GB2547662A - An optical network node - Google Patents

Abstract

An optical network node (320), such as an optical line terminal (OLT), comprising a plurality of interfaces (30, 32), each interface for exchanging optical signals between the optical network node and a different one of a plurality of passive optical networks (PONS). Each interface is capable of operating in a plurality of interface states and each interface comprises: a switch (220), a photodetector (224) a logic module (222). The switch (220) is for operating in a passing mode and a blocking mode, in which, in the passing mode, the interface is able to pass an optical signal to a one of the plurality of PONs, and in which, in the blocking mode, the interface is prevented from passing an optical signal to the one of the plurality of PONs. The photodetector (224) is for detecting an optical signal arriving from the one of the plurality of PONs at the interface. The logic module (222) is for controlling, in a first interface state, the mode of operation of the switch (220) in response to the detection of an optical signal arriving at the interface from the one of the plurality of PONs, so that the switch can be set to the blocking mode or the passing mode and so that, in the first interface state, the blocking mode is selected for the switch when an optical signal from the one of the plurality of PONs is detected at the interface; and so that, in the first interface state, the passing mode is selected for the switch otherwise. The optical network node thus provides protection switching in itself.

Description

An Optical Network Node

Field of the Invention

The present invention relates to passive optical networks and to an optical network node for passive optical networks.

Background US2014270770 describes a known Passive Optical Network (PON) in which Optical Line Terminals (OLT) are optically connected via a fibre cable to an Optical Splitter/Combiner (OSC). From the OSC, fibre optic cables connect to a plurality of Optical Network Units (ONUs).

Figure 1 shows a block diagram of a network 90 according to the prior art. The network 90 includes a PON 100, which includes passive optical elements. Cabling 105a and 105b provides a connection between a core network (not shown) and OLTs 110a, 110b, and PON 100. The PON 100 contains a main OLT 110a and a backup, OLT 110b. Additional elements of the PON 100 are the fibre optic cables 120a and 120b and 150a-c, which serve as interconnections between the PON 100 elements. The PON 100 also includes an OSC 140 and ONUs 190a-c. Main OLT 110a is connected via cabling 120a to the OSC 140. Second OLT 110b, is likewise connected via cabling 120b to the OSC 140. The OSC 140 is, in turn, connected via cabling 150a-c to the respective ONUs 190a-c. During normal operation, i.e. in the absence of a fault that affects communication between the main OLT 110a and OSC 140, main OLT 110a is in active state, e.g., the main OLT 110a is transmitting data. During normal operation, the main OLT 110a sends data to the ONUs 190a-c. During normal operation, the second OLT 110b is operating in standby state, e.g., the second OLT 110b is monitoring signals directed towards the core network but not transmitting data.

During abnormal conditions, for example in the presence of a cable fault (indicated by X in Figure 1) between the main OLT 110a and OSC 140, main OLT 110a can neither transmit data to nor receive data from ONUs 190a-c.

In the network of Figure 1, it is the Line Terminals at the OLTs 115a and 115b that control protection switching. Where status messages have stopped being received at Ethernet Switch 160 from main OLT, Ethernet Switch 160 asserts a fault condition for main OLT and switches traffic (both directions) from the affected main OLT over to the protection route via protection OLT. Where signal is lost in the OLT PON ports, they change from passing mode to blocking mode and from blocking mode to passing mode respectively, thus controlling the transmission of CCMs messages to and from the switch 160. Based on the port the switch is receiving these COM messages, it will choose the port through which it will send all the traffic for that particular Ethernet Connection. “GPON SFP Transceiver with PIC based Mode-Coupled Receiver”; Nesset, Farrow, Parkin (British Telecommunications) and Piehler (NeoPhotonics); 2012 ECOC Technical Digest; Optical Society of America, describes a mode-coupled-receiver (MCR) based transceiver for GPON. The paper describes implementation of the MCR using a photonic integrated circuit (PIC).

Known protected access systems are wasteful, requiring two ports for each PON with half of the ports normally not actively used but in standby, i.e. in an inactive state. There is a need to reduce the total number of ports required to provide adequate protection.

