Technical Field The present invention relates to passive optical networks and, in particular, to a method and system for providing resilience to passive optical networks.

Background Wavelength-division-multiplexed Passive Optical Networks (WDM-PONs) aim to bring the benefits of wavelength division multiplexing, in terms of increased fibre link capacity and protocol transparency, to a user in a cheaper and more scalable way than conventional optical networks. Passive optical networks are, at present, primarily used in residential access networks. However, the huge increase in mobile broadband over recent years has created a demand for low cost, scalable optical backhaul networks. For the reasons above, WDM-PON is an attractive alternative to for example WDM metro rings. It is important however that mobile backhaul networks have resilience, for example to optical fibre damage or cuts. Since each cell site will typically serve a large number of users, network failure is simply not acceptable. Furthermore, since backhaul networks may extend over very long distances, if fibre damage or cuts do occur, locating the point of failure along the optical fibre, and accordingly repairing that failure, may take a long time.

Conventional optical network protection mechanisms are however expensive, and/or not compatible for use in mobile backhauling where there are typically tight

requirements for synchronisation between upstream and downstream traffic. Summary

According to the invention there is provided a passive optical network comprising: an optical line terminal (OLT) connected to a plurality of optical network terminals (ONTs) by an optical fibre arranged in a ring. The optical line terminal comprises: an optical transmitter capable of generating a downstream signal at a first wavelength and at a second, different wavelength; and a passive optical band splitter having a first output port and a second output port each coupled to the optical fibre, wherein the passive optical band splitter is arranged to receive the downstream optical signal and, if the downstream optical signal is at the first wavelength, to pass the downstream signal to the first output port, wherein the first output port is coupled to the optical fibre such that the downstream signal is transmitted in a first direction around the ring, but, if the downstream signal is at the second wavelength, to pass the downstream signal to the second output port, wherein the second output port is coupled to the optical fibre such that the downstream signal is transmitted in a second direction around the ring opposite to the first direction. The passive optical network further comprises: a passive add drop multiplexer (ADM) configured to drop optical signals at the first and second wavelengths from the optical fibre to an optical drop path coupled to at least one of the plurality of optical network terminals; wherein the at least one optical network terminal is configured to receive the downstream signal and transmit an upstream optical signal; and wherein the passive add drop multiplexer is further configured to add the upstream optical signal to the optical fibre such that it is transmitted in an opposite direction around the ring with respect to the downstream signal dropped by the passive add drop multiplexer.

Embodiments of the present invention have the advantage of providing a cost-efficient WDM-PON ring having resilience in the event of optical fibre damage or cuts, which is suitable for use in mobile backhauling.

If a working link between the OLT and an ONT fails, advantageously, in embodiments of the present invention the downstream optical signal can instead be sent to the ONT in the other direction around the ring, through the same optical fibre, without requiring the use of expensive optical switches. This is achieved at the cost of assigning two wavelengths per downstream optical signal. However, the applicant has appreciated that this disadvantage may be outweighed by the above advantages, and furthermore that in applications such as mobile backhaul there may for example be spare wavelengths in the C and L bands.

Furthermore, since the upstream optical signal always travels along the same segment of the optical fibre as the downstream optical signal, whichever direction the

downstream optical signal travels around the ring (i.e. whether the PON is in a "working" or "protection" state) protection can be achieved using a single optical fibre. This is cheaper than known solutions which require multiple optical fibres. Moreover, synchronisation requirements between the upstream and downstream optical signals can be met, since the upstream and downstream optical signals will always travel similar distances. Thus, advantageously, a WDM-PON embodying the present invention is suitable for use in mobile or radio backhauling.

It should be appreciated that the passive optical splitter and the passive add drop multiplexers are passive in that they are optically passive. That is, they do not comprise any optically active components, they are unpowered without the need for a power source, and have a fixed (i.e. static) configuration of wavelengths which are split in each direction, dropped/added.

Preferably, the upstream optical signal is at the same wavelength as the downstream optical signal received by the at least one optical network terminal. This has the advantage of increasing the number of wavelengths which can be used for

downstream/upstream optical links. In addition, this further minimises the difference between the time taken by the downstream and upstream optical signals to travel through the optical fibre, since not only will the downstream and upstream optical signals always travel through the same segment of the optical fibre, but they travel along the same wavelength channel.

