GB2383919A

GB2383919A - Line extender

Info

Publication number: GB2383919A
Application number: GB0200163A
Authority: GB
Inventors: David John Sumner
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 2002-01-04
Filing date: 2002-01-04
Publication date: 2003-07-09
Anticipated expiration: 2022-01-04
Also published as: GB2383919B; GB0200163D0

Abstract

Line extender equipment for extending the range of an electrical telephone line is disclosed. A first interface (12) receives electrical telephone signals from a central office and pre-emphasises the signals such that they are in a form suitable for optical transmission. The pre-emphasised signals are then converted into optical signals and transmitted via optical fibre (13). A second interface (14) converts the optical signals into electrical signals and de-emphasises the electrical signals such that the signals are suitable for driving an electrical telephone line (56). The line extender equipment may allow the range of a telephone line to be extended for DSL services.

Description

LINE EXTENDER The present invention relates to line extenders for extending the range of an electrical telephone line, and to interfaces for use therein. The invention may be used in order to extend the range of telephone lines for carrying high speed data services such as digital subscriber line (DSL) services.

DSL services are used to provide high speed data to subscribers over existing electrical telephone lines (usually copper cable). However, DSL services are limited in the distance they can reach, and subscribers located several kilometres from the telephone exchange may not be able to receive DSL services.

It is anticipated that optical fibres will eventually replace copper cable in the subscriber loop, because of the large bandwidth offered by optical fibres.

However, installing optical fibre is expensive, and proposed optical transmission systems are not compatible with DSL. Operators may nonetheless prefer to install optical fibre rather than copper cable when connecting new subscribers or upgrading existing links.

According to a first aspect of the present invention there is provided an interface for converting electrical telephone signals which are suitable for driving an electrical telephone line into nonelectrical signals for transmission by non-electrical means, the interface comprising: means for pre-emphasising the electrical telephone signals, such that all of the electrical telephone signals are in a form suitable for non-electrical transmission; and means for converting the pre-emphasised signals

into non-electrical signals for transmission by nonelectrical means.

The present invention may allow all of the electrical telephone signals which are necessary to drive an electrical telephone line to be carried by nonelectrical means, such as optical fibre or a radio frequency link. This may allow the range of a subscriber link to be increased, by allowing at least part of the link to be non-electrical. For example, the interface may be used in line extender equipment which replaces some or all of the electrical telephone line between a central office and a subscriber, in order to allow DSL signals to be transmitted to a subscriber who might otherwise be unable to receive DSL services.

The electrical telephone signals may comprise a plurality of signals having different signal levels, and the pre-emphasising means may be arranged to reduce the difference in signal levels. This may reduce the noise or distortion that might otherwise occur in one or more of the signals. Preferably, the preemphasising means is arranged to attenuate signals at frequencies below 100 Hz relative to frequencies above 300 Hz. This can allow low frequency or dc signals, which usually have high signals levels, to have their signal levels brought more in line with those of other signals, which may facilitate transmission of the signals.

The pre-emphasising means may be arranged substantially to equalise the electrical telephone signals. For example, all of the signals may be equalised such that they have levels of the same order of magnitude, or such that the peak signal levels do not differ by more

than a factor of about ten or twenty.

The electrical telephone signals may comprise an information carrying signal and a dc line signal. The information carrying signal may be, for example, an inband telephone, facsimile or modem signal, while the dc line signal may be for powering a subscriber telephone. Alternatively or in addition, the electrical telephone signals may include broadband signals, such as DSL signals. Any type of DSL may be employed, such as symmetric DSL (SDLS), asymmetric DSL (ADSL), very high bit rate DSL (VDSL), rate adaptive DSL (RADSL) or any other type of DSL. The various types of DSL are sometimes referred to generically as xDSL.

The electrical telephone signals may include a line test signal. This can allow network management to be achieved by a traditional copper line test system and/or by a DSL network manager, which may avoid the need to integrate a new network management system into the network.

In a preferred embodiment, the non-electrical signals are optical signals, and optical transmission means are used to transmit the optical signals. Thus, in this embodiment, the pre-emphasised signals are converted into optical signals for optical transmission. The optical transmission means may comprise an optical waveguide such as optical fibre, or free-space optical transmission may be employed.

Preferably, analogue optical transmission is employed.

This may allow relatively simple components to be used for the interface, thereby reducing the cost. For example, intensity modulation, or frequency modulation,

or pulse width modulation may be employed to modulate the optical carrier, although other analogue modulation techniques could also be used.

