatus for determining a fault location in an optical ring network having a number of nodes. The apparatus comprises a processor and memory which contains instructions executable by said processor whereby the apparatus is operative to monitor a parameter of a traffic signal associated with each node of the optical ring network. In response to receiving an indication of a fault event, the apparatus is operable to compare a value of the parameter of each node before and after the fault event to determine the fault location. In another aspect there is provided a computer program comprising instructions which, when executed on a processor, causes the processor to carry out the methods described herein. The computer program may be stored on a non transitory computer readable media.

In another aspect there is provided an optical ring network having a number of nodes and apparatus as described herein.

Brief Description of the Drawings

For a better understanding of examples of the present disclosure, and to show more clearly how the examples may be carried into effect, reference will now be made, by way of example only, to the following drawings in which:

Figure 1 is a schematic illustration of an optical ring network according to an example;

Figure 2 is a schematic illustration of example parameter monitoring approaches according to an embodiment;

Figure 3 is a flow chart of a method of locating a fault in an optical ring network according to an embodiment;

Figures 4A, 4B and 4C illustrate example fault location scenarios in the optical ring network of Figure 1 ; and

Figure 5 is a schematic illustration of an apparatus for locating a fault in an optical ring network according to an embodiment.

Detailed Description

The following sets forth specific details, such as particular embodiments or examples for purposes of explanation and not limitation. It will be appreciated by one skilled in the art that other examples may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and/or discrete logic gates interconnected to perform a specialized function, ASICs, PLAs, etc.) and/or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, where appropriate the technology can additionally be considered to be embodied entirely within any form of computer-readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analogue) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions.

Figure 1 shows an example of an optical ring network 100 according to an embodiment. The network 100 comprises a hub or central office 105 coupled to a number of remote nodes

I I OA, 110B, 110C by an optical fiber 103. The optical fiber 103 may be a single working fiber (SFW) over which wave division multiplexing (WDM) technology is used to communicate between the nodes and hub. In other examples, references to the optical fiber 103 may indicate a plurality of fibers. Each node 110A, 110B, 110C may be allocated a different pair of wavelengths for downlink and uplink transmissions.

The optical fiber 103 can be divided into links L1 , L2, L3, L4 between two nodes 110A,

I I OB, 110C or a node 110A, 110C and the hub 105. In normal operation, downstream traffic signals 170D may propagate in a first direction D1 around the ring network and upstream traffic 170U may propagate in a second direction D2. In alternative embodiments dual or multiple optical fibers may be employed, and downstream and upstream traffic may travel in different directions to those indicated.

Were a fault event to occur, this may trigger protection switching in which some of the traffic signals may propagate in different directions to ensure continued communication with nodes downstream of the fault. For example if a fault were to occur in link L3, downstream signals intended for node 110C would no longer be able to propagate in direction D1 and protection switching may instead be arranged to propagate these traffic signals in direction D2 where there is no fault between the hub and node 110C. In alternative embodiments, protection switching may couple an isolated node to another fiber.

The nodes 110A, 110B, 110C may be radio access network, RAN, nodes connected to passive optical add/drop filters (not shown) on the SFW to form a Centralized Radio Access Network (CRAN) network domain. For example, the nodes 110A, 110B, 110C are remote radio units. The hub 105 may comprise a baseband processing unit. The RAN nodes may provide Third Generation Partnership Project (3PPG) wireless services to mobile subscribers. However other types of deployment may also be catered for such as Fiber to the Home for direct optical fiber provision to home and/or business subscribers.

The hub 105 comprises an optical transponder and multiplexing/demultiplexing unit 145 which converts between electronics signals in baseband circuitry 150 and optical signals in the fiber 103. A pair of wavelengths are allocated for the downstream and upstream traffic for each node 110A, 110B, 110C and a splitter 140 splits the wavelengths into the two ends of the fiber so that they may propagate in either direction D1 , D2 around the ring network. In normal operation the traffic signals carried on the respective wavelengths propagate through one end of the optical fiber 103, for example as shown the end connected to L1.

