WO1979000844A1

WO1979000844A1 - An arrangement for security systems

Info

Publication number: WO1979000844A1
Application number: PCT/SE1979/000068
Authority: WO
Inventors: R Svensson
Original assignee: Saab Scania Ab; R Svensson
Priority date: 1978-03-28
Filing date: 1979-03-28
Publication date: 1979-11-01
Also published as: SE7803437L

Abstract

In a security system, for status reporting from detectors (D1, D2, D3) set up in a protected local (P) to a monitoring centre (C), and monitoring the operational condition of communication lines (3) which are arranged to connect the detectors to the monitoring centre, an interrogation signal (F) is produced and transmitted from an interrogation unit (1) in the monitoring centre to an answering unit (2) connected to the interrogation unit and located in the detector. The answering unit replies by producing and sending back to the interration unit an answering signal (Sf) which is dependent on he interrogation signal. When alarm/sabotage status is indicated by the detector, the answering unit recognises this and sends an alarm/sabotage signal (sa/Ss) to the interrogation unit which is different from the answering signal (Sf) depending on the interrogation signal. In order to prevent or make unauthorised manipulation of the security system with a view to misleading the monitoring centre (C) extremely difficult, and to obtain a functionally-reliable arrangement in which both monitoring of the communication lines (3) and status reporting from the detectors (D1, D2, D3) is effected simultaneously with the aid of the same functional units by integrated communication, the interrogation signal (F) transmitted by the interrogation unit (1) is arranged to be transmitted continuously and also to be information-bearing with a stochastically varying content. The answering unit (2) is arranged to endow the answering signal with an information content which, in addition to the instantaneous information content of the interrogation signal, also depends on the actual status (a, s) of the detector, and to send back the answering signal continuously.

Description

An Arrangement for Security Systems

Technical Field

The present invention relates to an arrangement for security systems for status reporting from detectors set up in a protected location to a monitoring centre, and monitoring of the operational condition of communication lines which are arranged to connect the detectors to the monitoring centre. In the centre an interrogation unit is pro¬ vided for each detector, and an answering unit is arranged in each detector. Each interrogation unit is connected via a communication line to an answering unit co-acting with it. The interrogation unit comprises an interrogation signal transmitter and an answering signal receiver, and the answering unit has an interrogation signal receiver and an answering signal transmitter. The transmitter of the interro¬ gation unit is arranged to produce and send an interrogation signal to the answering unit, and the latter is arranged to receive the interro- gation signal and to produce and send back an answering signal to the interrogation unit, depending on the interrogation signal. When alarm/sabotage status is indicated by the detector the answering unit is arranged to recognise this and to produce and send back to the monitoring centre an alarm/sabotage signal which is different from the answering signal associated with the interrogation signal. Finally, the interrogation unit is arranged to receive answering signals and alarm/sabotage signals respectively, and to identify them.

State of the Art An arrangement of the above-mentioned type is shown and described in US Patent No. 4 044 351. For checking the communication line, the arrangement in the Patent sends from the interrogation signal trans¬ mitter of its interrogation unit an interrogation signal which con¬ sists of a tone pulse with a specific frequency F1 which recurs at random intervals of time. The time interval between the pulses can be up to 1--2 minute. The answering unit answers by sending back to the interrogation unit a second specific frequency F2 with the same time interval which characterises the interrogation signal. When there is alarm/sabotage status, status reporting is carried out by the answering

OMPI unit by means of a function which is separate from the communication line checking arrangement, in that the answering unit transmits to the interrogation unit an alarm/sabotage signal consisting of a constant tone with a third specific frequency F3.

The Technical Problem

Since the arrangement in the said Patent sends pulses with different time intervals and with a constant frequency content between the int rogation and answering units, it is relatively easy for an unauthori sed person to manipulate the equipment so as to prevent the indicati of alarm status when a break-in into the protected area is imminent. All that is required to simulate that the line is whole and to preven an alarm signal from being sent to the monitoring unit is for the answering signal frequency to be listened to and for an answering signal producing device to be made which, on receiving an interroga¬ tion signal, produces a false answering signal, and which can be con nected in the communication line between the interrogation unit and the answering unit during the time interval between interrogation signals, at the same time as the communication line is broken beyond the answering signal producing device.

The Object of the Invention

One object of the present invention is to remedy the said disadvanta by providing an arrangement of the type described in the introductio which, by continuous monitoring, prevents or makes very difficult an unauthorised manipulation with a view to misleading the monitoring centre in order to prevent the discovery of a projected break-in.

Another object of the invention is to provide a functionally-reliabl arrangement of the above-mentioned kind with which both communicatio line monitoring and status reporting from the detectors is effected simultaneously with the aid of the same functional units, by integra ted communication.

