WO2015096864A1

WO2015096864A1 - An optical line terminal

Info

Publication number: WO2015096864A1
Application number: PCT/EP2013/077991
Authority: WO
Inventors: Luca Giorgi; Filippo Ponzini
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2013-12-24
Filing date: 2013-12-24
Publication date: 2015-07-02

Abstract

The present invention provides an Optical Line Terminal (OLT) for an optical ring network. The OLT comprises a plurality of transmitters each configured to transmit a downstream signal at a respective wavelength. The OLT further comprises a multiplexer configured to wavelength division multiplex the downstream signals into a wavelength division multiplexed signal, an optical power splitter, and an optical switch having a first state and a second state. When the optical switch is in the first state, the wavelength division multiplexed signal is transmitted in a first direction around the optical ring. When the optical switch is in the second state, the wavelength division multiplexed signal is split by the optical power splitter into two wavelength division multiplexed signals and a first of the wavelength division multiplexed signals is transmitted in the first direction around the ring and the second of the wavelength division multiplexed signals is transmitted in a second, different direction around the ring. The OLT further comprises a controller configured to cause the optical switch to switch from the first state to the second state when a break in the ring is detected. There is also provided an optical ring network comprising the OLT, and a radio access network comprising the optical ring network.

Description

An Optical Line Terminal

TECHNICAL FIELD

The present invention relates to an Optical Line Terminal (OLT) for an optical ring network, an optical ring network comprising an OLT and a radio access network comprising an optical ring network.

BACKGROUND

Passive WDM (wavelength-division-multiplexed) optical ring networks are gaining

importance not only for use in urban scenarios but also for use in radio access networks.

Figure 1 illustrates an example of a passive WDM optical ring network 10. The WDM optical ring network 10 comprises an OLT 20 (which may also be referred to as an OLT hotel or hub) and a plurality of Optical Network Terminals (ONTs) 30. The OLT 20 is connected to each of the ONTs 30 by an optical fibre 40 which is arranged in a ring. Each of the ONTs 30 is coupled to the optical fibre 40 by a respective optical add drop multiplexer 50. The OLT hotel 20 comprises a plurality of OLT cards 21 . Each of the OLT cards 21 comprises a transceiver adapted to transmit a downstream signal to, and receive an upstream signal from, a respective one of the ONTs 30. Each of the transceivers is adapted to transmit its downstream signal at a different wavelength from the other one or more transceivers, and only the optical add drop multiplexer 50 associated with that transceiver is configured to drop signals at that wavelength. Thus, a one-to-one connection can be established between the OLT 20 and each of the ONTs 30.

When used in a radio access network, each of the ONTs 30 may be coupled to a respective Radio Equipment (RE) (not shown) which may send radio signals to and receive radio signals from a large number of user devices. The OLT hotel 20 may be coupled to a Radio Equipment Controller (REC) (not shown). The REC communicates with each RE via the optical ring network 10 and may also be connected to a communications network (not shown).

In particular in radio access networks, where each RE may have many users, but also in other networks, it is important to provide a reliable protection mechanism in the event that a break in the ring, for example in optical fibre 40, occurs.

Although not shown in Figure 1 , in typical configurations the OLT hotel 20 comprises a first WDM multiplexer coupled to a first end of the optical fibre 40 and a second WDM multiplexer coupled to the other end of the optical fibre 40. Each of the OLT cards 21 further comprises a switch which is operable to pass its downstream signal to either the first multiplexer or the second multiplexer, for transmission in either the clockwise or the anticlockwise direction around the ring.

In existing protection solutions, an O&M (Operations and Management) controller (also not shown) determines the location of the ring break and operates the switch in each OLT card 22 to cause each downstream signal to be transmitted in the necessary direction around the ring to reach its ONT 30. For example, if a fibre cut occurred between ONT 3 and ONT 4 shown in Figure 1 , instead of transmitting a downstream signal to ONT 3 in the anticlockwise direction around the ring (i.e. via the shortest route), the downstream signal will be transmitted in the clockwise direction around the ring. On the other hand, the downstream signal to ONT 4 will continue to be transmitted in the anti-clockwise direction around the ring.

A problem with this solution however is that it is expensive. Complex O&M management is required in order to determine in which direction to transmit each downstream signal and, furthermore, a dedicated optical switch is required per OLT card/wavelength to transmit each downstream signal in the determined direction.

The present invention aims to provide an improved protection mechanism for an optical ring network.

