Technical Field

Examples of the present disclosure relate to an optical network node and an optical network.

Background

A transport network is an important aspect for enabling network deployments, including 3G, 4G/LTE and 5G networks. Optical technologies and networks, for example based on wavelength division multiplexing (WDM), may serve a Radio Access Technology (RAN) network in both fronthaul and backhaul areas as these solutions may provide high bandwidth and low latency.

Summary

One aspect of the present disclosure provides an optical network node. The node is configured to operate in a first mode and a second mode. The node comprises a first optical routing apparatus configured to direct a first optical signal received from an optical network at a first port to an input of a first optical switching device in the first mode, and to direct an optical signal output from the first optical switching device to the first port in the second mode. The node also comprises a second optical routing apparatus configured to direct the output signal from the first optical switching device to a second port in the first mode, and to direct a second optical signal received from a network at the second port to the input of the first optical switching device in the second mode. The node is configured to transition from the first mode to the second mode upon determining that the first optical signal is not being received at the first port or on detection of a third optical signal received at a third port.

Another aspect of the present disclosure provides an optical network node. The node is configured to operate in a first mode and a second mode. The node comprises a transmitter configured to provide a first optical signal and a first optical routing apparatus configured to direct the first optical signal to an optical network from a first port in the first mode, and to direct the first optical signal to the optical network from the first port and a second port in the second mode. The node also comprises a receiver and a second optical routing apparatus configured to direct a second optical signal received from the network at a third port to the receiver, and to direct a third optical signal received from the network at a fourth port to the receiver. The node is configured to transition from the first mode to the second mode upon determining that the second optical signal is being received at the third port.

A further aspect of the present disclosure provides a method in an optical network node. The method comprises determining that a first optical signal is being received from an optical network at a first port or a third optical signal is not being received from the optical network at a third port, directing the first optical signal to an input of a first optical switching device, and directing an output signal from the first optical switching device to a second port. The method also comprises determining that the first optical signal is not being received from an optical network at a first port or the third optical signal is being received from the optical network at the third port, directing the optical signal output from the first optical switching device to the first port, and directing a second optical signal received from a network at the second port to the input of the first optical switching device.

A still further aspect of the present disclosure provides a method in an optical network node. The method comprises directing a first optical signal from a transmitter to an optical network from a first port and, in response to determining that a third optical signal is being received at a third port, directing the first optical signal to the optical network from a second port. The method also comprises directing a second optical signal received from the network at the third port to a receiver, and directing a third optical signal received from the network at a fourth port to the receiver.

Another aspect of the present disclosure provides an optical network comprising a hub node and a plurality of remote nodes. The hub node is configured to operate in a first mode and a second mode. The hub node comprises a transmitter configured to provide a first optical signal, and a first optical routing apparatus configured to direct the first optical signal to an optical network from a first port in the first mode, and to direct the first optical signal to the optical network from the first port and a second port in the second mode.

The hub node is configured to transition from the first mode to the second mode when at least one of the remote nodes transitions from the first mode to the second mode. Each of the plurality of remote nodes is configured to operate in a first mode and a second mode. Each remote node comprises a first optical routing apparatus configured to direct a first optical signal received from an optical network at a first port to an input of a first optical switching device in the first mode, and to direct an optical signal output from the first optical switching device to the first port in the second mode, and a second optical routing apparatus configured to direct the output signal from the first optical switching device to a second port in the first mode, and to direct a second optical signal received from a network at the second port to the input of the first optical switching device in the second mode. The remote node is configured to transition from the first mode to the second mode upon determining that the first optical signal is not being received at the first port or on detection of a third optical signal received at a third port. The remote nodes are arranged in series and comprise a first remote node and one or more subsequent remote nodes. The first port of the hub node is connected to the first port of the first remote node, the first port of each of the subsequent remote nodes is connected to the second port of its previous remote node, and the second port of a last one of the one or more

subsequent remote nodes is connected to the second port of the hub node.

A further aspect of the present disclosure provides an optical network comprising a hub node and a plurality of remote nodes. The hub node is configured to operate in a first mode and a second mode. The hub node comprises a receiver and a first optical routing apparatus configured to direct a first optical signal received from the network at a first port to the receiver, and to direct a second optical signal received from the network at a second port to the receiver. Each of the plurality of remote nodes is configured to operate in a first mode and a second mode. Each remote node comprises a first optical routing apparatus configured to direct a first optical signal received from an optical network at a first port to an input of a first optical switching device in the first mode, and to direct an optical signal output from the first optical switching device to the first port in the second mode, and a second optical routing apparatus configured to direct the output signal from the first optical switching device to a second port in the first mode, and to direct a second optical signal received from a network at the second port to the input of the first optical switching device in the second mode. The remote node is configured to transition from the first mode to the second mode upon determining that the first optical signal is not being received at the first port or on detection of a third optical signal received at a third port. The remote nodes are arranged in series and comprise a first remote node and one or more subsequent remote nodes. The first port of the hub node is connected to the first port of the first remote node, the first port of each of the subsequent remote nodes is connected to the second port of its previous remote node, and the second port of a last one of the one or more subsequent remote nodes is connected to the second port of the hub node.