In the event of a fault, an optical protection module (OPM) 130 may be arranged to switch the signals to and from the splitter through the other end of the fiber as well, that is the end connected to L4. As such, in the event of the protection scheme being used, a direction of propagation around the optical fiber 103 ring is reversed for at least one of the nodes 110A,

11 B, 110B. This allows the signals to reach all of the nodes even when there is a break or fault in the network preventing signal transmission across one of the links. In normal operation, the last link L4 of the fiber is connected to secondary port 135-2 of the OPM which is not coupled to the splitter 140. However, upon protection switching, the secondary port 135-2 is coupled to the splitter to enable wavelengths to circulate the fiber in both directions D 1 , D2.

Various mechanisms for detecting a fault may be employed. In one embodiment, a loss of a pilot tone on a wavelength separate from the traffic wavelengths may be used. In another embodiment, the pilot tone (or pilot signal) is on a same wavelength as the traffic. A pilot tone generator 125 is connected to an optical add/drop multiplexer 120X to inject the pilot tone onto the optical fiber 103. Another optical add/drop multiplexer 120Y at the other end of the fiber 103 drops or isolates the pilot tone from the other signals on the fiber and directs this to a first port 135-1 of the OPM 130. Loss of the pilot tone signal at the first port of the OPM may trigger the above described protection switching where the splitter 140 is coupled to both ends of the fiber 103. In other embodiments, loss of traffic carrying wavelengths may be used to trigger protection switching, or a signal indicating that a protection switching event has occurred may be used. The occurrence of a protection switching event is indicative of detection of a prior fault event by some mechanism.

The hub 105 also comprises an apparatus 155 for determining a fault location in the optical ring network. Alternatively, the apparatus 155 may be located in a different location. The apparatus 155 monitors one or more parameters of a traffic signal 170D, 170U associated with each node 110A, 110B, 110C. The parameter or parameters monitored may include received power of a wavelength carrying the traffic signal, a round trip delay of the traffic signal, and/or a quality parameter of the traffic signal, for example bit error rate (BER) or code violations (CV). Any or a combination of these parameters may be monitored depending on what is available within the network 100. Further additional or different parameters may be used which relate to traffic signals to respective nodes. When a fault event is detected, for example by loss of the pilot tone and/or by detection of a protection switching event, the apparatus compares a value of the parameter or parameters of each node before and after the fault event in order to determine the fault location as described in more detail below.

Figure 2 illustrates two methods of determining a value of a parameter before and after a fault event. Depending on the circumstances, one method may be more suitable for some types of monitored parameters than the other method. A timeline t is shown together with a fault event time 215 and a protection switching time 220. Protection switching of an optical ring network and settling of transient conditions will normally reliably follow fault event detection 215 within an expected time period, for example after a few seconds to 2 minutes. This is indicated by time period 232 which follows detection of the fault event and includes protection the switching event 220 and the settling time.

Values of the parameter 205 may be determined during predetermined or sequential time intervals 210 and recorded for later analysis. A predetermined time interval may be for example 15 minutes long. The determined value of the parameter 205 for each time interval 210 may be an average, median, a maximum or minimum or any suitable analysis of measurements of the parameter occurring during the time interval 210. For example, a measurement of the parameter 205 may be taken fifteen times during each time interval and the average taken as the value of the parameter to be used for later analysis. Valid and invalid time intervals and hence values of the parameter may be determined based for example on the variation of the measurements. If the measurements vary too much, for example over a threshold, then the time interval may be declared invalid. In Figure 2, invalid time intervals are indicated using horizontal lines and valid time intervals are indicated by diagonal lines. Transients associated with a fault event may cause parameter measurements to vary widely resulting an invalid time interval.

The comparison of the value of the parameter before and after the fault event 215 may be based on the respective value of the parameter stored for a valid time interval 225 before the fault event and a valid time interval 240 after the fault event. This approach to comparing values of the parameter before and after the fault event may be suitable for received power and/or return trip delay parameters. This may be because the length of optical fiber over which the traffic signals travel between the hub 105 and one or more nodes 110B, 110C changes when the direction D1 , D2 of the traffic signals changes. For example, traffic signals to node 110C travel via links L1 , L2, L3 during normal operation but following protection switching travel via only link L4 which may be much shorter than the combination of links L1 , L2, L3. Therefore, the attenuation of the wavelengths and the time to propagate to node 110C changes following a fault event. This change is shown by the change in the value of the parameter(s).