Summary of the Invention

According to a first aspect of the invention, there is provided an optical network node OLT (320) comprising a plurality of interfaces (30, 32), each interface for exchanging optical signals between the optical network node and a different one of a plurality of passive optical networks PONS (400). Each interface is capable of operating in a plurality of interface states and each interface comprises: a switch (220), a photodetector (224) and a logic module (222). The switch (220) is for operating in a passing mode and a blocking mode, in which, in the passing mode, the interface is able to pass an optical signal to a one of the plurality of PONs, and in which, in the blocking mode, the interface is prevented from passing an optical signal to the one of the plurality of PONs. The photodetector (224) is for detecting an optical signal arriving from the one of the plurality of PONs at the interface. The logic module (222) is for controlling, in a first interface state, the mode of operation of the switch (220) in response to the detection of an optical signal arriving at the interface from the one of the plurality of PONs, so that the switch can be set to the blocking mode or the passing mode and so that, in the first interface state, the blocking mode is selected for the switch when an optical signal from the one of the plurality of PONs is detected at the interface; and so that, in the first interface state, the passing mode is selected for the switch otherwise.

According to an embodiment, in the passing mode, the interface is able to receive an optical signal from a one of the plurality of PONs, and in the blocking mode, the interface is prevented from receiving an optical signal from the one of the plurality of PONs.

According to a further embodiment, the optical network node comprises a logic module for controlling, in a second interface state, the mode of operation of the switch so that, in response to the detection of an optical signal arriving at the interface from the one of the plurality of PONs in the second interface state, the passing mode is selected for the switch; and so that the blocking mode is selected for the switch otherwise.

According to a further embodiment, an interface changes from the first interface state to the second interface state when the passing mode is selected for the interface switch in the first interface state.

According to a further embodiment, the states of each one of the plurality of interfaces are independently changeable in response to the detection of an optical signal arriving at each interface from a different one of the plurality of PONs.

According to a further embodiment, the detected optical signal is intermittent and comprises periods of loss of signal in which periods of loss of signal of less than a maximum duration do not trigger the selection of the passing mode.

The present invention accordingly provides, in a second aspect, an optical network comprising the first optical network node (320) as set out, above, and a plurality of passive optical networks PONS (400); in which the first optical network node is connected by a plurality of optical fibres (210a, 210b) to the plurality of PONs; in which each of the plurality of optical fibres connects a different one of the plurality of interfaces (218, 220, 224) of the first optical network node with a different one of the plurality of PONs.

According to an embodiment, the plurality of PONs comprises at least one second optical network node (310) comprising a plurality of further interfaces, with one of the plurality of further interfaces for each one of the plurality of PONs, in which each PON is connected by optical fibre to a different one of the plurality of further interfaces.

The present invention accordingly provides, in a third aspect, a method of operating a optical network node (320) for an optical network, in which the optical network node comprises: a plurality of interfaces (30, 32); in which each interface comprises: a switch (220) for switching between a passing mode and a blocking mode, in which, in the passing mode, the interface is able to pass an optical signal to a one of a plurality of PONs, and in which, in the blocking mode, the interface is prevented from passing an optical signal to the one of the plurality of PONs; in which the interface can occupy a first interface state and a second interface state; and in which the method comprises in the first interface state: detecting an optical signal from one of the plurality of PONs arriving at one of the plurality of interfaces and when the optical signal from one of the plurality of PONS is determined to fall below a detection threshold; changing the switch to the passing mode.

According to an embodiment, the method comprises in the second interface state: detecting an optical signal arriving at the interface from the one of the plurality of PONs and when the optical signal from one of the plurality of PONS is determined to fall below a detection threshold, changing the switch comprised in the interface to the blocking mode.

The present invention accordingly provides, in a third aspect, a computer program element comprising computer program code to, when loaded into a computer system and executed thereon, cause the computer to perform the steps of the method set out above.

Brief Description of the Figures

In order that the present invention may be better understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a block diagram of a known system

Figures 2, 3, 4, 5, 6, 9, 10, 11, 12 and 13 show in block diagram form aspects of systems suitable for the operation of embodiments of the present invention;

Figures 7 shows state transitions according to embodiments of the present invention;

Figures 8 shows interface states according to embodiments of the present invention.