A still further aspect of the present disclosure provides an optical network comprising a hub node and a plurality of remote nodes. The hub node is configured to operate in a first mode and a second mode. The hub node comprises a transmitter configured to provide a first optical signal and a first optical routing apparatus configured to direct the first optical signal to an optical network from a first port in the first mode, and to direct the first optical signal to the optical network from the first port and a second port in the second mode.

The hub node also comprises a receiver and a second optical routing apparatus configured to direct a second optical signal received from the network at a third port to the receiver, and to direct a third optical signal received from the network at a fourth port to the receiver. The hub node is configured to transition from the first mode to the second mode upon determining that the second optical signal is being received at the third port. Each of the plurality of remote nodes is configured to operate in a first mode and a second mode. Each remote node comprises a first optical routing apparatus configured to direct a first optical signal received from an optical network at a first port to an input of a first optical switching device in the first mode, and to direct an optical signal output from the first optical switching device to the first port in the second mode, and a second optical routing apparatus configured to direct the output signal from the first optical switching device to a second port in the first mode, and to direct a second optical signal received from a network at the second port to the input of the first optical switching device in the second mode. Each remote node also comprises a third optical routing apparatus configured to direct a fourth optical signal received from an optical network at a fourth port to an input of a second optical switching device in the first mode, and to direct an optical signal output from the second optical switching device to the fourth port in the second mode, and a fourth optical routing apparatus configured to direct the output signal from the second optical switching device to a third port in the first mode, and to direct the third optical signal received from a network at the third port to the input of the second optical switching device in the second mode. The remote node is configured to transition from the first mode to the second mode upon determining that the first optical signal is not being received at the first port or on detection of the third optical signal received at a third port. The remote nodes are arranged in series and comprise a first remote node and one or more subsequent remote nodes. The first port of the hub node is connected to the first port of the first remote node, the first port of each subsequent remote node is connected to the second port of its previous remote node, and the second port of a last one of the one or more subsequent remote nodes is connected to the second port of the hub node. The fourth port of the hub node is connected to the third port of the first remote node, the third port of each subsequent remote node is connected to the fourth port of its previous remote node, and the fourth port of the last one of the one or more subsequent remote nodes is connected to the third port of the hub node.

Brief Description of the Drawings

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

Figure 1 is a schematic illustration of an example of an optical network node;

Figure 2 is a schematic illustration of the node of figure 1 in a first mode;

Figure 3 is a schematic illustration of the node of figure 1 in a second mode; Figure 4 is a schematic illustration of an example of an optical network;

Figure 5 is a schematic illustration of the network of Figure 4 in a fault scenario;

Figure 6 is a schematic illustration of the network of Figure 4 in an alternative fault scenario;

Figure 7 is a schematic illustration of an example of another optical network;

Figure 8 is a schematic illustration of an example of a hub node;

Figure 9 is a schematic illustration of another example of a hub node;

Figure 10 is a schematic illustration of an example of another optical network node;

Figure 11 is a schematic illustration of the network of Figure 7 in a fault scenario;

Figure 12 is a schematic illustration of an example of another optical network node;

Figure 13 is a schematic illustration of the network of Figure 7 in another fault scenario;

Figure 14 is a schematic illustration of an example of another optical network node;

Figure 15 is a schematic illustration of the node of Figure 14 in a second mode;

Figure 16 is a flow chart of an example of a method in an optical network node; and

Figure 17 is a flow chart of an example of another method in an optical network node. Detailed Description

The following sets forth specific details, such as particular embodiments or examples for purposes of explanation and not limitation. It will be appreciated by one skilled in the art that other examples may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and/or discrete logic gates

interconnected to perform a specialized function, ASICs, PLAs, etc.) and/or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, where appropriate the technology can additionally be considered to be embodied entirely within any form of computer-readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

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

Exploiting existing optical fibers for radio transport purposes may be of value for network operators. Such optical fibers may be present for example due to metropolitan

applications in urban areas. For example, a metropolitan area may include a small number of large fiber rings and by many more smaller rings.

Figure 1 shows an example of an optical network node 100. The node may be for example a remote node, for example a remote node that is located at a remote location from a hub node and exchanges data with the hub node via one or more optical fibers, and also in some cases also via one or more other nodes. The node 100 is configured to operate in a first mode and a second mode. The node 100 includes a first optical routing apparatus 102 configured to direct a first optical signal received from an optical network at a first port 104 to an input of a first optical switching device 106 in the first mode, and to direct an optical signal output from the first optical switching device 106 to the first port 104 in the second mode.

The node 100 also includes a second optical routing apparatus 108 configured to direct the output signal from the first optical switching device 106 to a second port 1 10 in the first mode, and to direct a second optical signal received from a network at the second port 1 10 to the input of the first optical switching device 106 in the second mode.

The node 100 is configured to transition from the first mode to the second mode upon determining that the first optical signal is not being received at the first port 104.

