EP2580884A1

EP2580884A1 - Optical access network

Info

Publication number: EP2580884A1
Application number: EP10725666.1A
Authority: EP
Inventors: Filippo Ponzini; Roberto Sabella
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2010-06-11
Filing date: 2010-06-11
Publication date: 2013-04-17
Also published as: WO2011154048A1

Abstract

An optical access network (5) comprises a node (40) and a plurality of optical terminal units (20, 30) optically connected to the node (40). An optical terminal unit (20) is electrically connected (13) to a plurality of end terminal units (12). In the downstream direction, an optical signal is transmitted over the access network to the first optical terminal unit (20). The optical signal comprises an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows (A, B, C, D) for the plurality of end terminal units (12). Traffic flows (A, B, C, D) are forwarded to the end terminal units (12) at the first optical terminal unit (20). A plurality of different operator networks (51,52,53) connect to the node (40). Traffic from the operator networks (51,52,53) is switched (80) such that it is applied to one of the frequency multiplexed traffic flows (A, B, C, D) corresponding to a required end terminal unit (12). Bandwidths of the traffic flows can be non-uniform. The access network (5) can be a wavelength division multiplexed access network (5).

Description

OPTICAL ACCESS NETWORK

TECHNICAL FIELD

This invention relates to optical access networks, such as Wavelength Division Multiplexed Passive Optical Networks (WDM-PON), and to carrying traffic over such networks.

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 Terminal (OLT) interfaces with a metro or carrier network. An arrangement of optical fibres and splitters connect to Optical Network Terminal units (ONT) deployed across a service area. There are various architectures for the access network, which fall under the term Fibre To The x (FTTx). The architectures differ in where the ONTs are located. In a Fibre To The Node (FTTN) architecture, ONTs are located at a street cabinet which can be several kilometres from subscriber premises. In a Fibre-To-The-Cabinet (FTTCab) or Fibre To The Curb (FTTC) architecture, ONTs are located at a street cabinet closer to the subscriber premises. In a Fibre-To-The-Building (FTTB) architecture, ONTs are located at the boundary of a subscriber premises, such as a basement of a block of apartments. Typically, in each of the architectures above there is an electrical connection between the ONT and each end terminal. In a Fibre To The Home (FTTH) architecture, ONTs are located at subscriber premises.

Various types of PON have been proposed. 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) typically use some form of time division multiple access technique, with the capacity of a fibre 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, called lambdas. A separate wavelength channel is allocated for communication between the Optical Line Terminal (OLT) and each ONT in the PON.

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. In many cases an access network will already be deployed with an operator, called the incumbent operator, owning and operating the access network. There is a problem of how to allow Other Licensed 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.

WDM-PONs are a good technology for an access network that is required to connect to multiple operator networks, as the traffic to each Optical Network Terminal unit (ONT) is carried by a separate wavelength channel. This allows traffic to be unbundled more easily at the Central Office of the access network, and it is not necessary to share the optical layer of the access network among different users, as in the other types of PON described above. This works for a Fibre To The Home (FTTH) access network architecture, where an ONT is located at a subscriber premises. However, in access network architectures such as Fibre To The Node (FTTN), Fibre- To-The-Cabinet (FTTCab), Fibre To The Curb (FTTC) and Fibre-To-The-Building (FTTB), where an ONT is located remote from a plurality of end terminals, there remains a problem of how to serve the end terminals.

SUMMARY

A first aspect of the present invention provides a method of operating a node in an optical access network. The access network comprises a plurality of optical terminal units optically connected to the node. At least a first optical terminal unit is electrically connected to a plurality of end terminal units. The method comprises, at the node transmitting an optical signal over the access network to the first optical terminal unit, the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows for the plurality of end terminal units. Additionally, or alternatively, the method comprises receiving an optical signal over the access network from the first optical terminal unit the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows from the plurality of end terminal units. An advantage of this aspect is that a full point-to-point "unbundling" of the traffic can be performed at the node. Traffic for the plurality of end terminal units is separated according to frequency. This allows a relatively simple circuit switching of traffic between subscriber terminals and operator networks across the range of FTTx scenarios. T he traffic fiows are independent, orthogonal signals, affording full transparency at the physical layer. Each traffic flow can use one or more of: a format, a bit-rate and a protocol which can be different from those used by other traffic flows.

Embodiments of the invention are useful where there is a need to unbundle traffic to/from a plurality of operator networks. The electrical domain is still an efficient and high bandwidth medium when used for short hauls, such as between a street cabinet, building or local node, and a terminal located in a premises.

Advantageously, the optical access network is a wavelength division multiplexed PON (WDM-PON). The WDM-PON has a set of different optical wavelength carriers, there being an optical wavelength carrier allocated to each of the optical terminal units. A different optical wavelength carrier is allocated to each of the optical terminal units. The wavelength channel carries the set of frequency- multiplexed traffic flows for the end terminal units connected to the optical terminal unit. The channel spacing between wavelength carriers in a WDM-PON is typically 100GHz, and the modulated optical wavelength carrier typically has a bandwidth of the order of a few GHz or more, depending on the number of frequency-multiplexed traffic flows.