As an alternative to optical transmission, wireless radio frequency transmission may be used, for example in the microwave band. Thus in an alternative embodiment the pre-emphasised signals are converted into radio frequency signals for wireless transmission.

The interface may further comprise means for converting non-electrical signals into electrical signals, and means for de-emphasising the electrical signals such that the electrical signals are in a form suitable for transmission over an electrical telephone line. This may allow the interface also to receive non-electrical signals which have been subject to pre-emphasis, and to convert those signals into electrical telephone signals for supply to a central office.

The invention extends to an interface which is arranged to receive the non-electrical signals from the interface in the first aspect of the invention. Thus according to a second aspect of the invention there is provided an interface for converting non-electrical signals which have been transmitted by non-electrical means into electrical signals, the interface comprising : means for converting the non-electrical signals into electrical signals; and means for de-emphasising the electrical signals such that the electrical signals are suitable for driving an electrical telephone line.

The interface may include a power amplifier for amplifying the electrical signals such that they can

drive an electrical telephone line.

Any of the features of the first aspect of the invention may also be applied to the second aspect, and vice versa.

According to a third aspect of the invention there is provided line extender equipment for extending the range of an electrical telephone line, the line extender equipment comprising an interface according to the first aspect and an interface according to the second aspect. The line extender equipment may employ optical communication, for example, and may further comprise an optical fibre. Such line extender equipment may allow progressive upgrading of the subscriber link, since the optical fibre which is laid as part of the line extender equipment may be used as part of an all-optical link at a later date.

According to a fourth aspect of the invention there is provided line extender equipment for extending the range of an electrical telephone line, the equipment comprising : means for pre-emphasising electrical telephone signals, such that the signals are in a form suitable for optical transmission; means for converting the pre-emphasised signals into optical signals; an optical fibre for transmitting the optical signals; means for converting the optical signals into electrical signals; and means for de-emphasising the electrical signals such that the signals are suitable for driving an electrical telephone line.

The line extender equipment in either the third or fourth aspect is preferably transparent to electrical telephone signals. In this way the range of a subscriber link can be increased by inserting the line extender equipment, without requiring other modifications to the network.

According to a fifth aspect of the invention there is provided a subscriber line for transmitting signals to a subscriber, the subscriber line comprising a section of electrical telephone line, and line extender equipment according to either of the third or fourth aspects.

Corresponding methods are also provided, and thus, according to a sixth aspect of the invention, there is provided a method of converting electrical telephone signals which are suitable for driving an electrical telephone line into non-electrical signals for transmission by non-electrical means, the method comprising: pre-emphasising the electrical telephone signals, such that all of the electrical telephone signals are in a form suitable for non-electrical transmission; and converting the pre-emphasised signals into nonelectrical signals for transmission by non-electrical means.

According to a seventh aspect of the invention there is provided a method of converting non-electrical signals which have been transmitted by non-electrical means into electrical signals, the method comprising: converting the non-electrical signals into electrical signals; and de-emphasising the electrical signals such that the electrical signals are in a form suitable for

driving an electrical telephone line.

According to an eighth aspect of the present invention there is provided an interface which converts electrical telephone signals which are suitable for driving an electrical telephone line into nonelectrical signals for transmission by non-electrical means, the interface comprising: a pre-emphasis circuit which pre-emphasises the electrical telephone signals, such that all of the electrical telephone signals are in a form suitable for non-electrical transmission; and a converter which converts the pre-emphasised signals into non-electrical signals for transmission by non-electrical means.

According to a ninth aspect of the invention there is provided an interface for converting non-electrical signals which have been transmitted by non-electrical means into electrical signals, the interface comprising : a converter which converts the non-electrical signals into electrical signals; and a de-emphasis circuit which de-emphasises the electrical signals such that the electrical signals are suitable for driving an electrical telephone line.

According to a tenth aspect of the invention there is provided line extender equipment for extending the range of an electrical telephone line, the equipment comprising: a pre-emphasis circuit which pre-emphasises electrical telephone signals, such that the signals are in a form suitable for optical transmission; an electro-optical converter (such as a laser or a light emitting diode) which converts the pre-emphasised

signals into optical signals; an optical fibre for transmitting the optical signals; an opto-electrical converter which converts the optical signals into electrical signals; and a de-emphasis circuit which de-emphasises the electrical signals such that the signals are suitable for driving an electrical telephone line.

