GB2239145A - International telephone circuits prevents speech transmission till answer signal starts call charging - Google Patents

Abstract

In an international telephone exchange a circuit operates on calls initiated through the circuit to control the transmission of speech along the line until an answer signal from the called party has been received to start charging for a call. The control may be strong attenuation of the speech or the addition of noise to render speech communication impossible. The controlling means may permit the passage of control signals used to set up a connection so that the subscriber can follow the setting up of the connection. This is intended to prevent fraud caused by deliberately masking the answer signal. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO INTERNATIONAL TELEPHONE CIRCUITS This invention relates to international telephone circuits. The circuit may be added to an existing telephone exchange or incorporated in the design of a new telephone exchange.

In order to set up a telephone call from one subscriber to another a telephone system uses signalling employing predetermined signals which are transmitted from one telephone system to another. First of all the calling subscriber initiating the call sends a calling signal to the exchange, and then some more signals (by dialling) identifying the called subscriber. The exchange reacts by sending a ringing signal to the called subscriber, either directly or through the medium of one or more other exchanges. Assuming that two or more exchanges are involved in the call, when the called subscriber answers the call an answer signal is sent to the calling subscriber's exchange which starts a meter for recording the charge to be made for the call.When the call is finished a clearing signal is sent from both subscribers to free the lines for other calls and to stop the meter recording the charge.

It has been found that some subscribers have made unauthorised alterations to the telephone circuit which have the effect of masking the answer signal so that it is not recognised by the calling subscriber exchange.

This means that although the connection between the subcribers has been set up so that they are able to communicate with one another, no charge is recorded because the meter apparently does not receive the answer signal in response to which it starts recording.

It is an object of the present invention to overcome this difficulty.

According to the present invention there is provided a circuit arrangement connectible to an international telephone circuit in an international telephone exchange including means effective on calls initiated by the subscriber to control the outgoing and the incoming transmission of speech or data along the circuit until an answer signal has been received from the called party for metering in respect of the call.

The controlling means may include means for strongly attenuating the transmission of voice frequency signals.

There may be provided means, such as filters, for permitting control signals to by-pass the controlling means. The filters may include a second controlling means responsive to a signal of a control signal frequency and of sufficient amplitude to allow the transmission of signals through the filters.

Alternatively the controlling means may include a noise generator connectible to the line to swamp the speech signals. The noise generated by the generator may have low levels at the frequencies of control signals.

Although a circuit arrangement for use with CCITT No.5 signals will be described in detail, the arrangement may be modified to operate with any form of telephone signalling.

Examples of circuit arrangements according to the invention will now be described with reference to the accompanying drawings, of which: FIGURE 1 shows in block diagrammatic form a first example of a circuit arrangement; FIGURE 2A, 2B, 2C, 3A, 3B, 3C, 4A, 4B, 4C. 4D, 4E, 4F and 5 together show in detail the first example of a circuit arrangement; and FIGURE 6 shows a second example of a circuit arrangement, also in block diagrammatic form.

Referring now to Figure 1, the circuit arrangement shown has a first path A for the transmission of voice frequency signals from left to right between terminals 1 and 2, and a second path B for the transmission of those signals from right to left between terminals 3 and 4.

The first path A starts from the terminal 1, passes through a gate-controllable amplifier 5 and goes to the terminal 2. The second path B starts from the terminal 3, passes through a gate-controllable amplifier 7 and goes to the terminal 4. A control signals detector 9 is connected to receive its input from the terminal 3 and produces outputs which are applied to the control inputs of the gate-controllable amplifiers 5 and 7. Signalling, signalling and tone by-pass circuits 6 and 8 are provided and are respectively connected to by-pass the gatecontrollable amplifiers 5 and 7. A special information tone (SIT) and recorded messages detector 10 is connected to control the amplifier 7 and by-pass circuit 8.

When connected in a telephone exchange the circuit arrangement of Figure 1 has terminals 1 and 4 on the left-hand side of the Figure connected to the trunk circuit (not shown) of the calling side, so that the first path A and the second path B respectively convey the outgoing signals from the calling exchange and the incoming signals to the calling exchange. The signals concerned are speech signals and control signals for the telephone system. The speech signals are in a frequency band from 300 Hz to 3400 Hz, and the control signals lie in narrow bands of 30 Hz bandwidth within the speech frequency band. In CCITT No.5 signals the "ANSWER" signal is of frequency 2400 + 15 Hz. A "RELEASE GUARD" signal is used to release the circuit connection and has two components, one of 2400 + 15 Hz and the other of 2600 + 15 Hz.The detector 9 responds to both these frequencies to control the blocks 5, 6, 7 and 8. To enable the control signals to be transmitted by the circuit arrangement when the gate-controllable amplifiers 5 and 7 are closed to the passage of signals, those gatecontrollable amplifiers are by-passed by the circuits 6 and 8 respectively, having narrow pass-bands which allow only the control signals to pass. Of course, if a speech signal were present then those components of it lying within the pass-bands would be passed by the circuits 6 and 8 but would not form intelligible communication.

