GB2061672A - Electro-optical telephone line interface unit - Google Patents

Abstract

An electro-optical interface allows data communication along a telephone line without electrically connecting the digital equipment thereto. A standard modem 16 and line connection unit 15 are connected to the lines 12. A logic-to-optical converter 18 converts the modem output to light signals and vice- versa. The light signals are fed along optical fibres 19, 25 connected between converter 18 and a connector 20. A corresponding mating connector 22 with fibres 23, 24 feed the signals to and from the digital equipment 11. <IMAGE>

Description

SPECIFICATION Electro-optical telephone line interface unit The present invention is concerned with an electro-opticai telephone line interface unit for digital data transmitted along telephone lines.

It is becoming increasingly common to use telephone lines, both public and private, for the transmission of digital data. Examples of this use are the view data systems either operated or proposed by various national telephone authorities, data links between computers, and facsimile transmissions over telephone lines.

When equipment is connected to either public or private telephone lines, it is important that precautions are taken to prevent the equipment from damaging the line system, causing interference in other lines or presenting any hazard to other users of the line system. For example, it is usual for national telephone authorities to insist that a piece of equipment may only be connected to the public switched telephone system if that equipment has been fully vetted and approved by the authority. Furthermore, it is normal for the authority to lay down standard tones to be used for data transmissions over the telephone lines. Typically, a pair of tones of different frequencies is employed to indicate a logic 0 or a logic 1 respectively. For different information rates (baud rates) different pairs of tones are employed.These tones are layed down by the telephone authority and must be used by all equipment sending digital information over the telephone line.

The need, hitherto, for any equipment desired to be connected to the telephone lines to be fully approved by the telephone authority has been a considerable disincentive to the use of telephone lines for data transmission.

Furthermore, with the emergence of view data systems, in which digital information transmitted over the telephone system is displayed on a domestic television receiver, there is the extra potential hazard resulting from the presence of very high supply voltages present in television receivers, typically of the order of 30 kilovolts.

The present invention provides an interface unit for interfacing equipment with a telephone line pair for receiving and/or sending digital information over the telephone line, whilst ensuring complete electrical isolation between the digital equipment and the telephone line and also ensuring that the correct tone codes are employed in the data transmission on the line.

Accordingly, an electro-optical telephone line interface unit for digital data transmitted along telephone lines, comprises a line connection unit adapted for connection to a telephone line pair for receiving and/or sending signals on said line pair, a demodulator and/ or modulator connected to demodulate signals received on said line pair, to reproduce corresponding logic level signals, and/or to modulate digital data logic level signals for sending on said line pair, a logic level to optical signal converter to produce optical signals corresponding to the received demodulated logic level signals and/or to convert optical signals into logic level signals for modulation and sending, and an optical connector adapted to co-operate with a mating connector associated with digital equipment intended to receive and/or send digital data on the telephone line pair, so that optical signals produced by said converter can be transmitted to said equipment, and/or optical signals from said equipment can be received by said converter, via said connector. Such an interface unit could be supplied by the telephone authority (for use on public switched lines) who could then ensure that the demodulator and/or modulator produced the standard tone codes which the authority required. Since the only connection with the digital equipment is via an optical connector, the problem of electrical isolation between the telephone line and the equipment is obviated. Furthermore, the unit does no more than convert optical signals from the digital equipment into the requisite tone signals for transmission on the telephone line and/or conversely receive such tone signals and convert them into optical signals for transmission to the digital equipment.The interface unit may be arranged to operate only at a single baud rate in which case the demodulator and/or modulator is arranged to recognise and/or produce the appropriate tones as designated for that information rate by the telephone authority. It is then necessary for the user of the digital equipment only to be able to receive and/or send the optical signals via the optical connector at the designated information rate. The protocol for receiving and/or sending data would be entirely under the control of the user of the digital equipment, in association with the digital equipment, at the other end of the telephone line, with which the data is being exchanged.

In a preferred arrangement, the optical connector of the interface unit comprises an optical fibre connector and is connected to said converter by one or more lengths of optical fibre. Such optical fibre connectors are now available on the market and would provide a simple and convenient way of making the digital data connection between the digital equipment and the interface unit. The digital equipment must, of course, be provided with a mating optical fibre connector which would normally be connected to the main body of the equipment via one or more further lengths of optical fibre. Furthermore, a further set of optical to logic level converters would be provided in the digital equipment for sending and/or receiving optical signals along the optical fibre connection.

