Optical access network

An optical access network comprises L wavelength division multiplexed access sub-networks. Each of the wavelength division multiplexed access sub-networks is arranged to use a set of wavelength channels. M optical line termination apparatus, each receive traffic from a respective operator network and output traffic on the wavelength channels. A wavelength routing apparatus comprises M sets of first ports and L second ports. Each set of first ports connects to a respective one of the optical line termination apparatus and each second port connects to an optical link of a respective one of the wavelength division multiplexed access sub-networks. The wavelength routing apparatus is arranged to route the set of wavelength channels between the sets of first ports and the second ports and to route different wavelength channels of the same wavelength to different ones of the second ports.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National stage of International Application No. PCT/EP2010/055723, filed Apr. 28, 2010, which is hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to wavelength division multiplexed (WDM) optical access networks, such as WDM passive optical networks (WDM-PONs).

BACKGROUND

Communications traffic at network edges is increasing over time due to the rising demand for a range of high-bandwidth services by business and residential customers. This rising demand places an increasing requirement on access networks to deliver those services.

One type of access network is a Passive Optical Network (PON). A PON typically has a central office (CO) at which apparatus called an Optical Line Termination (OLT) interfaces with a metro or carrier network. An arrangement of optical fibres and splitters connect the OLT with multiple Optical Network Termination units (ONTs). An ONT can be located at a subscriber premises in a Fibre To The Home (FTTH) system, or an ONT can be located at a roadside cabinet near to a group of premises in a Fibre To The Curb (FTTC) system. A PON is so-called because the optical transmission has no power requirements, or limited power requirements, once an optical signal is travelling through the network section connecting the ONT to the OLT.

Existing PONs are based on Asynchronous Transfer Mode Passive Optical Network (APON), Broadband PON (BPON), Gigagbit PON (GPON) and Ethernet PON (EPON) technologies as standardised by the International Telecommunications Union (ITU-T) and Institute of Electrical and Electronic Engineers (IEEE). Many of these PON technologies use some form of time division multiple access technique, with the capacity of a wavelength channel being shared in a time-divided manner across multiple ONTs.

More recently, Wavelength Division Multiplexed Passive Optical Networks (WDM PON) have been proposed. A WDM PON supports multiple wavelength channels. A separate wavelength can be allocated for communication between the Optical Network Unit (OLT) and each ONT in the PON.

In many cases an access network will already be deployed with an operator, called the incumbent operator, owning and operating the access network. In open markets, such as Europe, there is a regulatory requirement that a subscriber should be able to choose between a number of possible operators to provide their communications service. There is a problem of how to allow Other Local Operators (OLOs) to access the existing access network. This complicates the network equipment that must be provided, as an access network must be able to connect to one of a set of operator networks, as required by a subscriber.

SUMMARY

A first aspect of the present invention provides apparatus for use in an optical access network. The access network comprises L wavelength division multiplexed access sub-networks, where L≧2. Each of the wavelength division multiplexed access sub-networks is arranged to use a set of wavelength channels and has an optical link for carrying a signal comprising a multiplexed set of the wavelength channels. The apparatus comprises M optical line termination apparatus, where M≧1, each for receiving traffic from a respective operator network and for outputting traffic on the wavelength channels. The apparatus also comprises a wavelength routing apparatus comprising M sets of first ports and L second ports, each set of first ports for connecting to a respective one of the optical line termination apparatus and each second port for connecting to an optical link of a respective one of the wavelength division multiplexed access sub-networks. The wavelength routing apparatus is arranged to route the wavelength channels between the sets of first ports and the second ports and to route different wavelength channels of the same wavelength to different ones of the second ports.

An advantage of the apparatus is that it permits a full optical unbundling of the wavelengths used in multiple wavelength division multiplexed access sub-networks (e.g. WDM-PONS) to one, or multiple, operator networks. Different wavelength channels of the same wavelength can co-exist within the wavelength routing apparatus. This allows each of a plurality of wavelength division multiplexed access sub-networks to use a set of wavelength channels of the same wavelength, which has an advantage of allowing similar equipment to be installed in each wavelength division multiplexed access sub-network, thereby simplifying and reducing the overall cost of the equipment. The wavelength routing apparatus can route these wavelength channels to a respective optical line termination apparatus.

