GB2043314A - Line signaling apparatus - Google Patents

Abstract

In a method and a system for transmitting to a central station data relating to a condition, such as the security, of premises, a transmitter located at the premises transmits the data in a wide band spectrum of a.c. signals which also includes many signals which are not necessary to the recovery of the data. The transmitter responds only to a correct interrogation which is transmitted over a telephone line amongst a series of randomly or pseudo-randomly varying signals. <IMAGE>

Description

SPECIFICATION Line signalling apparatus This invention relates to a method and apparatus for signalling to remote positions, via lines, indications of certain conditions existing at protected premises.

It is well-known to use signalling systems for indicating remotely such information as to whether unauthorised entry has been made into the premises; whether personnel are within the premises; whether ambient conditions, such as temperature or humidity, are correct; or whether equipment, such as air conditioning plant, is operating correctly.

Known systems rely upon the changing of d.c.

levels to provide the remote indications. Alternatively, or additionally, the characteristics of a.c. signals, such as the frequency, duration or sequence of tones, are used.

In typical systems, a tone having a particular characteristic is sent to the remote position, and receipt of the tone causes the generation of other tones which may occur simultaneously or sequentially and which are related to the transmitted condition.

Such systems have, in particular, two drawbacks if security of the system is important. Firstly the spectrum of the signals is relatively simple and is predictably related to the data which are being transmitted. Secondly, the process of tone interchange or transmission occurs at regular, and therefore predictable, intervals.

The operation of such systems can be readily monitored, and illicit apparatus can be built to simulate safe operation of the equipment used on the premises. This equipment can then be attached to the line so that the remote position, e.g. a central security monitoring station, continues to receive normal signals, even though the premises are being tampered with.

It is an object of the present invention to provide a signalling method and system which relies on events which are extremely difficult, or even impossible, to predict, thereby providing a more secure system.

According to one aspect of the invention, a method of signalling between premises at which a condition is to be monitored and a station remote therefrom comprises feeding data representing the monitored condition to a first transmitter associated with the premises; transmitting over a line from a second transmitter at the remote station to the first transmitter randomly or pseudo-randomly varying signals including an interrogation to which the first transmitter is uniquely responsive, thereby causing the first transmitter to transmit the data over the line to the remote station, the data being represented by a.c. signals within a transmitted spectrum which also includes a substantial proportion of signals which are not necessary for data recovery; and recovering the data from the transmitted signals at the remote station.

According to another aspect of the invention, a system for signalling to a remote station a condition occurring at premises comprises means to monitor the condition; a first transmitter associated with the premises and coupled to the monitoring means; a second transmitter at the remote station and operative to transmit to the first transmitter, via a line, randomly or pseudo-randomly varying signals including an interrogation; means coupled to the first transmitter and uniquely responsive to the interrogation to cause the first transmitter to transmit via the line to the remote station data representing the monitored condition, the data being represented by a.c. signals within a transmitted spectrum which also includes a substantial proportion of signals which are not necessary for data recovery; and means at the remote station to recover the data from the transmitted signals.

The signals transmitted over the line in either or both of the directions may comprise tones, and the intervals between the tones may have variable durations. The durations can change pseudorandomly using a large number of different intervals which eventually recur in the same order, or can change completely randomly. Alternatively, or additionally, the lengths of the transmissions themselves can change randomly or pseudo-randomly. The introduction of illicit signals which would be acceptable to the remote station would be virtually impossible. The system would also be tolerant to interference which is cyclic in nature, since the transmitted signals are non-cyclic.

Alternatively, the frequency spectrum of the signals transmitted between the premises and the remote station may contain a number of frequencies each carrying the data to be transmitted, many of which frequencies are redundant. Such signals may be, for example, sideband signals of a modulated carrier. The station receiving the signals will expect to receive a particular one, or particular ones, of these frequencies, and it will be impossible to decide for certain, by intercepting the signals, which of the frequencies have to be received. The data may be represented by the phases, the periodic frequencies or the amplitudes of the tones, or by the number of tones present, or by any combination of these parameters.