Occasionally, recorded messages are required to be transmitted from the exchange to a subscriber. In order that such messages shall not be blocked by the closure of the amplifiers 5 and 7, those messages are conveyed by the by-pass circuit 8.

The circuit arrangement shown in Figure 1 is simplified so that the principle of its operation can be easily understood. A detailed form of the circuit arrangement will now be described.

The circuit arrangement to be described with reference to Figures 2A, 2B, 2C, 3A, 3B, 3C, 4A, 4B, 4C, 4D, 4E, 4F and 5 in terms of its functions has the following primary objectives to be met when connected in a 4-wire analogue line carrying Voice/Data Communications using CCITT - No.5 Signalling Functions in an international circuit connection.

(1) Compatibility and adaptability for use at any point in the analogue signal path in the CCITT - No.5 signalling circuit environment.

(2) Independence from the type of signalling devices used and the transmission requirements of systems involved and their interconnections.

(3) Transparency to the entire communication in both directions, involving Voice, Data, Recorded Messages, Information Tones, Service and Feedback Tones, Signalling Tones, Feedback Noise Level of the circuit etc., if and only if, a valid 'ANSWER' signal is received from the called subscriber and recognized by the international trunk switching centre concerned irrespective of whether the call is Incoming or Outgoing.

(4) Transparency to the entire communication in both directions, involving all entities as listed above except Voice and Data in their entire telephone spectral band (300 Hz - 3400 Hz), if a Valid 'ANSWER' signal is not received from the called subscriber till the elapse of CCITT specified 'ANSWER SUPERVISION TIME' set generally at the International Trunk Switching Centre or until either of a valid 'ANSWER' signal, or a 'RELEASE GUARD' signal is received from the called subscriber.

The circuit arrangement comprises four functional blocks.

(1) The Transmit Path Circuit (TPC) shown in Figures 2A, 2B and 2C.

(2) The Receive Path Circuit (RPC) shown in Figures 3A, 3B and 3C.

(3) The Control Decision Circuit (CDC) shown in Figures 4A to 4F.

(4) The Power Supply Circuit (PSC) shown in Figure 5.

The operation of these circuits will now be described with reference to the drawings. The circuits themselves are as shown in the drawings and the types of integrated circuits used are as follows: INTEGRATED CIRCUITS IC 1, 2, 3 Siliconix DG 308 ACJ Quad 6, 7, 8 CMOS compatible analogue switch IC 4, 9, 12 - Siliconix LMC 660 CN Quad CMOS operationai amplifier IC 5, 13 - Texas Instruments TLC 3702 CP Dual CMOS comparator IC 14, 16 - Signetics NE 567 Phase locked loop IC 22 - GE-RCA ICM 555 IPA CMOS timer IC 19 - MSI MC 14081B Quad CMOS 2-input AND gate IC 18, 23 - MSI MC 14071B Quad CMOS 2-input OR gate IC 20, 21 - MSI MC 14013B Dual CMOS D-type flip-flop IC 15 - MSI MC 14528B Dual CMOS monostable multivibrator IC 17 - NSC CD 40106 BCN Hex CMOS Schmitt trigger IC 24 - Texas Instruments 74 HC 158 Quad HCMOS 2-input multiplexer-inverting IC 26 - NSC LM 78 M05CT voltage regulator IC 27 - NSC LM 79 M05CT voltage regulator The operation of the transmit path circuit will now be described with reference to Figures 2A, 2B and 2C.

The exchange 2-wire transmit line connections named ETX and ETX' are protected against any lightning induced surge voltages by the Surge Suppressor SPT1, while the 2-wire line connections to Mux, LTX and LTX are protected by SPT2.

The analogue gates in IC1 and IC2 select a direct transparent path or an alternative non-transparent path for voice and data in the event of 'ANSWER' signal reception or masking of the 'ANSWER' signal respectively.

The gates of IC2 also provide for changing the direction of signals through the circuit in association with IC3. The gates of IC2 are opened when it is an outgoing call while the gates of IC3 come into effect for an incoming call. Gates of IC6 are activated to connect the line to the Control Decision Circuit (Figures 4A-4F) for an incoming call. The different symbols used to identify the different terminals also indicate the connections between circuits.

