WO1988005240A1

WO1988005240A1 - Communications systems

Info

Publication number: WO1988005240A1
Application number: PCT/GB1988/000012
Authority: WO
Inventors: Bikhu Ardeshir Unvala
Original assignee: Unvala Limited
Priority date: 1987-01-09
Filing date: 1988-01-08
Publication date: 1988-07-14
Also published as: GB8700458D0; EP0340231A1

Abstract

Two to nine telephones (TI1-TI9) are connected to a telephone cable (20) by control or signalling devices (CD1 to CD9), there being one per telephone. The devices (CD) together act as a telephone exchange distributed along the cable (20). The cable may be directly connected to an outside telephone company line (21). In one version, each signalling device (CD) is able to receive on only one frequency unique to that device, but is able to transmit on any selectable one of the reception frequencies of the devices (CD). Each device (e.g. CD1) responds to the selection by its telephone TI1 of another (e.g. TI2) of the telephones to signal to the device (CD2) associated with another telephone (TI2) to establish a two way communication link. Multiway links are also possible, as are links to the outside line. Preferably all signals transmitted and received by the devices (CD) are of magnitude less than the maximum magnitude of noise permitted in the external line by the telephone company.

Description

COMMUNICATIONS SYSTEMS

The present invention relates to communications systems. Illustratively, the invention relates to a telephone system and the invention and its background will be described herein in relation to a telephone system.

It is known to use in e.g. large offices having many telephones a private branch exchange (pabx) which allows the telephones to be used as an intercom system as well as to access the external line. A pabx is, however, very expensive and cannot usually be justified for use in e.g. a domestic dwelling or in a small office. Many domestic dwellings, ^"and small offices have several telephones and it is likely that the numbers of telephones per dwelling or small office will increase.

Thus according to one aspect of the present invention there is provided a telephone system in which a plurality of telephones are connected to a cable arrangement for transmitting telephone signals by one signalling device per telephone, the devices ^' cooperating one with another to act as a telephone exchange distributed about the cable arrangement to automatically establish a communication link between telephones in response to selection by one of the telephones of another of the telephones-

There is also provided a signalling device for use in a system as specified in said one aspect,the device having a port for connection to a cable arrangement for transmitting telephone signals, a port for connection to a telephone , and means which is cooperable with other such means to act as a telephone exchange distributed about the cable arrangement to automatically establish a communication link between telephones connected to the means in response to the selection by one of the telephones of another of the telephones.

If a telephone system is to be connected to an external network e.g. a public telephone network^' such as the network owned in the UK by British Telecom PLC, then the system must not interfere with the operation of that network to the detriment of other users.

According to another aspect of the present invention there is provided in a communication system for which there is a defined maximum noise limit representing the maximum amount of signal noise which may be injected into the system, a communication subsystem in which an information signal of magnitude less than the noise limit is transmitted and received.

Preferably the system of the said one aspect constitutes the subsystem of the another aspect.

In an embodiment of the system of the another aspect the noise limit is defined at a maximum frequency of the information signal, and the frequency of the information signal is greater than the said maximum frequency. Preferably the frequency of the information signal is greater than 10 MHz e.g. 20 MHz.

According to a further aspect of the present invention there is provided a communications system comprising

a plurality of information sources,

a signal transmission network,

a plurality of signalling devices coupling respective ones of the sources to the network,

each one of the devices being able to receive (or transmit) on one channel associated with itself, and transmit (or receive) on any one of a plurality of channels corresponding to the reception (or transmission) channels of the devices,

the devices being responsive to the selection, at the said source associated with one of the devices, of another of the devices to automatically form a communication link between the said one and another devices in which each of the devices forming the link receives (or transmits) on the said one channel associated with itself, and transmits (or receives) on the said one channel associated with the device to which it transmits (or from which it receives).

In addition there is provided a signalling device for use in a system according to the further aspect,

the device being arranged to receive (or transmit) on one channel associated with itself and transmit (or receive) on any selectable one of a plurality of channels corresponding to the reception (or transmission) channels of other such devices. the device being responsive to the selection at its associated information source of another such device to automatically form a communication link between itself and the said another such device in which the device receives (or transmits) on the said one channel associated with itself and transmits (or receives) on the said one reception (or transmission) channel associated with the another device or a further device intervening in the link.

