FR2674397A1 - Device for monitoring a subscriber line allowing detection and recording of electrical signals related to an illicit tapping - Google Patents

Abstract

The invention relates to subscriber telephone installations. This device for monitoring a subscriber line, allowing detection and recording of electrical signals relating to illicit tapping, is characterised especially in that it comprises a circuit for detecting the illicit tapping (6) which comprises a diode rectifier (15) and a circuit for memory-storage of the line DC voltage at rest (called reference voltage) (10), a circuit for filtering the ringing and dialling signals (11), a threshold comparator (12), a circuit for inhibiting the action of the local subscriber instrument (13), a circuit for detecting the unhooking of the local subscriber instrument (14) and the said monitoring device allowing the detection and recording of electrical signals related to an illicit tapping also comprises a device (5) for recording all the signals present on the subscriber line, from the moment when the "pirate" unhooks until he hangs up. The invention is applicable to subscriber instruments and to telephone answering machines.

Description

LINE MONITORING DEVICE
SUBSCRIBER ALLOWING DETECTION AND
THE RECORD OF ELECTRIC SIGNALS
RELATED TO AN ILLEGAL CONNECTION
The present invention relates to subscriber telephone installations, and more particularly in such installations, a monitoring device allowing the detection and recording of electrical signals linked to an illicit connection.

A subscriber telephone installation is connected to the telephone exchange by a subscriber line to which a "pirate" station can be connected in parallel on the said line. The object of the present invention therefore relates to a device for monitoring a subscriber line allowing the detection and recording of electrical signals present on a line on which a "pirate" station is illicitly connected, between the telephone exchange and the subscriber station.

The present invention therefore relates to a device for monitoring a subscriber line, disposed near the subscriber station, which first detects the illegal connection of the "pirate" station, then provides the legitimate subscriber with a recording of all the signals present on the line, these so-called signals being able to be the combination of so-called "voice frequency" signals, or decimal pulses, these two types of signals representing the subscriber number requested by the "pirate", and / or signals representative of the conversation of the "pirate" with his correspondent, or even all other types of signals present on the subscriber line.

More specifically, the monitoring device enabling the detection and recording of electrical signals linked to an illegal connection, comprises a circuit for detecting the illegal connection of the "pirate" and a circuit for recording signals transmitted by said pirate station or by the telephone exchange or by the pirate's correspondent ", said pirate being connected in parallel to the subscriber line, in an illegal manner.

The circuit for detecting the illegal connection of the "pirate" comprises, according to the invention, a circuit for detecting changes in the parameters of the subscriber line and a circuit for triggering the recording and resetting device of said device.

The signal recording device according to the invention comprises a recording device which records all the signals, whether for telephone signaling, speech or the like, and whether they are emitted by the "pirate", the correspondent of " pirate "or the telephone exchange, from the moment the" pirate "connected in parallel to the subscriber line.

Other characteristics and advantages of the present invention will appear on reading the following description of a particular embodiment, said description being made with the attached drawings in which: FIG. 1 is a block diagram subscriber line monitored by the monitoring device enabling the detection and recording of electrical signals linked to an illegal connection. As shown in FIG. 1, the monitoring device enabling the detection and recording of electrical signals linked to a illicit connection (4) on a subscriber line (3) with two conductors L1, L2 leading to a subscriber station (8), includes a circuit for detecting illicit connection (6) and a recording device (5 ) of all the signals present on the subscriber line, following the connection of the "pirate" station (7) in parallel on the line, between the telephone exchange and the surveillance device eg rmitting the detection and recording of electrical signals linked to an illegal connection (4).

FIG. 2 represents the block diagram of the illicit connection detection circuit (6). The subscriber line (3) with its two conductors L1 and L2, connects the subscriber station (8) to a telephone exchange not shown in the figure, via the monitoring device allowing detection and recording electrical signals linked to an illicit connection (4) of which the illicit connection detection circuit (6) is a part. The illicit connection detection circuit (6) comprises a bridge of four rectifying elements (15), a switching circuit in memory of the DC line voltage at rest (called reference voltage) (10>, a circuit for filtering ringing and dialing signals (11), a threshold comparator (12), a circuit for inhibiting station actions subscriber (13), a circuit for detecting the off-hook of the subscriber station (14).

The functions of the different circuits are as follows:
The illicit connection detection circuit (6) delivers, in the presence of a "pirate", an activation signal at point A intended for the recording device (5) and a suppression of this activation at point A when the line resumes its resting state. The signal available at point A can be used to trigger any alarm device external to the monitoring device allowing the detection and recording of electrical signals linked to an illegal connection.

The rectifier bridge (15) supplies the monitoring device enabling the detection and recording of electrical signals linked to an illicit connection (4), overcoming the variations in polarity that exist on the subscriber line.

