TECHNICAL FIELD

The present invention relates to a method and apparatus for determining the location of a fault along an optical fibre in a communications network.

BACKGROUND

Optical fibres are now used in all segments of networks, for example access, aggregation, metro and core networks. Telecom operators have already laid large numbers of optical fibres and are continuing to lay further optical fibres. These optical fibres may stretch up to several kilometres in length.

Maintenance of these optical fibres can contribute significantly to an operator’s operational expenditure (OPEX). In order to reduce OPEX, it is desirable for the operator to be able to quickly and easily determine the location of a fibre fault to enable a timely intervention at the precise geographic location where the fault has occurred. Fibre faults may be caused for example by lossy junctions or fibre damage such as fibre cuts. If no protection mechanism is present to re-route traffic from a faulty fibre, connection availability may be affected. Even where such a protection mechanism exists overall network performance may suffer until the fault is repaired.

A known technique to locate a fibre fault is to use an Optical Time Domain Reflectometer (OTDR). These devices operate by generating and emitting a light pulse down an optical fibre, and measuring the intensity and delay of any reflection (caused by a discontinuity produced by a fibre fault). These measurements enable the distance of the fibre fault down the length of the optical fibre to be calculated. This information, in combination with knowledge of the fibre layout, can be used to determine the precise geographic location of the fibre fault.

OTDRs may be used manually. For example, in response to an alarm on a management system indicating the occurrence of a fibre fault, a technician with an OTDR device may be sent to the Central Office (CO) where an end of the optical fibre is located to make the necessary measurements. This approach is however time consuming, delaying when the fibre fault can be repaired.

It is also known to use OTDRs automatically. For example, an OTDR test equipment may be located at a CO and coupled by an optical switch to a plurality of optical fibres to scan each optical fibre in turn. This solution enables the cost of the OTDR test equipment to be shared between the optical fibres. This solution is valid for large sites and for example data centre plants placed at the centre of a star topology of fibre connections, where the OTDR test equipment can be shared between a large number of fibres. However, where the number of fibres converging at the site decreases below a certain number, which can happen for example in medium to small Centralised Radio Access Networks (C- RAN) baseband hotels or aggregation sites, the cost of such an OTDR test equipment may no longer be justifiable. Transceivers with embedded OTDR functionality also exist, for example as produced by Optical Zonu Corporation™. These transceivers switch from“normal traffic mode” to“OTDR mode” when a Loss of Signal (LOS) is received at the transceiver. This technology may be suitable for use in single channel, Point-to-Point (PTP) connections. However, in Wavelength Division Multiplexed (WDM) links, where it would be necessary to equip each wavelength’s transceiver with OTDR capability, the Applicant has appreciated that this solution may not be cost effective.

SUMMARY

The present invention aims to provide an improved method and apparatus for determining the location of a fibre fault along an optical fibre in a communications network.

According to the present invention there is provided apparatus for determining the location of a fault along an optical fibre in a communications network. The apparatus comprises a port arranged to at least one of receive optical traffic and output optical traffic. The apparatus further comprises an Optical Time Domain Reflectometer, OTDR, operable to perform measurements for determining the location of a fault along an optical fibre. The apparatus further comprises an optical switch arranged to couple the port to a second optical fibre instead of to a first optical fibre when a fault is detected on the first optical fibre. The optical switch is further arranged to, when it couples the port to the second optical fibre instead of to the first optical fibre, selectively couple the OTDR to the first optical fibre, whereby the OTDR can perform measurements on the first optical fibre.

There is further provided a method of determining the location of a fibre fault along an optical fibre in a communications network. The method comprises coupling, by an optical switch, a port to a second optical fibre instead of to a first optical fibre when a fault is detected on the first optical fibre, wherein the port is arranged to at least one of receive optical traffic and output optical traffic. The method further comprises coupling, by the optical switch, when it couples the port to the second optical fibre instead of to the first optical fibre, an Optical Time Domain Reflectometer, OTDR, selectively to the first optical fibre, whereby the OTDR can perform measurements on the first optical fibre for determining the location of the fault along the first optical fibre.