Advantageously, the wavelength routing apparatus is arranged to route different wavelength channels of the same wavelength received from one of the optical line termination apparatus to different ones of the second ports. This allows an optical line termination (OLT) apparatus associated with a particular operator network to serve subscribers in a plurality of different wavelength division multiplexed access sub-networks, even where the subscribers use the same wavelength for their respective wavelength channel. The wavelength routing apparatus ensures that multiple wavelength channels of the same wavelength do not collide.

Advantageously, the wavelength routing apparatus is operable in downstream and upstream directions. In the upstream direction the wavelength routing apparatus is arranged to route wavelength channels between the second ports and the sets of first ports. Advantageously, the wavelength routing apparatus is arranged to route different wavelength channels of the same wavelength received from different ones of the second ports to one of the sets of first ports. The wavelength channels used in the upstream direction can be at different wavelengths to the wavelength channels used in the downstream direction.

Advantageously, the wavelength routing apparatus comprises L splitter/combiners each connected to a respective one of the second ports. Each splitter/combiner has a plurality of third ports and is arranged to combine and output on the second port signals received on the plurality of third ports. The wavelength routing apparatus also comprises M wavelength routing devices each connected to a respective one of the sets of first ports and also has fourth ports. Each wavelength routing device is arranged to route wavelength channels between the set of first ports and the fourth ports in dependence upon a wavelength of the wavelength channel and on which port of the set of first ports the wavelength channel is received. The wavelength routing apparatus also comprises links arranged to connect the fourth ports of the wavelength routing devices to the third ports of the L splitter/combiners. Each wavelength routing device can separately route different wavelength channels of the same wavelength to different ones of the fourth ports, thereby allowing an optical line termination (OLT) apparatus associated with a particular operator network to serve subscribers in a plurality of different wavelength division multiplexed access sub-networks, even where the subscribers use the same wavelength for their respective wavelength channel. The wavelength routing device ensures that multiple wavelength channels of the same wavelength do not collide.

An advantage of the apparatus is that it is readily scalable as additional operators require connection to the access network, as a further wavelength routing device and a further optical line termination apparatus can be added to connect to a new operator network. A further advantage of the apparatus is that it allows the possibility for Central Office equipment of different operators to be installed at different locations. For example, the OLT of one operator can be located remote from the OLT of another operator.

Advantageously, the wavelength routing devices are cyclic arrayed waveguide gratings. The cyclic arrayed waveguide gratings can be N×N cyclic arrayed waveguide gratings each having a set of N first ports and a set of N fourth ports.

Another aspect of the invention provides a method of operating apparatus in an optical access network comprising L wavelength division multiplexed access sub-networks, where L≧2. Each of the wavelength division multiplexed access sub-networks is arranged to use a set of wavelength channels. Each wavelength division multiplexed access sub-network has an optical link for carrying a signal comprising a multiplexed set of the channels. The apparatus comprises M optical line termination apparatus, where M≧1. Each optical line termination apparatus is connected to a respective operator network. The method comprises receiving traffic on wavelength channels from one of the optical line termination apparatus. The method further comprises routing the wavelength channels to the wavelength division multiplexed access sub-networks such that different wavelength channels of the same wavelength are routed to different wavelength division multiplexed access sub-networks.

DETAILED DESCRIPTION

FIG. 1shows an optical access network5according to a first embodiment of the present invention. The optical access network5comprises a plurality of access sub-networks10,11,12. Each access sub-network inFIG. 1is a WDM-PON10,11,12. The optical access network5connects to multiple operator networks51,52,53. Each WDM-PON10,11,12has a tree-like structure of fibres14emanating from a trunk fibre16. Each WDM-PON10,11,12can be used as an access network to serve Optical Network Termination units (ONTs)13. ONTs can be deployed at individual subscriber premises or at curbside cabinets, depending on the type of PON architecture. In an implementation where ONTs are deployed at curbside cabinets, electrical cables connect the ONT to terminals at subscriber premises. Each WDM-PON10,11,12can serve residential customers, commercial customers, wireless infrastructure (e.g. wireless base stations or access points), or any mix of these.

The optical access network5comprises apparatus41,42,43,110for interconnecting the wavelength division multiplexed passive optical networks (WDM-PON)10,11,12and operator networks51,52,53. Apparatus41,42,43,110can be installed at a Central Office (CO)100and, for clarity,FIG. 1shows this arrangement. However, an advantage of embodiments of the invention is that the apparatus41,42,43,110can be distributed across different physical locations. The multiple wavelength division multiplexed passive optical networks (WDM-PON)10,11,12each connect with the CO100. Central Office100interfaces with metro or core communication networks51,52,53belonging to different operators. The operators are different telco providers who can compete to offer a communications service to subscribers served by the WDM-PONs10,11,12. CO100comprises apparatus for each operator who wishes to provide a communication service to any of the subscribers served by the WDM-PONs10,11,12. For each operator, there is an optical line termination unit (OLT)41,42,43.

