WO2008003136A1

WO2008003136A1 - Multi-functional pon repeater

Info

Publication number: WO2008003136A1
Application number: PCT/AU2007/000928
Authority: WO
Inventors: Chang-Joon Chae
Original assignee: National Ict Australia Limited
Priority date: 2006-07-03
Filing date: 2007-07-03
Publication date: 2008-01-10

Abstract

A multi-functional repeater for optical communications. Upstream signals and downstream signals are separated into an upstream signal path and a downstream signal path. Each path is separately regenerated. The repeater further provides for integration of other networked services, such as wireless, satellite, cable TV, security or the like, to increase utilisation of a single optical infrastructure.

Description

"Multi-Functional PON Repeater"

Cross-Reference to Related Applications

The present application claims priority from Australian Provisional Patent Application No 2006903573 filed on 3 July 2006, the content of which is incorporated herein by reference.

Background of the Invention

Optical fibres are now being deployed to realize fibre to the home (FTTH), fibre to the premises (FTTP), or fibre to the node (FTTN). A passive optical network (PON) is generally considered a cost-effective deployment solution because of the low fibre installation cost and broad bandwidth capability. PON systems are designed to deliver services in digital foπnat in most cases, and thus do not and cannot deliver all types of access services, leaving the expensive fibre infrastructure underutilized.

Separately, implementation of broadband wireless networks has lead to deployment of many remote base stations or antennas which are themselves connected back to a central office by dedicated optical fibres. Another circumstance in which separate fibre deployment arises is for video signal transmission. Due at least in part to regulatory issues, cable television (CATV) operators are deploying separate optical fibres to transmit video signals to their customers to be more competitive. Yet another circumstance involving separate fibre deployment is in the distribution of received satellite signals from a shared dish to multiple dwellings by dedicated in-building cables. Further, separate fibre infrastructures are often needed for sensor networks, security networks, and the like.

The demand for higher bandwidth necessitated by data-intensive multimedia and realtime applications is increasing in the access network. To meet this bandwidth demand, a variety of access technologies are being introduced in the last mile access network. Among these solutions, passive optical networks (PONs) have become the most future proof technology for the delivery of broadband to users. Integrating video services delivery over an existing optical access network has become important since it could potentially reduce the cost of operation. Generally, video services such as cable TV and satellite TV are delivered over separate metal lines within the premises. This infrastructure is not scalable to support high bandwidth high definition image contents. Moreover, PONs can be extended for longer transmission distances to support a larger number of customers using the same infrastructure. In a scenario whereby multiple video channels have to be delivered to multi-dwelling units, apartment houses, or hotels, conventional schemes lead to an over investment. The deployment of separate dedicated fibres for each such application leads to over-built communication infrastructure, under-utilization of some or all fibres, and can lead to higher than necessary service fees for any or all such services.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Summary of the Invention According to a first aspect, the present invention provides a multi-functional repeater for optical communications, the repeater comprising: an upstream port and a downstream port; an upstream wavelength division multiplexer to pass a downstream wavelength from the upstream port to a downstream regenerator and to pass an upstream wavelength from an upstream regenerator to the upstream port; and a downstream wavelength division multiplexer to pass an upstream wavelength to the upstream regenerator and to pass a downstream wavelength from the downstream regenerator to the downstream port.

According to a second aspect, the present invention provides a method for multifunctional signal repeating in optical communications, the method comprising: passing downstream propagating signals at a downstream wavelength to a downstream regenerator; passing regenerated downstream signals from the downstream regenerator into a downstream optical fibre; passing upstream propagating signals at an upstream wavelength to an upstream regenerator; and passing regenerated upstream signals from the upstream regenerator into an upstream optical fibre.

The present invention thus involves wavelength division multiplexing to separate an upstream signal path from a downstream signal path, and provides for regeneration of downstream signals separate to regeneration of upstream signals. This configuration in turn provides for insertion and removal of signals at the regenerator which allows it to provide multiple functions .