Detailed Description of Embodiments

According to an embodiment, rather than having an OLT port dedicated to serving a single protection fibre for a single PON, a modified OLT is provided, so that the same OLT protection port is now able to serve protection fibres from multiple PONs: thus significantly reducing the OLT estate and related costs. A two-way (1:2) multiplexed PON port 200 according to an embodiment is shown, by way of example, in Figure 2. Multiplexed PON port 200 connects to multiple PONs. In the example shown, optical fibres 210 each connect to a different PON (as described, below, with reference to Figure 6). Multiplexed PON port 200 comprises optical transmitter 212 and optical receiver 214 each providing conversion between signals in optical form, to the left-hand side of the Figure, and signals in electrical form to the right-hand side. Multiplexed PON port 200 provides a plurality of optical interfaces (30, 32) with one optical interface for each of multiple optical fibres 210 impinging on port 200. Transmitter 212 and receiver 214 are connected, on the optical side, to an optical splitter and combiner (OSC) 216, which acts to combine optical signal received from multiple optical fibres 210 onto a single input to receiver 214 (typically, a photodiode) and to split a single optical output from transmitter 212 (typically, a laser diode) to the multiple optical fibres 210. OSC 216 comprises a plurality of optical diplexers (optical filters, combining two optical signals into a single output) 218, one per optical fibre 210. Each diplexer218 acts to pass to receiver 214, optical signals received from a PON over fibre 210. The diplexer also distributes a single beam to multiple optical fibres 210 (i.e. in the other direction) and, thereby, communicates to specific PONs, optical signals received from transmitter 212.

However, instead of connecting directly to each optical fibre 210, each diplexer 218 is connected to an optical fibre via an optical switch (SW) 220, which may comprise an optical amplifier, such as a silicon optical amplifier. Each optical switch 220 can exist in a passing mode or a blocking mode. In the passing mode, an optical signal issuing from the transmitter 212 is passed to the optical fibre 210 and an optical signal issuing from at least one of the ONUs is passed on to the receiver 214. In the blocking mode, an optical signal issuing from the transmitter is prevented from passing to the optical fibre 210. In the configuration shown in Figure 2, an optical signal issuing from any ONUs is prevented from passing on to the receiver 214 (although other configurations may be used, as shown in Figure 5). Each optical of switches 220 is controlled by logic, for example a state machine executing on a logic module 222, which may comprise hardwired logic, such as a programmed logic array integrated circuit or may comprise a processor executing software code stored in memory. Logic module 222 may comprise a single multi-tasking unit for controlling each interface of an OLT or may be implemented as a separate module for each interface. According to an embodiment, logic module 222 maintains a distinct state machine for each switch. As shown in Figure 7, the state machine for a particular switch determines if a switch is in passing or blocking mode. Each of optical fibres 210 is also optically coupled, on the PON side of switch 220, with a photodector (PD) 224, which provides to logic module 222 an indication when an optical signal is detected on the relevant optical fibre. In a standby state, optical switches 220 are in the blocking mode - so that, when in standby, the photodector 224 will only be able to detect optical signals originating from the PON.

With a multiplexed PON port configuration, multiple combinations of working and protection operation are possible for a single port 200, as shown in Figure 3. Figure 3 shows multiple combinations of working and protection operation for the case of a 1:2 multiplexed PON port 200, as shown by way of example in Figure 2, however, similar plural combinations are possible for the case of 1:3, 1:4 and other configurations of multiplexed PON port. As shown at (a) in Figure 3, both interfaces 30, 32 of the same multiplexed PON port 200 may be working, with both switches 220 in the passing mode, where by “working” is meant operative to exchange optical signals with a PON. Alternatively, as shown at (b) and (c), the multiplexed PON port may be partially working, with one interface working (32 in (b) and 30 in (c)) and the other in standby state (30 in (b) and 32 in (c)), where “standby” indicates that the switch is in the blocking mode so that the interface is not being used to exchange optical signals with a PON but is being held in reserve in case it is needed to provide service to the PON. With a partially working multiplexed PON port, one switch will be in the passing mode and the other in the blocking mode. Lastly, as shown at (d) in Figure 3, both interfaces 30, 32 may be in standby, with both switches in the blocking mode.

The multiplexed PON port 200 is not limited to the 1:2 model shown in Figure 2 but may server any number of optical fibres 210, including, but not limited to 1:3, 1:4, 1;5, 1:6, 1:12, 1:16. Figure 4 shows details of a four-way (1:4) multiplexed PON port 200 (i.e. excluding logic module 222). As shown in Figure 4, for example, multiplexed PON port 200 may comprise four optical diplexers 218 instead of the two shown in Figure 2 and optical splitter and combiner (OSC) 216 be designed with four channels instead of the two shown in Figure 2.