Such a system and signalling device may be used in the systems of the said one and another aspects.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 is a block schematic diagram of a telephone system according to an aspect . of the present invention,

Figures 2a, 2b, and 2c are diagrams illustrating the operation of aspects of the system,

Figure 3 is a block schematic diagram of a telephone control device according to another aspect of the present invention,

Figure 4 is a block schematic diagram showing a transmitter

Figure 5 is a block schematic diagram of a line/handset interface and

Figure 6 is a state diagram illustrating the operating states of the telephone system as implemented by the control logic in the telephone control devices.

Referring to Figure 1, there is shown a telephone system, comprising nine extension phones Til to 9 which are arranged in e.g. an office or a home. The number of phones may be 2 or more, and may be much ^' greater than nine, but a maximum of nine has the advantage that single digit dialling of all the extensions is then possible. Control boxes and Wiring

The phones TI 1 to 9 are shown in Figure 1 coupled by respective signalling or control devices CD 1 to 9 to a single pair of wires (or a coaxial cable) 20. Preferably, standard telephone company sockets SI to S9, e.g. British Telecom sockets together with BT wiring are used, and the control devices are in socket replicating boxes.

In principle, the control devices could be in any suitable boxes connected to the BT wiring; e.g. a box on which the telephone sits or even in the telephones themselves. The wiring may have any suitable configuration, e.g. star, branched, looped or any combination provided the wiring is continuous.

The socket SI is a master socket to which the external telephone company line 21 is connected. In a typical arrangement of extension phones, that socket has two wires coming in from the external line, and four wires out:

1) and 2) are the speech lines; 3) is an "anti-tinkle" line; and

4) is for a party line. The system described herein uses only lines 1 and 2, although it could be modified to use the "anti-tinkle" line 3 as well.

The control devices allow:

A) Internal Calls

1) any extension to call any other extension,

2) a further one or more of the remaining extensions to join in the call, and

3 ) any of the extensions involved in the call to hang up

4) several independent internal calls

B) External calls (incoming)

1 ) the presence of an incoming call from the external — line to be indicated at all of the extensions,

2) the call to be answered from any of the extensions,

3) the call to be transferred to any other extension, and

4) other extensions to join in the call and any extension to hang up as if it were an internal call as described above C) External Calls (outgoing)

1) any extension to access the external line to make an outside call,

2) transfer of the outside call, other extensions to join in the call, and hang up of extensions as described above.

Signal levels and Modulation Schemes

The telephone company e.g. British Telecom lay down regulations limiting the maximum amount of noise which users of its lines are allowed to introduce into those lines. In the UK the limit varies with signal frequency. At 10 MHz the limit is -70dBm, 10MHz being the maximum frequency with which a limit is associated in BS 3605:1982.. In accordance with one aspect of the present invention, the system uses control signals and speech signals for use internally, having a magnitude less than the noise limit at the frequency of the signals. In addition in the illustrative system described herein the frequency of the signals is above the maximum frequency for which a limit is associated. Preferably the frequency of the signals is chosen to be a frequency at which ambient noise levels are low; the illustrative system operates at approximately 20MHz. It should be appreciated that if the system is not connected to the telephone company line, or is used in a situation where noise limits are more relaxed or do not apply, signals of greater magnitude may be used.

The control and speech signals (for internal use only) are signals modulated onto a carrier of e.g. approximately 20 MHz. The form of modulation used in the illustrative system is frequency modulation (FM), because that allows reliable communication using the low signal levels ( <-70 dBm). In the- illustrative system the nine reception frequencies are in channels separated by 40 KHz each.

In principle other forms of modulation could be used especially where the noise limits are more relaxed- or do not apply. For example the signals could all be amplitude modulated. It is possible to use amplitude modulation on several different frequencies to avoid the need to synchronise the local oscillators.

The Control Devices (Fig 3)

The control devices CDl to CD9 each include a transmitter 22 and a receiver 23. In the illustrative system each of the nine receivers can receive signals only of reception frequency uniquely associated with that receiver, and all the nine transmitters can transmit on all of the nine reception frequencies, although it should be appreciated that any one of the transmitters can transmit on only one reception frequency at any one time.

In principle, each transmitter could be arranged to transmit on only one transmission frequency and the receivers could be arranged to receive all nine frequencies but that is not done in the illustrative system.