Embodiments of the present invention have the advantage that OTDR measurements for determining the location of a fault along an optical fibre can be performed automatically, when a fault in an optical fibre is detected. Thus, a timely OTDR measure can be made when needed. This can avoid an OTDR unnecessarily scanning an optical link and thus reduce OTDR aging. Furthermore, since the OTDR may only be used to make measurements on the faulty optical fibre once traffic has been switched from the optical fibre (and thus no interference is possible), the OTDR can advantageously, unlike prior art scanning systems, use the same wavelength as a wavelength used for optical traffic. This may allow efficient use of spectrum. Moreover, use of a data transport wavelength by the OTDR has the advantage that the wavelength can pass through selective components that might be present along the path and which might otherwise mask fibre interruption. Thus, the measurements made by the OTDR may be more reliable. Furthermore, advantageously, the solution is suitable for use with WDM links: only one OTDR (operating on one wavelength) is required even if the link is a WDM link. In addition, embodiments of the present invention enable only one OTDR to be used for measurements of multiple optical fibres: for example, the first optical fibre when it is the faulty optical fibre and traffic is switched to the second optical fibre, and vice versa. Furthermore, embodiments of the present invention may advantageously facilitate easy upgrade of optical systems via utilisation of existing protection switches.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows apparatus according to an embodiment of the present invention;

Figure 2 is a flow diagram showing logic steps according to a preferred embodiment of the present invention;

Figure 3 is a flow chart showing a method according to embodiments of the present invention; and Figure 4 shows a network node of a communications network,

DETAILED DESCRIPTION

Figure 1 illustrates apparatus 10 suitable for determining the location of a fault along an optical fibre in a communications network according to an embodiment of the present invention. The fault may for example be a lossy junction or a fibre interruption caused for example by damage such as a fibre cut.

The apparatus 10 comprises a port 12 arranged to at least one of receive optical traffic (which is to be transmitted over an optical link 13) and output optical traffic (which is received from the optical link 13). In a first configuration, the optical link 13 comprises a first optical fibre 24, which in this configuration may be referred to as a“working” optical fibre. Optical traffic to be transmitted over the optical link 13, to another network node (not shown), may be referred to as“uplink” optical traffic. Optical traffic received over the optical link 13, from another network node (not shown), may be referred to as “downlink” optical traffic. The optical traffic may be carried on one or more wavelengths. Each wavelength (or optical signal) may be modulated to carry data. In this preferred embodiment, the optical traffic is carried on a WDM optical signal. However, in other embodiments, the optical traffic could instead be carried on a single channel, for example over a High Speed grey interface. In this case, the optical traffic could be transmitted at speeds of 100, 200 or 400Gb/s or higher. Indeed the solution of the present invention may be advantageously used with high speed single channel links, as well as WDM links, since the solution of the present invention does not require the OTDR function to be integrated into the transceiver. Transceivers with integrated OTDR function, discussed above, are currently limited to those transceivers operating up to 1 Gb/s.

In this example, the port 12 is arranged to receive optical traffic. The port 12 is coupled to a WDM multiplexer 14 which is arranged to multiplex a plurality of wavelength channels received at respective ones of a plurality of input ports 16 into a single WDM optical signal which is output from an output port 18 and provided to port 12. In this example an internal optical link is shown connecting output port 18 of WDM multiplexer 14 and port 12. However, it should be appreciated that output port 18 and port 12 could be the same port, or the WDM multiplexer 14 could be located remote from port 12 for example at another node of the communications network. It should also be appreciated that, in another embodiment, where port 12 is arranged to output optical traffic which has been received over optical link 13, the WDM multiplexer 14 may, in addition or alternatively, be a WDM de-multiplexer. In this case, the WDM de-multiplexer (not shown) may be operable to receive the output optical traffic in the form of a WDM optical signal and de-multiplex the WDM optical signal into a plurality of optical signals (each at a respective wavelength) which are output from respective ones of a plurality of output ports.