Features of one aspect may be applied to any other aspect. Apparatus features may be applied to method aspects and vice versa.

Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which :- Figure 1 shows an overview of a conventional DSL system; Figure 2 shows line extender equipment according to a first embodiment of the present invention; Figure 3 shows the frequency response of a preemphasis network for use in the first embodiment; Figure 4 shows the frequency response of a corresponding de-emphasis network; Figure 5 shows an example of the electrical signals that are present on a electrical telephone line; Figure 6 shows how the signals of Figure 5 would be modified by a pre-emphasis network having the frequency response shown in Figure 3; Figure 7 shows a first example of the use of the line extender of the first embodiment; Figure 8 shows a second example of the use the line extender of the first embodiment ; Figure 9 shows an overview of a line extender in a

second embodiment of the invention; and Figure 10 shows parts of an interface in the second embodiment.

Overview of a conventional DSL system Figure 1 shows an overview of a conventional DSL system. A central office 1 receives telephone signals from the public switched telephone network (PSTN) 2.

The telephone signals are provided directly to a switch 3 in the central office. The central office 1 also receives data signals from the Internet 4 via an Internet service provider (ISP) 5. The data signals are provided to a digital subscriber line access multiplexer (DSLAM) 6 in the central office 1. The DSLAM 6 converts the data signals into DSL signals and provides them to switch 3. Switch 3 switches the telephone signals from PSTN 2 and the DSL signals from DSLAM 6 to the appropriate subscriber. The telephone and data signals are then transmitted simultaneously over an electrical subscriber line 7, which is typically a twisted copper pair.

At the subscriber's premises 8 a DSL modem 9 separates the telephone signals from the DSL signals and provides the telephone signals to a standard telephone or facsimile machine 10. The DSL modem 9 demodulates the DSL signals and provides them as data signals, for example to a computer. The DSL modem also converts data signals into DSL signals for transmission over subscriber line 7 to the central office 1, together with telephone signals from the telephone 10.

Transmission between the central office and the subscriber takes place bidirectionally over the subscriber line 7.

The DSL signals are transmitted between the central

office and the subscriber over the same wires as conventional telephone services, but in a different frequency band. By transmitting DSL signals over the wires that already exist between the central office and the subscriber, the subscriber is provided with a high speed data connection without the need to provide new wires or optical fibres to the subscriber's premises.

For example, ADSL may offer data rates of up to 1. 5Mbps in the downlink from the central office to the subscriber. However, the DSL signals are limited in the range that they can be transmitted. Thus, subscribers who are located more than about 5 km from the central office are usually unable to receive DSL services.

First embodiment Figure 2 shows line extender equipment according to a first embodiment of the present invention. The line extender equipment comprises first interface circuit 12, optical fibre 13, and second interface circuit 14.

The first interface circuit 12 is designed to interface with a central office via electrical wire 54, and the second interface circuit 14 is designed to interface with a subscriber via electrical wire 56, which may be a copper telephone line. First interface circuit 12 comprises hybrid circuit 16, pre-emphasis network 18, pre-amplifier 20, laser 22, duplexer 24, photodiode 26, pre-amplifier 28, de-emphasis network 30 and power amplifier 32. Second interface circuit 14 comprises hybrid circuit 36, pre-emphasis network 38, preamplifier 40, laser 42, duplexer 44, photodiode 46, pre-amplifier 48, de-emphasis network 50 and power amplifier 52.

In operation, hybrid circuit 16 separates the incoming and outgoing electrical signals from the central

office. The incoming signals are fed to pre-emphasis network 18. As will be explained, pre-emphasis network 18 pre-emphasises the electrical signals so that they are in a form suitable for optical transmission. The pre-emphasised signals are fed to pre-amplifier 20 and thence to laser 22. Laser 22 is a linear laser, and converts the electrical signals at its input into optical signals whose intensity corresponds to the level of the electrical signals. The optical signals from the laser 22 are fed to duplexer 24 which couples the optical signals from laser 22 into the optical fibre 13, and couples the optical signals from optical fibre 13 to photodiode 26. Duplexer 24 may be, for example, an optical circulator, a directional coupler, or any other suitable device for coupling and separating the respective optical signals.