In other embodiments of the invention, the optical access network can be a different type of PON, such as a Point to Point (P2P) Fibre Access Network, also known as a All Fibre (AF) PON, where a separate fibre is provided between the CO and each ONT.

A traffic flow can be carried by a modulated carrier, or a plurality of modulated carriers. The frequency multiplexed traffic flows can be formed by modulating a plurality of frequency carriers with the traffic using a digital multi-carrier modulation technique. The multiplexed combination of carriers is then used to modulate an optical wavelength carrier.

Advantageously, the number of carriers used to carry a traffic flow for a particular end terminal is varied according to traffic demand.

Advantageously, at least one of the following can be varied during operation: a bandwidth of one of the traffic flows; a number of traffic flows allocated to an end terminal unit. In this way, the system is flexible according to the traffic demands of the end terminal units. The bandwidth of a traffic flow can be varied by varying the number of frequency carriers used to carry the traffic flow. Alternatively, it is possible to vary the coding scheme and/or modulation scheme used to carry that traffic flow.

Another aspect of the invention provides apparatus for use at a node of an optical access network. The access network comprises a plurality of optical terminal units optically connected to the node. At least a first optical terminal unit is electrically connected to a plurality of end terminal units. The apparatus comprises a transmitter arranged to transmit an optical signal over the access network to the first optical terminal unit, the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows for the plurality of end terminal units. Additionally, or alternatively, the method comprises a receiver arranged to receive an optical signal from the access network, the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows from the plurality of end terminal units.

Another aspect of the invention provides a method of operating a first optical terminal unit in an optical access network. The access network comprising a plurality of optical terminal units optically connected to the node. The first optical terminal unit is electrically connected to a plurality of end terminal units. The method comprises receiving an optical signal over the access network, the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows carrying traffic for the plurality of end terminal units connected to the first optical terminal unit, converting the set of traffic flows to the electrical-domain and forwarding an electrical signal to each of the plurality of end terminal units. Additionally, or alternatively, the method comprises receiving a set of electrical signals from the plurality of end terminal units each electrical signal carrying traffic from a respective end terminal unit, forming an optical signal which comprises an optical wavelength carrier which is modulated to carry a set of frequency multiplexed traffic flows carrying the traffic, and transmitting the optical signal over the access network.

Another aspect of the invention provides a first optical terminal unit for use in an optical access network. The access network comprising a plurality of optical terminal units optically connected to the node. The first optical terminal unit is electrically connected to a plurality of end terminal units. The first optical terminal unit comprises a receiver arranged to receive an optical signal over the access network. The optical signal comprises an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows carrying traffic for the plurality of end terminal units connected to the first optical terminal unit. The receiver is arranged to convert the set of traffic flows to the electrical-domain. The first optical terminal unit also comprises an electrical interface arranged to forward electrical signals corresponding to the traffic flows to the plurality of end terminal units. Additionally, or alternatively, the first optical terminal unit comprises an electrical interface arranged to receive a set of electrical signals from the plurality of end terminal units, each electrical signal carrying traffic for a respective end terminal unit and a transmitter arranged to form an optical signal which comprises an optical wavelength carrier which is modulated to carry a set of frequency multiplexed traffic flows carrying the traffic. The transmitter is further arranged to transmit the optical signal over the access network.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 shows an embodiment of an optical access network comprising multiple WDM-PONs and connections to multiple operator networks;

Figures 2A to 2C show frequency-multiplexed traffic flows carried by a wavelength channel;

Figure 3 shows apparatus at a Central Office (CO) of the network of Figure 1; Figure 4 shows an Optical Line Terminal (OLT) apparatus of Figure 2;

Figure 5 shows an Optical Network Terminal (ONT) unit of Figure 2;

Figures 6A and 6B show a first example of apparatus for multiplexing and demultiplexing traffic flows;

Figures 7A and 7B show apparatus for multiplexing and demultiplexing traffic flows;

Figure 8 shows a method of operating apparatus at a Central Office node in the network of Figure 1;

Figure 9 shows a method of operating an optical terminal unit in the network of Figure 1; ,

6

Figures 10 and 11 show more detailed methods of operating apparatus at a Central Office node in the network of Figure 1;

Figures 12 and 13 show more detailed methods of operating an optical terminal unit in the network of Figure 1.

DETAILED DESCRIPTION

Figure 1 shows an optical access network 5 according to an embodiment of the invention. A Central Office (CO), also called a WDM-PON node 40, connects to at least one Wavelength Division Multiplexed Passive Optical Network (WDM-PON) 10, 11. In Figure 1 the CO 40 connects to two separate WDM-PONs 10, 1 1. CO 40 also connects to operator networks 51-53, which are transport networks (typically metro or core networks) of different telecommunication operators. Telco operators can compete to provide a communication service to subscribers within the WDM-PONs 10, 11. A Passive Optical Network is called "passive" 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.