In each WDM-PON10,11,12a set of wavelength channels, called lambdas λ, are allocated for communication between the Central Office100and ONTs13. In an advantageous scheme, a single lambda is allocated for communication between the Central Office100and a single ONT13. A set of wavelength channels are carried between the CO and a remote node12on a trunk fibre16, and then passively demultiplexed at a remote node15onto a set of fibres14. Each fibre14carries a single wavelength channel to an ONT13.

Network5supports communication in an upstream direction (i.e. from an ONT13towards an OLT41,42,43) and in a downstream direction (i.e. from an OLT41,42,43towards an ONT13). Bi-directional communication can be achieved in various ways, such as by the use of two wavelength channels to each ONT (i.e. one wavelength channel λDfor downstream communication and a different wavelength channel λUfor upstream communication) or by time-division multiplexed use of a single wavelength channel.

For clarity, in the following description the term “Central Office side”, or simply “CO side”, refers to the side of network apparatus nearest the operator networks51,52,53and the term “user side” refers to the side of network apparatus nearest the ONTs13.

Optical apparatus110, which will be called wavelength routing apparatus110, connects the WDM-PONs10,11,12and the OLTs41,42,43. The wavelength routing apparatus110works in the upstream and downstream directions. In the upstream direction, the wavelength routing apparatus110routes wavelengths between the WDM-PONs10,11,12and the OLTs41,42,43so that a particular wavelength channel is connected between a WDM-PON and an OLT of a required operator network. In the downstream direction, the wavelength routing apparatus110routes wavelengths between the OLTs41,42,43and the WDM-PONs10,11,12so that a particular wavelength channel is connected between an OLT of a required operator network and an ONT. The wavelength routing apparatus110can separately route multiple wavelength channels of the same wavelength. This has several applications. Firstly, it allows multiple OLTs41,42,43to use the same wavelength in different WDM-PONs. For example, OLT41can use a particular wavelength—say λ1—for a wavelength channel between the OLT41and WDM-PON10, OLT42can use λ1for a wavelength channel between the OLT42and WDM-PON11and OLT43can use λ1for a wavelength channel between the OLT43and WDM-PON12. Secondly, it allows a particular OLT to use the same wavelength for a wavelength channel serving each of a plurality of different WDM-PONs10,11,12. For example, a wavelength—say λ2—used for communication between OLT41and an ONT13in WDM-PON10can also be used for communication between OLT41and an ONT13in WDM-PON11and for communication between OLT41and an ONT13in WDM-PON12. Separation is maintained between multiple instances of the same wavelength within the OLTs and wavelength routing apparatus110, allowing each instance of the wavelength to carry different traffic.

FIGS. 1,3and4show an advantageous form of the wavelength routing apparatus110. Each trunk fibre16leading to a WDM-PON10,11,12connects to a respective splitter/combiner21,22,23. Splitter/combiner21comprises a port24on the user side of the device for connecting to trunk fibre16and a set of ports25on the CO side of the device21. In the upstream direction, splitter/combiner21functions as a splitter and replicates the set of signals (λ1-λN) received on fibre16onto each of the ports25on the CO side of the splitter. In the downstream direction, splitter/combiner21functions as a combiner and combines the set of signals received on ports25and transmits the combined signal on port24on the user side of the splitter. A wavelength routing device31,32,33connects to the user side of a respective OLT41,42,43. InFIGS. 1,3and4each wavelength routing device31,32,33is a cyclic arrayed waveguide grating (AWG)31,32,33. There is a set of ports34on the user side of each cyclic AWG and a set of ports35on the CO side of each cyclic AWG. The cyclic AWG can be an N port×N port device, or other sized devices can be used. Each CO side port35connects to a port on the user side of OLT41. Ports34on the user side of the cyclic AWG31connect, via fibres27, to ports25on the splitters21,22,23. The cyclic AWG31provides a wavelength multiplexing/demultiplexing function and a wavelength routing function. In the upstream direction, a set of wavelength signals (λ1-λN) received on a user side port34are demultiplexed onto the set of CO side ports35. In the downstream direction, a set of wavelength signals (λ1-λN) received on the set of CO side ports35are multiplexed and output on one of the user side ports34. Operation of the cyclic AWG is described in more detail later. A Cyclic AWG can carry a wavelength pair per port. One of the wavelengths in a wavelength pair is used for upstream communication and the other wavelength in a wavelength pair is used for downstream communication. The separation between the two wavelengths is fixed and does not depend on the considered port. Advantageously, the upstream and downstream wavelengths are located in different frequency bands. For example, the range 1530 nm-1560 nm can be used for downstream communication, and the range 1570 nm-1600 nm can be used for upstream communication. In this example, the offset between a downstream wavelength and the corresponding upstream wavelength, on the same AWG port, is 40 nm. Table 1, shown later, can be considered to define one of the wavelengths in the wavelength pair. The other wavelength in the wavelength pair is found by adding a fixed wavelength offset to the value in Table 1. The wavelength routing apparatus shown inFIGS. 1,3and4is passive and static. The term “static” means that the wavelength routing relationship between input ports of the cyclic AWG and output ports of the cyclic AWG, and between input ports of the cyclic AWG and output ports of the combiner, is static, and does not change.