Preferably, the optical network terminal is configured to use energy from the

downstream optical signal received by the at least one optical network terminal to transmit the upstream optical signal.

The optical transmitter may comprise two transmitters: a first optical transmitter and a second optical transmitter, the first optical transmitter being configured to generate the downstream optical signal at the first wavelength and the second optical transmitter being configured to generate the downstream optical signal at the second wavelength. Alternatively, the optical transmitter may comprise a single tuneable optical transmitter.

In an embodiment, the first wavelength and the second wavelength may be selected from a predefined first and second set of wavelengths respectively. Preferably, the first set of wavelengths comprises wavelengths equal to or lower than a predefined wavelength and the second set of wavelengths comprises wavelengths higher than the predefined wavelength. In a preferred embodiment, the optical line terminal is configured to transmit a plurality of downstream optical signals. In this case, each of the downstream optical signals may be generated by a respective optical transmitter at respective first and second wavelengths, whereby resilience is provided for each of the downstream links.

However, it is of course possible that only one or fewer of the downstream optical signals have the resilience capability offered by embodiments of the present invention.

In one embodiment, the optical line terminal further comprises: a multiplexer for multiplexing the plurality of downstream optical signals into a wavelength-division- multiplexed optical signal, the multiplexer being arranged to receive the downstream optical signal and pass the wavelength-division-multiplexed optical signal to the passive optical band splitter. That is, in this case, the downstream signal is received by the passive optical band splitter along with other downstream signals which make up the wavelength-division-multiplexed signal. In this case, the passive optical band splitter may be embedded in the optical fibre.

In an alternative embodiment, the optical line terminal further comprises: a first multiplexer for wavelength-division multiplexing downstream optical signals for transmission in the first direction around the ring, and a second multiplexer for wavelength-division multiplexing downstream optical signals for transmission in the second direction around the ring; wherein the first output port of the passive optical band splitter is coupled to the optical fibre via the first multiplexer and the second output port is coupled to the optical fibre via the second multiplexer. In this example, note that there may be a respective passive optical band splitter for each downstream optical signal having resilience according to embodiments of the present invention.

Preferably, the optical line terminal further comprises a controller configured to selectively cause the optical transmitter to generate the downstream optical signal at the first wavelength or at the second wavelength. The controller may be configured to change the wavelength at which the optical transmitter generates the downstream optical signal, if a transmission fails. This has the advantage that the OLT can automatically initiate protection, if for example it is determined that a "working" link has failed. Preferably, the optical line terminal comprises an optical receiver arranged to receive the upstream optical signal; wherein the controller is configured to determine whether a transmission has failed by determining whether the upstream optical signal has been received by the optical receiver. This has the advantage that the OLT can

automatically detect when a link has failed, and so automatically initiate protection if necessary.

Preferably, the controller is configured to periodically or continuously determine whether the upstream optical signal has been received by the optical receiver, whilst the downstream optical signal is being transmitted at the first or second wavelength. Thus, if after a link has been established it subsequently fails this is advantageously detected, and so protection can be initiated automatically if needed.

Preferably, the controller is further configured to cause the optical transmitter to turn off, if the downstream optical signal has been transmitted at both the first and second wavelengths and both transmissions have failed. This has the advantage that the OLT can save power, if for example the optical fibre is damaged along both links or if the respective ONT is offline. In this case, preferably, the controller is further configured to cause the transmitter to turn back on after a predetermined period. Thus, the optical transmitter can be automatically restarted.

There is also provided a radio access network comprising a backhaul network in the form of a passive optical network as described above. The radio access network may have a base band unit for a radio base station and a number of radio remote units, coupled to the base band unit by the passive optical network.

According to the present invention, there is also provided an optical line terminal for use in the passive optical network as described above. The optical line terminal comprises: an optical transmitter capable of generating a downstream signal at a first wavelength and at a second, different wavelength; and a passive optical band splitter having a first output port and a second output port each for coupling to the optical fibre. The passive optical band splitter is arranged to receive the downstream optical signal and, if the downstream optical signal is at the first wavelength, to pass the downstream signal to the first output port, but, if the downstream signal is at the second wavelength, to pass the downstream signal to the second output port, wherein the first output port is arranged for coupling to the optical fibre such that the downstream signal is transmitted in a first direction around the ring, and the second output port is arranged for coupling to the optical fibre such that the downstream signal is transmitted in a second direction around the ring opposite to the first direction.