The apparatus 10 further comprises an Optical Time Domain Reflectometer (OTDR) 20. This OTDR 20 is operable to perform measurements which can be used to determine the location of a fault along an optical link/optical fibre. The OTDR 20 may be any type of OTDR device. For example, the OTDR 20 may be a low cost OTDR board mounted or pluggable (e.g. in SFP format) such as those currently coming onto the market. Advantageously, in this example, the OTDR 20 is configured to use the same wavelength when performing measurements on the first optical fibre as a wavelength used for traffic on the first optical fibre. That is, the OTDR 20 is configured to emit a light signal at one of the one or more wavelengths on which the optical traffic is carried. In this example the OTDR 20 is configured to operate on a single wavelength.

As will be understood by those skilled in the art, the OTDR 20 may comprise an optical transmitter (not shown) operable to emit a light pulse down an optical fibre, and an optical detector (not shown) operable to detect/measure a reflection of the light pulse. Such a reflection may be caused by a discontinuity produced by a fault along the fibre. Thus, by determining the time taken for a reflection to arrive at the OTDR 20, the distance of the fault from the OTDR 20 may be calculated. That is, the distance of the fault along the length of the optical fibre may be calculated. In a preferred embodiment the OTDR 20 is arranged to provide measurement results to a management system (not shown), which may be remote from the apparatus 10. In this case, the calculation of the distance of the fault along the length of the optical fibre may be performed at the management system. In addition, the management system may use knowledge of the fibre layout to determine the geographical location of the fault. However, in other examples, these calculations could be performed elsewhere, for example at the node housing the OTDR 20.

In this example, it can be seen that the OTDR 20 is further configured to receive a control signal arranged to cause (or trigger) the OTDR 20 to perform measurements. More particularly, in this example, this control signal is arranged to cause the OTDR 20 to perform measurements after a fault on at least the first optical fibre 24 is detected, as will be explained further below. This control signal may be an electrical signal, an optical signal or a wireless signal.

The apparatus 10 further comprises an optical switch 22. This optical switch 22 may be referred to as a“protection switch”. In a first configuration, the optical switch 22 is arranged to couple port 12 to a first optical fibre 24, whereby the optical traffic received at port 12 can be transmitted over the first optical fibre 24 (or where optical traffic is output from port 12, that optical traffic is received over the first optical fibre 24). The optical switch 22 is further arranged to couple port 12 to a second optical fibre 26 instead of to the first optical fibre 24 when a fault is detected on the first optical fibre 24. In this manner, protection of the first optical fibre 24 can be achieved: optical traffic can be carried over the second optical fibre 26, in the event of a fault on the first optical fibre 24. In this case, the second optical fibre 26 becomes the“working” optical fibre.

In this example, there is circuitry 28 configured to detect a fault on the first optical fibre 20. This circuitry 28 may be referred to and is labelled in Figure 1 as“link monitoring” apparatus. As will be understood by the skilled person, this link monitoring apparatus 28 may take various forms, and may for example comprise one or more photodetectors as indicated in Figure 1. Typically, such link monitoring apparatus 28 is configured to detect/measure a portion of the optical power output from the monitored optical fibre. Where the link is a WDM link, this portion might be a portion of the aggregate optical power from a plurality of wavelengths.

This circuitry 28 may be configured to provide (or output) a control signal which is arranged to cause the optical switch 22 to couple the port 12 to the second optical fibre 26 instead of to a first optical fibre 24 when a fault is detected on the first optical fibre 24 by the circuitry 28. In this example, the link monitoring circuitry 28 is arranged to provide a control signal to a controller 30, which in turn provides a control signal to the protection switch 22. However, various arrangements are possible.

The optical switch 22 is further arranged to, when it couples the port 12 to the second optical fibre 26 instead of to the first optical fibre 24, selectively couple the OTDR 20 to the first optical fibre 24, whereby the OTDR 20 can perform measurements on the first optical fibre 24. In this example this configuration of the optical switch 22 is shown by dotted lines. Only one of port 12 and OTDR 14 is coupled to the first optical fibre 24 at any one time. In this example the optical switch 22 has a cross bar configuration. The optical switch 22 has two ports 32 and 34 coupled respectively to the first optical fibre 24 and the second optical fibre 26. The optical switch 22 has a further two ports, port 12 and port 36, port 36 being coupled to OTDR 20. In this example the optical switch 22 is a 2:2 optical switch. When port 12 is coupled to one of ports 32 and 34 (and thus to one of the first optical fibre 24 and the second optical fibre 26), port 30 is coupled to the other one of port 32 and 34 (and thus to the other one of the first optical fibre 24 and the second optical fibre 26).