SUMMARY

According to the present invention there is provided an Optical Line Terminal (OLT) for an optical ring network. The OLT comprises a plurality of transmitters each configured to transmit a downstream signal at a respective wavelength. The OLT further comprises a multiplexer configured to wavelength division multiplex the downstream signals into a wavelength division multiplexed signal, an optical power splitter, and an optical switch having a first state and a second state. When the optical switch is in the first state, the wavelength division multiplexed signal is transmitted in a first direction around the optical ring. When the optical switch is in the second state, the wavelength division multiplexed signal is split by the optical power splitter into two wavelength division multiplexed signals and a first of the wavelength division multiplexed signals is transmitted in the first direction around the ring and the second of the wavelength division multiplexed signals is transmitted in a second, different direction around the ring. The OLT further comprises a controller configured to cause the optical switch to switch from the first state to the second state when a break in the ring is detected. Thus, when a break in the optical fibre is detected, the WDM signal is duplicated and transmitted in both directions around the ring. Advantageously, this means that only a single switch is required to provide protection for all of the downstream signals/wavelengths.

Furthermore, complex O&M management is not required since it is not necessary to know the location of the ring break or switch each of the downstream signals/wavelengths individually. Whilst it is not usually desirable to transmit a downstream signal in both directions around a ring simultaneously, since this can cause signal in-band interference at the ONT, the inventors have appreciated that, because of the break in the ring, only one of the copies of the WDM signal will reach each ONT and therefore, according to embodiments of the present invention, signal in-band interference can be avoided.

In preferred embodiments of the present invention, the controller may be configured to detect a break in the ring and, in response, to cause the optical switch to switch from the first state to the second state. Alternatively, a separate device may detect that a break in the ring has occurred, and send an indication that there is a break in the ring to the controller. In a preferred example, the OLT may further comprise a plurality of receivers each configured to receive an upstream signal, and the controller may be configured to detect a break in the ring by monitoring the power of one or more of the upstream signals.

In a preferred embodiment of the present invention, the controller may further be configured to cause the optical switch to switch from the second state to the first state when the break in the ring is repaired.

In a preferred embodiment of the present invention, the controller may be configured to detect that the break in the ring has been repaired and, in response, to cause the optical switch to switch from the second state to the first state. For example, where the OLT comprises a plurality of receivers each configured to receive an upstream signal, the controller may be configured to detect that the break in the ring has been repaired by monitoring the power of one or more of the upstream signals. Alternatively, the controller may, for example, receive an indication from a systems operator that the break has been repaired.

In an embodiment of the present invention, the optical switch may be an open / close switch. Advantageously, this may provide a simple, cost effective implementation. However, other implementations are possible, as will be appreciated by those skilled in the art. In a preferred example, the first state of the optical switch may be an open state (in which a light signal can pass through the optical switch) and the second state of the optical switch is a closed state (in which a light signal cannot pass through the optical switch). In this preferred example of the present invention, the optical power splitter may have a first port, a second port and a third port. The first port is coupled to the multiplexer. The second port outputs, when the optical power splitter splits the wavelength division multiplexed signal into two wavelength division multiplexed signals, the first of the wavelength division multiplexed signals, and the third port outputs the second of the wavelength division multiplexed signals. The optical switch is coupled to the third port.

There is also provided an optical ring network comprising an Optical Line Terminal (OLT) as described above.

There is further provided a radio access network comprising an optical ring network as described above.

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 illustrates a WDM optical ring;

Figure 2 illustrates an example of an OLT according to an embodiment of the present invention;

Figure 3 illustrates the WDM optical ring in operation in a working mode according to an embodiment of the present invention; and

Figure 4 illustrates the WDM optical ring in operation in a protection mode according to an embodiment of the present invention.

DESCRIPTION

Figure 2 illustrates an example of an OLT 20 (which will also be referred to as an OLT hotel) according to a preferred embodiment of the present invention. The OLT hotel 20 comprises a plurality of OLT cards 21. Each of the OLT cards 21 comprises a transmitter 22 which is configured to transmit a downstream (light) signal at a particular wavelength (for receipt by its ONT (not shown)). Each of the transmitters 22 transmits its downstream signal at a different wavelength from the one or more other transmitters 22. In this example, each of the transmitters 22 comprises a laser 23 configured to generate a light signal at a particular wavelength, and a modulator 24 adapted to modulate the light signal to carry the

downstream traffic. The OLT 20 further comprises a WDM multiplexer 25, which is arranged to receive each of the downstream signals from the transmitters 22, at a respective input port. The WDM multiplexer 25 may be a passive device (requiring no power source) and, for example, be an array waveguide (AWG). The WDM multiplexer 25 is configured to frequency or wavelength multiplex each of the downstream signals into a WDM signal (or comb). This WDM signal is output at a common output port.

An optical power splitter 26 having three ports is arranged to receive the WDM signal at a first of its ports (port A). The optical power splitter 26 may be a passive device (i.e. requiring no power source). One of its other ports (port B) is connected or coupled to a first end of the optical fibre 40 (which is arranged in a ring). The other of its ports (port C) is connected or coupled to an optical switch 27, which in turn is connected or coupled to the other end of the optical fibre 40.