FIG. 2shows one of the optical line termination units (OLT)41ofFIG. 1in more detail. On one side, the OLT41comprises a set of ports35which connect to a cyclic AWG31. On the other side, the OLT41comprises an interface45to a metro or core network of that operator. InFIG. 2, each port35carries a wavelength channel λUused for upstream communication and a separate wavelength channel λDused for downstream communication. In the upstream direction, unit81forwards the wavelengths used for upstream communication from ports35to a receiver unit82. Each wavelength channel comprises an optical signal which is modulated in some way (e.g. by phase or amplitude/intensity modulation) with coded data. In one embodiment of the OLT41, each upstream wavelength channel is demodulated by receiver unit82. In the reverse direction, signals are received from interface45and an optical source84in transmitter unit83is modulated with the data signal to form a wavelength channel. A set of wavelength channels are combined by unit81and output from a port35of the OLT41. Optical sources84can be fixed wavelength sources which are connected to a required port of the transmitter unit83, such as by a manual or an automatic switching operation. More advantageously, optical sources84are tunable devices which can be caused to operate at a required wavelength. A tunable source84is associated with each port of the transmitter unit83. A transceiver comprises a combination of a receiver device in receiver unit82and a transmitter device in transmitter unit83. Control unit90controls operation of the optical sources84. Control unit90interfaces66with a network management unit200.FIG. 2shows one example form of the OLT41and it will be appreciated that it can take different forms. In an alternative embodiment, the OLT can be fully optical, with no electrical-to-optical conversion. The OLT can receive an optical signal from an operator network at a particular wavelength and either (i) forward it at the received wavelength or (ii) adjust the wavelength of the received optical signal to a wavelength required in the destination WDM-PON. Interface45can take various forms. Interface45can comprise functions such as an interface to a WDM or Dense Wavelength Division Multiplexed (DWDM) link used to connect to the operator network51and a switching function to switch signals between required channels of the OLT and required channels of the WDM/DWDM link.

The apparatus shown inFIG. 1supports full optical unbundling. Any wavelength (λ1-λN) from any one of the WDM-PONs10,11,12can connect, via a respective splitter21,22,23, and a cyclic AWG31,32,33, with an OLT41,42,43of a desired operator. For example, consider a subscriber in WDM-PON10is allocated wavelength λ1. This subscriber receives a communication service from operator network53. Wavelength λ1from WDM-PON10is split at splitter21onto a set of fibres27. One of these fibres27connects with an input port of cyclic AWG33. Wavelength λ1travels through the cyclic AWG33, and from an output port of the cyclic AWG33to OLT43of operator network53. Any OLT41,42,43of an operator can connect to one, or more, of the WDM-PONs10,11,12. Similarly, in the downstream direction, any of the OLTs41,42,43of an operator can connect, via a respective cyclic AWG31,32,33and a splitter21,22,23with any one of the WDM-PONs10,11,12.