The circuit for storing the DC line voltage at rest (called reference voltage) (10) stores the value of the DC line voltage at rest, that is to say when no direct current is not consumed on this line this reference value is not fixed, but follows the normal variations of the lines at rest imposed by the power supply of the telephone exchange.

The circuit for filtering the ringing and dialing signals (11) prevents these said signals from disturbing the monitoring and detection of variations in the DC line voltage.

The threshold comparator (12) delivers an active signal when the instantaneous line voltage is lower than the reference voltage delivered by the circuit for storing the DC line voltage at rest (10), by a certain fixed amount , this is to ignore variations in the line voltage at rest, considered normal by telephone systems.

The circuit inhibiting the actions of the subscriber station (13) will prevent the illicit connection detection circuit (6) from delivering an active signal to the recording device (5) when the drop in DC line voltage is due to off-hook. of the subscriber station (8), and more generally when the line voltage variations are due to the actions of this subscriber station.

The subscriber station pick-up detection circuit (14) detects the use of the subscriber station and prevents the delivery of an active signal to the recording device (5) by the illicit connection detection circuit (6 ). The operation of the monitoring device enabling the detection and recording of electrical signals linked to an illicit connection is as follows. In the normal state of the subscriber line, that is to say without illicit connection, and when idle state, crest to say when the subscriber station is on-hook and no ring signal from the exchange is present on the subscriber line, no active signal is delivered at point A by the circuit for detecting illegal connection (6). It is the same in the following cases: -The local subscriber unit (8) picks up and performs all the maneuvers to obtain a communication, then is in effective communication, then hangs up, and more generally whatever the maneuvers it performs after a hang-up and until a hang-up.

- A ring signal is sent by the central office to be routed to the local subscriber station (8).

When an "pirate makes an illicit connection, somewhere between the telephone exchange and the surveillance device allowing the detection and recording of electrical signals linked to an illicit connection, and in parallel on the subscriber line, the fact of picking up his phone, whatever the type, causes a drop in the line voltage which is detected by the illicit connection detection circuit (6), which causes the appearance of an active signal at point A, which causes the activation of the recording device (5), which can be located either in the immediate vicinity of the illicit connection detection circuit, or distant from this said device, in this case the active signal at A acts on its setting command input in so-called "remote" operation. The active signal at point A can also be used as the trigger signal of any external alarm device. The recording device (5), when it is activated, records all the signals present on the subscriber line, that is to say all the dialing and speech signals emitted by the "pirate" or by his correspondent or even emitted by the exchange and all this up to what the hacker hangs up. When the "pirate" hangs up his station, the illicit connection detection circuit (6) deactivates the signal at the point
A, which stops the operation of the recording device (5). The variations in line voltage caused by the "pirate's maneuvers, in particular its numbering, do not disturb the operation of the illicit connection detection circuit (6), this thanks to the circuit for filtering the ringing signals and numbering (11) .

Claims (4)

1. Device for monitoring a subscriber line allowing the detection and recording of electrical signals linked to an illicit connection, following the actions of a pirate station "(7) connected in parallel on said line, characterized in that it includes a circuit for detecting illicit connection (6) and a device for recording (5) the signals present on the subscriber line from the off-hook of said pirate station and until the hook-up of said pirate. 2. Device for monitoring a subscriber line allowing the detection and recording of electrical signals linked to an illicit connection according to claim 1 characterized in that the illicit connection detection circuit (6) of a "pirate" station (7) connected in parallel to the subscriber line comprises a diode rectifier bridge (15), a circuit for storing the DC line voltage at rest (called reference voltage) (10), a filtering circuit ringing and dialing signals (11), a threshold comparator (12) activating for a fixed value of voltage difference between the DC line voltage at rest (called reference voltage) and the instantaneous value of the voltage continuous line, it being understood that the activation of said comparator will take place only for a negative variation of the instantaneous line voltage relative to its rest value, a circuit inhibiting the actions of the local substation born (13), a circuit for detecting the off-hook of the local subscriber set (14>. 3. Device for monitoring a subscriber line allowing the detection and recording of electrical signals linked to an illicit connection according to claims 1 and 2 characterized in that the illicit connection detection circuit provides a control signal to the recording device (5), which is activated when the "pirate" is picked up and deactivated when the said Ilpiratell is hung up. 4. Device for monitoring a subscriber line allowing the detection and recording of electrical signals linked to an illicit connection according to the preceding claims there 3 characterized in that the control signal supplied by the detection circuit of the illicit connection (6), allows when activated, to activate the recording device (5), which then records all the electrical signals present on the line, whether they come from the "pirate" or the correspondent of this " pirate "or central office, and whatever the type of signals, speech or telephone signaling; this activated control signal also makes it possible to control any alarm system external to the device which is the subject of these claims.