Figure 5 shows a further embodiment according to which, switches 220 only act on signals from transmitter 212 for sending to the PONs: any signals received at multiplexed PON port 200 from a PON are not influenced by the switches and are passed to optical receiver 214. As shown in Figure 5, switches 220 are located on OSC 216, i.e. between optical diplexers 218 and transmitter 212. The photodiodes 224 of Figure 5 could be eliminated if the Rx 214 can distinguish the fibre 210 from which it is receiving signals.

An optical network incorporating the above main and protection OLTs is shown in Figure 6. Figure 6 shows a plurality of PONs 400, in which each PON 400 comprises an OSC 302 and one or more ONU 300. Each ONU may be connected to an item of CPE (not shown) over an Ethernet or other type of connection. Each OLT 310, 320 is connected at the core network side via Ethernet connections (Backhaul fibre) 420, 422 to Ethernet switch 610. Ethernet switch 610 comprises separate ports (612, 614) for exchanging signals according to Ethernet protocol with each of main OLT 310 and protection OLT 320. The two ports connecting the Ethernet switch 610 to the two OLTs are associated in a 1:1 subnetwork connection protection (SNCP) switching using ITU-T G.8031/Y.1342 “Ethernet linear protection switching”. Continuity Check Messages (CCMs) will flow over the Ethernet connections between the ONUs and Ethernet switch 610. Each CCM comprises a maintenance association end point (MEP) per connection.

The multiplexing technique employed in the downstream direction (from OLT to ONU) is typically TDM (the ONU will parse the PON frames only if addressed to one of its UNIs). In the upstream direction, the multiplexing technique employed is typically TDMA, with coordination and synchronisation between ONUs transmissions being handled by a MAC device sitting in the OLT or elsewhere. Upstream transmissions are enabled by using burst mode transmitters in the ONUs and burst mode receivers in the PON line cards. Downstream and upstream transmissions are multiplexed in the fibre using WDM.

Figure 6 shows two OLTs - main OLT 310 and protection OLT 320. Main OLT is conventional with a different OLT port for each connected PON. Protection OLT is different in design from main OLT and has a single, multiplexed port (comprising a plurality of interfaces) for all connected PONs. Typical designs for the interfaces of port 200 are shown in Figures 2, 3, 4 and 5.

In dual parented type B PONs, as shown at 400 in Figure 6, a 2:N OSC 302 is used, which has two input/output ports on the OLT side, from which two feeder fibres 210a, 210b connect the OSC to main OLT 310 and second OLT 320, respectively. Each port 20 on main OLT 310 is connected via cabling 210a to a different OSC 302. Each interface of port 200 on second OLT 320, is likewise connected via cabling 210b to a different OSC 302. The OSC 302 is, in turn, connected via cabling to respective ONUs 300.

Optical fibres 21 Oa make up a set of working links and optical fibres 21 Ob make up a set of protection (or fail-back) links. It follows from Figure 6 that main OLT 310 is used for normal working, while protection OLT 320 is used for protection, i.e. taking over operation when a fault is detected that puts main fibre 210a, port 20 or OLT 310 out of service.

Ethernet switch 610 comprises separate ports (612, 614) for exchanging signals according to Ethernet protocol with each of main OLT and protection OLT.

According to an embodiment, details (i.e. serial number, service profiles, etc.) of all ONUs of a PON are pre-registered and configured in both OLTs 310 and 320 by an element manager (i.e. a management function that allows network operators to configure nodes of the network - not shown).The PON ports 20 in main OLT 310 are initialised and configured in the “working” state and the interfaces of PON port 200 in protection OLT 320 (i.e. switches 220) are initialised and configured in the “Protection” state.

In operation, optical fibre 210b connection to one of PONs 400, which we shall refer to as PON 401 is monitored. Where optical signals are not detected from the PON 401 (i.e. “upstream” transmission) the logic module 222 is notified, which, triggers operation of the optical switch 220 on the same interface of port 200 to enable “downstream” transmission to the PON 401. According to an embodiment, the optical fibre is continually monitored. According to an embodiment, detection of an optical signal on the optical fibre requires the signal to be above the receiver threshold.