Values of the parameter may additionally or alternatively be determined during a sliding window 230 spanning the fault event 215. In this approach, one or more measurements of a parameter before the fault event 215 but within the sliding window 230 may be taken, and one or more measurements of the parameter may be taken after fault event 215 but still within the sliding window 230. The measurement(s) of the parameter taken after the fault event 215 may be within a post fault time period 235 following the settling time 232 but still within the sliding window 230. In an example the post fault time period 235 may be at least 2 minutes of a 10 minute sliding window 230, although both of these time periods are configurable. For some parameters, measurements after the fault event may alternatively or additionally be taken within the settling time 232. The comparison of a value of the parameter before or after the fault event may be based on a maximum or minimum measurement, for example a maximum measurement before the fault event and a minimum measurement after the fault event, or vice versa; although other options are available such as an average of the measurements before and after the fault event and within the sliding window. This approach to comparing values of a parameter before and after a fault event may be suitable for traffic quality parameters such as BER or CV as once the transients associated with protection switching subside the BER or other traffic quality parameter values may revert to similar levels and so significant differences in the value of such a parameter may not be seen in sequential time intervals 225 and 240 for example. However, differences in the value of such a parameter may be more apparent in the sliding window which spans the fault event and therefore captures volatility of the traffic signal quality measurements more clearly. However, in other embodiments either or both methods of comparing values of different parameters before and after a fault event could be used. If a valid measurement is not possible during the post fault time period 235, measurements during the next valid sequential time period 240 may alternatively be used, although this slows the determination of the fault location.

Similarly, a combination of the above parameter value measurement approaches is possible using both the sequential time intervals 210 and the sliding window 230. For example, measurements of received power may be taken during the sequential time periods 210 as described above and additionally during the sliding window. Measurements taken during the post fault time period 235 may then be used to determine the value of the parameter after the fault event and compared with the value of the parameter before the fault event determined from measurements during a valid sequential time period 225 before the fault event. This allows for a faster comparison of values of the parameter before and after the fault event and therefore a faster determination of the location of the fault. As above, if valid measurements cannot be determined during the post fault time period 235 then the embodiment may fall back to using measurements in the next valid sequential time period 240 as described above.

An analysis of the difference or change in the value of one or more parameters before and after the time of a fault event can be used to infer or determine the location of the fault. The value of the parameter(s) changes due to the use of the protection mechanism, i.e. protection switching to route the traffic signals on a different route or direction around the ring. Although there is a fault on a link, data traffic is still obtained by the nodes (assuming the protection mechanism is working). Thus, although the fault on the link stops the pilot signal, the protection applied to the data traffic allows the data traffic to continue between the nodes and the hub. The protection mechanism affects the one or more parameters of the traffic, e.g. received power, round trip delay and/or signal quality parameter received at the nodes or hub. This effect of the protection mechanism which is operating, following the fault, is used to determine the location of the fault. References to comparing a value of a parameter before and after a fault event may alternatively be considered as comparing a value of a parameter before and after a protection mechanism is operated.

For example if the difference in values before and after the fault event is below a threshold in all nodes, this may indicate that the fault is in the last link, L4 of Figure 1 , of the optical ring network because the traffic signals are unaffected, with downstream signals 170D continuing to propagate in a clockwise direction D1 even as far as the last node 110C. Similarly, upstream signals 170U are unaffected and continue to propagate from each node 100A,

110B, 110C in an anti-clockwise direction D2. This situation is illustrated in Figure 4B, and the fault location 480B may be a break in the fiber between node 110C and the hub 105. As the downstream and upstream traffic signals are unaffected, the distances the traffic signals travel between each node 110A, 110B, 110C and the hub are unaffected and therefore parameters such as received power and return trip delay will also be unaffected.

In another scenario illustrated in Figure 4A, a 480A fault is located in a link L2 between two nodes 110a and 110B. Following protection switching, the traffic signals 175U and 175D to the nodes 110B and 110C downstream of the fault location 480A propagate in a different direction. However, traffic signals 170U and 170D to and from the node 110A upstream of the fault propagate in the same direction. This means that parameters such as received power and round trip delay will likely be affected for nodes 110B and 110C downstream of the fault 480A as the distance between the nodes and the hub has changed, whereas these parameters are likely to be unaffected for the node 110A upstream of the fault location 410A as there has been no change in distance along the optical fiber of the optical ring network between the node 110A and the hub 105. Therefore, a change or difference in values of one or more parameters above a predetermined threshold in a series of nodes 110B, 110C can be used to infer that the fault is located before the first node 110B in the series. The series of nodes here refers to all nodes downstream of the fault or all nodes between the fault and the hub 105.