Additionally or alternatively, the node 100 is configured to transition from the first mode to the second mode on detection of a third optical signal received at a third port (not shown in Figure 1 ).

In some embodiments, the node 100 further comprises a first port optical signal detector configured to detect the first optical signal received at the first port, and the optical network node is configured to operate in the first mode when the first port optical signal detector detects the first optical signal, and the second mode when the first port optical signal detector does not detect the first optical signal. Additionally or alternatively, the node 100 may further comprise a third port optical signal detector configured to detect the third optical signal received at the third port, and the optical network node is configured to operate in the first mode when the third port optical signal detector does not detect the third optical signal, and the second mode when the third port optical signal detector detects the first optical signal.

Figure 2 shows an example of the node 100 of figure 1 in the first mode. A signal received at the first port 104 is provided to the input of the first optical switching device 106, whereas the output is provided to the second port 110. Figure 3 shows an example of the node 100 of figure 1 in the second mode. A signal received at the second port 110 is provided to the input of the first optical switching device 106, whereas the output is provided to the first port 104. In effect, in some embodiments, the direction of flow of an optical signal through the node 100 is reversed between the first and second modes. In some embodiments, the first optical switching device 106 may add signals and/or remove signals from the optical signal through the node 100. For example, the first optical switching device may be an add multiplexer, a drop multiplexer, or an add/drop multiplexer. In figures 2 and 3, the optical routing apparatus are not shown, and instead the effect of the optical routing apparatus is shown.

Figure 4 shows an example of an optical network 400. The optical network 400 comprises a hub node 402 and a plurality of remote nodes. In this example, there are two remote nodes 404 and 406, though in other embodiments there may be any number of remote nodes. The hub node 402 is configured to operate in a first mode and a second mode, and comprises a transmitter 408 configured to provide a first optical signal, and a first optical routing apparatus 410 configured to direct the first optical signal to an optical network from a first port 414 in the first mode, and to direct the first optical signal to the optical network from the first port 414 and a second port 416 in the second mode.

Each of the remote nodes 404 and 406 is configured to operate in a first mode and a second mode, and the hub node is configured to transition from the first mode to the second mode (i.e. to transition from not transmitting the first optical signal from the second port 416 to transmitting the first optical signal from the second port 416) when at least one of the remote nodes 404 and 406 transitions from the first mode to the second mode.

The first remote node 404 comprises a first optical routing apparatus 420 configured to direct a first optical signal received from an optical network (e.g. the first optical signal received from the first port 414 of the hub 402 over a fiber 422) at a first port 424 to an input of a first optical switching device 426 in the first mode, and to direct an optical signal output from the first optical switching device 426 to the first port 424 in the second mode. The first remote node 404 also comprises a second optical routing apparatus 428 configured to direct the output signal from the first optical switching device 426 to a second port 430 in the first mode, and to direct a second optical signal received from a network at the second port 430 to the input of the first optical switching device 426 in the second mode. The first remote node 404 is configured to transition from the first mode to the second mode upon determining that the first optical signal is not being received at the first port 424. Additionally or alternatively, the first remote node 404 is configured to transition from the first mode to the second mode on detection of a third optical signal received at a third port of the first remote node 404 (not shown in Figure 4).

The second remote node 406 comprises a first optical routing apparatus 440 configured to direct a first optical signal received from an optical network (e.g. the signal received from the second port 430of the first remote node 404 over a fiber 442) at a first port 444 to an input of a first optical switching device 446 in the first mode, and to direct an optical signal output from the first optical switching device 446 to the first port 444 in the second mode. The second remote node 406 also comprises a second optical routing apparatus 448 configured to direct the output signal from the first optical switching device 446 to a second port 450 in the first mode, and to direct a second optical signal received from a network at the second port 450 to the input of the first optical switching device 446 in the second mode. The second remote node 406 is configured to transition from the first mode to the second mode upon determining that the first optical signal is not being received at the first port 444. Additionally or alternatively, the second remote node 406 is configured to transition from the first mode to the second mode on detection of a third optical signal received at a third port of the second remote node 406 (not shown in Figure 4).

In some embodiments, the first remote node 404 and/or the second remote node comprise a node as shown in Figures 1-3.

The remote nodes are arranged in series. The second port of the second remote node 406 is connected to the second port 416 of the hub node 402 (e.g. by a fiber 452). Where there are more than the two remote nodes shown in the example network of Figure 4, the remote nodes may comprise a first remote node and one or more subsequent remote nodes. In such embodiments, the first port of the hub node 402 is connected to the first port of the first remote node, the first port of each of the subsequent remote nodes is connected to the second port of its previous remote node, and the second port of a last one of the one or more subsequent remote nodes is connected to the second port of the hub node.