Two optical interfaces emitting at the same wavelength (i.e. two wavelengths of the same value, each carrying different traffic for different customers) can coexist in the same OLT provided that they are sent to two different WDM-PONs10,11,12. The cyclic AWG maintains separation of the wavelengths at the OLT itself. This property exists for upstream and downstream directions of communication. Consider again the example of a wavelength λ1used for communication between OLT41and an ONT13in WDM-PON10. For the downstream direction of communication, traffic on wavelength λ1is received from OLT41on one of the ports35of AWG31and routed to an output port34of the AWG31which is connected to the splitter21connected to WDM-PON10. Also, (different) traffic on wavelength λ1is received from OLT41on another of the ports35of AWG31and routed to an output port34of the AWG31which is connected to splitter22connected to WDM-PON11. For the upstream direction of communication, traffic on wavelength λ1is received at port24of splitter21from WDM-PON10and forwarded to a port34of AWG31. AWG31routes wavelength λ1to a port35, which is connected to a receiver in OLT41configured to receive traffic on this wavelength. Also, (different) traffic on wavelength λ1is received at a port24of splitter22from WDM-PON11and forwarded to another port34of AWG31. AWG31routes wavelength λ1to a port35, which is connected to a receiver in OLT41configured to receive traffic on this wavelength. There are two instances of the wavelength λ1co-existing in the apparatus, but these are separately routed in the downstream direction between an OLT41and different WDM-PONs10,11, and in the upstream direction between WDM-PONs10,11and the OLT41. THis property of the wavelength routing apparatus110allows two or more of the WDM-PONs10,11,12to simultaneously use the same set of wavelengths (λ1-λN), and for any of the OLTs41,42,43to serve multiple ONTs13in different WDM-PONs operating on the same wavelength.

A further feature of the apparatus shown inFIG. 1is a monitoring arrangement for monitoring which wavelengths are in use in each of the WDM-PONs10,11,12. On the user side of each splitter21,22,23a small portion of the optical signal is tapped and coupled to a monitoring device61. The monitoring device61connects65with a management unit200. The monitoring device61can simply forward the optical signal tapped at the splitter21, for analysis at unit200or, advantageously, it is an Optical Spectrum Analyser (OSA) for analysing which wavelengths are in use and forwarding a data signal to the management unit200indicating which wavelengths are in use. Management unit200also connects, via a respective path66, with each of the OLT units41,42,43. Management unit200controls configuration of the system. Management unit200determines, from the information received via paths65, which wavelength channels are in use on each of the WDM-PONs10,11,12. This information is held as a look-up table in store220. Advantageously, this information is accessible by the OLTs41,42,43of different operator networks, thereby allowing the different operator networks to co-ordinate their use of wavelength channels. For example, if OLT41is already using wavelength λ1to serve an ONT in WDM-PON10, then other OLTs42,43will not use this wavelength channel to serve an ONT in WDM-PON10.

The wavelength routing unit110shown inFIG. 1uses cyclic AWGs (e.g. N×N AWGs). It will now be described how a cyclic AWG operates. A cyclic AWG operates as a static wavelength router. A cyclic AWG is a device having a set of input ports and a set of output ports. An N×N cyclic AWG will be considered (N input ports, N output ports). Similarly to a regular (non-cyclic) AWG, when a comb of N wavelengths is applied at one input port, the wavelengths in the comb are split so that each wavelength of the comb is present at a corresponding output port. If an equal comb is applied to a different input port, the wavelengths are split again among the output ports, but in a different order. This ensures that no output port experiences a collision of equal wavelengths coming from different input ports. Table 1 gives a simple example for a 10 channel AWG. Typical cyclic AWGs support 40 channels or more.

TABLE 1Operation of a cyclic AWGAWG Input Port12345678910AWG1λ1λ2λ3λ4λ5λ6λ7λ8λ9λ10Output2λ2λ3λ4λ5λ6λ7λ8λ9λ10λ1Port3λ3λ4λ5λ6λ7λ8λ9λ10λ1λ24λ4λ5λ6λ7λ8λ9λ10λ1λ2λ35λ5λ6λ7λ8λ9λ10λ1λ2λ3λ46λ6λ7λ8λ9λ10λ1λ2λ3λ4λ57λ7λ8λ9λ10λ1λ2λ3λ4λ5λ68λ8λ9λ10λ1λ2λ3λ4λ5λ6λ79λ9λ10λ1λ2λ3λ4λ5λ6λ7λ810λ10λ1λ2λ3λ4λ5λ6λ7λ8λ9
According to the table, from k=1, . . . 10:

λkis sent from input port1to output port k

λ□(k+1)mod 10is sent from input port2to output port k

λ(k+2)mod 10is sent from input port3to output port k

and so on.