For example, a PON system comprising a multi-functional repeater in accordance with the first aspect of the present invention may further comprise a system device connected to the multi-functional repeater such that downstream signals of a system wavelength are passed from the upstream wavelength division multiplexer to the system device, and such that signals of the system wavelength from the system device are regenerated and passed upstream by the upstream wavelength division multiplexer. The system wavelength is preferably spectrally distinct from the upstream wavelength and downstream wavelength to allow wavelength division multiplexed routing of signals to and from the system device. The system device could comprise one or more of: a wireless repeater for wireless communications, such as a mobile telephony base station; and a security camera for remote security monitoring.

Additionally or alternatively, a PON system comprising a multi-functional repeater in accordance with the first aspect of the present invention may further comprise a node device connected to the multi-functional repeater such that upstream signals of a node wavelength are passed by the downstream wavelength division multiplexer to the node device, and such that signals of the node wavelength from the node device are passed downstream by the downstream wavelength division multiplexer. The node wavelength is preferably spectrally distinct from the upstream wavelength, the downstream wavelength and the system wavelength, to allow wavelength division multiplexed routing of signals to and from the node device. The node device may comprise one or more of: a satellite signal transceiver for providing satellite connectivity to users of the node serviced by the multi-functional repeater; a cable TV repeater for providing cable TV service to users of the node serviced by the multifunctional repeater. The multi-functional repeater may further comprise a connection to return upstream signals at a local area networking wavelength in a downstream direction to effect local area networking amongst users of the node serviced by the multi-functional repeater. For example the principles of International Patent Application No. PCT/AU2005/001424 and/or those of International Patent Application No. PCT/AU2005/001355, the content of each of which is incorporated herein by reference, may be applied to effect such local area networking in the multi-functional repeater of the present invention.

It is envisaged that the multi-functional repeater would typically be remotely located from a central office, near a customer-side star coupler, where multiple services networks meet. Embodiments of the present invention may thus provide for improved utilization of fibre infrastructure by providing for an increased variety of services to be distributed via a single fibre distribution network. The invention may be particularly effective where the multi-functional repeater services multi-dwelling units, apartments, office buildings, shopping areas or other such high density precincts, whereby more advanced services may be effected and improved flexibility in service provision may be provided.

Brief Description of the Drawings

An example of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a system schematic illustrating how different service networks share a PON infrastructure by virtue of provision near a PON star coupler of a multifunctional repeater in accordance with the present invention;

Fig. 2 illustrates a multi-functional PON repeater in accordance with a first embodiment of the present invention;

Fig. 3 a illustrates a multi-functional PON repeater in accordance with a second embodiment of the present invention;

Fig. 4 illustrates a multi-functional PON repeater in accordance with a third embodiment of the present invention;

Fig. 5 illustrates a first access network utilizing multi-functional repeaters in accordance with the present invention; Fig. 6 illustrates a second access network utilizing multi-functional repeaters in accordance with the present invention; Fig. 7(a) is a system schematic illustrating a generic integrated repeater based optical access network architecture for video delivery utilising a multi-functional repeater in accordance with the present invention, while Figure 7(b) is a signal schematic illustrating provision of simultaneous RF SCM videos and data services; Fig. 8 is a circuit schematic of an experimental setup used to demonstrate the capabilities of the present invention, with accompanying spectral charts;

Fig. 9 comprises charts of measured BER curves for the upstream and downstream signals, respectively;

Fig. 10 is a chart illustrating the measured EVM values against received optical power;

Fig. 11 comprises plots illustrating the observed constellation diagrams and eye diagrams for the recovered QPSK signal; and

Fig. 12 is a plot of the observed constellation diagram for recovered 16-QAM data for received optical power of -20.81 dBm.

Description of the Preferred Embodiments

Figure 1 is a schematic of a system 100 illustrating how different service networks may share a PON infrastructure by virtue of provision near a PON star coupler 110 of a multi-functional repeater 120 in accordance with the present invention. Thus, the present invention envisages a unit 120 remotely located near the star coupler 110, where multiple services networks meet and are connected to customers 130a ... 130n.

The unit 120 functions as a point of access for multiple different services, offering customers more flexible choice and enabling provision of more advanced services.

Additional features including LAN emulation, bidirectional transmission for interactive video, services protection, and the like may be so implemented.