The fiber 452 is shown as a dashed line in Figure 4. This is because in the first mode, the first optical signal is not sent on the fiber 452 by the hub 402. The first mode may in some embodiments be referred to as a normal mode, or may be the mode of the nodes during normal operation of the network 400. In this mode, the signal from the hub 402 passes through the first remote node 404 and to the second remote node 406. Each remote node extracts appropriate signals from the signal from the hub, e.g. using WSS where WDM is used. In some embodiments, all remaining signals are extracted by the last remote node (the second remote node 406 in the example shown in Figure 4), and hence no signals are passed into the fiber 452 by the second remote node 406.

In some cases, a fault may develop on one or more of the optical fibers in the network 400. Figure 5 shows an example of the network 400 where the fiber 422 between the hub node 402 and the first remote node 404 has a fault 500, e.g. it has been cut. The optical routing apparatus of the remote nodes is no shown, and instead the effect of these apparatus are shown. The first remote node 404 detects that no signal is being received at its first port 424. Therefore, the first remote node 402 switches to the second mode, where signals from port 430 are passed to the input of the optical switching device 426, and the output of optical switching device 426 is passed to port 424. Similarly, the second remote node 406 detects that no signal is being received at its first port 444 and switches to the second, where signals from port 450 are passed to the input of the optical switching device 446, and the output of optical switching device 446 is passed to port 444. The hub 402 also switches to the second mode, where the first optical signal from the hub is also transmitted into fiber 452 to the second port 450 of the second remote node 406.

As a result, in the second mode, the signal from the hub is transmitted through the second remote node 406 to the first remote node 404, and therefore both remote nodes receive the signal and can extract the appropriate signals therefrom. There is no interference of the duplicate signals from the hub 402 transmitted through the respective fibers 422 and 452 as the signal in the fiber 422 does not reach the first remote node 404 due to the fiber fault 500.

Figure 6 shows the network 400 in an alternative fault scenario, where there is instead a fault 600 in the fiber 442 between the first and second remote nodes 404 and 406. The optical routing apparatus of the remote nodes is no shown, and instead the effect of these apparatus are shown. In this case, the first remote node 404 continues to operate in the first mode, as it is still detecting the signal received at its first port 424 from the hub 402. Therefore, the signal at the first port 424 is passed to the input of the optical switching device 426. However, due to the fault 600, the signal is not passed from the first remote node 404 to the second remote node 406. Instead, the second remote node 406 detects that the signal is no longer being received at its first port 444, and switches to the second mode. In this mode, the signal from the hub 402 via fiber 452 is received at the second port 450 (as the hub 402 has also switched to the second mode), and is provided to the input of the optical switching device 446. Therefore, both first and second remote nodes 404 and 406 can extract the appropriate signals from the signal from the hub 402, although in this example scenario the remote nodes do not all receive the signal from the hub 402 via the same path.

Figure 7 shows an example of another optical network 700. The optical network 700 includes a hub node 702 and a plurality of remote nodes 704, 706 and 708. In the example shown, there are three remote nodes, though there may be any number of remote nodes in other examples. The hub node 702 is connected to the first remote node 704 by a pair of optical fibers 710 and 712. The first remote node 704 is connected to the second remote node 706 by another pair of optical fibers 714 and 716. The second remote node 706 is connected to the third remote node 708 by another pair of optical fibers 718 and 720. The third remote node 708 is connected to the remote node 702 by another pair of optical fibers 722 and 724. Each pair of optical fibers may allow bidirectional optical signals to be exchanged between the nodes at each end of the pair, for example using wavelength division multiplexing. In some embodiments, the hub node 702 and the remote nodes 704, 706 and 708 may operate in a first mode and a second mode. For example, the first mode may be associated with normal operation of the network 700. In this mode, the hub 702 may exchange signals with remote node 704 using the pair of fibers 710 and 712. For example, the hub 702 may transmit optical signals to the first remote node 704 using fiber 710. The remote node 704 may extract signals from the signals from the hub 702, for example using wavelength selective switching (WSS) if the optical network 700 uses wavelength division multiplexing (WDM). The remainder of the signal not extracted by the first remote node 704 is passed to the second remote node 706 via fiber 714. Similarly, the second remote node 706 extracts signals and passes the remainder to the third remote node 108 via fiber 718. The third remote node 708 may then extract signals. In some embodiments, the third remote node 708 may extract all remaining signals, and as such may pass no signal into fiber 722.

Conversely, in some embodiments, the third remote node 708 may generate optical signals and pass these signals to the second remote node 706 via fiber 720. The second remote node 706 may add signals to the signals from the third remote node 708 and pass the aggregated signals to the first remote node 704 via fiber 716. The first remote node 704 may add further signals and pass the aggregated signals from the remote nodes 704, 706 and 708 to the hub 702 via fiber 712. In this example, the fiber 724 is unused. In some examples, therefore, in the first mode, the pair of fibers 722 and 724 are unused, e.g. carry no or substantially no optical signals.