There is further provided a method of operating the optical line terminal as described above, comprising: using the optical transmitter to generate the downstream signal at the first wavelength; and, if the transmission fails, using the optical transmitter to transmit the downstream signal at the second wavelength.

There is further provided a method of installing a passive optical network as described above. The method comprises: installing a passive add drop multiplexer configured to drop optical signals at the first and second wavelengths from the optical fibre to an optical drop path to be coupled to at least one of the plurality of optical network terminals, and to add an upstream optical signal from the at least one optical network terminal to the optical fibre such that it is transmitted in an opposite direction around the ring with respect to the downstream signal dropped by the passive add drop multiplexer; and coupling the at least one optical network terminal to the passive add drop multiplexer.

The method may further comprise installing a passive optical band splitter having a first output port and a second output port, wherein the passive optical band splitter is arranged to receive the downstream optical signal and, if the downstream optical signal is at the first wavelength, to pass the downstream signal to the first output port but, if the downstream signal is at the second wavelength, to pass the downstream signal to the second output port; coupling the first output port to the optical fibre such that the downstream signal is transmitted in a first direction around the ring; and coupling the second output port to the optical fibre such that the downstream signal is transmitted in a second direction around the ring opposite to the first direction.

The method may further comprise installing a controller at the OLT configured to selectively cause the optical transmitter to generate the downstream optical signal at the first wavelength or at the second wavelength. Brief Description of the drawings

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

Figure 1 shows a schematic view of a WDM-PON in a ring according to a first embodiment of the present invention;

Figure 2 shows a preferred implementation of an ONT shown in Figure 1 ;

Figure 3 is a flow chart illustrating a method of operating the WDM-PON in a ring according to an embodiment of the present invention;

Figure 4 shows a preferred implementation of an OLT unit according to the first embodiment;

Figure 5 shows a schematic view of a WDM-PON in a ring according to a second embodiment of the present invention; Figure 6 shows a preferred implementation of an OLT unit according to the second embodiment;

Figure 7 is a flow chart illustrating a method of providing resilience to a WDM-PON in a ring according to a preferred embodiment of the present invention;

Figure 8 is a flow chart showing a method of installing a WDM-PON ring according to embodiments of the present invention; and

Figure 9 is a schematic view of a radio access network embodying the present invention.

Detailed Description of the Embodiments of the Invention

A schematic view of a WDM-PON in a ring 10 according to first embodiment of the present invention is shown in Figure 1 . WDM-PON 10 has a ring topology and comprises an OLT 20 (which may also be referred to as a central office) and several ONTs 30 connected by a single optical fibre 40 arranged in a ring. In this example there are three ONTs 30, but there may be many more. In this example, each of the ONTs 30 is coupled to optical fibre 40 by a respective passive add drop multiplexer (ADM) 50. However, alternatively, more than one ONT may be coupled to each ADM 50. Although Figure 1 shows that each ONT 30 is close to its respective ADM 50, it should be appreciated that this is not necessarily the case, and each ONT 30 may for example be several kilometres away from its respective ADM 50. In this embodiment, OLT 20 comprises a plurality of OLT modules or units 25, each configured to transmit a downstream optical signal to a respective one of the ONTs 30. Thus, in this example there are three OLT modules 25.

In this example, each OLT module 25 is configured to generate its downstream signal at two different wavelengths: a first wavelength and a second, different wavelength. These first and second wavelengths are different from each other and also different from the wavelengths transmitted by the other OLT modules 25, whereby, by appropriately configuring the ADMs 50, a 1 :1 channel can be set up between each OLT module 25 and its respective ONT 30.

In this example, as illustrated in Figure 1 , each of the first wavelengths are selected from a first set of wavelengths, wavelengths n, where n = 1...K 2. Each of the second wavelengths are selected from a second set of wavelengths, wavelengths m, where m = K/2+1 ...K. However, of course, other wavelength plans are possible.