In this embodiment, control circuitry is configured to cause the OTDR 20 to perform measurements on the first optical fibre after the optical switch 22 has selectively coupled the OTDR 20 to the first optical fibre 24. Thus, the OTDR 20 may advantageously be turned off (or be inactive) before a fault is detected and it is required to take measurements. In this example, circuitry 28 is configured to provide a control signal which is arranged to cause the OTDR 20 to perform measurements after a fault is detected on the first optical fibre 24 by the circuitry 28. This control signal may be referred to as an OTDR control signal. This control signal may in the form of an electrical or optical signal, or even a wireless signal. Use of an optical signal may be advantageous as the OTDR 20 may then be an (off the shelf) OTDR equipped transceiver, where the optical control signal is configured to replicate a LOS in order to trigger the OTDR 20. For example, by splitting part of the light input to the link monitoring apparatus 28 (received from the first optical fibre) and delivering it directly into a transceiver-OTDR receive port, the OTDR 20 may be triggered automatically by optical feedback. This may provide an elegant, effective implementation. Further, circuitry 28 may be configured to provide a control signal arranged to cause the OTDR 20 to perform the measurements a predefined delay after a fault is detected on the first optical fibre 24 by the circuitry 28. Such a delay may ensure that the OTDR 20 has been coupled to the first optical fibre 24 before the OTDR 20 performs measurements, i.e. scans the optical fibre 24. This may, advantageously, ensure that the OTDR measurements are reliable. In some examples the OTDR 20 and circuitry 28 may be integrated in the same unit. In other examples, the feedback between the circuitry 28 and OTDR 20 may be provided via a backplane wire or by the system controller for example.

In this example, the optical switch 22 is further arranged to couple the port 12 to the first optical fibre 24 instead of to the second optical fibre 26 when a fault is detected on the second optical fibre 26. This switching may be performed subsequently from initial switching as described above, where traffic is switched from the first optical fibre 24 to the second optical fibre 26 (and thus where the second optical fibre 26 is the“working” optical fibre). As can be seen in Figure 1 , in this example, the link monitoring apparatus 28 is coupled to both the first optical fibre 24 and the second optical fibre 26 and is arranged to detect a fault on both the first optical fibre 24 and the second optical fibre 26 respectively. In this example, the optical switch 22 is further arranged to, when it couples the port 12 to the first optical fibre 24 instead of to the second optical fibre 26, selectively couple the OTDR 20 to the second optical fibre 26, whereby the OTDR 20 can perform measurements on the second optical fibre 26. In this example, the circuitry 28 is configured to provide a control signal which is arranged to cause the OTDR 20 to perform measurements on the second optical fibre 26 after a fault is detected on the second optical fibre 26 by the circuitry 28. Again, preferably, the control signal causes the OTDR 20 to perform measurements a predefined delay after the fault is detected on the second optical fibre 26. This may ensure that the optical switch 22 has coupled the OTDR 20 to the second optical fibre 26 before the OTDR 20 performs measurements. The control signal may be an electrical, optical or wireless signal as described above.

More generally, as indicated above, the optical switch 22 may be arranged to couple the OTDR 20 to the second optical fibre 26 when it is in its first configuration, i.e. when the optical switch 22 couples port 12 to the first optical fibre 24. In this case, the OTDR 20 may be triggered to perform measurements on the second optical fibre 26, when a fault is detected on the second optical fibre 26, even where the second optical fibre 26 is acting as the“protection” optical fibre at that time (i.e. not carrying the optical traffic and thus no switching of optical traffic is required).

Referring to Figure 4, the apparatus 10 may be comprised within a network node 400 of a communications network, for example but not exclusively in a Central Office, CO. The network node 400 may be coupled to one or more other network nodes (not shown). Advantageously, the apparatus 10 may be used in a Centralised Radio Access Network, C-RAN, or in an Internet Protocol over WDM network, for example in the access portion of the network. These are examples where a limited number for channels/fibres converge at the same site. However, the apparatus 10 may be used in other types of communications network, as will occur to those skilled in the art.