Figure 8 shows an example of a hub node 800. The hub node 800 comprises a transmitter 802 configured to provide a first optical signal, and a first optical routing apparatus 804 configured to direct the first optical signal to an optical network from a first port 806 in the first mode, and to direct the first optical signal to the optical network from the first port 806 and a second port 808. The hub node 800 also includes a receiver 810 and a second optical routing apparatus 812 configured to direct a second optical signal received from the network at a third port 814 to the receiver, and to direct a third optical signal received from the network at a fourth port 816 to the receiver. In some examples, the signals received at the third ports may not interfere if a signal is received at only one of the ports 814 or 816, they represent different wavelengths in WDM. The hub node 800 is configured to transition from the first mode to the second mode upon determining that the second optical signal is being received at the third port.

Thus, in some embodiments, the hub node 800 transmits from the first port 806 in the first mode, but does not transmit from the second port 808 in the first mode. As a result, there is no interference for signals transmitted from both ports 806 and 808 in other parts of the optical network. The hub node 800 transmits from the second port 808 in the second mode.

In some embodiments, the node 800 is configured to transition from the second mode to the first mode upon determining that the second optical signal is not being received at the third port 814. For example, if the hub node 800 is connected to one or more remote nodes that operate in first and second modes, such as those shown in Figure 1 or 10, where there is a fault in the network, signals may be received at both ports 814 and 816, whereas once the fault is repaired, and the remote nodes return to the first mode, a signal may no longer be received at the third port 814. Therefore, detection of no signal at the third port 814 may return the hub 900 to the first mode, and transmission from the second port 808 may cease.

Figure 9 shows an example of an embodiment of a hub node 900. Components that are similar or identical to those in Figure 8 are given like reference numerals. In the example hub node 900, a first optical amplifier 902 is located between the transmitter 802 and the first optical routing apparatus 804. The first optical amplifier 902 may be configured to compensate for any losses in the first optical routing apparatus 804 and/or any other components. The first optical routing apparatus 804 comprises a beam splitter 904 that splits the signal from the first optical amplifier 904. The split signal is sent on one path to the first port 806 and on another path to the second port 808 via an optical switch (e.g. shutter) 906. The switch 906 allows the signal to pass through in the second mode, and prevents the signal passing through in the first mode. Hence, the optical switch 906 is configured to direct the first optical signal to the second port 808 in the second mode. The signal may not be transmitted from the port 808 in the first mode. The hub node 900 also includes a second optical amplifier 908 between the second optical routing apparatus 810 and the receiver 810, and configured to compensate for losses in the second optical routing apparatus 810 and/or any other components. The second optical routing apparatus 810 comprises a coupler 910 that directs signals received from ports 814 and 816 to the second optical amplifier 908. In some examples, the coupler 910 may comprise a beam splitter configured as a coupler.

Figure 10 shows an embodiment of an optical network node 1000. The node 1000 may be for example a remote node, for example a remote node that is located at a remote location from a hub node and exchanges data with the hub node via one or more optical fibers, and also in some cases also via one or more other nodes. The node 1000 is configured to operate in a first mode and a second mode. The node 1000 includes a first optical routing apparatus 1002 configured to direct a first optical signal received from an optical network at a first port 1004 to an input of a first optical switching device 1006 in the first mode, and to direct an optical signal output from the first optical switching device 1006 to the first port 1004 in the second mode.

The node 1000 also includes a second optical routing apparatus 1008 configured to direct the output signal from the first optical switching device 1006 to a second port 1010 in the first mode, and to direct a second optical signal received from a network at the second port 1010 to the input of the first optical switching device 1006 in the second mode. In some examples, components 1002-1010 are similar or identical to corresponding components 102-110 of the node 100 shown in Figure 1.

The node 1000 further comprises a third optical routing apparatus 1012 configured to direct a fourth optical signal received from an optical network at a fourth port 1014 to an input of a second optical switching device 1016 in the first mode, and to direct an optical signal output from the second optical switching device 1016 to the fourth port 1014 in the second mode. The node 1000 also includes a fourth optical routing apparatus 1018 configured to direct the output signal from the second optical switching device to a third port 1020 in the first mode, and to direct the third optical signal received from a network at the third port to the input of the second optical switching device in the second mode.

The node 1000 is configured to transition from the first mode to the second mode upon determining that the first optical signal is not being received at the first port 1004.

Additionally or alternatively, the node 1000 is configured to transition from the first mode to the second mode on detection of a third optical signal received at the third port 1020.

In some embodiments, one path (for example the path that includes the first optical switching device 1006) may be used for“downstream” communications, i.e.

communications from a hub node. Therefore, the first optical switching device may comprise for example an optical drop multiplexer for extracting signals from the signals from the hub node. The other path may therefore be used for“upstream”

communications, i.e. communications to a hub node. Therefore, the second optical switching device 1016 may comprise for example an optical add multiplexer. In some embodiments, both upstream and downstream communications may pass through a single optical switching device, which may be for example an optical add/drop multiplexer. Figure 10 shows arrows through the components that represent optical signal direction in the first mode (solid arrows) and the second mode (dashed arrows).