Each WDM-PON 10, 1 1 comprises a trunk fibre 16, or fibres, which connect the CO 40 to a remote node 15. Remote node 15 connects, via fibres 14, to Optical Network Terminals (ONT) 20, 30 deployed in the service area of the WDM-PON. Typically, there is a single fibre 14 between the Remote Node 15 and ONT 20, 30 for each ONT. The ONT 20, 30 terminates the optical path of the access network. An ONT can be installed at a subscriber premises, such as a home or business premises. This scenario is typically called Fibre To The Home (FTTH) or Fibre To The Premises (FTTP). A ONT is shown in Figure 1 by ONT 30. Alternatively, an ONT can be installed at a unit which serves a plurality of premises. A unit can be positioned at a streetside cabinet or can serve an apartment building. The ONT comprises optical-to- electrical conversion and an electrical interface 13 to each of a plurality of terminals 12 located at premises. This scenario is typically called Fibre To The Node (FTTN), Fibre To The Curb (FTTC), Fibre To The Cabinet (FTTCab) or Fibre To The Building (FTTB). In the following description ONT 20 represents an ONT which serves a plurality of subscriber terminals.

In a WDM-PON, a set of optical wavelength carriers are used to serve ONTs. Each ONT 20, 30 is served by a different wavelength carrier. The wavelength carriers are also called wavelength channels, or lambdas (λ), and the term lambdas will be used in the following description. In the downstream direction, remote node 15 demultiplexes lambdas received on trunk fibre 16 and outputs lambdas on different ones of the fibres 14. In the upstream direction, remote node 15 receives lambdas on the plurality of fibres 14, multiplexes them, and outputs the multiplexed combination of lambdas on trunk fibre 16. Bi-directional communication is typically provided by two separate lambdas, with one lambda for downstream communication and one lambda for upstream communication. Another possible scheme is time-division multiplexing of a single lambda between downstream communication and upstream communication.

In Figure 1, traffic for a plurality of terminal units 12 connected to an ONT 20 is transported as a set of frequency multiplexed traffic flows which are carried by modulating the lambda serving the ONT 20.

Figures 2A to 2C show some possible options for the set of frequency multiplexed traffic flows carried by a lambda. Figure 2A shows four frequency multiplexed traffic flows A-D. Each of the traffic flows A-D comprises a frequency carrier 91-94 modulated with data. In Figure 2 A the bandwidth of each traffic flow A- D is equal, and there is one modulated carrier per traffic flow: modulated carrier 91 is a traffic flow A for a first terminal 12; modulated carrier 92 is a traffic flow B for a second terminal 12, and so on. The bandwidths of each modulated carrier do not have to be equal. Figure 2B shows non-uniform bandwidths. The bandwidth of traffic flow D is greater than the bandwidth of an individual one of the flows A-C. There can be more than a single modulated carrier per traffic flow. Figure 2C shows a non-uniform allocation of modulated carriers to traffic flows: traffic flows A and D each comprise a single modulated carrier; traffic flow B comprises two modulated carriers; traffic flow C comprises three modulated carriers. The frequency multiplexing of traffic flows can be achieved using an analogue multiplexing technique, or digital multiplexing technique, as described in more detail below. Multiple traffic flows may carry traffic for a particular end terminal, such as different communication services.

Figure 3 shows an overview of the Central Office (CO) 40. An Optical Line

Terminal (OLT) unit 61-63 is provided for each WDM-PON. The OLT connects to the trunk fibre 16 serving that WDM-PON. Each OLT terminates the optical path and comprises optical-to-electrical conversion of the lambdas and traffic flows. Each OLT „

o outputs a set of electrical-domain signals 81 corresponding to the traffic carried by the lambdas. For a lambda which carries a set of frequency-multiplexed traffic flows, an OLT outputs a set of electrical-domain signals 81 corresponding to the individual traffic flows carried by that lambda. A switching matrix 80 connects to the OLTs 61- 63 and to an operator network interface 85-87 for each operator network 51-53. Electrical-domain signals 81, 82 are passed between the switching matrix 80 and OLTs 61-63, and between the switching matrix 80 and operator network interfaces 85-87. Switching matrix 80 switches traffic between WDM-PONs and operator networks, such that traffic from an operator network 51-53 is switched to a lambda (and a particular traffic flow on a lambda) which serves a terminal that has subscribed to that operator network. Similarly, switching matrix 80 switches traffic between WDM- PONs and operator networks, such that traffic from a lambda (and a particular traffic flow on a lambda) is switched to an operator network 51-53 to whom that terminal has subscribed. The use of separate lambdas per ONT 30, and frequency multiplexed traffic flows per ONT 20 allows full, transparent, unbundling at the CO 40.