The system 100 of Figure 1, with the benefit of the functionality of the multi-functional repeater 120, may thus provide for increased utilisation of community owned or accessible fibres, and/or may provide for multiple users such as multiple optical network units (ONUs) in a multi dwelling unit (MDU) to share a single satellite dish and re-distribution system 140, a common CATV regenerator 142, a common wireless repeater / base station 144, and/or common security/health systems such as surveillance cameras 146.

In addition, although not shown in figure 1, new value-added services can be implemented by appropriate use of the remote box 120 and different networks. For example, bidirectional transmission capabilities needed for interactive TV may be implemented via combined use of wireless/wired links and CATV/Satellite links.

The remote box 120 is provided with many interfaces to allow for multiplexing of multiple services. From the customer's perspective, customers can have easy access to any kind of services networks at the remote box 120.

An optical line terminal (OLT) 150 is located at the central office where one or more service provider signals may be multiplexed.

Fig. 2 illustrates a multi-functional PON repeater 200 in accordance with a first embodiment of the present invention. In this embodiment, only two wavelengths, λ and λ^u, are used, being standard-based PON wavelengths. Coarse wavelength division multiplexers 220, 240 provide for a dedicated processing path for downstream signals at wavelength λ^d, and a separate dedicated processing path for upstream signals at wavelength λ^u.

A signal regenerator is provided in the downstream path, comprising an optical to electrical receiver 222, an electrical signal combiner and regenerator 224 and an electrical to optical transmitter 226. Signal regeneration is similarly provided in the upstream path, by way of optical to electrical receiver 242, electrical signal combiner and regenerator 244 and electrical to optical transmitter 246. In each signal combiner and regenerator 224, 244, a range of processing functionality may be required such as signal fonnat conversion, signal multiplexing, signal monitoring, and the like. A processing unit 230 provides for inter-path connectivity, discussed in more detail in the following.

Provision of distinct upstream and downstream signal regeneration paths thus provides for signal regeneration along the optical signal path between the OLT and the ONUs, via star coupler 210. Further, this configuration further provides for repeaters for multiple services to be connected to the multi-functional PON repeater (MFPR) 200, whether via optical fibres, coaxial cables, Ethernet cables, or the like. Such signals can be combined and extracted at each regenerator 224, 244 in the electrical domain using any of the numerous known techniques for multiplexing. The star coupler 110 may be l x N or m x N where N and m are integers. For an m x N^' star coupler, the CWDM 240 may instead be effected by optical isolators. In addition to the optical services provided from the OLT 250 to the multiple ONUs 252a ... 252n via normal PON operation, other services may be distributed to the ONUs 252a ... 252n by injecting their signals into the downstream signal path at combiner 224. For example, signals from a cable TV repeater 260 or satellite repeater 262 may be inserted into the downstream path at combiner 224 for distribution to the ONUs 252a ... 252n. Moreover, return signals from ONUs 252a...252n may be delivered back to the cable TV repeater 260 and/or satellite repeater 262 by appropriate extraction by combiner 244. Such signals can be combined and extracted at each regenerator 224, 244 in the electrical domain using any of the numerous known techniques for multiplexing.

Moreover, a feeder fibre between the OLT 250 and MFPR 200 may be utilised for multiple services, as the signal combiner 244 is configured to provide upstream multiplexing of signals not only from ONUs 252a...252n, but also of signals from system devices such as a wireless repeater or base station 270 and security surveillance camera network 272. Downstream communication from the OLT 250 to the system devices 270, 272 is effected by extraction of such signals by combiner 224. Such signals can be combined and extracted at each regenerator 224, 244 in the electrical domain using any of the numerous known techniques for multiplexing. System devices 270, 272 may themselves have optical fibre feeds, in which case an optical/electrical interface 274 need merely be provided to allow for electrical domain routing by combiners 224 and 244. Thus, the MFPR 200 of the present embodiment provides for feeder fibre between the OLT 250 and MFPR 200 to be utilised not only for communication with the ONUs 252a...252n, but also for communication between OLT 250 and system devices 270, 272 without requiring any change to the. normal PON operation of ONUs 252a...252n. Furthermore, due to the signal regeneration capabilities of the signal combiner 244, the local system devices 270, 272 need only produce signals of sufficient power to reach the MFPR 200, and need not produce signal power sufficient for transmission to the distant OLT 250. Similarly, signals from OLT 250 to system devices 270, 272 may be of reduced power provided appropriate regeneration is applied by signal combiner 224.