The Solution of the Problem

This is accomplished according to the invention in that this is en¬ dowed with the following characteristics: the interrogation signal produced by the interrogation unit transmitter is information-bearing and is sent continuously; a ran¬ dom number generator is connected to the transmitter and is arranged to vary the said information content stochastically; the answering unit has a signal converter connected in between its receiver and its transmitter which is arranged to be influenced by the different status conditions prevailing at the detector and to endow the answering sig¬ nals with an information content which, in addition to the informa¬ tion content of the interrogation signal at the time, is also depen¬ dent on the actual status of the detector, and to send this answering signal continuously; the interrogation unit comprises a signal conver¬ ter which is connected to the transmitter and is arranged to convert the transmitted interrogation signal to a reference signal, the infor¬ mation content of which is in a specific relationship to the instan¬ taneous information content of the transmitted interrogation signal; a signal comparator is connected to the receiver and the signal con¬ verter and arranged to identify the alternative answering signals by comparing these with the reference signal.

Description of the Figures The arrangement according to the invention will be described in more detail below with reference to the attached Drawing which illustrates the invention schematically in the form of two block diagrams.

Figure 1 shows fundamentally a security system arranged according to the invention, in which the three detectors in a protected area are connected to a monitoring centre.

Figure 2 illustrates fundamentally in more detail the various functio¬ nal blocks of the arrangement according to the invention and their connection with each other.

In the following description of the Figures, three alternatively constructed arrangements according to the invention will be described, which are imagined to operate with interrogation and answering sig¬ nals of a different character. Thus, in each case the blocks in the system are constructed in a slightly di-fferent way and carry out slightly different technical functions. These technical constructions and their interconnections, which are of a conventional nature, are not shown in the Drawings, but are only described. With reference firstly to Figure 1 in the Drawing, one large block C designates a monitoring centre and the other large block P designate a protected local located peripherally realtive to the monitoring centre. The local is protected by three detectors D1, D2 and D3 respectively. The monitoring centre has an interrogation unit 1 for each detector. Each detector has an answering unit 2 and an alarm contact 2a and a sabotage contact 2s, these detector contacts being arranged to break when there is alarm or sabotage status. The con¬ tacts of the detector are connected by lines 7 and 8 respectively to the answering unit. Each answering unit is connected to a communica¬ tion line 3 for double-direction communication with its interrogatio unit.

The functional units of the interrogation unit and the answering uni may be seen in more detail in Figure 2. The interrogation unit has a interrogation signal transmitter 9 which is connected to the communi cation line 3, a random number generator 10 which is connected to th interrogation signal transmitter 9, an answering signal receiver 12 which is connected to the communication line, a signal comparator 13 connected to the transmitter, and a signal converter 11 connected in between the transmitter and the signal comparator.

For its part, the answering unit comprises an interrogation signal receiver 4 connected to the communication line 3, an answering signa transmitter 6 connected to the communication line, and a signal con¬ verter 5 connected in between the receiver and the transmitter. The lines 7, 8 of the contacts 2a, 2s are connected to the signal conver ter.

A First Alternative Embodiment of the Arrangement According to the Invention

The first alternative arrangement is imagined to operate with a direct current signal which is transmitted continuously from the interrogation unit as an interrogation signal, and an alternating current signal with variable frequency which is transmitted continuo ly from the answering unit as an answering signal. The answering uni 2 in this case is a so-called voltage-controlled oscillator (VC0) which has two separate control inputs, represented on the Drawing by an interrogation signal receiving and amplifying block 4 and the sig¬ nal converting block 5 connected thereto, and a signal output shown as an answering signal transmitting block 6 connected to the signal converting block. When an unauthorised presence in the protected local is indicated, or when an opening is indicated in the cover of the detector, the signal converting block 5 receives status signals a or s from the contacts 2a or 2s respectively, for alarm or sabotage respectively.

The interrogation signal transmitter 9 of the interrogation unit has a number of different load resistances which can be connected indivi¬ dually by means of analogue gates. The random number generator 10 is arranged to control the analogue gates of the transmitter so that load resistances are connected continuously, following on after one another without any time interval between them and varying in a random manner (stochastically). The signal converter 11 consists of a voltage-con¬ trolled oscillator of the same type as in the answering unit. The answering signal receiver 12 consists of an input amplifier of a con- vεntional type. The signal comparator 13 comprises the units, a binary computer, a short-term memory (latch), and a de-coder.