In this embodiment, each of the OLT modules 25 are coupled by respective optical paths to a common multiplexer 26, such as a passive array waveguide grating (AWG). Multiplexer 26 is configured to multiplex the downstream signals generated by the OLT modules 25 into a wavelength division multiplexed signal. Multiplexer 26 is, in turn, coupled to a passive optical band-splitter 27 which is arranged to receive the wavelength division multiplexed signal and, depending on the wavelength of each of the downstream signals, as will be described below, to split the wavelength division multiplexed signal into two signals for transmission in different directions around the ring or to direct the wavelength-division-multiplexed signal in either the first or second direction around the ring (clockwise or anticlockwise). Passive optical band-splitter 27 has an input port A at which the downstream optical signals are received (as part of the wavelength-division-multiplexed signal) and a first output port B and a second output port C which are each coupled to the optical fibre 40. In this case, passive optical band-splitter 27 is embedded in optical fibre 40 such that the first and second ports are directly connected to the optical fibre 40. However, it should be appreciated that this is not necessarily the case, and the first and second output ports B and C could be coupled to the optical fibre 40 by any optical

transmission medium, such as an optical fibre, and in some cases by optical components each of which allows the transmission of optical signals.

Note that although ports A, B and C are referred to respectively as input and output ports, in this embodiment passive optical band-splitter is also configured to combine upstream optical signals which may be received from the optical fibre (from either direction around the ring) into a single upstream wavelength-division-multiplexed signal, which is passed to multiplexer 26. Thus, in this case, upstream optical signals may be received at each of the first and second output ports B and C and a single upstream wavelength-division-multiplexed signal is output from input port A.

Multiplexer 26 is further configured to de-multiplex the single upstream wavelength- division-multiplexed signal, and pass the upstream optical signals to their respective OLT modules 25.

Passive optical band-splitter 27 is pre-configured to direct downstream signals at any of the first set of wavelengths, in this case wavelengths n, in a first direction around the ring (in this example anticlockwise as illustrated in Figure 1 ) and to direct downstream signals at any of the second wavelengths, in this case wavelengths m, in the other direction around the ring (in this example clockwise as illustrated in Figure 1 ).

Thus, in this example, if a downstream signal is at its first wavelength it is passed by passive optical band-splitter 27 from input port A to first output port B, which is coupled to the optical fibre such that the downstream optical signal is transmitted anticlockwise around the ring. On the other hand, if the downstream signal is at its second wavelength, it is passed by passive optical band splitter 27 from input port A to second output port C, which is coupled to the optical fibre such that the downstream optical signal is transmitted clockwise around the ring. Thus, in this example, by virtue of passive optical band splitter 27, if a downstream signal is generated at its first wavelength it is sent anticlockwise around the ring, and if it is generated at its second wavelength it is sent clockwise around the ring.

As will be appreciated by the skilled person passive optical band-splitters are a known type of component which may have various configurations typically comprising a number of optical filters. The ADMs 50 coupled to the ONTs 30, which may also be referred to as "taps", are configured to "drop" the first and second wavelengths associated with their respective ONT 30 (that is, to remove those wavelengths from the optical fibre 40) to an optical drop path 55 coupled to the ONT 30. The ADMs 50 allow any other wavelengths to continue through the optical fibre 40. In addition, the ADMs 50 are configured to "add" a corresponding upstream signal from their ONT 30 to the optical fibre such that it travels in a reverse or opposite direction around the ring to that travelled by the downstream optical signal dropped by that ADM, and received by its ONT 30. That is, as illustrated in Figure 1 , if the downstream signal travelled clockwise around the ring, the corresponding upstream signal is sent anticlockwise around the ring by the ADM 50 and vice versa.

In this example, it is seen that each ADM 50 has four ports, and two optical drop paths 55. One of the optical drop paths 55 carries the downstream signal (and corresponding upstream signal) if the downstream signal is dropped whilst traveling in the clockwise direction around the ring (i.e. in this example if it is at the second wavelength) and the other optical drop path 55 carries the downstream signal (and corresponding upstream signal) if the downstream signal is dropped whilst traveling in the anticlockwise direction (i.e. in this example, if it is at the first wavelength). However, it should be appreciated that alternatively, for example, the upstream optical signals could be passed to the ADMs 50 by optical add paths separate from the optical drop paths 55. Various configurations are possible.

Again, as will be appreciated by the skilled person, passive ADMs 50 are a known type of component, which typically comprise a configuration of optical filters, and AMDs 50 may be configured in a number of ways. It should be appreciated that, although in this embodiment, the ADMs 50 are illustrated as single modules, each ADM 50 may alternatively include a number of modules, which need not be integrated together, and which may for example be manufactured separately. These modules may be coupled together by conventional couplings such as plug-in optical connectors or fibre splices, or be stand-alone modules.