Yet another functionality is effected by the MFPR 200, in the form of local area networking within local area network 280. Such networking is effected by the signal combiner 244 extracting upstream local area networking signals transmitted by ONUs 252a...252n and passing such signals to the signal combiner 224 for injection back into the local area network 280 in a downstream direction. Once again, such local area networking signals can be combined and extracted at each regenerator 224, 244 in the electrical domain using any of the numerous known techniques for multiplexing.

Notably, communication links between node devices 260, 262 and ONUs 252a...252n, and between system devices 270, 272 and OLT 250, may each be made bidirectional. For example, interactive TV may be effected by bidirectional communications between cable TV repeater 260 and the ONUs 252a...252n.

Fig. 3 illustrates a multi-functional PON repeater 300 in accordance with a second embodiment of the present invention. Once again, the MFPR 300 is adapted to regenerate signals between an OLT 350 and OMUs 352a...352n, by providing a separate downstream signal path with regenerator 324 for downstream signals at wavelength λ , and a separate upstream signal path having regenerator 344 for pupstream signals at wavelength λ^u.

The MFPR 300 further comprises a star coupler 310 adapted to multiplex node services such as signals to and from satellite receiver 362 and cable TV regenerator 360, for the benefit of nodes 352a...352n. Such additional services are optically CWDM multiplexed and thus connected to the MFPR 300 by optical feeders 364. To permit CWDM multiplexing the node and system services are at wavelengths λ , λ², λ³ and λ which are distinct from each other and are distinct from λ^d and λ^u.

Similarly, the MFPR 300 comprises a CWDM 390 for optically multiplexing system services such as a wireless repeater / base station 370 and security cameras 372. Thus the MFPR again provides for regeneration of standard PON signals and further provides for integration of multiple node and system services to improve utilisation of both the long-haul and last-mile portions of a single optical infrastructure.

Fig. 4 illustrates a multi-functional PON repeater 400 in accordance with a third embodiment of the present invention. The MFPR 400 comprises a reconfigurable or fixed optical add-drop module (OADM). OADM may include optical amplification and maintenance functions. The REG component of the MFPR 400 comprises two TRX units (not shown) connected back-to-back in the same manner as the TRX units 324 and 344 of Figure 3. The MFPR 400 further comprises a processing unit connected to and controlling the OADM, which can have access to each wavelength channel via optical taps. By such a design of the signal processing unit and the OADM the MFPR 400 provides corresponding functionality as already described with respect to Figure 3. In alternative embodiments a similar function may be obtained by replacing the OADM with a small-scale optical cross-connect.

The embodiments of Figures 2 to 4 thus allow a PON infrastructure to be shared by multiple service networks, creating a great cost advantage compared to the current industry practise. Moreover, in the embodiments described customers do not need to buy multiple separate modems to receive various services. In turn, customers pay less and are more inclined to adopt a greater number of broadband services at the lower prices enabled by improved infrastructure utilisation. Still further, builders and/or building owners may be motivated to take this approach because it adds additional value to the property, as merely being connected with fibre increases the property value.

Another advantage afforded by the signal regeneration of upstream signals is that the feeder between the MFPR and OLT may be made longer than 20 km, and depending on the optical power at the regenerator could be up to 60km or more. Furthermore, multiple such MFPRs may be placed along the feeder to achieve longer distances, for example in providing such optical services to rural environments.

In this context, Fig. 5 illustrates a first access network 500 utilizing multi-functional repeaters 502a ... 502d in accordance with the present invention. The network 500 comprises an OLT 550 at a substantial distance (up to 20km or more) from a first MFPR 502a and accompanying star coupler. The network downstream from MFPR 502a is split into three by a star coupler, with a first branch servicing a multi-dwelling unit serviced by another MFPR 502b and accompanying star coupler, a second branch servicing office buildings having accompanying MFPR 502c and star coupler, and a third branch servicing clustered residences having accompanying MFPR 502d and star coupler. Each MFPR 502 provides not only PON functionality but also permits the addition of other services such as wireless antennas connected to MFPR 502a and 502d, satellite dish connected to MFPR 502b, and security cameras connected to MFPR 502c. Fig. 6 illustrates a second access network 600 utilizing multi-functional repeaters in accordance with the present invention. In the network 600 an optical backbone network is leveraged, by including MFPRs in accordance with the present invention, to provide wireless subnets servicing urban and/or rural communities.