The functioning of the arrangement is as follows. The interrogation signal transmitter 9 of the interrogation unit 1 loads the interroga- tion signal receiver 4 of the answering unit 2 and its own signal converter 11 with the load resistances which are varying stochasti¬ cally in time, whereupon the interrogation signal F is produced. The interrogation signal receiver of the answering unit receives the interrogation signal F which is supplied to the signal converter 5. If no status signal a, s is coming from the detector contacts 2a, 2s via lines 7 and 8 to the signal converter, the interrogation signal F is converted to an answering signal Sf, which is the answering signal expected from the answering unit by the interrogation unit for the status "all in order", i.e. when neither alarm nor sabotage is indi- cated by the detectors. Upon reception of a status signal a, the sig¬ nal converter 5 converts the interrogation signal F to an alarm answe¬ ring signal Sa, the frequency of which differs from that expected of the answering signal Sf. Upon the alternative receipt of a second

OMPI wiPO A signal s, the signal converter in this case converts the interroga¬ tion signal to a sabotage answering signal Ss, the frequency of whic differs in another predetermined way from that expected of the answe ring signal Sf and the alarm answering signal Sa. The alternative answering signals Sf, Sa, Ss are transmitted by the answering signal transmitter 6 via the communication line to the interrogation unit answering signal receiver 12, and after amplification in the latter are supplied to the signal comparator 13.

From the signal converter 11 a reference signal Sr is also supplied to the signal comparator, this reference signal having an informatio content which is in a specific relationship to that expected of the answering signal Sf. The reference signal is compared by the signal comparator with the three alternative answering signals Sf, Sa and S The signal comparison is based on the concept that the period time f the reference signal Sr is selected greater by a specific factor tha the period time for the answering signals Sf, Sa and Ss from the answering unit. In this way, the period time of the reference signal Sr can determine the time interval during which a period calculation of the answering signals Sf, Sa and Ss is carried out. At the end of the calculation period time, the calculated period number is trans¬ mitted to the short-term memory and alarm A or sabotage S respective are indicated for the answering signals Sa or Ss respectively, the calculated period number of these differing from the calculated peri number of the answering signal Sf.

An unauthorised manipulation of the communication line 3 between the interrogation signal transmitter 9 and the interrogation signal rece ver 4 either influences the load connected to the interrogation sign receiver or disturbs the period counting of the answering signals going on in the signal comparator 12, which gives a counted period number which is either less or greater than the period number for th expected answering signal Sf, or alternative signals Sa and Ss, and in both cases is indicated with the signal K as a fault on the commt-i nication line. . A Second Alternative Embodiment of the Arrangement According to the Invention

The second alternative arrangement is imagined to operate with alter¬ nating current signals which are composed of at least two frequency components for both the interrogation signal and the answering signal. In this case, the interrogation signal transmitter 9 of the interro¬ gation unit 1 consists of a so-called touch-tone generator which in this case has eight binary control signal inputs, and is arranged to transmit the interrogation signal F continuously, depending on the signals at the control inputs corresponding to a binary code, said interrogation signal F being a combination of, for example, two of eight selected frequencies. The random number generator 10 is arranged to control stochastically the binary signal inputs so that a connec¬ tion varying in time in a random manner is obtained without pause for two out of the eight frequencies. The signal converter 11 of the interrogation unit is arranged to convert in a predetermined way a certain chosen binary code to another such code. The answering signal receiver 12 of the interrogation unit comprises a detector and a decoder which is arranged to convert incoming alternating current signals to a binary code. The signal comparator 13 is arranged to compare two binary codes.

The interrogation signal receiver 4 of the answering unit is construc¬ ted in the same way as the answering signal receiver 12 of the interro- gation unit. The signal converter 5 is of the same type as that of the interrogation unit, but has here in addition the two signal inputs for status signals a and s. The conversion by the signal converter of the incoming bynary code to another binary code is arranged to occur dependent on the said status signals. The answering signal transmitter 6 is of the same kind as the interrogation signal transmitter 9 of the interrogation unit.

The functioning of the arrangement is as follows. The interrogation signal transmitter of the interrogation unit sends out the interroga- tion signal F which is detected by the answering unit interrogation signal receiver 4 as a binary code. The binary code is converted in the signal converter 5 depending on the possible status signals a, s and the code obtained controls the transmission by the

signal transmitter 6 of the corresponding alternative answering signa Sf, Sa, Ss. The answering signal is detected by the answering signal receiver 12 of the interrogation unit as a binary code. The binary code corresponding to the interrogation signal F is converted by the signal converter 11 to a reference code, this reference code being in a specific relationship to the answering signal code of the answering signal Sf expected when the status "all in order" prevails at the de¬ tector. The reference code is compared in the signal comparator with the three alternative answering signal codes and the signal comparato indicates alarm A or sabotage 5 for the codes corresponding to the answering signals Sa and Ss, which differ in a specific way from the reference code.

An unauthorised manipulation of the communication line 3 causes dis- turbances in the transmission of information, which are indicated by the signal comparator with the signal K, as a fault on the communica¬ tion line.