In this example, the optical switch 27 has a first state which is an open state (in which no light signal can pass through the optical switch 27) and a second state which is a closed state (in which a light signal can pass through the optical switch 27).

When the optical switch 27 is in the open state, the input WDM signal will simply pass through the optical power splitter 26 to port B, and be transmitted in only one direction around the ring (in this example, in the anticlockwise direction).

However, when the optical switch 27 is in the closed state, the optical power splitter 26 splits the received WDM signal into two (i.e. into two copies, a first WDM signal and a second WDM signal). Each of the WDM signals comprises all of the downstream signals from the transmitters 22 but is at only a fraction of the power of the input WDM signal. For example, the first WDM signal and the second WDM signal may each be at half the power of the input WDM signal. One of the copies (the first WDM signal) is output at port B and is transmitted in a first direction around the ring (in this example, in the anticlockwise direction). The other copy (the second WDM signal) is output at port B, passes through optical switch 27 and is transmitted in a second, different direction around the ring i.e. in the opposite direction around the ring (in this example, in the clockwise direction).

The OLT 20 further comprises a controller 28 which is configured to control optical switch 27, for example by sending a control signal to the optical switch 27 to cause it to switch between the first and second states (in this example between the open and close states). In this example, the controller 28 causes the optical switch 27 to transition or switch from the open position, the first state (which it is in when the OLT 20 is in "working mode") to the closed position, the second state, when a break in the ring, for example in optical fibre 40, is detected (which prevents the WDM comb traversing that portion of the optical ring 40).

Figures 3 and 4 illustrate examples of a passive WDM ring network 10 comprising an OLT 20 embodying the present invention. The WDM ring network 10 further comprises a plurality of ONTs 30, which are each coupled to the optical fibre 40 by a respective optical add drop multiplexer 50.

In Figure 3 the WDM ring network 10 is in "working mode" (optical switch 27 is in the open position). In this case, as illustrated in Figure 3, the WDM downstream comb is transmitted in only one direction around the ring, the anticlockwise direction. As indicated in Figure 3, each of the optical add drop multiplexers 50 drop a respective one of the downstream signals from the optical fibre 40. The ONT 30 coupled to the optical add drop multiplexer 50 receives the downstream signal and transmits an upstream signal. The upstream signal may be at a different wavelength to the downstream signal, or at the same wavelength as the downstream signal.

In this example, the optical add drop multiplexer 50 then adds the upstream signal to the optical fibre, such that the upstream signal is transmitted in the opposite direction around the ring to the direction travelled by the downstream signal. Thus, in this example, each upstream signal is transmitted in the clockwise direction around the ring. Advantageously, this means that the upstream signals travel along the same path as the downstream signals, which is desirable in many applications such as radio access networks where there are strict synchronisation requirements between upstream and downstream signals. However, in other examples, it is possible that the upstream signals may travel in the same direction as the downstream signals around the ring.

Referring again to Figure 2, when the optical switch 27 is open (in the first state), the upstream signals are received at port B of the optical power splitter 26, and are passed to port A of the optical power splitter 26. In this example, the WDM multiplexer 25 is also a WDM de-multiplexer, and the WDM de-multiplexer 25 splits the upstream signals into a plurality of signals, each at a respective wavelength. Each of the upstream signals is then passed to a respective one of the OLT cards 21 , where it is received by a respective receiver 29 (which may comprise, for example, a photodiode).

As indicated by the dotted lines, the controller 28 may be configured to detect a break in the optical ring, for example in optical fibre 40, by monitoring the power of one or more of the upstream signals received by the receivers 29. For example, if the power of the upstream signal received from ONT 1 drops this may indicate that there is a break in the optical fibre 40. Note that it is not necessary for the controller 28 to determine the location of the break in the ring.

If and when the controller 28 detects a break in the ring, the controller causes the optical switch 27 to switch from the open position (the first state) to the closed position (the second state).

Figure 4 shows the WDM ring network 10 in "protection mode" (optical switch 27 in the closed position). In this example, a break in the optical fibre 40 has occurred in the portion of the optical fibre between ONT 2 and 3. As explained above, the downstream WDM comb is transmitted in both directions around the ring. However, because of the break in the optical fibre 40, only one of the copies of the WDM comb will reach each ONT 30. Thus, in- band signal interference is avoided.

As before, the ONT 30 will again receive its downstream signal, dropped by its optical add drop multiplexer 50, and transmit an upstream signal, which is added to the optical fibre 40 by the optical add drop multiplexer 50. In this case the upstream signal needs to travel in the opposite direction around the ring to the downstream signal, in order to reach the OLT 20. Thus, in this example, the upstream signals transmitted by ONTs 1 and 2 travel in the anticlockwise direction around the ring, and the upstream signals transmitted by ONTs 3 and 4 travel in the clockwise direction around the ring. The upstream signals still therefore travel along the same path as their respective downstream signals. The upstream signals are then received by the receivers 29.