While an MFPR in accordance with the present invention requires electric power at the MFPR location remote from the OLT, the node based nature of the MFPR allows commercial power lines to be drawn, and preferably with a battery back-up. The overlaying of multiple services over a single infrastructure may further provide for a more reliable network, and/or performance-enhancement of the standard PON network by the existence of other type of networks. For example, wireless networks can be temporarily used as back-up networks in case of a feeder cable cut or breakdown of a wired network somewhere in its backbone.

Fig. 7(a) is a system schematic illustrating a generic integrated repeater based optical access network architecture for video delivery utilising a multi-functional repeater in accordance with the present invention. The generic integrated optical access network to support data, voice and video services has a 1 by N splitter to support a larger number of customers. To enable a larger split (N < 256) in the network, a remote repeater (MFPR) is used that regenerates the upstream and downstream signals. The repeater needs to be housed close to a power source for an example, at a basement of a building or at a street cabinet to provide services for the residents of an apartment house or a multi-dwelling unit. In this setup, cable TV and satellite TV signal distribution links can be overlaid on the downstream link at the repeater to be delivered to the customers.

Time multiplexing downstream signals with the baseband video signals at the central office in Figure 7a would cause delay for the transmitted signals while increasing various types of overheads for the downstream channel and therefore limits the number of simultaneous video channels. As the number of network splits and transmit distance increases, this problem becomes even more pronounced. Thus the present invention proposes video services delivery for the repeater based optical access network using RF subcarrier multiplexed transmission, whereby the chosen RF carrier is placed outside the bandwidth of the baseband downstream data as shown in Fig. 7(b), which is a signal schematic illustrating provision of simultaneous RF SCM videos and data services. Moreover, these wavelength channels can be different between the two segments of the network to facilitate the use of different standard components. For example, between the ONU and remote node λ_d = 870 nm and λ_u = 850 nm are chosen for cheaper ONU transceivers. The received video channels at the repeater are upconverted to the designated RF frequency and then electrically combined with the downstream signals before the transmission to the customers.

Fig. 8 is a circuit schematic of an experimental setup used to demonstrate the capabilities of the present invention, with accompanying spectral charts. A 1.25 Gb/s downstream signal of 2³¹-1 PRBS NRZ data was directly modulated onto downlink carrier, λ_d and transmitted to the repeater through a 10 km fiber. At the repeater, the downstream signal was detected using a 1.25 Gb/s receiver. A 4.096 Msymbols/s quaternary phase shift keyed (QPSK) data was generated using a vector signal generator and upconverted onto a RF carrier at 1.7 GHz. This signal was then electrically combined with additive white gaussian noise (AWGN) generated from an electrical noise source that simulated multiple video channels.

The detected downstream signal and RF signals were electrically combined and directly modulated onto a downstream wavelength and transmitted to the customers. The first inset chart of Fig. 8 shows the RF spectra of composite signals containing downstream signal and the RF video signals before the optical modulation. The downstream wavelength was passed through a 4 by 4 star coupler and detected using a photo receiver. The received signals were split using an electrical splitter, and the downstream signal was recovered using a low pass filter (LPF) while the RF signals were separated fr.om the downstream signal using a bandpass filter (BPF), and the QPSK signal was recovered using a demodulator. In the upstream direction, 1.25 Gb/s 2³¹-1 PRBS NRZ data was directly modulated onto the uplink carrier, λ_u and transmitted to the repeater, where it was regenerated and transmitted to the CO OLT through the 10 km fiber. CWDM couplers were used at the repeater to separate λ_u and λ_d before the regeneration of the signals.