Photodiode 26 converts the incoming optical signals from optical fibre 13 into electrical signals whose level corresponds to the intensity of the optical signals. The output of the photodiode 26 is fed via pre-amplifier 28 to de-emphasis network 30. Deemphasis network 30 de-emphasises the electrical signals to restore them to a level suitable for transmission over a copper line. The de-emphasised signals are then fed to copper line 54 via power amplifier 32 and hybrid circuit 16.

Second interface circuit 14 converts incoming and outgoing electrical signals on copper line 56 into incoming and outgoing optical signals on optical fibre 13. The components in the second interface circuit 14 operate in a similar way to the corresponding components in the first interface circuit 12.

The signals which are typically carried by an

electrical telephone line include speech or data signals in the band from 300 Hz to 3 kHz, and a SOV dc power supply which is used to power the subscriber's telephone. DSL signals, if present, are transmitted in a radio frequency (rf) band up to 30 MHz. Table 1 shows examples of the various signals-which may be transmitted over an electrical telephone line, together with their approximate signal levels.

Table 1

signal band typical level power supply (on hook) dc 50V off hook dc 5V loop-disconnect dialing near dc 50V tone DTMF dialing 697-1633Hz ImW ("0. 7V) ringing 16Hz 70V two-way speech 300Hz-3kHz ImW ("0. 7V) in-band data signals 300Hz-3kHz ImW (-0. 7V) ISDN 300Hz-140kHz ImW ("0. 7V) DSL rf up to 30MHz 25mW (-3. SV) line test signals dc 50V The line extender equipment of the present embodiment is arranged to be transparent to the rest of the network. In order to achieve this, the electrical signals sent to and received from the copper line 56 are in substantially the same format as the electrical signals received from and sent to the copper line 54.

Thus the line extender equipment of the present embodiment is capable of receiving and transmitting all of the signals shown in Table 1.

Referring to Table 1, it can be seen that the various signals have quite different signal levels. If the

electrical signals were converted directly to optical signals, the lower-level signals, such as speech or data signals or DSL signals, might be subject to distortion or noise. Thus the electrical signals are pre-emphasised prior to conversion to optical signals in order at least partially to equalise the levels of the various electrical signals. When the optical signals have been transmitted through the optical fibre and converted back into electrical signals, they are de-emphasised in order to restore the levels of the original electrical signals.

Figure 3 shows the frequency response of a pre-emphasis network for use in the present embodiment. The preemphasis network acts to attenuate signals in the band from dc to 100 Hz by 34 dB relative to signals with a frequency of 300 Hz and higher. The effect of this pre-emphasis is to reduce the levels of the dc signals, which were previously at 50V, to around IV, so that they are at approximately at the same level as the other signals. This enables the linear range of the lasers 22, 42 in the interfaces 12,14 to be better exploited.

Figure 4 shows the frequency response of a corresponding de-emphasis network. The de-emphasis network amplifies signals in the band from dc to 100 Hz by 34 dB relative to signals at frequencies above 300 Hz. This restores the electrical signals to the levels which they were at prior to pre-emphasis.

The design of filter networks to achieve the frequency responses shown in Figures 3 and 4 would be apparent to the skilled person, and so further details of such networks are not given.

Figure 5 shows an example of the electrical signals that might be present on a electrical telephone line.

Figure 6 shows how the signals of Figure 5 would be modified by a pre-emphasis network having the frequency response shown in Figure 3. A de-emphasis network having the frequency response shown in Figure 4 would convert the signals of Figure 6 back to the form shown in Figure 5.

Pre-emphasis networks and de-emphasis networks having different frequency responses to those shown in Figures 3 and 4 may be used where appropriate. For example, the pre-emphasis network may attenuate signals above 3 kHz to a certain extent, so that the DSL signals have substantially the same signal levels as the speech signals, and the de-emphasis network may then amplify signals above 3 kHz by the same amount. In general, the pre-emphasis network reduces differences in the levels of the electrical signals, and the pre-emphasis network restores the signals to their original levels.

All of the components in the interfaces 12,14 have a response down to zero frequency (dc), to enable the dc line signals to be transmitted. In this embodiment a resistive bridge is used for the hybrid circuits 16, 36, although other suitable components could also be used. The interfaces 12,14 are dc-coupled throughout.

The second interface 14 may be required to deliver several watts of power at 50V dc to the copper line 56.