In some scenarios such as that illustrated in Figure 4C, it may be more difficult to localize the fault down to one location and instead analysis of the difference in value of one or more parameters may instead infer one of two fault locations. For example, the analysis may indicate a fault location 480C1 in L2 between nodes 110a and 110B or fault 480C2 located between node 110C and the hub 105. This may occur when the difference in values of a parameter at node 110B downstream of the first potential fault location 480C1 is above the predetermined threshold but where the difference in the value of the parameter at the or each node 110C further downstream of the first potential fault location 480C1 is or are below the predetermined threshold. On the one hand, the difference in values of the parameter of node 110B being above the threshold indicate that there is a fault immediately upstream at fault location 480C1 , similar to the situation in Figure 4A. On the other hand, the difference in values of the parameter of node C being below the threshold indicate that there is a fault between this node and the hub at 480C2, similar to the situation in Figure 4B. Therefore, in this situation, it is not clear from the change in parameter values which of the inferred fault locations 480C1 , 480C2 corresponds to the actual fault location and two potential fault locations may be indicated and require further investigation. Figure 3 illustrates a flowchart of a method of determining a fault location in an optical ring network according to an embodiment. The method 300 may be implemented by any suitable apparatus and optical ring network, for example apparatus 155 and optical ring network 100 of Figure 1. The apparatus 155 may be a dedicated hardware module forming part of the hub 105, software running on a generic processing platform, or the apparatus may be implemented at a remote location for example in a cloud computing environment where parameter values and fault indication signals are forwarded from the hub to the remote location.

At 305, the method 300 monitors a parameter of a traffic signal associated with each node of the optical ring network. One or more parameters may be monitored, including for example received power, round trip delay and/or a signal quality parameter such as BER or CV.

These different parameters correspond to different protocol layers operating in the optical ring network and therefore provide flexibility in terms of utilizing any available parameter which may differ depending on network implementation. Using more than one parameter, if available, also make the fault determination method more robust and reliable.

The parameter(s) may be monitored using either or both of the approaches described with respect to Figure 2. In one approach, a value for the or each parameter 205 is determined for each sequential time interval 210. This may be an average of measurements taken over the time interval for example. These values are stored for subsequent analysis but comparing a value in a (valid) time interval before the fault event and a time interval after the fault event. In the other approach, measurements of the or each parameter are taken over a sliding window 230 which moves with time. When a fault event is indicated, a measurement within the sliding window taken before and after the fault event are used in the subsequent fault location determination analysis. In another alternative, measurements from one of the sequential time intervals 225 before the fault event may be compared with measurements from a time period 235 within the sliding window but after the fault event.

At 310, the method determines an indication of a fault event and if so, moves onto 315. If no fault event is indicated, the method moves back to 305 to continue monitoring the parameter(s). The indication of a fault event may be a loss of a pilot tone added to the optical ring network. Other mechanisms for indicating a fault event may alternatively and/or additionally be used, for example a loss of a traffic signal or a protection switching event which implies a fault has been detected. Activation of protection switching may indicate that a fault event has occurred a few milliseconds prior. The indication of a fault event may be a signal from an OPM to the apparatus 155. At 315, the method compares a value of the or each parameter of each node before and after the fault event. In an embodiment this may be implemented by calculating a change or difference between the respective values before and after the fault event so that this can be compared with a threshold. The difference in values for each parameter is calculated for each node in the optical ring network. The method then compares the differences in parameter values of each node with a predetermined threshold.

At 320, the method determines whether the different in the values of the or each parameter do not exceed the threshold in any of the nodes. In this case, at 325, the fault location 480B is determined to be between the last node 110C in the downstream direction (D1) and the hub 105, as shown in Figure 4B. If this is not the case, the method moves to 330. The downstream direction is a first direction D1 corresponding to the direction of propagation of downstream traffic signals 170D as illustrated in Figure 1. This contrasts in the embodiment with an upstream or second direction D2 corresponding to the direction of propagation of upstream traffic signals 170U in normal operation, but which may carry some downstream traffic signals (175D) in the second direction D2 following protection switching; depending on the location of the fault.