Each of the control devices has a power supply 24 which in the illustrative system is a battery. In order to conserve power, each control device is powered-up intermittently e.g. for 10ms every 2s under the control of an intermittent actuation circuit 25. In addition each device may be powered-up automaticaly when its associated phone is used, e.g. the hand piece is removed from its "hook". Furthermore each device is powered-down automatically when use of its associated phone ceases although, power-down may be delayed depending on the circumstances.

The Control Signals

The control signals are tones in the frequency range lKHz to 3 KHz which are frequency modulated onto the 20 MHz carrier. Each tone is transmitted in a short burst of preset length e.g. 12 cycles. The tones are transmitted in sets called messages, many of which include tones representing the identity of the sender (i.e. the channel unique to the sender) and the identity of the intended recipient (i.e. the channel unique to the recipien ), and a tone representing a command.

There are two sorts of message: Incoming message of the form From, To, Command; and outgoing messages of the form Frequency; From, To, Command.

Another message mentioned here is: Con, Con, Con (meaning connect). Commands include Discon (which means disconnect) and Conn (which means connect).

The messages will be described in more detail in the the following.

"from" and "to" in the above messages may not be the actual sender and actual recipient of the tone but may represent other channels as will become apparent in the following.

State Diagram

Figure 6 is a state diagram, diagrammatically illustrating all states of one control device (called

A) of the illustrative system, as will be described in the following. The diagram gives only a general out line of some of the procedures which can be incorporated into the system; it does not necessarily exhaustively illustrate every possible detail of operation. Those skilled in the art could produce other procedures. State diagrams are understood by those skilled in the art. Thus Fig.6 will not be described in detail herein. Some of the more important procedures disclosed in it will be discussed with reference to the simplified diagrams of Figs. 2a, 2b and 2c, but should be also apparent from Fig. 6.

2 Way Internal calls (Figures 2a)

Consider the situation in which extension A (eg extension 1) calls extension X (eg extension 2) as shown in Fig 2a. Use of the phone TI ie causing it to go "off hook"(OH) associated with control device CDl (Fig. 1) causes it to be powered-up with the transmitter off and the receiver unmuted. Dialling is then enabled, the number X = '2' of an internal phone is dialled, a dialling decoder 2 and a control logic unit 27 adjust the carrier frequency of the transmitter to the reception frequency X= '2' . In addition, the logic unit 27 causes a "mute" to be applied via an interface 29 to the phone TI. A tone generator 28 under the control of logic unit 27 causes

3 control tones to be transmitted on the frequency X (as frequency modulations of the carrier). The mute ensures the caller does not hear the tones.

The 3 control tones transmitted on frequency X represent:

identity of the sender (eg extension 1) identity of the receiver (eg extension 2) command connect, (ie the message has the form X: A, X, Conn. )

The tones are transmitted by the wires 20 to all the control devices CD, but only the receiver 23 of control device X= CD2 will respond to its particular reception frequency X= '2' . The 3 tones are transmitted in a sequence each tone lasting 12 cycles until either extension 1 hangs up or extension 2 responds.

Extension^' X= 2 is actuated by its intermittent actuator 25 for 10ms every 2s. If it then detects a tone, it remains actuated. Upon detection of any of the tones by a tone decoder 30, the control logic causes the mute to be applied to the phone TI2 and actuates a bleeper 31 eg a piezo electric resonator to indicate the call. Then upon detection of the identity-of-sender tone by the tone decoder, the control logic 27 adjusts the carrier frequency of the transmitter of extension '2' so that it transmits on reception frequency '1 ' . The control logic and tone generator of extension X= 2 then cause its transmitter to transmit the con/con/con set of tones to extension A=l. Extension A=l responds to that by cancelling the mute at extension 1 and initiating the ringing signal to be heard at A. If the user X picks up the handset, then extension X then transmits X, A, Conn to extension A, if user X answers the phone.

A removes the ringing signal and 2-way communication is established.

The users of the two extensions can then speak to each other via a modulated RF carrier, speech from the phones TI 1 and 2 being supplied by the interfaces to the transmitters via lines 32 and (demodulated) speech from the receivers being supplied to the phones via lines 33 and the interfaces 29.