As mentioned above, each of the ONTs 30 are configured to receive the downstream optical signal and transmit a return upstream optical signal. An example configuration of an ONT 30 according to a preferred embodiment of the present invention is illustrated in Figure 2. In this preferred embodiment, the ONT 30 transmits its upstream signal at the same wavelength as the downstream signal, by using "wavelength reuse" technology. This is a known technology, which enables the ONT 30 to use part of the energy of the downstream signal to generate the upstream signal. Typically, phase modulation is applied to the upstream signal, so that any reflections with the downstream signal interfere incoherently, which mitigates the optical reflection penalty and cross talk between the links, although various configurations are possible.

Using "wavelength reuse" technology has the advantage that the ONTs 30 can be cheap in comparison to alternative ONTs 30 that use fixed or tuneable lasers, and furthermore that the ONTs 30 are "colourless". If an ONT 30 receives its downstream signal at the first wavelength, it will transmit the upstream signal at the first wavelength, whereas if the ONT 30 receives the downstream signal at the second wavelength, the ONT 30 will transmit the upstream signal at the second wavelength. Thus,

advantageously, only one type of ONT 30 is needed for all of the ONTs 30.

Referring to Figure 2, ONT 30 comprises an optical coupler 31 which is configured to receive optical signals from both optical drop paths 55 and to transmit a first portion of the incoming optical signal to an optical receiver 32 and a second portion of the incoming optical signal to an reflective element 33, which reflects an upstream signal back to optical coupler 31 . Reflective element 33 may comprise, for example, a reflective semiconductor optical amplifier as described in the applicant's patent application publication number WO201 1/134536, to which a drive signal is applied to cause the reflective semi-conductor optical amplifier to apply upstream modulation to the same wavelength received for the downlink, although other configurations are possible as will be appreciated by those skilled in the art. In this example, optical coupler 31 is further configured to transmit the upstream signal to its ADM 50 along the same optical drop path from which the downstream optical signal was received, although as mentioned above alternative configurations are possible.

Figure 3 illustrates a method of operating OLT 20 to provide resilience to WDM-PON ring 10 according to a preferred embodiment of the invention. The method comprises, at step 60, transmitting a downstream optical signal to be sent to a respective ONT 30 at a first wavelength and, at step 70, if the transmission fails, transmitting the downstream signal to be sent to the respective ONT at a second, different wavelength, such that the downstream signal is transmitted over an alternative optical path. As illustrated in Figure 1 by the arrows, by way of example, a downstream optical signal transmitted by OLT module 1 to ONT 1 may initially be transmitted, or attempted to be transmitted, anticlockwise around the ring, which provides the shortest path between OLT 20 and ONT 1. This is referred to, and labelled in Figure 1 , as the "downstream worker direction". Thus, OLT module 1 generates the downstream optical signal at the first wavelength, and the downstream optical signal is directed in the anticlockwise direction around the ring to ONT 1 by passive optical band-splitter 27.

If the downstream optical signal reaches the ADM 50 coupled to ONT 1 30, that ADM 50 "drops" the downstream optical signal to an optical drop path 55 coupled to ONT 1 30. ONT1 receives the downstream optical signal and transmits a return upstream optical signal. The ADM 50 "adds" the upstream optical signal to the optical fibre 40 such that it travels in the opposite direction to that in which the downstream optical signal travelled. That is, in this example, the upstream optical signal travels in the clockwise direction around the ring. This is labelled as the "upstream worker direction".

If, however, this link fails, for example because of fibre damage to the section of optical fibre 40 between OLT 20 and ONT 1 or a cut, OLT unit 1 instead generates the downstream optical signal at the second wavelength. This means that, by virtue of passing through passive optical wavelength/band splitter 27, the downstream signal travels in the clockwise direction around the ring. This direction is labelled the

"downstream protection direction".