Figure 2 is a flow diagram showing logic steps according to a preferred embodiment of the present invention. At 200 link monitoring is performed. In particular, in this example, both the first optical fibre 24 and the second optical fibres 26 are monitored for fibre faults. In the event of a“worker” failure, i.e. a fault on the“working” optical fibre, at 210 the optical/protection switch 22 switches the optical traffic to the“protection” optical fibre, which becomes the“working” optical fibre. The optical switch 22 also switches the OTDR 14 to the faulty optical fibre. Then, at 220 a scan of the faulty optical fibre by the OTDR 14 is triggered. Alternatively, in the event that a“protection failure” is detected, i.e. a fault on the “protection” optical fibre is detected, the method proceeds directly to step 220, i.e. triggering the OTDR 14 to scan the faulty optical fibre, in this case the“protection” optical fibre. As indicated at 230, after the OTDR 14 scan, monitoring of the faulty optical fibre continues. Once the failure is cleared, the system logically returns to step 200 (link monitoring,“no fail state”). It should be appreciated that when the method returns to step 200 the fibres designated as“worker” and“protection” fibres may have changed.

Figure 3 is a flow chart showing a method of determining the location of a fibre fault along an optical fibre in a communications network according to preferred embodiments of the present invention. The method comprises 300 coupling, by an optical switch, a port to a second optical fibre instead of to a first optical fibre when a fault is detected on the first optical fibre. The port is arranged to at least one of output optical traffic and receive optical traffic. That is, the port may be arranged to receive optical traffic. Alternatively, the port may be arranged to output optical traffic. Alternatively, the port may be arranged to both receive optical traffic and output optical traffic. The optical traffic may be carried on one or more wavelengths. In some examples, the optical traffic is carried on a wavelength division multiplexed, WDM, optical signal. The method further comprises 310 coupling, by the optical switch, when it couples the port to the second optical fibre instead of to the first optical fibre, an Optical Time Domain Reflectometer, OTDR, selectively to the first optical fibre, whereby the OTDR can perform measurements on the first optical fibre for determining the location of the fault along the first optical fibre. The OTDR may emit an optical signal at one of the one or more wavelengths used to carry the optical traffic.

The method may further comprise 330 sending measurement results of the OTDR to a management system.

According to some embodiments, the method may further comprise causing 320 the OTDR to perform measurements on the first optical fibre after the optical switch has selectively coupled the OTDR to the first optical fibre. The method may comprise causing the OTDR to perform measurements on the first optical fibre after a fault is detected on the first optical fibre, for example by providing a control signal to the OTDR. This control signal may be an electrical or wireless signal or, advantageously, an optical signal. In particular, the method may advantageously comprise causing the OTDR to perform measurements on the first optical fibre a predefined delay after a fault is detected on the optical fibre (allowing time for the switch to occur). In some embodiments, the method may comprise detecting a fault on the first optical fibre and, in response to detecting the fault on the first optical fibre, causing the OTDR to perform measurements.

Embodiments of the present invention have the advantage that OTDR measurements for determining the location of a fault along an optical fibre can be performed automatically, when a fault in an optical fibre is detected. Thus, a timely OTDR measure can be made when needed. This can avoid an OTDR unnecessarily scanning an optical link and thus reduce OTDR aging. Furthermore, since the OTDR may only be used to make measurements on the failed optical fibre once traffic has been switched from the optical fibre, the OTDR can advantageously, unlike prior art scanning systems, use the same wavelength as a data transport wavelength. This may allow efficient use of spectrum. Moreover, using a data transport wavelength for OTDR has the advantage that the wavelength can pass through selective components that might be present along the path and which might otherwise mask fibre interruption. Thus, the measurements made by the OTDR may be more reliable. Furthermore, advantageously, the solution is suitable for use with WDM links: only one OTDR is required even if the link is a WDM link. In addition, embodiments of the present invention enable only one OTDR to be used for measurements of multiple optical fibres: for example, the first optical fibre when it is the faulty optical fibre and traffic is switched to the second optical fibre, and vice versa. Furthermore, embodiments of the present invention may advantageously facilitate easy upgrade of optical systems via utilisation of existing protection switches.