At 330, the method determines whether a series of nodes have a difference in values of the or each parameter above the threshold. In other words, the method determines whether all nodes 110C in the downstream direction after a first node 110B in the downstream direction have a parameter difference above the threshold. In this case, at 335, the fault location 480a is determined to be before the first node 110B in the downstream direction with a parameter difference above the threshold as shown in Figure 4A.

Otherwise, at 340, the method determines the fault location 480C1 , 480C2 to be either before the first node 110B in the downstream direction with a parameter difference above the threshold or after the last node in the downstream direction as shown in Figure 4C.

The likelihood of a single fault location being determined may be improved by using differences in values for two or more parameters. This may minimize the number of times the method arrives at 340. Furthermore, using two parameters increases robustness, for example, if measurement of one parameter fails, or accuracy.

The predetermined threshold used to compare the difference in values of the parameter before and after the fault event may be determined experimentally and/or calibrated depending on the parameter used as noted below and/or certain aspects of the optical ring network such as its size

In an embodiment, a combined change score may be derived from changes the values of a plurality of parameters, with the change score used to determine the fault location. This may provide more accurate localization and reduce the likelihood of ambiguity between two possible fault locations; the situation of Figure 4C.

Table 1 below illustrates calculating a change score for a number of nodes SG_A - SG in an optical ring network. Columns for respective scores for change of difference in values before and after a fault event for received power DR, return trip delay ARTD, and bit error rate ABER are shown. These scores may be added to provide a change score, with an example provided for node SG_A.

Table 1

The change scores for each parameter may be determined based on the level of difference between the values of the parameter before and after a fault event, for example whether they fall within certain ranges, e.g. above and/or below one or more thresholds. Table 2 below illustrates how the change score may be determined for each parameter.

Table 2

“Averaged on SG ” means that the value of the parameter is calculated taking into account the measurements of the parameter reported on all the services belonging to the i-th Service Group. A Service Group (SG) refers to the wavelengths added/dropped to a node and used for uplink and downlink communications between the node and the hub. In an SFW system this may be two wavelengths, one for the uplink and one for the downlink. The change in value of each parameter may be determined by averaging the change in values from each wavelength in the node’s SG, or by selecting the largest change in value in the SG and comparing this with the threshold. Alternative calculations of the values used to calculate the change scores could be used, for example medium and aggregate.

The change score for each parameter may be determined according to whether the difference in values of the parameter before and after the fault event exceed one or more change score thresholds. For example a change score of 0 is allocated to the received power parameter if the change in values is below a first received power change parameter threshold PwThrl , change score is 1 if it is between PwThrl and a second higher received power change parameter threshold PwThr2 and change score is 2 where this is above the second threshold PwThr2. Similarly, the change scores for other parameters can be allocated depending on respective thresholds. These may be determined experimentally and/or based on expert experience with other optical ring networks.

The received power may be measured by a transceiver without high accuracy but with a great stability (tens of dBm) and therefore this parameter is useful when comparing the value before and after the fault event. Example change score threshold may be:

. PwThn= 1 dBm

. PwThr₂= 3 dBm

These thresholds may calibrated case by case, for example according to the accuracy of the measurements available.

The Round Trip Delay (RTD) parameter is the time taken by a special packet sent in the frame at Hub to reach a node and return to the Hub. In case where the received power measurements are uncertain, the use of a second parameter like RTD may resolve the fault location determination. RTD measurements may be implemented with high accuracy, that is in terms of tens of nanoseconds. Considering that 5 ns corresponds to 1 meter of optical fiber, we can say that differences of 100 meters in the optical paths can be well measured and, based on this assumption, the following threshold may be used in an example implementation:

. RTDThn = 500 ns (corresponding to a path length variation of ~ 100 m)

. RTDThr₂ = 2500 ns (corresponding to a path length variation of ~ 500 m)

As with the received power parameter, these thresholds may be refined according to the RDT measurement accuracy available and to the actual lengths of the links L1 - L4 in the optical ring network which are known by design.

