Process for protecting a remote monitoring system against sabotage and a system using this process

• US 4,536,748 A
• Filed: 04/28/1983
• Issued: 08/20/1985
• Est. Priority Date: 04/30/1982
• Status: Expired due to Fees

• Current Assignee: Compagnie Europeenne de Teletransmission CETT
• Original Assignee: Compagnie Europeenne de Teletransmission CETT
• Inventors: Tonello, Marc
• Primary Examiner(s): Crosland, Donnie L.
• Application Number:
  US06/489,355
• Time in Patent Office: 845 Days
• Field of Search: 340/506, 340/505, 340/511, 340/514, 340/517, 340/518, 340/825.07, 340/825.1, 340/825.14, 340/825.54, 340/825.34, 340/870.09, 340/870.16 , 340/870.17, 340/870.21
• US Class Current: 340/506
• CPC Class Codes: G08B 26/002 only replying the state of the sensor
• CPC Class Codes: G08B 29/08 Signalling of tampering with the line circuit

Abstract

Each sensor of the system is connected to a central station and is interrogated cyclically. In turn, the sensors generate over a bus line a variable amplitude signal synthesized from data stored in a read only memory. Each sensor has a read only memory with different data and thus generates a different waveform during successive intervals of time. Any modification in the waveform of the signal transmitted by a sensor is interpreted as an alarm due to the detection of an intrusion, for example, or due to an attempt at sabotaging the sensor or the bus lines. The complexity of the waveforms synthesized by the sensors makes a simulation very difficult intended to neutralize the remote monitoring system.

Claims

1. A remote monitoring system comprising a central station and a plurality of sensors wherein each sensor comprises:

an alarm input terminal;

a synchronizing signal and clock signal separator having one input connected to the central station and having a first and a second output supplying respectively said synchronizing and said clock signal;

a counter having a reset input and a clock input connected respectively to the first and to the second output of the separator and having outputs;

a read only memory having address inputs connected respectively to the outputs of the counter, and having outputs, for supplying data for synthesizing a waveform;

a digital-analog converter having inputs connected respectively to the outputs of the read only memory and having an ouput, for supplying a variable amplitude signal;

an analog gate having an input connected to the output of the converter, a first control input connected to an output of the memory, a second control input connected to the alarm input terminal and having an output connected to the central station for transmitting the variable amplitude signal to the central station when the sensor is interrogated and when there is no alarm to be transmitted.

2. A remote monitoring system including a central station and a plurality of sensors, wherein said central station comprises:

an interrogation signal generator having an output connected by a first line to all of said sensors;

counting means having an input connected to the output of said generator and having outputs;

a read only memory having address inputs connected respectively to the outputs of the counting means and having outputs, for supplying data characteristics of a succession of waveforms identical to waveforms transmitted successively by all of said plurality of sensors of the system, in the absence of an alarm;

comparator means having inputs connected respectively to the outputs of the read only memory and an input connected to all of said sensors by a second line, and an output, for supplying an alarm signal when a sucession of waveforms received by the central station on said second line differs from said succession of waveforms identical to waveforms transmitted successively by all of said plurality of sensors of the system in the absence of an alarm.

3. A process for protecting a remote monitoring system against sabotage, wherein said system includes a central station connected to a plurality N of sensors, comprising the steps of:

sequentially testing each of said N sensors by feeding from said central station to said N sensors, an interrogation signal formed by a sychronizing pulse followed by a train of periodic pulses whose number P is at least equal to N and counting the pulses in each sensor which follow any of said synchronization pulses and triggering the transmission of a variable amplitude signal when the count has reached a predetermined value associated with each sensor and chosen from the whole numbers between 1 and P;

transmitting, from a tested sensor to the central station of the system, a signal of variable amplitude formed by a waveform synthesized from data stored in said tested sensor;

authenticating said signal of variable amplitude when said signal arrives at said central station by sampling said signal and comparing the value of each sample with a reference value.