In normal operation, a medium access control logic in main OLT 310 periodically instructs ONUs on a PON to send a status report or other information (e.g. information on traffic queues) upstream. When the protection OLT port 200 does not detect upstream transmission on one of the PONs 400 (say PON 401), this is taken as an indication of a fault with the connection of this PON 401 via main OLT 310 and this prompts the interface for PON 401 on the protection OLT port 200 to become active, i.e. change to passing mode the switch on that interface and take control of ONUs on the PON 401. The switches on the interfaces for the other PONs 400, except PON 401, remain in blocking mode while the other PONs 400 remain working with main OLT 310.

Each interface of port 200 on protection OLT 320 may occupy a number of states, as shown in Figure 7. Figure 7 shows a state machine running on logic module 222 for an interface on multiplex port 200. Initialisation is the initial state after power-up and may be selected by intervention from the management system (element manager), where necessary. This state is exited under control of the management system. Standby is a state in which the port 200 interface monitors the PON for received optical signals while keeping its switch 220 in blocking mode. When entering this state, the logic module 222 starts a timer Thold, and port 200 interface is not allowed to change to the working state until the timer has expired. Where loss of signal (LOS) is detected (i.e. insufficient optical power from a PON, is received at a port 200 interface on protection OLT 320), the logic module 222 starts timer Tfail. Where timer Tfail expires and timer Thold has already expired, the logic module 222 will change the interface to the working state. When entering the working state, the logic module 222 starts the timers Thold and Tract. The port 200 interface changes switch 220 to the passing mode and assumes control of the ONUs in the PON. The port 200 interface can only enter the working state from the protection state, and is not allowed to change back to the working state until the timer Thold expires. Where the timer Tract expires without the port 200 interface being able to take control of at least one ONU, the port 200 interface will change from the working state to the initialisation state and an alarm will be raised. Otherwise, after timer Thold has expired, the port 200 interface will change back to protection state when LOS has been detected. By way of example, the OLT can detect when an ONU is under its control, when a response to an instruction sent by the OLT is received at the OLT from the ONU. The port 200 interface can always be brought to initialisation state from the working state through a reset from the management system, e.g. even if timer Thold has not yet expired.

Figure 8 provides more detailed information on the operation of the timers referred to above, according to an embodiment of the invention. The timers that are used in a state are only meaningful while the port 200 interface stays in that state. Once the interface changes to a different state, the timers are cleared.

Figures 9 and 10 show details of the operation of the optical network of Figure 6 (shown simplified with only PON 401, which is one of the PONs 400 of Figure 6). PON 401 connects to both main OLT and protection OLT, with main OLT being the default working OLT and protection OLT being the protection OLT. Figure 9 shows operation in absence of a fault, with traffic being exchanged between the MEPs via main OLT 310. In the absence of a fault, protection OLT 320 listens to upstream optical power from all ONUs 300 on PON 401 including optical power associated with CCMs generated by ONUs 300 on PON 401. The path through protection OLT 320 is interrupted (the switch 220 is in blocking mode) as long as optical signals are detected from the PON 401. As a result, CCMs received at protection OLT 320 are not forwarded to the Ethernet switch 610. Figure 10 shows the same simplified details as Figure 9 from the optical network of Figure 6. Figures 10 and 11 shows operation in presence of a fault. When a fault occurs on PON 401 for example in a fibre running between OSC 302 and main OLT 310, the ONUs 300 on PON 401 will stop sending CCMs to the network. Protection OLT 320 will detect the LOS - i.e. the absence of optical signal from the PON 410. The LOS condition is detected by protection OLT 320 when photodetector 224 does not detect any or sufficient optical power for a set time period (Tfail), for example for the duration of 4 consecutive PON frames or 0.5ms. On detecting LOS, protection OLT 320 enables switch 220 on the relevant optical interface, i.e. the interface connected to PON 401, and changes that interface to the “working” state where the switch 220 is in the passing mode. The relevant optical interface on protection OLT 320 becomes active, taking over control of all ONUs in the PON 401 and restoring the traffic flow on the protection route. The flow of CCMs resumes over the protection route back to the Ethernet switch 610. When the Ethernet switch detects loss of CCM being received on the working port 612, it raises the event signal fail for that port. When the Ethernet switch starts receiving CCMs on the protection port 614, it will change the condition of that port from Fail to OK and will start transmitting and receiving through that port, thus re-establishing the Ethernet connection with the ONUs through OLT 320.