In some embodiments, the hub node 702 of the network 700 shown in Figure 7 comprises the hub node 800 or 900 shown in Figures 8 and 9 respectively, and the remote nodes 704, 706 and 708 may comprise the node 1000 shown in Figure 10. In this case, fiber 710 may connect the first port of the hub 702 to the first port of the remote node 704, and fiber 712 may connect the fourth port of the hub 702 to the third port of the remote node 704. Fiber 714 may connect the second port of remote node 704 to the first port of remote node 706. Fiber 716 may connect the fourth port of remote node 704 to the third port of remote node 706. Fiber 718 may connect the second port of remote node 706 to the first port of remote node 708. Fiber 720 may connect the fourth port of remote node 706 to the third port of remote node 708. Fiber 722 may connect the second port of remote node 708 to the second port of the hub 702. Fiber 724 may connect the fourth port of remote node 708 to the third port of the hub 702. Figure 11 shows an example of the network 700 of Figure 7 in an example fault scenario. In this example scenario, fibers 710 and 712 have developed a fault 1 100, e.g. they have been cut. Pairs of fibers may in some cases develop a fault substantially simultaneously if they are co-located for at least a portion of their respective lengths. In this scenario, the first remote node 704 detects that no signal is being received at the first port, and switches to the second mode. In the second mode, the first remote node 704 assumes a received optical signal is received at its second port, and that any transmitted signal should be sent from the fourth port. Thus, the direction of fibers 714 and 716 have reversed compared to Figure 7.

The second remote node 706 subsequently detects that no signal is being received at its first port, as a signal is no longer being transmitted by the first remote node 704 over fiber 714. Therefore, the second remote node 706 switches to the second mode. The direction of fibers 718 and 720 is thus show as reversed compared to Figure 7. Similarly, the third remote node 708 switches to the second mode. In this mode, the third remote node 708 transmits signals (including signals from the other remote nodes 704 and 706) from its fourth port to the third port of the hub 702 over fiber 724, and it expects to receive a signal from the hub 702 at its second port over fiber 722 from the second port of the hub 702.

At this stage, the hub 702 detects that a signal is being received at its third port. The hub therefore transitions to the second mode, causing an optical signal transmitted from the first port of the hub 702 to also be transmitted from its second port. The third remote node 708 thus receives the signal at its second port. This signal may pass through the remote nodes 708, 706 and 704, with each remote node extracting appropriate signals. As a result, even though there is a fault 1 100 in the fibers 710 and 712, nodes in the network 700 have switched mode and all remote nodes are able to communicate (i.e. transmit and receive) with the hub 702. The switching from the first to the second mode can be done automatically in each node upon detection of the presence or absence of an optical signal received at a port, as appropriate. When the fault 1 100 is repaired, the signal transmitted from the first port of the hub 702 again reaches the first port of the first node 704. In some embodiments, the nodes in the network 700 may be reset to the first mode manually. In this case, one or more of the remote nodes (e.g. the first remote node 704) may include optical switches to simulate the fault 1 100. Figure 12 shows an example of an embodiment of an optical network node 1200 such as for example a remote node. Components that are identical or similar to components in the node 1000 of Figure 10 are given like reference numerals. The node 1200 includes an optical switch 1202 between the first port 1004 and the first optical switching device 1006. The switch is closed in the first mode and open in the second mode. This ensures that, when the fault is repaired and the node 1200 is still in the second mode, the signal received at the first port 1004 from the hub node does not interfere with the signal from the hub node that is transmitted from the second port of the hub node and reaches the node 1200 via a different path. Similarly, an optical switch 1204 is between the third port 1020 and the second optical switching device 1016. The switches 1202 and 1204 may be open in the second mode and closed in the first (e.g. normal) mode. In the example shown, the switch 1202 is located between the first port 1004 and the first optical routing apparatus 1002, and the switch 1204 is located between the third port 1020 and the fourth optical routing apparatus 1018. However, in other embodiments the switches 1202 and 1204 may be in any suitable position to simulate a fiber fault. The switches may additionally or alternatively comprise other suitable components such as optical shutters.

In some embodiments, one or more of the nodes may switch automatically back to the first mode. For example, a remote node may be configured to transition from the second mode to the first mode upon detection of the optical signal received at the first port (e.g. the signal from the hub is being received at the first port, either directly or via another remote node) or upon determining that the third optical signal is not being received at the third port (e.g. the signal from a remote node is no longer being received as that node has switched back to the first mode). The hub node may return to the first more when it determines that a signal is no longer being received at its third port, e.g. because the“last” remote node is no longer transmitting from its fourth port as it has returned to the first mode. Figure 13 shows the network 700 of Figure 7 in an alternative fault scenario. In this scenario, there is a fault 1300 such as a break or cut to the fibers 714 and 716 between the remote nodes 704 and 706. Here, the first remote node 704 remains in the first mode as it is still receiving a signal from the hub 700 via fiber 710. However, the signal is not passed to the second remote node 706 via the fiber 714. Therefore, the second remote node 706 transitions to the second mode, and this also causes the third remote node 708 to transition to the second mode. As a result, the first remote node 704 communicates with the hub 702 via fibers 710 and 712, whereas the second and third remote nodes 706 and 708 communicate with the hub 702 via the fibers 722 and 724. The hub 702 is in the second mode as it is receiving a signal from the third remote node 708 at its third port. Therefore, all of the remote nodes are still able to communicate with the hub node 702. In this scenario, before the fault 1300 is repaired, the hub receives a signal from the first remote node 704 and also receives via a different path a signal from the third remote node 708. However, these two signals may in some embodiments not interfere as they represent different wavelengths in a WDM system, and thus these two signals may for example be combined using a beam combiner before being passed to a receiver.