A Third Alternative Embodiment of the Arrangement According to the Invention

The third alternative arrangement is imagined to operate with direct current signals. The interrogation signal transmitter 9 of the inter¬ rogation unit 1 and the random number generator 10 are of the same kind as in the first alternative arrangement. In this case, the answering unit 2 is a direct current amplifier with two separate control inputs, shown as an interrogation signal receiver 4 and the signal converter 5 with a signal output represented by the answering signal transmitter 6. The signal converter 5 and the answering signal transmitter 6 are arranged to convert in a predetermined way the interrogation signal F received by the interrogation signal receiver to a direct current answering signal, the magnitude Sf, Sa, Ss of which depends on the status "all in order" at the detector, or the alternative status signals alarm a and sabotage s respectively.

The answering signal receiver 6 of the interrogation unit is a con¬ ventional input amplifier. The signal converter 11 is an amplifier which converts the interrogation signal received from the interroga¬ tion signal transmitter to a reference signal Sr, the magnitude of

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A\> .₇. Wv.Iπ which is in a specific relationship to the magnitude of the anticipated answering signal Sf when the status "all in order" prevails at the detector. In the signal comparator 13 the magnitude of the reference signal Sr is compared with the magnitudes of the three alternative answering signals Sf, Sa, Ss. The signal comparator is designed to indicate alarm A or sabotage S for the signal magnitudes corresponding to the signals Sa and Ss.

An unauthorised manipulation of the communication line 3 influences the direct current signal coming into the interrogation unit so that it becomes less, or alternatively greater, than the three alternative answering signals Sf, Sa, Ss, and this is indicated by the signal comparator with the signal K, as a fault on the communication line.

Claims

1. An arrangement for security systems for status reporting from detectors (D1 , D2, D3) set up in a protected location (P) to a monitoring centre (C), and monitoring of the operational condi¬ tion of communication lines (3) which are arranged to connect the the detectors to the monitoring centre. In the centre an interro¬ gation unit (1) is arranged for each detector, and an answering unit (2) is arranged in each detector. Each interrogation unit is connected via a communication line to an answering unit co-acting with it. The interrogation unit comprises an interrogation signal transmitter (9) and an answering signal receiver (12), and the answering unit has an interrogation signal receiver (4) and an answering signal transmitter (6). The transmitter of the interro¬ gation unit is arranged to produce and send an interrogation sig¬ nal (F) to the answering unit, and the latter is arranged to receive the interrogation signal and to produce and send back to the interrogation unit an answering signal (Sf) depending on the interrogation signal. When alarm/sabotage status (a, s) is indi¬ cated by the detector the answering unit is arranged to recognise this and to produce and send back to the monitoring centre an^* alarm/sabotage signal (Sa/Ss) which differs from the answering signal associated with the interrogation signal. Finally, the interrogation unit is arranged to receive the answering signal and the alarm/sabotage signals respectively, and to identify them The arrangement is characterized in that the interrogation signal (F) produced by the transmitter (9) of the interrogation unit (1) is informationbearing and is transmitted continuously, that a random number generator (10) is connected to the transmitter (9) and is arranged to vary stochastically the said information con¬ tent, and that the answering unit has a signal converter (5) con- nected between its receiver (4) and its transmitter (6), this converter being arranged to be influenced by various status con¬ ditions prevailing at the detactor (D1 , D2, D3) and to endow the answering signal with an information content which, in addition to the instantaneous information content of the interrogation signal, is also dependent on the actual status (a, s) of the detector, and to transmit this answering signal (Sf, Sa, continuously, and that the interrogation unit comprises a signal converter (11) which is connected to the transmitter and is arranged to convert the transmitted interrogation signal (F) to a reference signal (Sr), the information content of which is in a specific relationship to the instantaneous information content of the transmitted interrogation signal, and that a signal comparator (13) is^* connected to the receiver (12) and the signal converter (11) and is arranged to compare the alternative answering signals (Sf, Sa, Ss) with the reference signal (Sr) in order to identify them.

2. An arrangement according to Claim 1, characterized in that the interrogation, signal (F) transmitted by the interrogation signal transmitter (9) in the interrogation unit (1) and the alternative answering signals (Sf, Sa, Ss) sent back by the answering signal transmitter (6) in the answering unit (2) are all direct current signals which differ in magnitude from one another.

3. An arrangement according to Claim 1, characterized in that the interrogation signal (F) transmitted and the alternative answering signals (Sf, Sa, Ss) sent back are all different alternating cur¬ rent signals containing one or more frequencies.

4. An arrangement according to Claim 1, characterized in that the interrogation signal (F) is of one kind, such as a direct current signal, for example, and the alternative answering signals (Sf, Sa, Ss) are all of another kinds, such as alternating current signals which differ in frequency from one another, for example.