Figure 4 shows one of the OLT units 61 of Figure 3. For each uplink and downlink lambda pair, OLT 61 has apparatus 70 for processing the traffic flows of that uplink/downlink lambda pair. Apparatus 70 has electrical-domain processing stages and optical domain processing stages. Considering the downstream transmission path, the electrical-domain processing comprises a multiplexer 71 which receives signals 82 from the switching matrix 80. Each signal can represent traffic for a particular ONT 30, or traffic for a particular terminal 12 served by an ONT 20. The latter case will be considered in detail. Multiplexer 71 frequency multiplexes the traffic flows 82 together by a suitable scheme (analogue, digital) and then forwards the frequency- multiplexed combination of traffic flows to an optical modulator 73. The optical domain processing comprises an optical laser source 72 generating an optical wavelength carrier at a required wavelength. As described above, in a WDM-PON each ONT 20, 30 is allocated a wavelength carrier of a particular wavelength value. The optical modulator 73 modulates the optical wavelength carrier signal (lambda) with the frequency multiplexed data and outputs a modulated optical wavelength carrier signal. The modulation of the optical wavelength carrier can use any type of modulation, such as intensity (amplitude) modulation, phase modulation or frequency modulation. The modulated lambda is applied to an optical combiner 74. Other instances of apparatus 70 perform similar processing for other uplink/downlink lambdas. In the downstream direction of transmission, optical combiner 74 combines lambdas received from different instances of apparatus 70. In the upstream direction of transmission, optical combiner 74 separates received lambdas and forwards them to different instances of apparatus 70.

Considering the upstream direction of transmission, optical combiner 74 receives a plurality of lambdas. In a transmission scheme where downstream and upstream communication are at different wavelengths, the upstream lambdas will be at a wavelength which is offset from those used for downstream communication. The upstream lambdas are forwarded 75 to an optical receiver 74. Optical receiver 76 detects/demodulates the modulated optical signal, i.e. it outputs, in the electrical- domain, a signal representing the set of frequency-multiplexed traffic flows carried by the lambda. Receiver 76 outputs an electrical-domain signal to a demultiplexer 77. The set of frequency-multiplexed traffic flows are demultiplexed at the demultiplexer 77 and output as electrical-domain signals 81 to the switching matrix 80.

Figure 5 shows one of the ONT units 20 of Figure 1. The ONT 20 has electrical-domain processing stages and optical domain processing stages. In the downstream direction of transmission, a combiner 100 receives the downstream lambda and forwards it 101 to an optical receiver 102. The lambda carries a frequency-multiplexed set of traffic flows for the terminals served by the ONT 20. Optical receiver 102 detects/demodulates the optical signal, i.e. it outputs an electrical- domain signal 103 corresponding to the frequency-multiplexed set of traffic flows carried by the lambda. Signal 103 is applied to a demultiplexer 104. Demultiplexer 104 outputs a set of electrical signals 105. Each electrical signal 105 carries traffic for one of the terminals 12 connected to the ONT 20, and is applied to a line interface 106. Line interface 106 can include an appropriate termination for the electrical line 13 and an amplifier. Each line interface 106 connects to a line 13 to a subscriber terminal 12. In the upstream direction of transmission, line interfaces 106 each receive an electrical signal from a subscriber terminal 12 served by the ONT 20. The signals are applied to inputs of a multiplexer 1 1 1. Multiplexer 1 1 1 frequency multiplexes the signals and outputs an electrical-domain signal 1 12 representing the frequency multiplexed combination of traffic flows to an optical modulator 1 14. A laser source 113 outputs an optical wavelength carrier at a wavelength which has been allocated for upstream communication from that ONT 20. Optical modulator modulates the optical signal generated by source 113 with the frequency multiplexed traffic flows 112 and outputs _{1 Q} the modulated signal (lambda) to combiner 100 for transmission towards the OLT. The modulation of the optical wavelength carrier can use any type of modulation, such as intensity (amplitude) modulation, phase modulation or frequency modulation. The type of modulation used to modulate the wavelength carrier in the upstream direction of transmission can be different to that used in the downstream direction. Where a single wavelength is used for upstream and downstream communication, in a non time- divided manner, then different types of modulation can be used for the upstream and downstream directions of transmission to avoid interference. Advantageously, the ONT is "colorless", meaning that the same type of apparatus is provided at each ONT in the WDM-PON. This has benefits of reduced cost, as the transceiver can be manufactured in high volume and a limited range of spare parts is needed.

As described above, there are various ways in which the traffic flows can be carried by a lambda. The detail of the processing performed by the multiplexer 71 and demultiplexer 77 of OLT 61 , and the multiplexer 1 11 and demultiplexer 104 of ONT 20, will vary according to the frequency multiplexing scheme that is used to combine the traffic flows. Figures 6A to 7B show example apparatus for performing frequency multiplexing of the traffic flows.