Resistors R1, R2, R3 and R4 together with the forward resistances 'RF' of the analogue gates on both the limbs provide 600 n resistive line terminations on the exchange lines ETX and ETX' in the case of 'ANSWER' signal masking. Moreover, the resistors R1,R3 or R2,R4 together with RF and R5 provide 600 Q line balancing to earth from both the limbs individually. In addition R3,R5 or R4,R5 also function as a voltage divider to reduce the signal voltage to approximately half of its value.

The driving point impedance on the line side is provided by the resistors in the 'PI' networks constituted by R25, R27 and R29 serving LTX and R26, R28 and R30 serving LTX'. The impedance seen from LTX to LTX' into the circuit is fixed at 600 n by these networks.

The networks also provide impedance matching between the line side and the circuit side.

The frequency impulse transfer (FIT) function is simulated by an active network designed to realise the FIT mathematical model. The active network contains the active components, IC4, IC5, D1, ZD1 and ZD2 and the associated passive resistors, trimmers and capacitors as shown in Figures 2A, 2B and 2C. The overall function of the circuit is to pass the signalling tones F1 (2400 + 15 Hz) and F2 (2600 + 15 Hz) with 'unity gain' while providing strong attenuation for both voice and data throughout the entire telephonic spectral range (300 Hz - 3400 Hz).

The resistors R8,Rll in combination with the capacitors C3,C5 and C4,C6 provide for the peak response of an input signal having a frequency (2500 Hz) which can be set by the trimmers R6',R7'. Capacitors C1,C2 and C7,C8 provide coupling and d.c. isolation. The band of the peak response (2385 Hz - 2615 Hz) can be set by trimmers R12' and R13'. The threshold level of the detector formed by D1 and IC5 can be set by the trimmer R16. The primary purpose of R19 is to shift the d.c.

level of the output signals from IC4 so that the signal swings always lie in a range of values to avoid any signal clipping by ZD1 and ZD2, the main purpose of which is to hold the output d.c. level of IC4 at VR (2.4V) and thereby provide a flow of contact wetting currents in both the limbs. R23 and R24 together with R27 and R28 limit the wetting currents and the zener currents. This wetting current is introduced in order to overcome any noise generated by contact resistances anywhere in the transmission path. The overall gain from the input to the output (i.e. from ETX, ETX' to LTX, LTX') can be varied from zero to more than 100 according to the transmission requirements in case of any need to compensate for signal power losses brought in by the deterioration of cable qualities or signal senders due to ageing or other factors. The gain for normal connections devoid of quality deterioration should be unity.

All parts of the receive path circuit (RPC) function the same way as those of the transmit path circuit (TPC) with few exceptions in FIT function. The receive isolation circuit is shown in Figures 3A, 3B and 3C.

In the receive path, the peak response impulse frequency is set to 420 Hz by trimmers R36' and R37' in association with R38 and R41, C12, C14 and C13, C15 and the peak response band is adjustable from 340 Hz to 500 Hz which represents the CCITT allotted band for all Service and Feedback tones, using the trimmers R42' and R43'. The circuit comprising R44, R51 and R54, IC10 and IC11 provides for automatic signal boost in case of attenuated service or feedback tones received from the called subscriber. This is done by varying the feedback signal levels to IC9 and countering the feedback resistance offered by R52' and R53'. The feedback noise is generated using a noise generator to synthesize 'Rain Effect' to keep the calling subscriber alert during the setting up of the call.Any call progress tone appearing on the line also lies in the peak response band and swamps the 'Rain Effect' noise so that it can be heard by the caller. The level of this 'Rain Effect' noise is limited to lie within CCITT specified noise level by R56 and R57 while R57 can be trimmed to arrive at a pleasing effect. With the variation of the passive circuit components constituting the noise generator (D3, TR1 and the passive components connected to them), a more pleasant rhythmic effect can be generated as an option.

This will provide a pleasing effect while the subscriber is waiting for the call to be set up.

The control decision circuit (CDC) contains analogue and digital circuits and incorporates a very high input impedance combined Attenuator and Isolator, Signal Tone Detectors, Validation Timers, a Sequence Control Circuit, and a Gate Control Logic Circuit. The control decision circuit is shown in Figures 4A to 4F.