When either user hangs up (goes on hook) the control logic causes the tone generator to transmit a sequence of 3 tones sender/recipient disconnect, eg X:A, X, Disconn if A hangs up, each of 12 cycles duration. That causes the control logic at the recipient (X) to revert t:o a power off condition. The control logic at the sender causes its control device to power down once the message has been transmitted.

If extension X is engaged when A calls, A produces an internally engaged signal, turns off its transmitter and receiver and when A goes on hook (OH) reverts to a power off condition. If X fails to answer after X has transmitted Con, Con, Con to A, when A hangs up (OH), A transmits X:A, X, Disσonn and A and X power down.

3 Way Internal Calls (Figure 2b)

Referring to Figure 2b, there is shown a schematic diagram of a three way conversation. It should be recalled that each extension can receive on only its own reception frequency, and that, at any one time, each transmitter can transmit on only one frequency.

Thus extension 1 transmits to extension 2 on reception frequency '2' , extension 2 transmits to extension 3 on reception frequency '3' and extension 3 transmits to extension 1 on reception frequency '1'. In order for, e.g. speech from extension 1 to be relayed to extension 3, it is demodulated in extension 2 and remodulated in extension 2 onto reception frequency 3, even though that involves a reduction of the signal to noise ratio.

2 Way to 3 Way call (Figure 2c)

The manner in which the three way conversation is set up and altered (e.g. one of the extensions hangs up) is illustrated in the accompanying state diagram: Fig. 6 and in Fig. 2c.

Assume an internal call has already been set up between extension A=l and extension X=2 as ' described above. Assume extension 1 wants extension Y=3 to join in the call.

Extension A=l dials the number of extension Y=3. Then A mutes its receiver and transmits on frequency X; A, Y, Conn.

Extension X receives the set of tones and applies the mute, and forwards on frequency Y: A, Y, Conn.

Extension Y=3 receives the set of tones, ringing starts at Y, which also transmits on channel A=l to extension A=l the set of tones: Con, Con, Con; and if user Y answers extension Y, Y sends to A=l on channel A=l: Y, A, Conn. A forwards to X, (Y, A, Conn) and the 3 way call is established. If Y is engaged, X changes its transmission frequency back to A and two way conversation is resumed.

If Y is not engaged but fails to answer after transmitting Con, Con, Con to A, then:

A cancels ringing by sending to X:

A, Y, Discon.

X forwards to Y: A, Y, Discon,

Y stops ringing and;

A and X resume their call.

Extension hangs up

Assume now that extension X=2 hangs up (OH) Extension X transmits on frequency Y: X, Y, Discon; X powers down;

Extension Y responds by retransmitting to A; X, Y, Discon. Extension A responds by changing its transmission frequency to Y. Extension A can then speak to extension Y.

External Calls (incoming)

If there is an incoming external call on the external line 21, its presence is initially indicated by the normal ringing signal on the line 21. In each of the control devices, a ring detect circuit indicates the presence of a ring signal (RI) to the control logic and powers up (R2) the device and actuates the bleeper e.g. a piezo-electric resonator. All the control devices can thus indicate the presence of the call.

Any of the phones, eg A=TI can be used to pick up the incoming external call. Any of the other phones, eg X can then be connected to the call as if it were an internal call. For that purpose, the code number of the other extension which is to join the call is dialled. The dialling decoder 26 and the control logic 27 at the original extension respond to that code by causing the interface to put a dummy load on the BT line to hold it and the procedure described above for an internal call is then automatically followed. If the phone that originally picked up the call dials a zero, the control logic responds to the zero dialling to cause the interface circuit to reconnect the outside line to it and also transmits to the other phone X the message on frequency X, A, X, Discon which re-enables dialling and mutes the receiver at X. X then dials '0' to also directly access the outside line.

In order to transfer the outside call to another extension, eg X, the call is held at the extension eg

A, which originally picked it up by dialling the code

(1-9) for the other extension. The other extension X, is called as if a normal internal connection was being made to inform the other extension X that there is an external call for it. The other extension X then dials '0'. The control logic and tone generator respond to the zero to automatically transmitting a set of tones on frequency A: X, A, Discon and to cause the interface circuit at X to connect X to the external line. X, A, Discon causes the extension A to assume the dialling enabled state. 'A' then powers down when A hangs up, leaving X connected directly to the line. External calls (outgoing)

To make an outgoing call from a particular extension a code Q_ is dialled and the control logic 27 responds by causing the interface circuit to connect the phone TI to the external line for a period of sixty seconds. In this sixty seconds all dialling is routed to outside lines. When the call is finished, the handset is replaced.