Configuration

One example implementation of configuring the access network will now be described. A configuration unit210performs the method. The configuration unit210can form part of the management unit200, or can comprise a further functional unit of the access network5. As a further alternative, each OLT41,42,43can have a dedicated configuration unit210. Each OLT41,42,43uses three different tables. A first table (Table 1 above) indicates what wavelength corresponds to each output port/input port pair of the N×N cyclic AWG. To explain this table, consider that a signal at wavelength λ2is applied to input (CO side) port2. The signal is emitted on output (user side) port1. Similarly, if wavelength λ2is received at output (user side) port1it will emerge on input (CO side) port2. Other instances of signals at the same wavelength λ2can be applied to other input ports of the OLT and they are emitted on output ports according to Table 1. So, a signal λ2applied to input (CO side) port3is emitted from output (user side) port10and a signal λ2applied to input (CO side) port4is emitted from output (user side) port9.

A second table (Table 2 below) indicates what wavelengths are currently used at that OLT. An example is shown below.

TABLE 2λ1λ2λ3λ4λ5λ6λ7λ8λ9λ10AWG1xxxxOutput2Port3x4x5xx67x8910xx
where “x” indicates the wavelength indicated in column is already used on the WDM-PON corresponding to the AWG output port indicated in the column. In this example, the WDM-PON corresponding to output (user side) port1already uses the wavelengths λ1, λ3, λ6and λ7. Table 2 can be populated using the wavelength monitoring function described earlier. Before allocating a wavelength, Table 2 is updated using the latest information220which has been obtained using the wavelength monitoring function. If operator networks share information about which wavelengths they have assigned in each WDM-PON, then this shared information can be used to populate Table 2, without needing to perform the wavelength monitoring function.

A third table (Table 3 below) indicates what input (CO side) ports are already in use on the N×N cyclic AWG:

TABLE 3AWG Input Port12345678910xxxx
where “x” indicates the input port is busy and cannot be used.

Now consider an example in which an operator receives a subscription request for the WDM-PON corresponding to AWG output (user side) port1of its OLT. The configuration module checks Table 2 for the available wavelengths on the WDM-PON connected to that port, which are λ2, λ4, λ5, λ8, λ9, λ10in this example. The configuration module then selects one of these wavelengths, e.g. λ2. Using Table 1, the configuration module determines which input port corresponds to λ2□ on the output port1. Stated another way, the configuration module determines which input (CO side) port a signal at λ2must be applied to in order for it to emerge on output port1corresponding to the required WDM-PON. In this example, it is input port2. The configuration module determines, using Table 2, if input port2is free. In this example it is free, so a transceiver can be connected to this input (CO side) port. The tables shown above can be held as data structures in a data store220at the management unit200or at some other data store which is accessible by the configuration unit210. In this example the wavelength λ2is selected. The tables are updated to indicate that λ2is now in use. No other instance of λ2can be used by the OLT to serve an ONT in the same WDM-PON. λ2is marked as “in use” in Table 2 for output port2, and for all other output ports which are connected to the same WDM-PON. Also, the corresponding Table 2 held for other OLTs is updated to indicate that λ2is “in use” on any output ports which connect to the WDM-PON where λ2has just been allocated. This prevents other OLTs from allocating the same wavelength. λ2can still be used by the OLT to serve an ONT in any another WDM-PON where λ2is not already in use.

InFIG. 1there are multiple WDM-PONs10,11,12and multiple AWGs31,32,33and OLTs41,42,43connecting to operator networks51,52,53. Each passive splitter21,22,23is shown with a single link27to a cyclic AWG. There can be multiple links27between different splitter ports25of a particular splitter21and different output (user side) ports34of a particular cyclic AWG31. Providing multiple links27allows a greater opportunity for routing a required wavelength between an OLT41and a WDM-PON10. Referring again to the configuration example described above, consider that the WDM-PON is connected to output (user side) ports1and2of the cyclic AWG. There are now more opportunities that an input (CO side) port on the AWG will be free. An operator network51can connect to multiple OLT units to further increase capacity. When there is only a single link27between an OLT31and a splitter21serving a WDM-PON, the cyclic AWG will, by itself, prevent any more than one instance of a wavelength being output to a given WDM-PON. Referring to Table 1, only once instance of wavelength λ1will be output to port1. When there are multiple links27between an OLT31and a splitter21serving a WDM-PON, there is a possibility that multiple instances of a wavelength can be output to a given WDM-PON. Referring to Table 1, consider that output (user side) ports1and2connect to splitter21serving WDM-PON10. Wavelength λ1could be used on input ports1and10of the cyclic AWG. Configuration module210will ensure that only one instance of the same wavelength is used in WDM-PON10by using the information held in Table 2. Other OLTs can also access Table 2 to ensure that only one instance of the same wavelength is used by any of the OLTs41,42,43in WDM-PON10.