A fault in the fibers 718 and 720 may in some embodiments similarly cause the first and second nodes 704 and 706 to remain in the first mode, and cause the third remote node 708 and the hub node 702 to switch to the second mode. Thus in some embodiments a fault in any pair of fibers between two nodes will cause appropriate mode switching of nodes in the network such that all remote nodes are still able to communicate with the hub node 702.

Figure 14 shows an example embodiment of an optical network node 1400, such as for example a remote node. The node 1400 includes a first optical switching device 1402 and a second optical switching device 1404. The node 1400 also includes a first optical routing apparatus 1406 configured to direct a first optical signal received from an optical network at a first port 1408 to an input of a first optical switching device 1402 in the first mode, and to direct an optical signal output from the first optical switching device 1402 to the first port 1408 in the second mode. The node 1400 further comprises a second optical routing apparatus 1410 configured to direct the output signal from the first optical switching device 1402 to a second port 1412 in the first mode, and to direct a second optical signal received from a network at the second port 1412 to the input of the first optical switching device 1402 in the second mode.

The first optical routing apparatus 1406 in the example shown comprises a first optical switch 1414 and a first optical coupler 1416. The switch 1414 connects the first port 1408 to a first input of the coupler 1416 in the first mode as shown. The output of the coupler 1416 is provided to the input of an optical amplifier 1418 that may compensate for losses in any of the components in the first optical routing apparatus 1406 and/or any other components. The output of the optical amplifier 1418 is provided to the input of the first optical switching device 1402. Thus, the first optical switch 1414 is configured to direct the first optical signal received from the optical network at the first port 1408 to a first input of the optical coupler 1416 in the first mode.

The second optical routing apparatus 1410 comprises a second optical switch 1420 that connects the output of the first optical switching device 1402 to an optical circulator 1422 in the first mode as shown. The circulator 1422 provides the signal from the switch 1420 to the second port 1412. Thus, the second optical switch 1420 is configured to direct the optical signal output from the first optical switching device 1402 to the second port 1412 in the first mode, and the optical circulator is configured to direct the optical signal output from the first optical switching device to the second port in the first mode.

The node 1400 includes a third optical routing apparatus 1430 configured to direct a fourth optical signal received from an optical network at a fourth port 1432 to an input of the second optical switching device 1404 in the first mode, and to direct an optical signal output from the second optical switching device 1404 to the fourth port 1432 in the second mode. The node 1400 also includes a fourth optical routing apparatus 1434 configured to direct the output signal from the second optical switching device 1404 to a third port 1436 in the first mode, and to direct the third optical signal received from a network at the third port 1436 to the input of the second optical switching device 1404 in the second mode.

The third optical routing apparatus 1430 comprises a second optical circulator 1438 and a second optical coupler 1440, such that a signal received at the fourth port 1432 is passed to a first input of the coupler 1440. Thus, the second optical circulator 1438 is configured to direct the fourth optical signal received from the optical network at the fourth port 1432 to a first input of the second optical coupler 1440 in the first mode. The output of the coupler 1440 is provided to the input of the second optical switching device 1404.

The fourth optical routing apparatus 1434 comprises a third optical switch 1442 and a fourth optical switch 1444, and an optical amplifier 1446 that compensates for any losses in the fourth optical routing apparatus 1434 and/or any other components. The output of the second optical switching device 1404 is provided to the optical amplifier 1446. The third optical switch 1442 and fourth optical switch 1444 are configured to connect the output of the optical amplifier 1446 to the third port 1436 in the first mode. Thus, the third optical switch 1442 is configured to direct the optical signal output from the second optical switching device 1404 to the fourth optical switch 1444 in the first mode, and the fourth optical switch 1444 is configured to direct the optical signal output from the first optical switching device 1402 to the third port 1436 in the first mode.