The traffic signal quality is usually measured in terms of Bit Error Rate, deduced from FEC (when available) or from CV (Code Violations counter for 8B/10B frames) or in terms of a Hi BER (in case of 64B/66B frames). An example change score threshold for BER is:

. ABERThr = 6 (i.e. 6 decades of differences in BER)

If a Code Violation counter is instead available, the threshold should be calibrated on this parameter. For example, a score of 2 if a loss of signal alarm has been detected or 0 if not.

As noted above with respect to Table 1 , the change scores for each parameter may be added together to generate a combined change score for each node. The combined changes scores of each node may then be compared against a predetermined score threshold S_THr corresponding to the predetermined threshold described with respect to Figure 3. The comparison of the combined change scores for each node with this score threshold may then be used to determine the fault location as described with respect to Figure 3. The following example tables illustrate the scenarios of Figures 4B, 4A and 4C respectively, and where S_THr = 2. In this case, nodes 110B and 110C both have change scores which exceed the threshold and therefore correspond to a fault location before node 110B.

In this case, none of the nodes have change scores which exceed the threshold and therefore correspond to a fault location after node 110C.

In this case, node 110B has a change scores which exceed the threshold but the subsequent node 110C does not which corresponds to a fault location either before node 110B or after node 110C. Figure 5 illustrates an apparatus according to an embodiment. The apparatus 500 may be the apparatus 155 for locating a fault in an optical communication link of Figure 1 or any other suitably configured apparatus. This may be implemented in dedicated hardware in the hub 105 or in software operating on a generic processing environment within the hub. In a further alternative, the apparatus may be configured in a cloud environment communicable with the hub. The apparatus 500 may be configured to perform the method 400 of Figure 4 or other methods for determining the location of a fault by comparing values of one or more parameters of a traffic signal of each node before and after a fault event.

The apparatus 500 comprises processing circuitry 510 (e.g. a processor) and a memory 530 in communication with the processing circuitry 510. The memory 530 contains instructions 535 which when executed by the processor 510 cause the processor to carry out methods of the embodiments. The memory 530 may also be used to store values and/or measurements of the parameters. At example method is illustrated which may be performed by the apparatus 500 to determine a location of a fault in an optical ring network.

At step 550, the method monitors one or more parameters of a traffic signal associated with each node in optical ring network. Examples parameters include one or more of the following: received power, round trip delay, signal quality parameters such as BER. At step 555, the method determines whether there has been an indication of a fault event. This may be indicated by loss of a pilot tone or wavelength used for traffic signals, and/or a protection switching event. If there is no such indication, the method continues to monitor the parameters, otherwise the method moves onto step 560.

At step 560, the method determines the location of the fault by comparing a value of the or each parameter before and after the fault event. This may be achieved by determining a difference between the values before and after the fault event and comparing these to respective thresholds. In some embodiments, change scores may be determined for a plurality of parameters and combined to calculate a combined change score which may be used to determine the fault location. The differences and/or change scores may be used to determine which parameters have been affected by the fault event and subsequent protection switching.

The location of the fault can be determined by analyzing which nodes have been sufficiently affected by the fault as indicated by their parameter values before and after the fault event changing by a threshold amount. If no nodes have been significantly affected this indicates that only the last link in the optical ring network has been affected and therefore the fault may be determined to be in this link. If a series of nodes along the ring between the hub and intermediate node have been sufficiently affected, this indicates that the fault is in the link immediately upstream of the intermediate node most distant from the hub. Where only some or none of the nodes between the intermediate node and the hub have been affected, then the fault may be immediately upstream of the intermediate node but may also be in the last link.

Once the location of a fault has been determined, maintenance crews may be dispatched to investigate and/or repair the link corresponding to the determined location. This provides the advantage that the amount of the optical ring network that must be investigated to closely localize and repair the fault is reduced. Embodiments may provide a number of other advantages including not requiring any additional hardware and being suitable for passive as well as active and semi-active nodes. The solution may be implemented in software and therefore easily installed on existing optical ring network equipment, may be installed in one location only such as the hub, and does not require complex monitoring and/or signaling at remote nodes. By being able to use different types of parameter, the solution is adaptable to whatever monitoring may be available at respective networks and where available can utilize two or more parameters to add robustness. Whilst the embodiments have been described with respect to a SFW Fronthaul system, they are not restricted to such an application and may, for example, equally be applied to dual fiber working optical ring networks.