FIG. 3shows an optical access network comprising a single WDM-PON and connections to multiple operator networks51,52,53. The splitter21can comprise a single, or multiple, links27between splitter ports25and a particular one of the cyclic AWGs31,32,33. Multiple links are shown between ports25and each of OLT41and OLT42.FIG. 3typically represents an early stage of access network deployment. Additional WDM-PONs can be added to arrive at the arrangement shown inFIG. 1.

FIG. 4shows another embodiment of an optical access network comprising multiple WDM-PONs and connections to multiple operator networks. The number of operator networks is less than the number of WDM-PONs. Again, the splitter21can comprise a single, or multiple, links27between splitter ports25and a particular one of the cyclic AWGs31,32,33. The number of splitter ports25should be large enough to enable one OLT to serve users belonging to different WDM-PONs. The limit case is when one operator serves N users, one user per WDM-PON, by means of the same OLT.

FIG. 5shows a general method of operating the apparatus of the access network. At step201wavelength channels are received from OLTs. There can be one OLT or several OLTs. At step202wavelength channels are routed to WDM-PONs using the wavelength routing apparatus110. Different wavelength channels of the same wavelength value are routed to different ones of the WDM-PONs.

Apparatus installed at the Central Office100can be modified as additional WDM-PONs are added to the access network. Apparatus installed at the Central Office100can be modified as additional operator networks are added. An advantage of this arrangement is that the addition of a new WDM-PON or operator network does not require a significant change to the apparatus which has already been installed. Various scenarios for modifying the access network5will now be considered with reference toFIGS. 6 to 9.

Adding a New Subscriber

Consider that a new subscriber wishes to obtain service in the access network from a particular operator network (e.g. network51). The subscriber will be served by one of the existing WDM-PONs10,11,12. Assume the new subscriber can be served by WDM-PON10. Depending on the type of access network, a new ONT13is installed at the subscriber premises (FTTH), or cabling is installed between an existing ONT13in the WDM-PON and a new terminal in the subscriber premises (FTTC). No additional apparatus is required in the CO.FIG. 6describes the method. At step211a request for service is received for a subscriber in one of the WDM-PONs. At steps212,213the configuration process described above is performed to find an available wavelength between the OLT41of operator network51and the new subscriber in WDM-PON10. When an available wavelength and input (CO side) port of the AWG31is found, a transceiver in OLT41is connected to the input (CO side) port35. Step214checks if there is a free input port. If a free input port is found at step214, the method proceeds to step216and a transmitter operating at the available wavelength is connected to the input port. As described above, this can comprise manually, or automatically, forming a connection between a source operating at the required wavelength and the required input port35of the AWG, or by controlling a tunable source connected to the input port to operate at the required wavelength. If a free input port is not found at step214, the method proceeds to step215. A connection is not possible using the existing apparatus. An upgrade path may be used to provide service, as described later. The upstream path is configured at the same time. Typically, the upstream wavelength will be a fixed offset from the downstream wavelength. A receiver is connected to the AWG port.

Adding a Connection to a New Operator Network

Referring toFIG. 1, consider that the CO already connects to operator networks51,52. A new operator network53wishes to connect to the access network5.FIG. 7describes the method. At step221an OLT43is provided for the new operator. The OLT is connected to a free set of ports of the wavelength routing apparatus. Advantageously, the set of ports is created by installing an additional AWG33at the CO100. At step222, for each of the ONTs that the new operator network requires connection to, wavelength channels are routed to the WDM-PONs. Advantageously, one or more links27are added between splitters21,22,23of the WDM-PONs10,11,12and the new AWG33. The configuration process described above is performed to find available wavelengths to serve each of the ONTs in the WDM-PONs10,11,12which require service from the new operator network53. When an available wavelength and input (CO side) port of the AWG33is found, a transceiver in OLT43is connected to the input (CO side) port35. As described above, this can comprise a manual or automatic switch between a source operating at that wavelength and the required input port35of the AWG, or by controlling a tunable source connected to the input port to operate at the required wavelength. The upstream path is configured at the same time. Typically, the upstream wavelength will be a fixed offset from the downstream wavelength. A receiver is connected to the AWG port. Step222is repeated for each ONT requiring service by the new operator network.