Fig. 9 shows the bit-error-rate (BER) measurements for the upstream and downstream signals of the experiment shown in Figure 8. For the upstream signal, less than 0.4 dB penalty was observed when the signals were passed through the repeater compared to back-to-back (B-B) measurements. For the downstream data, no penalty was observed when the RF SCM signals were added with the downstream signals. Fig. 10 shows the measured error vector magnitude (EVM) for the recovered 4.096 Msymbols/s QPSK data. For the received optical power of -31.81 dBm, an EVM of approximately 16.67% was measured. The graph shows that RF SCM signals did not suffer crosstalk penalty from the downstream signals.

Fig. 11 shows the observed constellation diagrams and the eye diagrams for the recovered QPSK signal. The left constellation diagram shows the QPSK signals for the B - B case while the right constellation diagram was observed when the received optical power was at -31.81 dBm. Eye diagrams for I (or Q) vector were observed for the above scenarios and it shows clear opening. As the bandwidth requirements for the digital video channels increases, higher order modulation formats have to be adapted without changing the allocated frequency spectrum. The experiment was repeated for 4.096 Msymbols/s 16-quadrature amplitude modulation (QAM) video channel transmissions and the resulting constellation diagram is shown in Fig. 12. As can be seen from the constellation diagram, 16-QAM data signals can be recovered error-free after the transmission.

As the number of ONUs that can supported in the extended repeater based optical access network is large, the number of RF' SCM based video signals may be limited by the power budget. As the number of splits in the SC increases, the splitting loss of the SC also increases. For the calculations, the transmitted optical power, distribution fiber length, attenuation of fiber, WDM coupler loss, and sensitivity for 2.5 GHz optical receiver are chosen to be 0 dBm, 5 km, 0.2 dB/km, 0.75 dB, and -32 dBm respectively. The losses of the 128-split SC and 256-split SC are approximately 24 dB and 27 dB. Under these circumstances, power margins of 6.25 dB and 3.25 dB can be obtained for the video channels. As the bandwidth of the downstream data increases, the total downstream bandwidth increases and therefore higher bandwidth PD is required to receive both signals. As the noise bandwidth of the PD increases, the sensitivity decreases and leads to tighter power budget. Higher bandwidth video channel transmissions can be performed by using higher order modulation formats.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A multi-functional repeater for optical communications, the repeater comprising: an upstream port and a downstream port; an upstream wavelength division multiplexer to pass a downstream wavelength from the upstream port to a downstream regenerator and to pass an upstream wavelength from an upstream regenerator to the upstream port; and a downstream wavelength division multiplexer to pass an upstream wavelength to the upstream regenerator and to pass a downstream wavelength from the downstream regenerator to the downstream port.

2. The multi-functional repeater of claim 1 further comprising a connection to return upstream local area networking signals in a downstream direction to effect local area networking amongst users of the node serviced by the star coupler.

3. The multi-functional repeater of claim 1 or claim 2, further comprising at least one system device port, whereby downstream signals of a system wavelength are passed to the system device port, and such that upstream signals of the system wavelength are passed from the system device port upstream from the repeater.

4. The multi-functional repeater of claim 3 wherein the system wavelength is spectrally distinct from the upstream wavelength and downstream wavelength.

5. The multi-functional repeater of claim 3 or claim 4 wherein the system device port is adapted to pass signals from at least one of: a wireless repeater for wireless communications, such as a mobile telephony base station; and a security camera for remote security monitoring.

6. The multi-functional repeater of any one of the preceding claims, further comprising at least one node device port, whereby upstream signals of a node wavelength are passed to the system device port, and such that downstream signals of the node wavelength are passed from the node device port downstream from the repeater.

7. The multi-functional repeater of claim 6 wherein the node wavelength is spectrally distinct from the upstream wavelength and downstream wavelength.

8. The multi-functional repeater of claim 6 or claim 7 wherein the node device port is adapted to pass signals from at least one of: a satellite signal transceiver; and a cable TV repeater.

9. A PON system comprising at least one multi-function al repeater in accordance with any one of claims 1 to 8.

10. A method for multi-functional signal repeating in optical communications, the method comprising: passing downstream propagating signals at a downstream wavelength to a downstream regenerator; passing regenerated downstream signals from the downstream regenerator into a downstream optical fibre; passing upstream propagating signals at an upstream wavelength to an upstream regenerator; and passing regenerated upstream signals from the upstream regenerator into an upstream optical fibre.