When the break in the optical ring has been repaired, the controller 28 switches the optical switch back from the second state (the closed position) to the first state (the open position), i.e. back to "working mode", in which the WDM signal is only transmitted in one direction around the ring.

In a preferred embodiment of the present invention, the controller 28 may detect when the break in the optical fibre 40 is repaired (i.e. when the optical ring is functioning again as normal). For example, by monitoring the power of one or more of the upstream signals, received by the receivers 29. Alternatively, for example, the controller 28 may receive an input from a systems operator indicating that the ring has been repaired.

Thus, advantageously, embodiments of the present invention enable protection to be achieved in the event of a break in the ring, for example in optical fibre 40, in a simple, cost efficient manner. There is no need for each of the OLT cards 21 to comprise a dedicated optical switch. Instead only one optical switch is required (a common optical switch), and an optical power splitter which is a relatively cheap component. Further complex O&M management is not required, since in the event of a break in the optical fibre 40, the WDM comb is simply transmitted in both directions around the ring.

In the preferred embodiment of the present invention described above, the optical switch is an open / close switch. However, it should be appreciated that, in other embodiments, the optical switch may be a different type of optical switch. For example, in some embodiments the optical switch could be a two-way optical switch, in which, when the optical switch is in the first state, an input light signal is passed to a first output port of the optical switch and, when the optical switch is in the second state, the input light signal is passed to a second, different output port of the optical switch.

The downstream / upstream signals may carry traffic at any bit rate, using any protocol.

In a preferred embodiment of the present invention, the WDM ring 10 may be used in a radio access network. The OLT 20 may be coupled to a Radio Equipment Controller (REC) (not shown) and each of the ONTs 30 may be coupled to a respective Radio Equipment (RE) (not shown). In this embodiment, the downstream / upstream signals may comprise CPRI (Common Protocol Radio Interface) traffic. However, the WDM ring 10 may be used in any other type of network, for example in a fibre to the home network, and the downstream / upstream traffic may be, for example, Ethernet, OTN (Optical Transport Network) or SDH or any other type of traffic.

Claims

1 . An Optical Line Terminal (OLT) for an optical ring network, the OLT comprising:

a plurality of transmitters each configured to transmit a downstream signal at a respective wavelength;

a multiplexer configured to wavelength division multiplex the downstream signals into a wavelength division multiplexed signal;

an optical power splitter;

an optical switch having a first state and a second state, wherein, when the optical switch is in the first state, the wavelength division multiplexed signal is transmitted in a first direction around the optical ring, and, when the optical switch is in the second state, the wavelength division multiplexed signal is split by the optical power splitter into two wavelength division multiplexed signals and a first of the wavelength division multiplexed signals is transmitted in the first direction around the ring and the second of the wavelength division multiplexed signals is transmitted in a second, different direction around the ring; and

a controller configured to cause the optical switch to switch from the first state to the second state when a break in the ring is detected.

2. An OLT according to claim 1 , wherein the controller is configured to detect a break in the ring and, in response, to cause the optical switch to switch from the first state to the second state.

3. An OLT according to claim 2, further comprising a plurality of receivers each

configured to receive an upstream signal, wherein the controller is configured to detect a break in the ring by monitoring the power of one or more of the upstream signals.

4. An OLT according to any preceding claim, wherein the controller is further configured to cause the optical switch to switch from the second state to the first state when the break in the ring is repaired.

5. An OLT according to claim 4, wherein the controller is configured to detect that the break in the ring has been repaired and, in response, to cause the optical switch to switch from the second state to the first state.

6. An OLT according to claim 5, further comprising a plurality of receivers each configured to receive an upstream signal, wherein the controller is configured to detect that the break in the ring has been repaired by monitoring the power of one or more of the upstream signals.

7. An OLT according to any preceding claim, wherein the first state of the optical switch is an open state and the second state of the optical switch is a closed state.

8. An OLT according to claim 7, wherein the optical power splitter has a first port, a second port and a third port, wherein the first port is coupled to the multiplexer, the second port outputs the first of the wavelength division multiplexed signals and the third port outputs the second of the wavelength division multiplexed signals, wherein the optical switch is coupled to the third port.

9. An optical ring network comprising an Optical Line Terminal (OLT) according to any preceding claim.

10. A radio access network comprising an optical ring network according to any of claims 9 to 13.

1 1 . A radio access network according to claim 14, further comprising a plurality of Radio Equipment (RE) and a Radio Equipment Controller (REC), wherein the OLT is coupled to the REC and each of the ONTs is coupled to a respective RE.