Figures 11 and 12 show details of the operation of the optical network of Figure 6 (shown simplified with only two PONs 401 and 402, which are two of the PONs 400 of Figure 6). PONs 401 and 402 share a single port 200 on OLT 320. As shown, port 200 comprises two interfaces 30, 32. In Figures 11 and 12, the interaction between two PONs is shown. PON 401, as before, comprises OSC 302, and fibres 210a and 210b. PON 402 comprises OSC 304, and fibres 211a and 211b. We now describe operation in the absence of a fault with reference to Figure 11. For PON 401, in the absence of a fault OLT 310 is in the working state (ON) and OLT 320 (interface 30) is in the standby state (OFF). For PON 402, in the absence of a fault OLT 312 (interface 32) is in the working state and a third OLT (not shown but connected to fibre 211b) is in the standby state. We now describe operation in the presence of a fault with reference to Figure 12. When a fault occurs on PON 401, for example in a fibre running between OSC 302 and main OLT 310, OLT 320 (interface 30) takes over in the working state (ON). Hence, OLT 320 is now in the working state for both PONs 401 and 402. In the presence of a fault on PON 401, the single port 200 on OLT 320 acts as the working port for both PONs.

Figure 13 is a block diagram of a computer system suitable for the operation of embodiments of the present invention. In particular, the computer system of Figure 13 is suitable for implementing the or each logic module 222. A central processor unit (CPU) 910 is communicatively connected via a data bus 920 to a memory 912, storage 914, user interface 916 and communications interface 918 and/or other components found in electronic computing devices. The storage 914 can be any read/write storage device such as a random access memory (RAM) or a non-volatile storage device. An example of a non-volatile storage device includes a disk or tape storage device. Computer system 90 may also include input/output devices (not shown) such as a keyboard, a pointing device and a display communicating with processor 910 via user interface module 916. Computer system 90 may also, via communications interface module 918 (which may comprise a plurality of network interfaces), be connected to or have the capability for connection to one or more communications network, such as an optical or electrical wired, wireless or hybrid connection.

We have described a N:1 OLT configuration, where a modified PON port according to this invention can protect N working PON ports, thus reducing the number of ports required to provide adequate protection. The proposed system thus reduces the overall cost of providing protection in a PON system. A plurality of interfaces forms a multiple-input optical port on the optical network node. According to an embodiment, the plurality of interfaces comprises a modified MCR.

Insofar as embodiments of the invention described are implementable, at least in part, using a software-controlled programmable processing device, such as a microprocessor, digital signal processor or other processing device, data processing apparatus or system, it will be appreciated that a computer program for configuring a programmable device, apparatus or system to implement the foregoing described methods is envisaged as an aspect of the present invention. The computer program may be embodied as source code or undergo compilation for implementation on a processing device, apparatus or system or may be embodied as object code, for example.

Suitably, the computer program is stored on a carrier medium in machine or device readable form, for example in solid-state memory, magnetic memory such as disk or tape, optically or magneto-optically readable memory such as compact disk or digital versatile disk etc., and the processing device utilises the program or a part thereof to configure it for operation. The computer program may be supplied from a remote source embodied in a communications medium such as an electronic signal, radio frequency carrier wave or optical carrier wave. Such carrier media are also envisaged as aspects of the present invention.

It will be understood by those skilled in the art that, although the present invention has been described in relation to the above described example embodiments, the invention is not limited thereto and that there are many possible variations and modifications which fall within the scope of the invention. According to an embodiment multiplexed PON port 200 may be implemented as a modified mode-coupled-receiver (MCR). An MCR includes an optical power splitter after the transmitter and an almost loss-less optical power combiner before the receiver. This configuration is equivalent to having a PON splitting stage at the transceiver to which a number of fibres are connected. The MAC running the PON to which such transceiver is connected makes no distinction among the fibres it is connected to, i.e. transmits to all and receives from all without distinction.