Once an external call has been established it can be transferred to other extensions after expiry of a dialling by-pass time. Other extensions can also join the call as described above.

The receiver

The receiver 23 comprises a Plessey SI.6653 low power RF/IF/AF single chip mixer, IF amplifier and detector. Although the Plessey chip also incorporates an oscillator it is not used in this embodiment of the invention.

Instead an oscillator 35 (external to the Plessey chip) is provided which produces a signal of frequency 455KHz higher or lower than the nominal reception frequency of the particular extension.

The transmitter

The transmitter 22 comprises a phase-locked-loop (PLL) Frequency Synthesizer and an adjustable reference frequency divider 36.

The PLL comprises, as is known, a phase comparator 40 a loop filter 41, a voltage controlled oscillator 42 and a frequency divider 43.

In order to adjust the frequency of the transmitter to match the reception frequency of the extension to which it transmits, the frequency divider 43 is adjustable in integer steps.

Adjustable frequency dividers 43 are well known. The control logic unit controls the frequency divider so that the output frequency of the VCO equals the desired carrier frequency.

Tones from the tone generator 28 and audio from the interface circuit are applied to the VCO to frequency modulate its output. The Interface circuit (Fig 5)

The line 20 or 21 is connected to a splitter network 50 which separates the RF signals from the conventional signals on the telephone company line. Downstream of the splitter 50 is a fullwave rectifer and surge arrester circuit 52 as known in the art. When making an external call, the interface circuit is powered from the telephone company line via the network 50 and the circuit 52. When making an internal call, the interface circuit is powered from a battery 56. To switch from battery to telephone company line powering, a switching circuit 57 is provided. The circuit is an electronic switching circuit, controlled via a control input 57, by the control logic 27 which in turn is responsive to the intermittent actuator 25.

The phone TI is coupled between switch 57 and the OV rail via a series load resistor 51 of nominally loΩ. resistance. The total resistance in series with the phone TI is limited by telephone company regulations to, eg 10Q . In practice, because of impedance in the switching circuit 57 and the rectifier 52, the resistance of resistor 51 may be slightly less than 10-2. Internal calls

In order to make an internal call, the handset is lifted off its hook. The current flowing through the 180 _»^ resistor is sensed by a power on sensor 55 which produces a power on signal indicating the handset is off the hook. The control logic responds to that signal by maintaining power to the control device. 'Ringing' of an internal call is audibly indicated by a buzzer 31 controlled by the control logic 27.

The tones which are transmitted as described above are received by the interface circuit at the input RXAF. 33. The tones are passed via an amplifying circuit to the tone decoder 30 (Fig. 3). Speech from the handset varies the current in the resistor 51 and an amplifier 58 sends the resulting signals TX audio to the VCO of the transmitter 22 via a path 32 and a controllable signal combiner 65 as will be described hereinbelow.

Demodulated speech received from the receiver 23 is input at RXAF (line 33). The signals are passed by an amplifier 53, to the handset. A mute 59 controlled by the control logic is applied to prevent a user hearing the tones which are transmitted and receiver noise when no carrier is present.

3 Way Conversations

As described above in circumstances where there is a three (or more) way conversation, speech and tones are received from one control device and transmitted to another via an intervening control device. For that purpose, the interface circuit comprises a further output TT audio from the amplifier 53 to a controllable signal combiner 65 which adds the speech from the telephone instrument TI to the speech received from another control device via the input RXAF (33) for transmission to a further control device via an output 66.

External calls

An incoming external call is indicated by the ringing signal. The ringing signal is detected by the ring detector 61 upstream of the full wave rectifier. The detector actuates a buzzer 31. The ringing detector 61 has two outputs; RI and R2 (which differ only in amplitude) for indicating (RI) the ring to the logic circuit and for causing (R2) the intermittent actuation circuit to power-up the control device.

When the handset is lifted after the ringing is indicated, the switching curcuit 57 connects the phone TI to the line 21 to be powered from the line.

If an outgoing external call is to be made, the outside code 0 is dialled and the dialling decoder and control logic respond to it to actuate the switch. 57 to connect the phone TI to the line 21.