Thus the power amplifier 52 is of sufficient rating to deliver such power levels. If required, one or both of the interfaces 12,14 can incorporate separate power amplifiers to amplify different frequency bands, to avoid the need to provide a single wide band, dccoupled high power amplifier. For example, a separate

power amplifier might be provided to amplifier the DSL signals, which are at rf frequencies.

The line extender of the present embodiment uses baseband transmission to transmit intensity modulated

optical signals over the optical fibre. Linear lasers ) p' and photodiodes are used to convert the electrical signals into optical signals and vice versa. Linear lasers and photodiodes have been developed for use in transmitting multiple television signals, and such devices are suitable for use in the present embodiment.

Lasers with external modulators may be used instead of directly modulated lasers, if desired. Where appropriate, light emitting diodes (LEDs) may be used instead of lasers.

Figure 7 shows a first example of how the line extender of the present embodiment might be used. In this example, the line extender extends from a central office 60 to a cabinet 66 which is located part of the way along a subscriber link. Referring to Figure 7, central office 60 comprises switch 3 and DSLAM 6, which are the same as in a conventional central office. Line extender 62 comprises first interface 12, optical fibre 13, second interface 14 and power supply unit 64.

First interface 12 is located within central office 60, while second interface 14 and power supply unit 64 are located within cabinet 66. Cabinet 66 may be located, for example, in the subscriber's street, or at any other convenient location.

In operation, the switch 3 outputs signals which are in a format suitable for transmission over an electrical telephone line. The signals which are for transmission to subscriber 8 are fed via electrical wire 68 to first interface 12. As explained above, first interface 12

converts the electrical signals into optical signals for transmission over optical fibre 13. The thus transmitted optical signals are converted back into electrical signals by second interface 14 and then transmitted via a section of electrical wire 56 to the subscriber 8. Signals from the subscriber to the central office are transmitted upstream in a similar way. In this example, the section of electrical wire 56 is wire which is already in place between the cabinet 66 and the subscriber's premises 8.

Power supply unit 64 is provided in the cabinet 66 in order to supply power to the interface 14. The cabinet 66 is connected to a source of ac power, and this is converted by power supply unit 64 into dc power for supplying the second interface 14. The power supply unit 64 is of sufficient rating to power the interface 14, which may need to supply several watts of power to the copper line 56. Power supply unit 64 also includes a back-up battery (not shown) for providing lifeline services should the ac power source fail.

Although only a single line extender 62 is shown in Figure 7, further line extenders may be provided in a similar way for other subscribers. Such line extenders may share the same cabinet 66 and power supply unit 64, where appropriate.

The arrangement shown in Figure 7 is suitable for cases where DSL cannot reach a subscriber who is located far from the central office. This arrangement can allow future upgrade to full digital operation when demand grows to justify the cost.

Figure 8 shows a second example of how the line extender of the present embodiment might be used. In

this example, the second interface 14 and the power supply unit 64 are located at the subscriber's premises 70, and the optical fibre 13 extends from the central office 60 to the subscriber. However, the line extender 62 presents an electrical interface to both the switch 3 and the DSL modem 9, and thus functions in the same way as copper cable from the point of view of these devices. Switch 3 and DSL modem 9 therefore do not need to be modified for optical transmission.

The arrangement shown in Figure 8 might be suitable for new dwellings, where the cost of installing fibre is similar to the cost of installing copper cable.

Installing optical fibre means that the system can be easily upgraded to a fully optical network in the future. The arrangement of Figure 8 might also be appropriate where electromagnetic interference (EMI) problems would otherwise arise due to rf DSL signals being transmitted over copper cables.

Since the line extender described above uses low cost optical components and does not require modification of the rest of the network, it can be installed at low cost.

Second embodiment In a second embodiment of the invention, a plurality of subscriber lines are extended using a single optical fibre. Figure 9 shows an overview of a line extender in the second embodiment. Referring to Figure 9, line extender 80 comprises first interface 82, optical fibre 84, second interface 86 and power supply unit 88. The first interface 82 is located at a central office 90, and the second interface 86 is located together with power supply unit 88 at a cabinet 92.

In operation, the first interface 82 receives a plurality of electrical signals from switch 3. Each of these electrical signals is destined for a different subscriber. As will be explained below, the first interface 82 converts the plurality of electrical signals into optical signals for transmission over a single optical fibre 84. The second interface 86 converts the optical signals into a plurality of electrical signals for transmission via a plurality of separate copper lines to a plurality of subscribers.

Signals are also transmitted from the subscribers to the central office in a similar way.