In the combined attenuator and isolator the operational amplifiers of IC12 with their associated passive components provide complete isolation of CDC from the transmit and/or the receive 4-wire lines and function to attenuate the complex input signal to the required minimum level for tone detection. R68, R69 and R70 provide together a gain of unity for the signal which is then attenuated to the required level by using the trimmer R73. The signal tones while appearing at the input of C22 and C23 with the specified absolute power levels to be maintained at the International Trunk Maintenance Centre (ITMC) are reduced in level to the minimum required levels for tone detection by the tone detectors, with a small allowance for tolerance which can be set by R73. The functions of C22, C23, and C24, C25 and C27 are to provide coupling and d.c. isolation.

The signal tone detectors are functionally realized using two phase locked loops respectively constituted by IC14,IC16 and their associated circuits. The loop including IC14 detects F1 (2400 Hz) while the loop including IC16 detects F2 (2600 Hz) in their CCITT specified bands of variation (i.e. + 15 Hz). R75', R76, C26 and R79', R80, C28 provide for setting the centre frequency of detection respectively at F1 and F2 accurately by adjusting the respective trimmers R75',R79'.

The locking and capture ranges of the phase locked loop are fixed by C31,C32 for the F1 detector and C37,C38 for the F2 detector to accommodate the + 15 Hz variation specified for the signal tones and can be altered using R73. The detection band centring is achieved through R81,R82 for the F1 detector and R89,R90 for the F2 detector . R83, R85, C33, C34 and R86, R88, C35, C36 suppress any spurious spikes which may occur at the respective outputs of phase locked loops through feedback and bypass filtering. R84 and R87 are the output pull-up resistors. Since the output of each phase locked loop is slow to rise because of the spike suppression circuits, a Schmitt trigger is employed to reshape the output pulses into fast rising ones.

As discussed earlier, since R73 provides a fair control on the detection band as well as the adjustment to the minimum signal power level for the detection, it can be manipulated to provide a compromise so as to tune the circuit to be proof against an imitation of the 'ANSWER' signal (signal Tone F1) which would not be detected by the call metering circuits in the International Trunk Switching Centre. Signal imitations which occur during Voice/Data Communications cannot trigger the detectors since the signal power level in the band of 2400 Hz to 2600 Hz due to such an imitation lies far below the minimum detection level set by R73 (i.e beyond -17 dB). This ensures the immunity of the detection circuit to such extraneous factors.

A higher level of protection against any false triggering of the detectors due to spurious, random noise of sufficient power level and spurious spikes is provided by timers using the monostable multivibrators of IC15.

There are two such timers. The first timer times out after 30 ms of the trigger which represents the presence of either F1 or F2 or both while the second one times out after 100 ms. These timings are fixed to validate the signals according to the minimum time specifications for the recognition of the line signals specified by CCITT (i.e. 40 + 10 ms and 125 + 25 ms). Since different signal recognition timings are specified for the different states of the signalling sequence, these timers are enabled and disabled according to the signalling states so that at any time only one timer is active. The timings are set accurately by R77',C27 and R78',C30. The outputs of these timers validate the detected signals from the phase locked loops before they cause any state transition in the control logic circuit.

The sequence control circuit and the gate control logic circuit are built by ICs: IC12, IC13, IC18, ICl9, IC20, IC21, IC22, IC23 and IC24. Initially, by power on reset provided by R106, C46 and the part of IC17, the flip flops IC20 and IC21 are cleared and remain in logical state 0. The output J from It 21 when 0 represents the direct transparent connections, and when 1 represents the alternative non-transparent connections in both directions only to voice and data.When the circuit is in its 'Idle' state the arrival of the Proceed-to-Send (PTS) signal, which is the F2 signal, makes the state change to logical 1 in IC20 and the first part of IC21. This state change in IC21 triggers a timer which times out another 5 seconds and transfers the logical 1 state to the second part of IC21. Thus the circuit is transparent to the complete sequence of dialled digits, sent using MFC tones (Register Signalling), for about 5 sec from the time PTS is received and then becomes non-transparent by the shift of the logical 1 to the second stage of IC21 by the timer output from IC22. The flip flops flip back to their initial state in the event of receiving the F1 signal (ANSWER) to make the circuit transparent to communication or the F1 + F2 (RELEASE GUARD) to make the circuit idle.

When the circuit is in initial idle state, if the signal Fl is received (i.e., SEIZURE or DUAL SEIZURE) the direction of circuit is changed by the control signals generated by the component circuits of IC18, IC19, IC20, IC23 and IC24.

IC12 and IC13 with their associated circuits have a peak response over the band of Special Information Tones and Signalling Tones and also account for the average energy content of the 'Recorded Messages' received from the called exchange. This circuit functions in such a way that it opens only the receive path and that only during the presence of any one of the tones or recorded messages.