The lambda can carry a single traffic flow for each of the N end terminals connected to the ONT. In this case, the multiplexer 71 , 1 1 1 receives an input from each of the N line interfaces 106 and multiplexes the inputs to form a multiplex of N traffic flows. Each traffic flow in the multiplexed signal can be carried by a modulated carrier, or a plurality of modulated carriers. In an alternative embodiment, there can be more than one traffic flow for at least one of the N end terminals. The multiplexer 71, 1 1 1 forms a multiplex of K traffic flows, where K>N. Where an end terminal subscribes to multiple traffic flows, the multiple traffic flows can be combined (in the downlink direction) as part of the processing performed at the demultiplexer 104 or at the line interface 106. Similarly, the multiple traffic flows can be separated (in the uplink direction) at the line interface 106 or as part of the processing performed at the multiplexer 1 1 1. Each traffic flow in the multiplexed signal can be carried by a modulated carrier, or a plurality of modulated carriers.

Figures 6A and 6B show a frequency multiplexing scheme in which carriers are individually modulated and then combined to form a frequency multiplexed output signal. Figure 6A shows apparatus at the multiplexer 71 , 1 1 1 . A data signal representing traffic for (or from) a particular end terminal 12 is applied to a modulator 131, which modulates a carrier signal having a centre frequency ft with the data. The modulator 131 can use a phase modulation scheme, such as Quadrature Phase Shift Keying (QPSK), an amplitude modulation scheme such as Quadrature Amplitude Modulation (QAM) or a frequency modulation scheme such as Frequency Shift Keying (FSK). Other input signals 82 modulate respective carriers ft - fk. The set of modulated carriers are combined 132 to form a multiplexed signal for use in modulating an optical source. Figure 6B shows apparatus at the demultiplexer 77, 104. An input signal, representing a frequency multiplexed combination of traffic flows, is received at an input 134 and applied to a bank of filters 136. Each filter 136 is tuned to a particular centre-frequency of one of the carriers ft - fN used in the multiplexing scheme. Each filter outputs a filtered signal to a respective demodulator 137. Each demodulator outputs a data signal corresponding to traffic for (or from) a particular end terminal 12. The frequency multiplexed traffic flows are each separated by a guard band. The size of the guard band is selected as a trade-off between hardware costs and spectral efficiency.

The apparatus shown in Figures 6A and 6B can be implemented in the analogue domain, in the digital domain, or a combination of the analogue domain and digital domain. For a digital domain implementation of Figure 6A, the modulation 131 of each carrier and combination 132 of modulated carriers is performed in the digital domain by digitally processing signal samples representing the carriers. The combined signal is then converted to the analogue domain by a digital-to-analogue converter 133. For a fully digital domain implementation of Figure 6B, the multiplexed signal received 134 from the optical receiver is converted to the digital domain by an analogue-to-digital converter 135. The filtering 136 of frequency bands and demodulation 137 of the signal in each filter pass-band is performed in the digital domain, by digitally processing signal samples obtained by the ADC 135. A traffic flow for a particular end terminal 12 can comprise a single one of the modulated carriers, or a plurality of the modulated carriers.

The bandwidth of each modulated carrier can be equal, or at least one of the modulated carriers can have a different bandwidth from others. The bit-rate of each carrier can be selected as a trade-off between the switching complexity and the flexibility in terms of bandwidth. Typical values can range from 50Mbit/s to 500Mbit/s, offering a practical bandwidth upgrade from conventional Digital Subscriber Line (DSL) solutions. An example of a first implementation is an initial ^ aggregated bandwidth per wavelength of 2.5Gbit/s, compatible with data rates used by small form-factor pluggable (SFP) equipment, and able to accommodate up to 40 carriers each at 50Mbit/s. For optical systems operating at 10G and beyond, it will be possible to accommodate a higher number of carriers or, as an alternative, increase the bit-rate per carrier up to 200Mbit/s. A traffic flow can be carried by a single modulated carrier or by a plurality of modulated carriers.