Figure 10 shows parts of the first interface 82 in the second embodiment. The interface 82 comprises hybrid circuits 102,104, 106, pre-emphasis networks 108,110, 112, pre-amplifiers 114,116, 118, laser diodes 120, 124,126, wavelength division multiplexer 128, duplexer 130, wavelength division demultiplexer 132, photodiodes 134,136, 138, pre-amplifiers 140,142, 144, deemphasis networks 146,148, 150, and power amplifiers 152,154, 156.

The interface 82 receives the plurality of electrical signals from a plurality of electrical lines 158,160, 162. The hybrid circuits 102,104, 106 separate the incoming and outgoing electrical signals on each electrical line. The incoming signals are fed to respective pre-emphasis networks 108,110, 112. Each of the pre-emphasis networks has the same function as the pre-emphasis network described above with reference to the first embodiment, and so is not described further. Signals from the pre-emphasis networks are fed to pre-amplifiers 114,116, 118 and thence to respective laser diodes 120,124, 126.

Each of the lasers 120, 124, 126 is a linear laser, and' converts the electrical signals at its input into optical signals whose intensity corresponds to the level of the electrical signals. However, each of the lasers 120,124, 126 produces an optical signal having a different wavelength. These optical signals are then combined in wavelength division multiplexer 128. The output of wavelength division multiplexer 128 is fed to duplexer 130 which couples the optical signals into optical fibre 84. Duplexer 130 also separates incoming and outgoing optical signals.

Incoming optical signals from the optical fibre 84 are fed by duplexer 130 to wavelength division demultiplexer 132, which separates optical signals having different wavelengths. The various optical signals are then fed to respective photodiodes 134, 136,138, which convert the optical signals into electrical signals.

The outputs of the photodiodes 134,136, 138 are fed via pre-amplifiers 140,142, 144 to respective deemphasis networks 146,148, 150. The de-emphasis networks 146,148, 150 have the same function as the de-emphasis network described above with reference to the first embodiment. The outputs of the emphasis networks are fed to respective copper lines 158,160, 162 via power amplifiers 152, 154, 156 and hybrid circuits 102,104, 106.

Second interface 86 is constructed in a similar way to first interface 82.

By wavelength division multiplexing the signals which are carried on the optical fibre 84, a single optical fibre can be used to extend the line of two or more

subscribers. This may reduce the cost per subscriber of the line extender.

In the second embodiment, multiplexing techniques other than wavelength division multiplexing may be used instead. For example, time division multiplexing, code division multiplexing, polarisation multiplexing, or any other type of multiplexing could be used, with the appropriate changes to the system hardware.

In either of the above embodiments, duplex operation over the fibre link may be achieved through the use of isolators and directional couplers, or circulators, or wavelength multiplexing or polarisation multiplexing may be used, or separate fibres may be used for the uplink and downlink, or any other form of multiplexing may be used.

In the above embodiments the line extender is transparent to the rest of the network. Thus, network management can be achieved by a traditional copper line test system and by the DSL network manager. This avoids the need to integrate a new network management system into an existing network.

The line extenders described above do not depend on a knowledge of the type of signals being used on the copper line, and do not require modification or adjustment should these change in the future. Thus, any type of signal which can be carried by a copper telephone line can be carried by the line extender without a requirement to know the details of the signalling and line conditions employed. For example, plain old telephone services (POTS), ISDN, DSL, and Ethernet signals may be carried as well as the ringing and battery information and the line test information.

At a future date, the extender can be removed and additional optical fibre installed to cover the full distance to the subscriber. In this way, the operator can start to invest in optical fibre now, and can fully utilise the asset in the future. Since the line extender consists of simple components, it can be installed at relatively low cost, compared to a fully optical digital link.

The transparent analogue techniques described above may have the advantage of low cost, simplicity, and easy integration into existing networks. By contrast, digital optical systems have the disadvantages of high power consumption, expensive firmware, standardisation delay and cost, network management integration difficulties, the need for multiple interfaces, and latency.

Use of the line extender described above to carry DSL signals avoids radio interference and EMI problems that might otherwise arise if the signals were carried over a copper line.

It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention. For example, although intensity modulation has been given as an example of an analogue optical transmission technique, other techniques may be used instead. For example, a carrier or pulse modulation system could be used, whereby the amplitude of the optical signals remains substantially constant and information is carried by, for instance, frequency modulation or pulse width modulation. In such cases, it is still possible to apply pre-emphasis and deemphasis in order to convey ringing and DC conditions.