Line Hold

If it is desired to hold the line 21, the dialling decoder and the control logic respond to a hold code e.g. the number of another extension to activate a load hold circuit 62 to place a line holding load across the line 21.

In order that the user of the system is reassured that the system is working when he or she first pick-ups the hand set, a sound generator 64 is provided to inject sound signals into the phone TI via amplifier 53. The generator 64 is controlled by the control logic. The user will hear all dialling tones at low volume.

It is important to separate received and transmitted speech, ie avoid cross-talk, in the interface circuit especially in view of the possibility of retransmitting speech around a loop. The separation is achieved by several means in the interface circuit.

Firstly, the telephone instrument TI itself usually includes a hybrid circuit for that purpose.

Secondly, received speech is fed to the telephone instrument via the "top" (as shown in Fig.5) terminal 70, whilst transmitted speech current originating at the telephone is fed out to the load resistor 51 at the "bottom" terminal 71.

Some-cross talk however will remain. To eliminate that cross -talk, a small proportion of the received speech signal at input 33 RXAF is combined with the transmitted speech signal from the load resistor 51 in the amplifying circuit 58 so as to cancel the remaining cross-talk. Cancellation occurs due to phase inversion in the amplifying circuit 58 which comprises an inverting operational amplifier 58' and feed back circuits 58" as known in the art.

The control logic unit

The control logic unit may be a microprocessor, or an uncommitted logic array .

Dialling

Whilst reference has been made to dialling a code it should be appreciated that that term covers loop break dialling using a rotary dial or using a push-button system, and also covers the production of multi-frequency tones to represent the code. The dialling decoder (which is known in the art) needs to be chosen to suit the form of dialling used.

Modifications

Whilst the various aspects of the invention have been illustrated herein with respect to a telephone system at least some of the aspects can be applied to other systems. For example, the technique of using very low level signals can be applied to the transmission of any information over any sort of signal line, such as alarm information of a burglar alarm system transmitted over the electrical mains cabling in a building.

In the telephone system, speech information could be replaced by other information provided it occupies the same or less bandwidth as the speech information.The telephones could then be replaced by other telephonic devices which act as information sources, such as facsimile machines,modems,etc although it should be appreciated that the term telephonic device as used herein does not cover equipment such as telephone exchanges.

The system could be used as a network linking computers or computer terminals. If the cabling of the network is entirely independent of the telephone company cabling, then the signals used need not comply with telephone company regulations. For instance, the system of the invention could be used to link computers via mains wiring using high level signals.

It will be apparent to those skilled in the art that although the foregoing describes various functions defined by the control logic and performed by the illustrative telephone system of the invention, many other functions could be performed by appropriately modifying the control logic.

The control or signalling devices CD1-9 as described herein are each adjusted at manufacture to their own respective reception frequencies and would be sold as nine differently marked devices.

Although the telephone system of Figure 1 is described as having continuous wiring, other possibilities exist such as capacitively coupling the . RF signals, or transmitting them through free space e.g. by use of radio channels or other free space transmission methods.

Although the system has been described mainly with respect to frequency division multiplex other multiplex systems could be used in principal, e.g. time division multiplex.

Claims

CLAIMS :

1. A telephone system in which a plurality of telephones are connected to a cable arrangement for transmitting telephone signals by one signalling device per telephone which cooperate one with another to act as a telephone exchange distributed about the cable arrangement to automatically establish a communication link between telephones in response to selection by one of the telephones of another of the telephones .

2. A system according to claim 1, wherein each signalling device comprises a transmitter arranged to transmit on a single transmission channel of frequency specific to that device, .a receiver operable to receive on any selected one of a plurality of reception frequencies corresponding to the transmission channel frequencies of the devices in the system, and control means for selecting the reception frequency.

3. A system according to claim 1, wherein each signalling device comprises a receiver arranged to receive on a single reception channel of frequency specific to that device, a transmitter operable to transmit on any selected one of a plurality of transmission frequencies corresponding to the receptive channel frequencies of the devices in the system, and control means for selecting the transmission frequency.

4. A system according to claim 2 or 3, wherein the transmitters are arranged to transmit, and the receivers are arranged to receive, amplitude modulated signals.

5. A system according to claim 3 wherein the transmitters are arranged to transmit, and the receivers are arranged- to receive, frequency modulated signals.