It should be noted that the above-mentioned examples illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative examples without departing from the scope of the appended statements. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the statements below. Where the terms, “first”, “second” etc. are used they are to be understood merely as labels for the convenient identification of a particular feature. In particular, they are not to be interpreted as describing the first or the second feature of a plurality of such features (i.e. the first or second of such features to occur in time or space) unless explicitly stated otherwise. Steps in the methods disclosed herein may be carried out in any order unless expressly otherwise stated. Any reference signs in the statements shall not be construed so as to limit their scope.

Claims

1. A method of determining a fault location in an optical ring network having a number of nodes, the method comprising: monitoring a parameter of a traffic signal associated with each node of the optical ring network; in response to receiving an indication of a fault event, comparing a value of the parameter of each node before and after the fault event to determine the fault location.

2. The method of claim 1 , wherein the parameter is one or more of the following: received power; round trip delay; signal quality parameter.

3. The method of claim 1 or 2, wherein the indication of a fault event corresponds to a loss of signal event and/or a protection switching event.

4. The method of claim 3, wherein the loss of signal event is a loss of pilot tone, the pilot tone using a pilot wavelength which is different to a traffic wavelength used for the traffic signal.

5. The method of any one preceding claim, wherein the parameter is monitored using measurements of the parameter during sequential time intervals; and wherein the value of the parameter used before the fault event is determined from a time interval before the fault event and the value of the parameter used after the fault event is determined from a time interval after the fault event.

6. The method of any one of any one preceding claim, wherein the parameter is monitored during a sliding time interval spanning the fault event.

7. The method of claim 6, wherein the values of the parameter compared before and after the fault event are maximum and minimum values of the parameter.

8. The method of any one preceding claim, comprising monitoring a plurality of parameters of the traffic signal associated with each node and calculating a change score for each node using the difference between the value of each parameter before and after the fault event.

9. The method of any one preceding claim, wherein determining the fault location is dependent on which nodes have a change in the value of the parameter before and after the fault event which is above a threshold.

10. The method of claim 9, wherein the traffic signals for each node correspond to downstream traffic signals in a first direction in the optical ring network, and wherein the fault is determined to be before a first node in the first direction having a change in the value of the parameter before and after the fault event which is above the threshold when the nodes after the first node also have respective changes in the value of the parameter before and after the fault event which is above the threshold.

11 . The method of claim 9, wherein the traffic signals for each node correspond to downstream traffic signals in a first direction in the optical ring network, and wherein the fault is determined to be after a last node in the first direction when none of the nodes have a change in the value of the parameter before and after the fault event which is above the threshold.

12. The method of claim 9, wherein the traffic signals for each node correspond to downstream traffic signals in a first direction in the optical ring network, and wherein the fault is determined to be before a first node in the first direction having a change in the value of the parameter before and after the fault event which is above the threshold or after a last node in the first direction when at least one of the nodes after the first node has a change in the value of the parameter before and after the fault event which is below the threshold.

13. Apparatus for determining a fault location in an optical ring network having a number of nodes, the apparatus comprising a processor and memory, said memory containing instructions executable by said processor, whereby said apparatus is operative to: monitor a parameter of a traffic signal associated with each node of the optical ring network; responsive to receiving an indication of a fault event, comparing a value of the parameter of each node before and after the fault event to determine the fault location.

14. The apparatus of claim 13, wherein the indication of a fault event corresponds to a loss of signal event and/or a protection switching event.

15. The apparatus of claim 14, operative to apply a pilot tone to the optical ring network, wherein the apparatus is configured to detect a loss of signal event as a loss of pilot tone.

16. The apparatus of any one of claims 13 to 15, operative to monitor a plurality of parameters of the traffic signal associated with each node and to calculate a change score for each node using the difference between the values of each parameter before and after the fault event.

17. The apparatus of any one of claims 13 to 16, operative to determine the fault location dependent on which nodes have a change in the value of the parameter before and after the fault event which is above a threshold.

18. An optical ring network having a number of nodes and an apparatus for determining a fault location according to any one of claims 13 to 17.

19. A computer program comprising instructions which, when executed on a processor, cause the processor to carry out the method of any one of claims 1 to 12.

20. A computer program product comprising non-transitory computer readable media having stored thereon a computer program according to claim 19.