In the simplest system envisaged digital data is either only received or only transmitted by the digital equipment along the telephone line in which case a single demodulator, or modulator respectively, is required in the interface unit and only a single optical fibre is required to connect the digital equipment to the interface unit via the optical connector.

However, the interface unit may provide for both reception and transmission of data on the telephone line- in which case both a modulator and demodulator are provided and two interconnecting optical fibres may be used for the transmission and reception channels. Still further, the interface may be arranged to allow transmission and/or reception at more than one baud rate in which case further modulators and/or demodulators may be provided and corresponding further optical fibres.

Examples of the present invention will now be described with reference to the accompanying drawing which shows a block schematic diagram of an interface unit incorporating the present invention interfacing digital equipment with a telephone line pair.

Referring to the drawing, an interface unit 10 is connected between digital equipment 11 and a pair 1 2 of telephone lines. In the example shown the telephone lines 1 2 are part of a public switched telephone system and are connected to a pair 1 3 of lines leading to the Telephone Exchange. The lines 1 2 may be associated with a normal telephone receiver and are shown connected to a hand set 14.

In the example illustrated, the interface unit 10 is arranged for both the transmission and reception of data by the digital equipment 11 at two different baud rates. The telephone lines 1 2 are connected in the interface unit 10 to a line connection unit 1 5 which provides the usual line isolation and matching required for connecting equipment to telephone lines. For this purpose standard telephone line interfaces may be used such as specified in the United Kingdom in Post Office Technical Specifications 26 and 30. These interfaces provide the usual safety disconnection barriers.

The line connection unit is connected to a modem 1 6. In the present example the modem 1 6 provides for the demodulation and modulation of data at each of two different baud rates. As explained previously, data transmission on the telephone line can take place at a number of baud rates permitted by the telephone authority and each of these rates uses a different pair of signalling tones.

Thus, the demodulators in modem 1 6 can simply comprise band pass filters tuned to the signalling tones for the respective baud rates.

Thus, each demodulator comprises filters tuned to the signalling tones for the respective baud rate. The demodulators further comprise detectors for sensing whether tones at the filter frequencies are present and for generating a logic 1 or a logic 0 in response to the tones as appropriate. The detectors may provide the logic level signals in any desired logic system, e.g. suitable for TTL logic circuits.

The logic level signals produced by the demodulators are sent on lines 1 7 and 18 to a logic-to-optical converter unit 1 8 and thence as corresponding optical signals along optical fibres 1 9 to the digital equipment 11 via an optical fibre connector 20.

The modem 1 6 also comprises a pair of modulators for generating from logic level signals provided on lines 21 appropriate tone signals for transmission on the telephone line 12. One of the lines 21 is selected for logic level signals at one of the two specified baud rates and supplies the logic level signals to a respective one of the modulators which is adapted to generate the appropriate tones for that baud rate. The modulators may comprise, for example, phase locked loop tone generators tuned to the respective tone frequencies or altenatively the tone frequencies may be generated from a crystal controlled master clock frequency which is divided down appropriately.

Modems of the kind suitable for modem 1 6 are already well known in connection with data signalling over telephone lines and known modems would be suitable in the interface unit 10.

The logic level signals generated by the demodulators in the modem 1 6 and supplied on the lines 1 7 to the converter 1 8 are converted therein to corresponding optical signals. For this purpose, the converter 18 may include light generators such as light emitting diodes or semi-conductor lasers driven in response to the logic level signals received from the demodulator. The light emitted by the LED's or semi-conductor lasers is directed into the ends of optical fibres 1 9 for transmission along the fibres 1 9 to the connector 20. The connector 20 is provided at an outer wall of the interface unit so that it can be interconnected with a mating connector 22 associated with the digital equipment 11.

As mentioned previously, optical fibre connectors which would be suitable as the mating connectors 20 and 22 are commercially available. For example, 4 or 8 way fibre connectors from ITT would be suitable. The optical signals conducted along the fibres 1 9 to the connector 20 are fed into the ends of the corresponding fibres 23 running from the connector 22 to the digital equipment 11 so that the optical signals are fed to the equipment 11 for reconversion into logic level signals and appropriate use.