If the downstream optical signal reaches the ADM 50 coupled to ONT 1 30, again, that ADM 50 "drops" the downstream optical signal to an optical drop path 55 coupled to ONT 1 30. ONT1 receives the downstream optical signal and transmits an upstream optical signal to be sent to the OLT 20. As before, the ADM 50 "adds" the upstream optical signal to the optical fibre 40 such that it travels in the opposite direction through the optical fibre around the ring to the direction in which the downstream optical signal travelled around the ring. That is, in this case, the upstream optical signal travels in the anticlockwise direction around the ring. This is labelled as the "upstream protection direction".

Thus, advantageously, protection can be achieved in the event that optical fibre 40 is damaged or cut along the working link between OLT 20 and ONT1 simply by changing the wavelength at which the OLT module 25 generates the downstream signal from the first wavelength to the second wavelength (or vice versa), without requiring expensive optical switches. Of course, if both the working and protection link fail, then a link nonetheless cannot be established.

An example configuration of an OLT module 25 is shown in Figure 4. In this example, OLT module 25 comprises two transmitters 21 : one configured to transmit the downstream signal at the first wavelength and the other configured to transmit the downstream signal at the second wavelength.

In this case, the two transmitters 21 are fixed transmitters, whereby they can only generate the downstream optical signal at a particular wavelength. Each transmitter 21 is coupled to multiplexer 26 by a respective optical path 22. It should be appreciated however that, in an alternative embodiment, the two transmitters 21 could be replaced either by two tuneable transmitters or by a single tuneable transmitter, which may be coupled to multiplexer 26 by a single optical path 22. Although the advantage of using fixed transmitters is that these are presently cheaper than tuneable transmitters.

The OLT module 25 further comprises a controller 23 configured to selectively cause the transmitter 21 to generate the downstream optical signal at the first wavelength or at the second wavelength. This may be achieved, in this example, by sending a signal to each of the two transmitters 22 to turn them on and off respectively, or, where the transmitter 22 comprises a single tuneable transmitter, sending a signal to the transmitter 22 to cause it to tune to the first or second wavelength as desired.

A schematic view of a second WDM-PON 10 according to a preferred embodiment of the present invention is illustrated in Figure 5. The difference between this

embodiment and that of Figure 1 lies in OLT 20. In this embodiment, instead of having only one multiplexer 26, there are two multiplexers 26: a first multiplexer 26, labelled a "protection" multiplexer, which is configured to wavelength-division multiplex downstream signals for transmission in the clockwise direction around the ring, and a second multiplexer 26, labelled a "worker" multiplexer, which is configured to wavelength division multiplex downstream signals for transmission in the anticlockwise direction around the ring. In this example these multiplexers 26 are coupled directly to optical fibre 40, the first multiplexer 26 being coupled to the optical fibre 40 such that that its wavelength-division multiplexed signal is transmitted anticlockwise around the ring, the second multiplexer 26 being coupled to the optical fibre 40 such that its wavelength-division multiplexed signal is transmitted clockwise around the ring. In addition, in this embodiment, an optical passive band-splitter 27 is not shown in

Figure 5, but is present within the OLT 20, one being in each of the OLT modules 25 as illustrated in Figure 6 which shows an example implementation of one of the OLT modules 25 of Figure 5. In this case, each of the OLT modules 25 is coupled to the first multiplexer 26 and the second multiplexer 26 by respective optical paths 22, and more particularly the first output port B of the passive optical band-splitter 27 in each OLT module 25 is coupled to the first multiplexer 26 and the second output port C of the passive optical band-splitter 27 in each OLT module 25 is coupled to the second multiplexer 26. Referring to Figure 6, in this embodiment, each of the OLT modules 25 comprises a single tuneable optical transmitter 21 , which is coupled by an optical path to the input port A of the passive optical band-splitter 27. Preferably, the passive optical band splitters 27 in the OLT modules 25 are the same and, for example, as in the first embodiment, are configured to direct wavelengths n in a first direction around the ring and wavelengths m in the other direction around the ring. If the downstream signal generated by the tuneable transmitter is at its first wavelength, the passive optical band splitter 27 passes the downstream signal from input port A to output port B, which is coupled to the first "worker" multiplexer 26 (such that it is transmitted in the anticlockwise direction around the ring). However, if the downstream signal is at the second wavelength, the passive optical band-splitter 27 passes the signal from input port A to output port B, which is coupled to the second multiplexer 26 (such that it is transmitted in the clockwise direction around the ring). Thus, in this manner, a protection link can also be established, by virtue of passive optical wavelength splitter 27 (and the respective ADM 50), simply by changing the wavelength of the downstream signal, and without the use of expensive optical switches. It should be appreciated that, in this embodiment, since each of the ADMs 50 have a fixed configuration, whereby each ADM is configured to drop a certain first and second wavelength, instead of controller 23 knowing the particular first and second wavelength assigned to a downstream signal, each OLT module 25 may be configured to determine these first and second wavelengths automatically. In this case, the controller may cause the tuneable optical transmitter 21 to transmit its downstream signal sequentially through either the first or second set of wavelengths, as desired, until a link is established. This is sometimes referred to as a handshake.