Adding a New WDM-PON

A new WDM-PON can be added to the access network. Referring toFIG. 1, consider that WDM-PONs10and11already exist.FIG. 8describes the method for adding a new WDM-PON12to the access network. Apparatus for the new WDM-PON12is deployed at step231. This comprises ONTs13, fibres14, a remote node15and trunk fibre16for the new WDM-PON. At step232the trunk fibre is connected to a free port24on the wavelength routing apparatus110. Advantageously, the port24is provided by adding a splitter23to the wavelength routing apparatus110. Links27are added between splitter23and each of the AWGs31,32,33. For each of the operator networks that the new WDM-PON requires connection to, wavelength channels are configured and routed to the operator networks at step233. The configuration process described above is performed to find available wavelengths to serve ONTs in the new WDM-PON13. ONTs in the new WDM-PON12can be served by any of the operator networks51,52,53. When an available wavelength and input (CO side) port of the respective AWG31,32,33is found, a transceiver in respective OLT41,42,43is connected to the input (CO side) port35of the AWG. The upstream path is configured at the same time. Typically, the upstream wavelength will be a fixed offset from the downstream wavelength. A receiver is connected to the AWG port. This is repeated for each ONT requiring service in the new WDM-PON.

Transferring an Existing Subscriber to a New Operator

Consider that an existing subscriber wishes to obtain service in the access network from a different operator network. Consider a subscriber in WDM-PON10wishes to transfer from operator network51to operator network52. No additional apparatus is required in the CO.FIG. 9describes the method. At step241arequest to transfer service is received. The configuration process described above is performed to determine if the current wavelength (e.g. λ1) that is used between the ONT13and OLT41can be used between the OLT42of the new operator network52and the subscriber in WDM-PON10. This is because the operating wavelength of the ONT13is fixed at the deployment, when the ONT13is connected to a particular one of the ports14of the remote node15. A change of the operating wavelength of the ONT would require a change at the remote node15. Step243checks if there is a free input port. If a free input port is found at step243, the method proceeds to step244and a transmitter in OLT42, operating at the wavelength, is connected to the input port of AWG32. The transceiver in OLT41is disconnected from the input (CO side) port35of AWG31and the tables held in store220are updated to reflect the new configuration. If a free input port is not found at step243, the method proceeds to step245. A connection is not possible using the existing apparatus. An upgrade path may be used to provide service, as described later. Typically, the upstream wavelength will be a fixed offset from the downstream wavelength. A receiver is connected to the same port of AWG32.

Channel Upgrade Method

In the method described above, the configuration unit210determines which wavelengths are not already used on the WDM-PON and determines if, for at least one of these wavelengths, there is a free input (CO side) port of the AWG from where it is possible to send the wavelength to the corresponding output (user side) port. If the check is positive, the operator plugs an optical transceiver (e.g. a SFP) emitting at that wavelength on the AWG input (CO side) port. If the check is negative, the configuration module repeats until a free port is found on other OLTs owned by the operator. If no free port is found on any OLT of the operator, the apparatus ofFIG. 1does not permit a connection.

FIG. 10shows an alternative apparatus which allows additional connections. An additional splitter60is added upstream of the 1:N splitter21. InFIG. 4this additional splitter60is a 1:2 splitter to minimise losses, although the splitter can split a signal a larger number of ways. One port61of the new splitter60is connected to the N×N AWG31, as in the apparatus ofFIGS. 1 to 3. The other port62of splitter60is used for expansion. For example, it can be connected to an additional N×N cyclic AWG131, with an associated OLT141. It may be necessary to compensate for the additional insertion loss introduced by the splitter60by means of optical amplifiers64.

InFIGS. 1,3and4, the wavelength routing apparatus110which routes wavelengths between WDM-PONs10,11,12and OLTs41,42,43of different operator networks can be co-located at a central office CO100. However, the AWG of one operator network can be located at a different physical location from the AWG of another operator. This allows further flexibility. Advantageously, the OLTs41,42,43of different operators share configuration data210, thereby still allowing the OLTs41,42,43of different operator networks to co-ordinate their use of wavelength channels.