6. A system according to claim 5 wherein the control means of each signalling device is operable to cause the transmission of control signals for controlling the operation of the other signalling devices, the control means of each signalling device being responsive to the received control signals.

7. A system according to claim 6 wherein the control means of each signalling device is responsive a) to its associated telephone to produce the control signals, and b) to control signals received from another of the signalling devices to transmit further control signals or retransmit the received control signals.

8. A system according to claim 6 wherein the control means of any one of the signalling devices is responsive to the selection by the telephone associated therewith of another of the signalling devices to transmit on the reception frequency of the another of the signalling devices a control signal identifying the said one of the signalling devices

• which causes the transmitter of the another of the. signalling devices to transmit on the reception frequency of said one of the signalling devices.

9.. A system according to claim 8, wherein the control means of the said one of the signalling devices is responsive to the additional selection by its associated telephone of a further one of the signalling devices to transmit on the reception frequency of the said another one of the signalling devices another control signal identifying the said one of the signalling devices and identifying the said further one of the signalling devices, the control means of the said another one of the signalling devices being responsive to the said another control signal to retransmit the said another control signal on the reception frequency of the further one of the signalling devices, and the control means of the said further one of the signalling devices is responsive to the another control signal to cause its transmitter to transmit on the reception frequency of said one of the signalling devices.

10. A system according to claim 9, wherein the control means of any one of the signalling devices is responsive to its associated telephone terminating communication to produce a control signal indicating the termination of communication and identifying that signalling device and the signalling device to which it was transmitting during the communication.

11. A system according to claim 10 wherein the cable arrangement comprises a twisted-pair for transmitting the telephone signals.

12. A system according to any preceding claim wherein the cable arrangement includes socket connectors for the connection thereto of the signalling devices, the signalling devices are housed in socket replicating boxes which plug into the said sockets, and the telephones are plugged into the said boxes.

13. A system according to claim 3 wherein the cable arrangement is connected to an external telephone network, each of the signallaing devices including means for connecting its associated telephone to the network via the cable arrangement in response to selection of the network by its associated telephone.

,

14. A system according to claim 13 wherein the external network is a public telephone network.

15. A system according to claim 14, wherein there is defined for the external network a maximum noise limit representing . the maximum magnitude of signal noise which may be injected into the network, all the signalling devices of the system being arranged to transmit and receive information signals of magnitude less than the noise limit over the communication link established between one another.

16. A system according to claim 15 wherein each of the said signals have a magnitude of less than -70dBm.

17. A system according to claim 15 wherein there is a maximum signal frequency associated with the noise limit defined for the external network, the frequency of the said transmitted and received signals being greater than the said maximum frequency.

18. A system according to claim 17 wherein said frequency of said transmitted and received signals is greater than 10MHz.

19. A system according to claim 14 wherein each of the signalling devices includes means, responsive to the control means, for selectively applying an external line holding load to the cable arrangement.

20. A system according to claim 13 wherein each of the signalling devices includes means for coupling the input of the transmitter to the cable arrangement and for coupling the output of the receiver to the cable arrangement.

21. A signalling device for use in a system as specified in claim 1, 2 or 3 the device having a port for connection to a cable arrangement for transmitting telephone signals, a port for . connection to a telephone, and means which is cooperable with other such means to act as a telephone exchange distributed about the cable arrangement to automatically establish a communication link between telephones connected to the means in response to the selection by one of the telephones of another of the telephones.

22. A device according to claim 21 wherein the device comprises a receiver arranged to receive on a single reception channel of frequency specific to that device, a transmitter operable to transmit on any selected one of a plurality of transmission frequencies corresponding to the receptive channel frequencies of the devices in the system, and control means for selecting the transmission frequency.

23. A device according to claim 22 wherein the transmitter is arranged to transmit, and the receiver is arranged to receive, frequency modulated signals.

24. A device according to claim 23 wherein the control means of the signalling device is operable to cause the transmission of control signals for controlling the operation of the other signalling devices, the control means of the signalling device being responsive to received control signals.

25. A device according to claim 24 wherein the control means of the signalling device is responsive a) to its associated telephone device to produce the control signals, and b) to control signals received from another signalling device to transmit further control signals or retransmit the received control signals.