In a preferred embodiment, the OLT modules 25 each further comprise an optical receiver 24 arranged to receive a corresponding upstream optical signal from its ONT 30. The controller 23 is coupled to the optical receiver 24.

In more detail, referring to Figure 4, in this embodiment, the optical paths 22

connecting the respective transmitters 21 to multiplexer 26 each comprise an optical circulator 28. The optical circulators 28 are configured to transmit the downstream signals received from the transmitters 21 to multiplexer 26, but transmit upstream signals received from multiplexer 26 to a different optical path connected to a common power coupler 29. The power coupler 29 couples the upstream optical signals via an optical path to optical receiver 24 which may for example comprise a photodetector. In the embodiment of Figure 6, on the other hand, it is the optical path between the tuneable transmitter 21 and the passive optical band splitter 27 which comprises an optical circulator 28. In this case, optical circulator 28 is configured to transmit signals received from the transmitter 21 to the passive optical band splitter 27 but to transmit signals received from the passive optical splitter 27 to optical receiver 24.

Preferably, controller 23 comprises a processor configured to determine whether a corresponding upstream signal has been received by the optical receiver. This may be achieved in a number of ways, for example using a Loss of Signal (LOS) signal generated by the receiver 24 which indicates whether optical power is present or not. Note that the processor may comprise any kind of numeric or analog circuitry, integrated to any degree, and not limited to general purpose processors, digital signal processors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs) or any other processor. This enables the OLT 20 to determine whether a transmission has failed automatically and, if so, to initiate protection. That is, if the transmitter 21 is transmitting the downstream signal at the first wavelength, the controller may cause the transmitter to transmit the downstream signal at the second wavelength instead and vice versa. Figure 7 is a flow chart illustrating a method of providing resilience or protection to a WDM-PON ring according to a preferred embodiment of the present invention. In this embodiment, a connection flag is stored in a memory device which indicates whether a link is established (either a "working" or "protection" link). In this example, if a

"protection" or "working" link is established, the flag is set to 1 , whereas if a "protection" or "working" link is not established, the flag is set to 0. Of course this is only one example.

If a link is not presently established (and the connection flag is at 0), at step 100, the downstream optical signal is transmitted at the first wavelength. This may involve referring to Figure 4, turning transmitter 1 on, or referring to Figure 5 tuning transmitter 21 to generate the downstream optical signal at the first wavelength. Then, after a predetermined time (which is at least as long as time needed by the downstream signal to reach the ONT and the upstream signal to reach the OLT), at step 1 10, it is determined whether a corresponding upstream signal has been received by the OLT 20. If so, the "working" link is established, and at step 120 the connection flag is set to 1 .

It should be appreciated that in the embodiment of Figure 5, where the transmitter 21 is tuned to the first wavelength associated with the ONT 30 automatically, steps 100 and 1 10 may be repeated for each of the first set of wavelengths, up until a "working" link is established. Or, until all of the first set of wavelengths have been tried.

At step 130, it may then be determined, continuously or periodically, whilst the working link is established, whether an upstream signal has been received by OLT 20. In this way, if the "working" link subsequently fails, advantageously, this is detected.

If a "working" link has not been established, or if the working link is established but then subsequently fails, at step 140, the downstream optical signal is transmitted at the second wavelength. Thus, in the example of Figure 4, transmitter 1 is turned off and transmitter 2 is turned on. In the example of Figure 5, transmitter 21 is tuned to generate the downstream optical signal at the second wavelength. Similarly, after a predetermined time (which is at least as long as time needed by the downstream signal to reach the ONT and the upstream signal to reach the OLT), at step 150, it is determined whether an upstream signal has been received by the OLT 20. If so, a "protection" link is established, and at step 160 the connection flag is set to 1.