The activity of the circuit is displayed by light emitting diodes CAD1 and CAD2. CAD1 glows green and indicates the genuine usage of the international circuit, whereas CAD2 glows red and thus indicates either a fraudulent attempt at usage or the timing of the Answer Supervision interval before communication is established.

"Test Access Points" TAP1 to TAP4 are provided so that maintenance staff, in case of circuit failure, can send signals to identify the fault location and isolate the faults from external circuits.

The power supply circuit is shown in Figure 5. IC26 together with its Filter/Decoupling capacitors supply +5V at 500 mA regulated from a +12V input, whereas IC27 and its associated capacitors provide -5V at -500 mA regulated from a -12V input. The activity of the power supply is indicated by light-emitting diodes PAD1, and PAD2, with PAD1 indicating the +5V power and PAD2 indicating the -5V power. The POWER bus and the AND bus are decoupled/filtered separately to isolate spurious transitions and voltage spikes due to ground returns, reflections, etc. The separate power and ground conductors feed Analogue Circuits and Digital Circuits individually and the grounds are made common at the point of Telecommuncation Earth to give each individual circuit a low resistance return path.

Figure 6 shows an alternative circuit arrangement for blocking speech communication using noise produced by a white noise generator 50 to drown the speech. A notch filter 51 is provided to attenuate strongly the components of the noise in the frequency band or bands used by the control signals of the telephone system, so that the control signals can still be transmitted satisfactorily. It is important that at least the parts of the telephone system traversed by the control signals and the noise together include no non-linear circuits because such circuits will cause cross-modulation of the noise components outside the control signal bands and thereby produce noise signals within those bands. This difficulty could be reduced by employing a comb filter to filter the noise. A single noise generator can supply several lines.

The filtered noise is applied to the subscriber's lines through individual controllable amplifiers, only one of which is shown and has the reference 52. The amplifier 52 is controlled by the output signals detector like the detector 9 (Figure 1), but in this case the amplifier 52 is blocked when the "ANSWER" signal is received so that the noise is no longer applied to the line.

Although the invention has been described with reference to specific examples using CCITT No.5 System Signalling, it is not limited to that form of signalling but may be used with other forms. In addition, other features of the circuits described can be adapted to suit different kinds of telephone circuits from the one to which specific reference is made.

Claims (13)

CLAIMS:

1. A circuit arrangement connectible to an international circuit in an international telephone exchange including means effective on calls through the circuit to control the outgoing and the incoming transmission of speech or data along the line until an answer signal has been received from the called exchange for charging purposes in respect of the call.

2. A circuit arrangement according to claim 1 wherein the controlling means is also effective on calls received unless a valid answer signal is sent to the other party.

3. A circuit arrangement according to claim 1 or 2 wherein the controlling means includes means for strongly attenuating the transmission of signals of speech frequencies.

4. A circuit arrangement according to claim 3 wherein the controlling means includes means for permitting the transmission along the line of control signals for the telephone system lying in one or more specific frequency bands.

5. A circuit arrangement according to claim 4 wherein the means for permitting the transmission of control signal includes filter means capable of passing the control signals connected to by-pass the attenuating means.

6. A circuit arrangement according to claim 4 wherein the means for permitting the transmission of control signals includes a signal path, capable of passing a narrow band of frequencies including the frequency of a control signal, and means responsive to a signal in the narrow band or one of the narrow bands of frequencies having an amplitude greater than a threshold value.

7. A circuit arrangement according to claim 6 wherein the signal path when selected has a gain substantially equal to unity.

8. A circuit arrangement according to any one of claims 4 to 7, wherein the control signals for the telephone system includes the answer signal lying in a first frequency band and the release guard signal having two components, one lying in the first frequency band and the second lying in a second frequency band.

9. A circuit arrangement according to any preceding claim including means for producing a contact wetting current when the transmission of speech is attenuated so as to reduce noise which would otherwise be generated by mechanical contacts.

10. A circuit arrangement according to any one of claims 4 to 9, including means for passing recorded messages.

11. A circuit arrangement according to claim 1 or 2 wherein the controlling means includes means connectible to the line for generating noise having frequency components extending over the speech transmission bandwidth of the line.

12. A circuit arrangement according to claim 11 wherein the noise generating means includes meansfor filtering out of the noise those components which lie in one or more narrow frequency bands occupied by control signals of the telephone system

13. A circuit arrangement connectible to an international circuit in an international telephone exchange substantially as herein described and as illustrated by Figure 1 or Figures 2A, 2B, 2C, 3A, 3B, 3C, 4A, 4B, 4C, 4D, 4E, 4F, 5 and Figure 6, of the accompanying drawings.