As an alternative to analogue optical transmission techniques, the electrical signals may be digitised and transmitted over the optical fibre using digital transmission techniques. In this case, the digital signals are restored to analogue signals prior to onward transmission over electrical cables.

As an alternative to optical fibre transmission, wireless optical transmission, or wireless radio frequency transmission (for example, at frequencies of around 75 GHz) could be used instead.

Claims

CLAIMS 1. An interface for converting electrical telephone signals which are suitable for driving an electrical telephone line into non-electrical signals for transmission by non-electrical means, the interface comprising: means for pre-emphasising the electrical telephone signals, such that all of the electrical telephone signals are in a form suitable for non-electrical transmission; and means for converting the pre-emphasised signals into non-electrical signals for transmission by nonelectrical means.
2. An interface according to claim 1, wherein the electrical telephone signals comprise a plurality of signals having different signal levels, and the preemphasising means is arranged to reduce the difference in signal levels.
3. An interface according to claim 1 or 2, wherein the pre-emphasising means is arranged to attenuate signals at frequencies below 100 Hz relative to frequencies above 300 Hz.
4. An interface according to any of the preceding claims, wherein the pre-emphasising means is arranged substantially to equalise the electrical telephone signals such that all of the signals have levels of the same order of magnitude.
5. An interface according to any of the preceding claim wherein the electrical telephone signals comprise an information carrying signal and a dc line signal.

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6. An interface according to any of the preceding claims, wherein the electrical telephone signals include broadband signals.
7. An interface according to any of the preceding claims, wherein the electrical telephone signals include DSL signals.
8. An interface according to any of the preceding claims wherein the electrical telephone signals include a line test signal.
9. An interface according to any of the preceding claims wherein the pre-emphasised signals are converted into optical signals for optical transmission.
10. An interface according to claim 9, wherein analogue optical transmission is employed.
11. An interface according to any of claims 1 to 8 wherein the pre-emphasised signals are converted into radio frequency signals for wireless transmission.
12. An interface according to any of the preceding claims, further comprising: means for converting non-electrical signals into electrical signals ; and means for de-emphasising the electrical signals such that the electrical signals are in a form suitable for transmission over an electrical telephone line.
13. An interface for converting non-electrical signals which have been transmitted by non-electrical means into electrical signals, the interface comprising :

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means for converting the non-electrical signals into electrical signals; and means for de-emphasising the electrical signals such that the electrical signals are in a form suitable for driving an electrical telephone line.
14. An interface according to claim 12 or 13, including a power amplifier for amplifying the deemphasised electrical signals such that they can drive an electrical telephone line.
15. Line extender equipment for extending the range of an electrical telephone line comprising an interface according to any of claims 1 to 12 and an interface according to claim 13 or 14.
16. Line extender equipment according to claim 15, the equipment employing optical communication, and further comprising an optical fibre.
17. Line extender equipment for extending the range of an electrical telephone line, the equipment comprising: means for pre-emphasising electrical telephone signals such that the signals are in a form suitable for optical transmission; means for converting the pre-emphasised signals into optical signals; an optical fibre for transmitting the optical signals; means for converting the optical signals into electrical signals; and means for de-emphasising the electrical signals such that the signals are suitable for driving an electrical telephone line.

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18. Line extender equipment according to any of claims 15 to 17, wherein the equipment is transparent to electrical telephone signals.
19. A subscriber line for transmitting signals to a subscriber, the subscriber line comprising: a section of electrical telephone line; and line extender equipment according to any of claims 15 to 18.
20. A method of converting electrical telephone signals which are suitable for driving an electrical telephone line into non-electrical signals for transmission by non-electrical means, the method comprising : pre-emphasising the electrical telephone signals, such that all of the electrical telephone signals are in a form suitable for non-electrical transmission; and converting the pre-emphasised signals into nonelectrical signals for transmission by non-electrical means.
21. A method of converting non-electrical signals which have been transmitted by non-electrical means into electrical signals, the method comprising: converting the non-electrical signals into electrical signals; and de-emphasising the electrical signals such that the electrical signals are in a form suitable for driving an electrical telephone line.
22. Apparatus substantially as described herein with reference to and as illustrated in the accompanying drawings.
23. A method substantially as described herein

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with reference to the accompanying drawings.