Again, in the embodiment of Figure 5, steps 140 and 150 may be repeated for each of the wavelengths in the second set of wavelengths until a "protection" link is

established, or until all of the second set of wavelengths have been tried.

At step 170, it may then be determined, continuously or periodically, whilst the

"protection" link is established, whether a corresponding upstream signal has been received by OLT 20. Thus, in this way, similarly, if the "protection" link subsequently fails, this is also advantageously detected.

If a protection link cannot be established or subsequently fails, at step 180, the transmitter 21 is turned off, and the connection flag is set to 0. This means that, in the embodiment illustrated in Figure 4, transmitter 2 is turned off such that both

transmitters 1 and 2 are off. Note that transmitter 21 is not turned back on immediately. Only after waiting a predetermined time period T at step 190 is the transmitter turned back on at step 100 and the process re-started. Thus, if a link cannot be established in either the working or protection direction, perhaps because the ONT 30 is offline, advantageously, the transmitter 21 is turned off, thereby saving power.

A method of installing a WDM-PON ring 10 according to embodiments of the present invention is shown in the flow chart of Figure 8. In order to upgrade a particular downstream link, the method may comprise at step 200 installing a passive ADM 50 configured to drop the first and second wavelengths assigned to that link, and to add a return upstream optical signal to the optical fibre such that it travels in an opposite direction to the downstream optical signal dropped by the ADM 50; and at step 210 coupling an ONT 30 to the passive ADM 50. As mentioned previously, since in preferred embodiments, the ONTs 30 are colourless, this means that any of the ONTs 30 may be coupled to the passive ADM 50. In addition, the method may further comprise at step 220 installing a passive optical band splitter 27 at the OLT 20 having a first output port and a second output port, wherein the passive optical band splitter 27 is arranged to receive the downstream optical signal and, if the downstream optical signal is at the first wavelength, to pass the downstream signal to the first output port but, if the downstream signal is at the second wavelength, to pass the downstream signal to the second output port. At step 230 coupling the first output port to the optical fibre 40 such that the downstream signal is transmitted in a first direction around the ring; and at step 240 coupling the second output port to the optical fibre such that the downstream signal is transmitted in a second direction around the ring opposite to the first direction. Of course steps 220 to 240 may be performed before steps 200 to 210. The method may further comprise, where necessary, installing an optical transmitter 21 capable of generating the downstream optical signal at the first and the second wavelength.

The method may further preferably comprise installing a controller 23 at the OLT 20 configured to selectively cause the optical transmitter 21 to generate the downstream optical signal at the first wavelength or at the second wavelength.

A WDM-PON ring 10 embodying the present invention may be used in a radio access network, often referred to as a mobile backhaul network. For example, as illustrated in Figure 9, OLT 20 may be coupled to a base station 220 and each of the ONTs 30 to a respective remote radio unit (RRU) 230. The base station may be any kind of base station and not limited to a particular wireless protocol or frequency and not limited to being located in a single location. The base station can encompass distributed base stations having functions at different locations or shared functions in a computing cloud shared between multiple base stations.

In a preferred embodiment, the base station 220 is a centralised base station, such as a CRAN base station, which comprises a plurality of base station units (BSUs), each coupled to respective OLT modules 25 in OLT 20 (which are often also referred to as DU modules).

Base stations 200 often use the CPRI (common public radio interface) protocol to send data to remote radio units 230, and for example each BSU may be configured to transmit one or more downstream signals carrying CPRI data to its respective RRU 230 via the WDM-PON. Each RRU transmits a corresponding CPRI uplink to the BSU via the WDM-PON.

Thus, the WDM-PON ring 10 embodying the present invention is particularly advantageous for use in a radio access network, since not only is protection provided in the event of fibre damage or cuts in a cost-efficient way, but the uplink and downlink traffic travels along the same segment of the optical fibre, whether the WDM-PON 10 is in working or protection mode. This is particularly advantageous since many protocols, such as the CPRI protocol, have strict synchronisation requirements, whereby a differential delay between the time taken by uplink and downlink traffic to traverse the network cannot be tolerated.

It should be appreciated, however, that the WDM-PON ring 10 of embodiments of the present invention may nonetheless be used in any access network, where resilience is required in the event of optical fibre damage or cuts.