26. A device according to claim 24 when appended to claim 3, wherein the control means of the signalling device is responsive to the selection by the telephone associated therewith of another signalling device to transmit on the reception frequency of the another signalling device a control signal identifying the said signalling device which causes the transmitter of the another signalling device to transmit on the reception frequency of said signalling device.

27. A device according to. claim 26 wherein the control means of the signalling device is responsive to the additional selection by its associated telephone of a further one of the signalling devices to transmit on the reception frequency of the said another signalling device another control signal identifying the said signalling device and identifying the said further signalling device, the control means being responsive to such another control signal when received thereby to retransmit the said another control signal on the reception frequency of the further signalling device if the control means is not of said further signalling device, the control means being responsive to such a received control signal to cause its transmitter to transmit on the reception frequency of said signalling device if the control means is of the further signalling device.

28. A device according to claim 26 wherein the control means of the signalling device is responsive to its associated telephone terminating communication to produce a control signal indicating the termination of communication and identifying that signalling device and the signalling device to which it was transmitting during the communication.

29. A device according to claim 21 housed in a socket replicating box.

30. A device according to claim 21 which comprises means for selectively connecting the said port for connection to a telephone to the port for connection to the cable arrangement in response to selection of the cable arrangement by the telephone.

31. A device according to claim 22 wherein the transmitter is arranged to transmit and the receiver arranged to receive signals of magnitude less than -70dBm.

5

32. A device according to claim 31 wherein the frequency of the said signals is greater than 10MHz.

33. A device according to claim 21 wherein each of 10 -the signalling devices includes means, responsive to the control means, for selectively applying an external line holding load to the cable arrangement.

34. A device according to claim 21 wherein the i.5 signalling device includes means for coupling the input of the transmitter to the cable arrangement and for coupling the output of the receiver to the cable arrangement.

20 35. A device according to claim 21 comprising ringing signal detector means and means responsive to detection of the ringing signal to indicate the presence thereof.

36. A device according to claim 30 comprising means for passing ringing signals from the port for connection to the cable arrangement to the port for connection to the telephone.

37. In a communication system for which there is a defined maximum noise limit representing the maximum amount of signal noise which may be injected into the system, a communication subsystem in which an information signal of magnitude less than the noise limit is transmitted and received.

38. A subsystem as specified in claim 37 wherein the said magnitude is less than -70dBm.

39. In a system as specified in claim 37 for which the noise limit is defined at a maximum frequency of the information signal, a subsystem as specified in claim 3 in which the frequency of the information signal is greater than the said maximum frequency.

40. A subsystem as specified in claim 39 wherein the said frequency of the information signals is greater than 10 MHz.

41. A subsystem as specified in claim 40 wherein the said frequency of the information signals is in the region of 20 MHz.

42. A subsystem as specified in claim 41 wherein the said frequency of the information signal is in the range 24 to 26 MHz.

43. A subsystem as specified in claim 39 wherein the information signal comprises a frequency modulated carrier signal.

44. A subsystem as specified in claim 39 wherein the information signal comprises an amplitude modulated carrier signal.

45. A communications system comprising

a plurality of information sources,

a signal transmission network,

a plurality of signalling devices coupling respective ones of the sources to the network, each one of the devices being able to receive (or transmit) on one channel associated with itself, and transmit (or receive) on any one of a plurality of channels corresponding to the reception (or transmission) channels of the devices,

the devices being responsive to the selection, at the said source associated with one of the device, of another of the devices to automatically form a communication link between the said one and another devices in which each of the devices forming the link receives (or transmits) on the said one channel associated with itself, and transmits (^"or receives) on the said one channel .associated with the device to which it transmits (or from which it receives).

46. A signalling device for use in a system according to claim 45

the device being arranged to receive (or transmit) on one channel associated with itself and transmit (or receive) on any selectable one of a plurality of channels corresponding to the reception (or transmission) channels of other such devices,

the device being responsive to the selection at its associated information source of another such device to automatically form a communication link between itself and the said another such device in which the device receives (or transmits) on the said one channel associated with itself and transmits (or receives) on the said one reception (or transmission) channel associated with the another device or a further device intervening in the link.

47. A telephone system in which a plurality of telephonic devices are connected to a transmission arrangement for transmitting telephonic signals by respective signalling devices which cooperate one with another to act as a telephone exchange distributed about the cable arrangement to automatically establish a communication link between the telephonic devices in response to selection by one of the telephonic devices of another of the telephonic devices.