Figures 7A and 7B show a frequency multiplexing scheme which uses multi- carrier modulation. Suitable modulation schemes are Discrete multi-tone modulation (DMT) and Orthogonal Frequency Division Multiplexing (OFDM). In these types of scheme, the modulated carriers are often called "tones". Figure 7A shows apparatus at the multiplexer 71 , 1 1 1. A first stage 121 separates a serial steam of data 13 into parallel data words, with one (or more) data words being allocated to each carrier (tone). A mapper 122 maps the data word to a complex value selected from a constellation of possible values. This is typically a form of Quadrature Amplitude Modulation (QAM). Mapper 122 outputs a parallel set of values. Each output determines the complex value of one of the frequency carriers in the modulation scheme. The set of values received from mappers 122 are then transformed using a suitable frequency domain-to-time domain transform 123, such as the Inverse Fast Fourier Transform (IFFT). This transforms the set of frequency-domain data to the time-domain. The IFFT processing block 123 outputs digital samples of an output signal in the time domain. The time-domain signal is applied to a digital-to-analog converter (DAC) 23 for conversion to an analog signal. Figure 7B shows apparatus at the demultiplexer 77, 104. An input signal, representing a frequency multiplexed combination of traffic flows, is received at an input and applied to an analog-to-digital converter (ADC) 125. Subsequent processing occurs in the digital domain. The digital signal is transformed using a suitable time domain-to-frequency domain transform 126, such as the Fast Fourier Transform (FFT). This transforms the set of time-domain data to the frequency-domain. FFT 126 outputs a set of complex values to symbol detector stages 127. Each symbol detector stage 127 is arranged to detect the constellation value of a signal, representing the value of one of the frequency carriers used in the modulation scheme. The constellation value corresponds to a data value. Data values are output to a parallel-to-serial converter 128, to form an output signal. In this scheme, a traffic flow for a particular end terminal is distributed across a group of the ^ modulated carriers. As an example, traffic flow C in Figure 2C is carried by a group of three carriers which are modulated in the manner just described.

The traffic flow for each end terminal 12 is carried by a number of carriers (tones) that depends on the chosen provider and subscription. The "tones" can be of fixed bandwidth and digitally managed, but each user can receive more than one tone to satisfy its bandwidth and service needs. Single tone bandwidth can be selected taking into account the available technology and costs and it should be a trade-off between the point-to-group switching at the OLT side and the bandwidth assignment flexibility. For example a bandwidth of 20Mbit/s for each tone appears as a good trade-off in terms of maximum number of served users (up to 100 with a SFP 2.5Gbit/s link) and switching costs at the central office.

This scheme is more flexible as it is possible to change the number of carriers used to carry the data of a traffic flow. Control signalling between the multiplexer 71 in the OLT 61 and the demultiplexer 104 in the ONT 20 signals how data is allocated to carriers. The total number of carriers can be automatically adjusted to fit the transmission requirements of the end-to-end link, formed by the optical part of the access network (trunk fibre 16 and fibre 14) and the electrical part of the access network 13. Bandwidth can be assigned on demand. For example, the number of carriers allocated to a traffic flow to a particular end terminal 12 can be increased during heavy file transfer or Internet Protocol Television (IPTV) services and reduced during web browsing. Dynamic tone allocation can also allow a customer to subscribe to different services with different operators at the same time. For example, a customer can subscribe to a voice service with operator #1, a data service with operator #2, and an IPTV service with operator #3.

Referring again to Figures 4 and 5, a controller 78, 1 18 in each of the OLT 61 and ONT 20 controls allocation of bandwidth to each of the carriers/tones of the traffic flows. Signalling between controllers 78, 1 18 can be carried by an overhead of the wavelength channel.

In both schemes, the independence of each traffic flow provides a fully transparent network. It is possible to transport traffic of different operator networks 51-53 according to the protocols and/or service requirements of those operators. Traffic flows on a particular lambda can comprise a mix of different protocols and/or service requirements. ^

The switching matrix 80 can also support a "one-to-group" switching function at the level of a traffic flow, which is useful where multicasting is required to end terminals 12 across the access network 5.

Methods of operating nodes in the access network will now be described with reference to Figures 8 to 13. Figure 8 shows steps of a method performed by apparatus, such as an OLT, at a CO of the access network. Step 201 comprises transmitting an optical signal over the access network to an optical terminal unit which is electrically connected to a plurality of end terminals. The optical signal comprises an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows for the plurality of end terminal units. Step 202 comprises receiving an optical signal over the access network from an optical terminal unit which is electrically connected to a plurality of end terminals. The optical signal comprises an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows from the plurality of end terminal units.

Figure 9 shows steps of a method performed by an optical terminal (e.g. an

ONT) deployed in the access network. Step 205 comprises receiving an optical signal over the access network at an optical terminal unit which is electrically connected to a plurality of end terminals. The optical signal comprises an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows for the plurality of end terminal units. At step 206 the traffic flows are converted to the electrical-domain and forwarded to the plurality of end terminal units. Step 207 comprises receiving electrical signals from a plurality of end terminals connected to the optical terminal unit. Each signal carries traffic from an end terminal. The method forms an optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency-multiplexed traffic flows and transmits the optical signal over the access network. In the case of a WDM-PON, the optical signal is a wavelength carrier dedicated to the optical terminal unit.