Correspondingly, logic level signals to be sent by the equipment 11 for transmission on the telephone lines 1 2 are converted in the digital equipment 11 into optical signals in a manner similar to the converter 18, and fed into the fibres 24 for transmission to the connector 22 and thence by the fibres 25 to the converter 1 8. In the converter 18, the incoming optical signals from the equipment 11 are converted back into corresponding logic level signals for example by using photosensitive devices such as a photo voltaic or photosensitive semi-conductor devices. The logic level signals produced thereby are then transmitted along the lines 21 to the modem 1 6 for modulation and transmission on the telephone line 12.

Various additional features may be included in the interface unit 10. For example, the unit may include an auto-dialler system whereby a predetermined telephone number may be dialled automatically on receipt by the unit of a specific command to connect the interface unit via the telephone lines 1 2 with a chosen remote equipment, such as a computer. Autodiallers for this purpose are known and in the present case the command may be generated by a "dial" button provided on the interface unit. Alternatively, the command may be provided by a signal supplied to the interface unit from the digital equipment 11, for example as on optical signal via the connector 20.

It is also normal for the modem 1 6 and line connection unit 1 5 to be switchable into an operative condition from a standby condition.

The modem 1 6 is normally placed in a standby condition when not required to avoid spurious responses which might result from speech signals for example when the telephone line 1 2 is used for normal speech communication. Again, the modem 1 6 and line connection unit 1 5 may be switched into operation from standby by pressing a "data" button located on the interface unit 1 0. Also, another common method is the so-called "auto answer" mode on leased or private lines etc. Alternatively, the unit may be switched into operation by a remote command signal for for example as supplied from a telephone answering machine adapted to respond to a computer on an incoming call.In this way, a remote computer could place itself in communication with the digital equipment 11 automatically via the usual telephone answering machine with suitable modification which would respond to receipt of the normal computer tone produced by the remote computer by sending the command signal to activate the interface unit 10 ready for data exchange between the equipment 11 and the remote computer. The advantage of this arrangement is that the equipment 11 can be called up by the remote computer on a standard telephone number, e.g. one also used for voice communication. If a human caller dials the number, the telephone answering machine will operate in the usual way and the interface unit 10 is not activated.

The optical signals may be transmitted along the fibres 23 using any known system of coding light, such as on-off switching or two frequency (i.e. two colour) switching. Further, the signals may be multiplexed, either by time or by light frequency and the light frequencies may be split up or separated on decoding by one or more appropriate filters.

Optionally a data and/or flag checking unit may be provided to generate standard checking flags such as "quality", "data set ready", "carrier detect", "data valid", etc., for use by the digital equipment 11. The checking unit may be connected directly to the equipment 11, or if preferred optically connected to the interface unit 10 via a further set of optical fibres.

Claims (6)

1. An electro-optical telephone line interface unit for digital data transmitted along telephone lines, comprising a line connection unit adapted for connection to a telephone line pair for receiving and/or sending signals on said line pair, a demodulator and/or modulator connected to demodulate signals received on said line pair, to reproduce corresponding logic level signals, and/or to modulate digital data logic level signals for sending on said line pair, a logic level to optical signal converter to produce optical signals corresponding to the received demodulated logic level signals and/or to convert optical signals into logic level signals for modulation and sending, and an optical connector adapted to co-operate with a mating connector associated with digital equipment intended to receive and/or send digital data on the telephone line pair, so that optical signals produced by said converter can be transmitted to said equipment, and/or signals from said equipment can be received by said converter, via said connector.

2. An interface unit as claimed in claim 1, wherein the optical connector of the interface unit comprises an optical fibre connector and is connected to said converter by one or more lengths of optical fibre.

3. An interface unit as claimed in claim 2 and having a single demodulator, or modulator, for only reception, or only transmission respectively of data by the digital equipment along the telephone lines, and a single length of optical fibre between the connector and the converter.

4. An interface unit as claimed in claim 2 and having both modulator and demodulator with two optical fibres, for both reception and transmission.

5. An interface unit as claimed in claim 2 having more than one modulator and/or more than one demodulator with corresponding optical fibres for transmission and/or reception at more than one band rate.

6. An electro-optical telephone line interface unit substantially as hereinbefore de scribed with reference to and as illustrated in the accompanying drawing.