The scope of the present invention includes any novel features or combination of features disclosed herein. The applicant hereby gives notice that new claims may be formulated to such features or combination of features during prosecution of this application or of any such further applications derived therefrom. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the claims.

Claims (15)

1. An optical network node comprising: a plurality of interfaces, each interface for exchanging optical signals between the optical network node and a different one of a plurality of passive optical networks (PONS); in which each interface is capable of operating in a plurality of interface states; in which each interface comprises: a switch for operating in a passing mode and a blocking mode, in which, in the passing mode, the interface is able to pass an optical signal to a one of the plurality of PONs, and in which, in the blocking mode, the interface is prevented from passing an optical signal to the one of the plurality of PONs; a photodetector for detecting an optical signal arriving from the one of the plurality of PONs at the interface; and in which the optical network node comprises logic for controlling, in a first interface state, the mode of operation of the switch in response to the detection of an optical signal arriving at the interface from the one of the plurality of PONs, so that the switch can be set to the blocking mode or the passing mode and so that, in the first interface state, the blocking mode is selected for the switch when an optical signal from the one of the plurality of PONs is detected at the interface; and so that, in the first interface state, the passing mode is selected for the switch otherwise.

2. The optical network node of claim 1, in which in the passing mode, an interface of the plurality of interfaces is able to receive an optical signal from a one of the plurality of PONs, and in which, in the blocking mode, the interface is prevented from receiving an optical signal from the one of the plurality of PONs.

3. The optical network node of any of claims 1 to 2 comprising logic for controlling, in a second interface state, the mode of operation of the switch so that, in response to the detection of an optical signal arriving at the interface from the one of the plurality of PONs in the second interface state, the passing mode is selected for the switch; and so that, in the second interface state, the blocking mode is selected for the switch otherwise.

4. The optical network node of claim 3, in which an interface changes from the first interface state to the second interface state when the passing mode is selected for the interface switch in the first interface state.

5. The optical network node of any of claims 1 to 4 in which the states of each one of the plurality of interfaces are independently changeable in response to the detection of an optical signal arriving at each interface from a different one of the plurality of PONs.

6. The optical network node of any of claims 1 to 5 in which the plurality of interfaces forms a multiple-input optical port on the optical network node.

7. The optical network node of any of claims 1 to 5 in which the plurality of interfaces comprises a modified MCR.

8. The optical network node of any of claims 1 to 7, in which the detected optical signal is intermittent and comprises periods of loss of signal in which periods of loss of signal of less than a predetermined duration do not trigger the selection of the passing mode.

9. The optical network node of claim 8, in which a period of loss of signal corresponds to a period during which the signal falls below a detection threshold.

10. The optical network node of any of claims 1 to 9, in which each switch is an optical amplifier.

11. An optical network comprising the first optical network node of any of claims 1 to 10 and a plurality of passive optical networks (PONS); in which the first optical network node is connected by a plurality of optical fibres to the plurality of PONs; in which each of the plurality of optical fibres connects a different one of the plurality of interfaces of the first optical network node with a different one of the plurality of PONs.

12. The optical network of claim 11, in which the plurality of PONs comprises at least one second optical network node comprising a plurality of further interfaces, with one of the plurality of further interfaces for each one of the plurality of PONs, in which each PON is connected by optical fibre to a different one of the plurality of further interfaces.

13. A method of operating a optical network node for an optical network, in which the optical network node comprises: a plurality of interfaces; in which each interface comprises: a switch for switching between a passing mode and a blocking mode, in which, in the passing mode, the interface is able to pass an optical signal to a one of a plurality of PONs, and in which, in the blocking mode, the interface is prevented from passing an optical signal to the one of the plurality of PONs; in which the interface can occupy a first interface state and a second interface state; and in which the method comprises in the first interface state: detecting an optical signal from one of the plurality of PONs arriving at one of the plurality of interfaces and when the optical signal from one of the plurality of PONS is determined to fall below a detection threshold; changing the switch to the passing mode.

14. The method of claim 13, comprising in the second interface state: detecting an optical signal arriving at the interface from the one of the plurality of PONs and when the optical signal from one of the plurality of PONS is determined to fall below a detection threshold, changing the switch comprised in the interface to the blocking mode.

15. A computer program element comprising computer program code to, when loaded into a computer system and executed thereon, cause the computer to perform the steps of a method as claimed any of in claims 13 to 14.