Figure 10 shows steps of a method of transmission performed by apparatus at a CO of the access network. At step 210 traffic is received from operator networks. Typically, the CO is connected to a plurality of operator networks. At step 21 1 , the traffic is switched to inputs of OLTs corresponding to the traffic flows that connect to required end terminals. At step 212 an OLT forms, for each ONT connected to that OLT, an optical signal comprising an optical wavelength carrier which is modulated to carry traffic for the ONT. For an ONT which is electrically connected to a plurality of _j end terminals, the OLT forms an optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency-multiplexed traffic flows for the end terminals. At step 213 each OLT transmits the optical signal to the ONT. Optionally, at steps 214, 215, a change is made to an aspect of the transmission. At step 214 the bandwidth of a traffic flow is varied. The bandwidth of a traffic flow can be varied by varying the number of frequency carriers used to carry the traffic flow. Alternatively, it is possible to vary the coding scheme and/or the modulation scheme used to modulate a frequency carrier which carries that traffic flow. At step 215 there is a change in a number of traffic flows allocated to an end terminal. This can arise when an end terminal subscribes to a new communications service, or terminates an existing communications service.

Figure 11 shows steps of a method of receiving performed by apparatus at a CO of the access network. At step 220 an OLT receives an optical signal from an ONT. The optical signal comprises an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows from an ONT. At step 221 the OLT demodulates the set of traffic flows from the wavelength carrier and then demultiplexes the set of traffic flows. At step 222 the traffic is switched to required operator networks. Optionally, at step 223, the bandwidth of one of the set of frequency multiplexed traffic flows is varied. Optionally, at steps 223, 224, a change is made to an aspect of the transmission. At step 223 the bandwidth of a traffic flow is varied. The bandwidth of a traffic flow can be varied by varying the number of frequency carriers used to carry the traffic flow. Alternatively, it is possible to vary the coding scheme and/or the modulation scheme used to modulate a frequency carrier which carries that traffic flow. At step 224 there is a change in a number of traffic flows allocated to an end terminal. This can arise when an end terminal subscribes to a new communications service, or terminates an existing communications service.

Figure 12 shows steps of a method of transmission performed by an optical terminal unit of the access network. At step 230 the ONT receives an optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows. At step 231 the ONT demodulates the wavelength carrier to extract the set of frequency multiplexed traffic flows. At step 232 the ONT demultiplexes the set of traffic flows. At step 233 the ONT transmits the demultiplexed traffic flows to end terminals electrically connected to the ONT. _Λ ,

16

Figure 13 shows steps of a method of receiving performed by an optical terminal unit of the access network. At step 240 the ONT receives electrical-domain signals from end terminals connected to the ONT. At step 241 the ONT multiplexes the received signals to form a set of frequency multiplexed traffic flows. At step 242 the ONT modulates a wavelength carrier with the set of frequency multiplexed traffic flows. At step 243 the ONT transmits the modulated wavelength carrier to an OLT.

Advantages of embodiments of the invention are full unbundling capacity for any WDM-PON FTTx solutions. Each operator network can benefit from the "point- to-point" capability of WDM and FDM solutions. Another advantage is modularity, especially for the digital implementation. The apparatus required at the OLT 61 and ONTs 20 scales with the real traffic demand coming both from the incumbent and the OLOs, according to a "pay as you grow" model. An associated advantage is that the incumbent operator, which is expected to deploy the first WDM PON systems, only needs to deploy a limited amount of apparatus. Combining optical and electrical orthogonal channels allows the use of point-to-point links not only in FTTH but also in FTTN, FTTCab, FTTC and FTTB scenarios, allowing unbundling in all FTTx architectures.

In the description of the Central Office (CO), an electrical-domain switching matrix is described. However, it is also possible to switch traffic in the optical domain between operator networks 51-53 and OLTs 61-63.

A WDM-PON has been described in detail. In other embodiments of the invention, the optical access network can be a different type of PON, such as a Point to Point (P2P) Fibre Access Network, also known as a All Fibre (AF) PON, where a separate fibre is provided between the CO and each ONT. Each fibre between the CO and an ONT carries an optical signal. For an ONT which is connected to a plurality of end terminal units 12, the optical signal carries a set of frequency-multiplexed traffic flows, as described above. As each ONT is connected to the CO by a physically separate fibre, all of the ONTs can receive an optical wavelength carrier of the same value. Similarly, all of the ONTs can transmit an optical wavelength carrier of the same value. On each fibre to an ONT, the optical wavelength carrier is modulated to carry a set of frequency-multiplexed traffic flows for the end terminals connected to that ONT.

Modifications and other embodiments of the disclosed invention will come to mind to one skilled in the art having the benefit of the teachings presented in the _j foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. A method of operating a node in an optical access network, the access network comprising a plurality of optical terminal units optically connected to the node and wherein at least a first optical terminal unit is electrically connected to a plurality of end terminal units, the method comprising, at the node, at least one of:

transmitting an optical signal over the access network to the first optical terminal unit, the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows for the plurality of end terminal units;

receiving an optical signal over the access network from the first optical terminal unit, the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows from the plurality of end terminal units.