Thus, in the first mode, a signal received at the first port 1408 is passed through the first optical switching device 1402 and to the second port 1412, and a signal received at the fourth port 1432 is passed through the second optical switching device and to the third port 1436. In some embodiments, the first optical switching device 1402 extracts signals 1450 from a signal received at the first port 1408 in the first mode and passes the remainder to the second port 1412, and thus in some examples the first optical switching device 1402 may be a drop multiplexer. In some embodiments, the second optical switching device 1404 adds signals 1452 to a signal received at the fourth port 1432 in the first mode and passes the aggregated signals to the third port 1436, and thus in some examples the first optical switching device 1402 may be an add multiplexer. The node is configured to transition from the first mode to the second mode upon determining that the first optical signal is not being received at the first port 1408 or on detection of a third optical signal received at the third port 1436. Thus, the node includes a first monitor 1454 to determine if a signal is being received at the first port 1408, and a second monitor 1456 to determine if a signal is being received at the third port 1436. In some examples, each monitor includes a beam splitter to ensure that any signals received at the opposite port (e.g. 1414 or 1432) are not detected. For example, a beam splitter may split a signal received at the first port 1408 between the first monitor 1454 and the first optical routing apparatus 1406. Thus any signal that is received at the second port 1412 and passed through the first optical switching device 1402 to the first port in the second mode 1408 is not detected by the first monitor 1454.

In the second mode, the direction of the signals is reversed as illustrated in Figure 15, which shows the node 1400 in the second mode. In the second mode, the switches 1414, 1420, 1442 and 1444 have switched. Thus, the switches 1414 and 1420 connect the output of the first optical switching device 1402 to the first port 1408. The switch 1442 connects the output of optical amplifier 1446 to the circulator 1438 and hence to the fourth port 1432. The switch 1444 connects the third port 1436 to the second input of coupler 1440. In addition, the circulator 1422 passes any signal received at the second port 1422 to the second input of coupler 1416. Hence, for example, the first optical switch 1414 is configured to direct the optical signal output from the first optical switching device 1420 to the first port 1408 in the second mode, and the first optical coupler 1416 is configured to receive, at a second input, the second optical signal received from the network at the second port 1412 in the second mode. The second optical switch 1420 is configured to direct the optical signal output from the first optical switching device 1402 to the first port 1408 in the second mode, and the optical circulator 1422 is configured to direct the second optical signal received from the network at the second port 1412 to the input of the first optical switching device 1402 in the second mode. The second optical circulator 1438 is configured to direct the optical signal output from the second optical switching device 1404 to the fourth port 1432cvin the second mode, and the second optical coupler 1440 is configured to receive, at a second input, the third optical signal received from the network at the third port 1436 in the second mode and to provide an output from the second optical coupler 1440 to the input of the second optical switching device 1404. The third switch 1442 is configured to direct the optical signal output from the second optical switching device 1404 to the fourth port 1432 in the second mode, and the fourth switch 1444 is configured to direct the third optical signal received from the network at the third port 1436 to the input of the first optical switching device 1404 in the second mode.

In the example node 1400, the specific configuration of the optical routing apparatus 1406, 1410, 1430 and 1434 are merely examples and these apparatus may have any suitable configuration. In an alternative configuration, for example, the switch 1444 may be replaced by a circulator.

In the configuration shown, the node 1400 may be able to switch from the first mode and the second mode automatically, and may also be able to switch from the second mode to the first mode automatically. However, in some embodiments where the switch from the second mode to the first mode is performed manually in response to an external trigger, the node 1400 may include apparatus to simulate a fiber fault (e.g. so as to avoid interference within the node 1400 if signals are being received on multiple ports). For example, the node 1400 may include a switch or shutter between the first port 1408 and the switch 1414, and also a switch or shutter between the third port 1436 and the switch 1444.

Figure 16 is a flow chart of an example of a method 1600 in an optical network node such as for example a remote node that is remote from a hub node. The method 1600 comprises, in step 1602, determining that a first optical signal is being received from an optical network at a first port or a third optical signal is not being received from the optical network at a third port. Next, in step 1604, the method 1600 comprises directing the first optical signal to an input of a first optical switching device, and in step 1606, directing an output signal from the first optical switching device to a second port. Step 1608 comprises determining that the first optical signal is not being received from an optical network at a first port or the third optical signal is being received from the optical network at the third port. Step 1610 comprises directing the optical signal output from the first optical switching device to the first port, and step 1612 comprises directing a second optical signal received from a network at the second port to the input of the first optical switching device

In some embodiments, the method 1600 comprises, in response to determining that the first optical signal is being received from the optical network at the first port or the third optical signal is not being received from the optical network at the third port, directing a fourth optical signal received from an optical network at a fourth port to an input of a second optical switching device, and directing an output signal from the second optical switching device to the third port. In some embodiments, the method 1600 comprises, in response to determining that the first optical signal is not being received from an optical network at the first port or the third optical signal is being received from the optical network at the third port, directing the optical signal output from the second optical switching device to the fourth port, and directing the third optical signal received from a network at the third port to the input of the second optical switching device.

Figure 17 is a flow chart of an example of a method in an optical network node such as for example a hub node. The method 1700 comprises, in step 1702, directing a first optical signal from a transmitter to an optical network from a first port. In step 1704, the method comprises, in response to determining that a third optical signal is being received at a third port, directing the first optical signal to the optical network from a second port. Step 1706 comprises directing a second optical signal received from the network at the third port to a receiver, and step 1708 comprises directing a third optical signal received from the network at a fourth port to the receiver.

In some embodiments, the remote nodes may be for example Radio Equipment (RE) and the hub node may be a baseband processing node.

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