2. A method according to claim 1 wherein the step of transmitting a set of frequency multiplexed traffic flows comprises transmitting a set of frequency multiplexed carriers and wherein each traffic flow is carried by one of: a different carrier of the set of frequency multiplexed carriers and a different subset of the set of frequency multiplexed carriers.

3. A method according to claim 1 or 2 further comprising receiving traffic from one of a plurality of different operator networks and switching the traffic such that it is applied to one of the frequency multiplexed traffic flows, corresponding to a required end terminal unit of the traffic.

4. A method according to any one of the preceding claims wherein each of the frequency multiplexed traffic flows has a bandwidth and the bandwidths are nonuniform.

5. A method according to any one of the preceding claims further comprising varying at least one of the following during operation:

a bandwidth of one of the traffic flows;

a number of traffic flows allocated to an end terminal unit.

6. A method according to any one of the preceding claims further comprising receiving traffic for the plurality of end terminal units connected to the first optical terminal unit, and forming the set of frequency multiplexed traffic flows.

7. A method according to any one of the preceding claims wherein the optical access network is a wavelength division multiplexed access network using a plurality of different optical wavelength carriers, there being an optical wavelength carrier allocated to each of the optical terminal units.

8. Apparatus for use at a node of an optical access network, the access network comprising a plurality of optical terminal units optically connected to the node and wherein at least a first optical terminal unit is electrically connected to a plurality of end terminal units, the apparatus comprising at least one of:

a transmitter arranged to transmit an optical signal over the access network to the first optical terminal unit, the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows for the plurality of end terminal units;

a receiver arranged to receive an optical signal from the access network, the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows from the plurality of end terminal units.

9. Apparatus according to claim 8 wherein the optical access network is a wavelength division multiplexed access network using a plurality of different optical wavelength carriers, there being an optical wavelength carrier allocated to each of the optical terminal units, and wherein at least one of:

the transmitter is arranged to transmit an optical signal comprising an optical wavelength carrier allocated to the first optical terminal unit;

the receiver is arranged to receive an optical signal comprising an optical wavelength carrier allocated to the first optical terminal unit.

10. A method of operating a first optical terminal unit in an optical access network, the access network comprising a plurality of optical terminal units optically connected to the node, and wherein the first optical terminal unit is electrically connected to a plurality of end terminal units, the method comprising at least one of:

receiving an optical signal over the access network, the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows carrying traffic for the plurality of end terminal units connected to the first optical terminal unit, and converting the set of traffic flows to the electrical-domain and forwarding an electrical signal to each of the plurality of end terminal units;

receiving a set of electrical signals from the plurality of end terminal units, each electrical signal carrying traffic from a respective end terminal unit, forming an optical signal which comprises an optical wavelength carrier which is modulated to carry a set of frequency multiplexed traffic flows carrying the traffic, and transmitting the optical signal over the access network.

11. A method according to claim 10 wherein the step of forming an optical signal comprises modulating a set of frequency multiplexed carriers and wherein each traffic flow is carried by one of: a different carrier of the set of frequency multiplexed carriers and a different subset of the set of frequency multiplexed carriers.

12. A method according to claim 10 or 1 1 further comprising varying at least one of the following during operation:

a bandwidth of one of the traffic flows;

a number of traffic flows allocated to an end terminal unit.

13. A method according to any one of claims 10 to 12 wherein the optical access network is a wavelength division multiplexed access network using a plurality of different optical wavelength carriers, there being an optical wavelength carrier allocated to each of the optical terminal units.

14. A first optical terminal unit for use in an optical access network, the access network comprising a plurality of optical terminal units optically connected to the node, and wherein the first optical terminal unit is electrically connected to a plurality of end terminal units, the first optical terminal unit comprising at least one of: (a) a receiver arranged to receive an optical signal over the access network, the optical signal comprising an optical wavelength carrier which has been modulated to carry a set of frequency multiplexed traffic flows carrying traffic for the plurality of end terminal units connected to the first optical terminal unit, the receiver being arranged to convert the set of traffic flows to the electrical-domain; and

an electrical interface arranged to forward electrical signals corresponding to the traffic flows to the plurality of end terminal units;

(b) an electrical interface arranged to receive a set of electrical signals from the plurality of end terminal units, each electrical signal carrying traffic for a respective end terminal unit; and

a transmitter arranged to form an optical signal which comprises an optical wavelength carrier which is modulated to carry a set of frequency multiplexed traffic flows carrying the traffic, and to transmit the optical signal over the access network.

15. Apparatus according to claim 14 wherein the optical access network is a wavelength division multiplexed access network using a plurality of different optical wavelength carriers, there being an optical wavelength carrier allocated to each of the optical terminal units, and wherein at least one of:

the receiver is arranged to receive an optical signal comprising an optical wavelength carrier allocated to the first optical terminal unit;

the transmitter is arranged to an optical signal comprising an optical wavelength carrier allocated to the first optical terminal unit.