A large number of signal combinations will be present in the transmission, only a few of which are relevant to the transmitted data. The transmitted signals would be difficult to analyse, but even if a full analysis were to be effected, using sophisticated equipment, it would be impossible to judge with certainty which of the many signals were the relevant ones. If the signals were fed to a transducer, the human ear and brain would not be able to distinguish the important signals from the resultant sound, which would appear to be merely random noise.

Since only a part of the whole signal is needed to convey the data, it is not necessary to receive and decode the whole signal in order to recover the data.

Hence, even though parts of the signal may be obliterated by interference, it may still be possible to receive the data correctly.

In known systems the signals are encoded to represent particular data to be transmitted. For example, the fact that the premises are secure will be represented by a particular combination of signals, and the encoding will be the same for all premises connected to the remote station. It will be relatively easy to determine the code and to produce an illicit encoding apparatus which can be connected to the line to cover up unauthorised entry into the premises.

In an embodiment of the present invention, the data are encoded differently and arbitrarily at the various premises and are suitable decoded at the remote station. Even if an intruder were able to obtain a duplicate of the equipment used at the premises, and connect such equipment to the line in place of the true equipment, he would not know the key to the coding in the true equipment.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block schematic diagram of equipment which is to be installed at the premises to be protected, Figure 2 is a block schematic diagram of remote station equipment, and Figure 3 indicates a typical frequency spectrum used in the transmission of data over a line interconnecting the premises and the remote station.

Referring to Figure 1, the premises equipment is connected to the central station (i.e. the 'remote' station) equipment (Figure 2) via a telephone line 1 and an interface 2. The central station equipment transmits a number of interrogation tones (8, in the present embodiment) at pseudo-random intervals and with varying durations.

The interrogation tones are fed to a tone receiver 3 and thence to a tone decoder 4 which responds to the correct receipt of a predetermined tone frequency, or combination of tone frequencies, to cause a return signal decoder 5 to feed to alarm equipment 6 an instruction to feed back data relating to certain conditions existing at the premises.

The alarm equipment 6 can include means to monitor various conditions as mentioned above, such as whether the premises are secure, and receipt of the return signal data causes the equipment 6 to feed out, on lines 7, a coded representation of the requested data. For given data these coded representations can be the same for all premises equipments connected to the system. However, the codes can then be arbitrarily 'scrambled' in an encoding field 8, so that the resulting coded representations for given data from each equipment are quite different from those from all other premises equipments.

The 'scrambled' (i.e. arbitrarily encoded) signals are fed to a premises state output unit 9, which is triggered by a signal from the decoder 4 to feed the signals to a premises signal transmitter 10. This transmitter produces a wideband signal based on a 22KHz carrier, which signal includes the data and any or all of the redundant signals and obscuring signals mentioned previously. The data bits are represented by amplitude coding of the wideband signal by modulating signals of varying durations. The bits are made up of anumber of 'sub-bits', and it is necessary to receive only a fraction of this number in order to be able to determine correctly the character of the transmitted data.

Referring now to Figure 2, at the heart of the central station equipment is a group interval signal generator 11 which determines the intervals between successive interrogation tones. The generator 11 includes a pseudo-random scan generator comprising a clocked shift register with a number of feedback loops fed off at various positions along the register. The output of the shift register is used to trigger an interrogation tone generator 12 to provide the interrogation tones in turn at pseudo-random intervals. The tones are fed via an interface 13 to the line 1.

The interrogation tones are pulse-width modulated by a return signal encoder 14 in response to return signal data fed to the encoder 14 from a premises state priority detector 15. The number of cycles of the particular tone in successive tone bursts is therefore dependent upon the return signal data. The tones may, for example, have two different durations.

When the premises equipments respond to their respective interrogation tones, the wideband signal is received by a receiver 16 to which a signal is fed from the generator 11 to determine the correct time interval from a particular premises equipment's response. The data bits are separated from the rest of the signal and are fed via a premises signal decoder 17 to a decoding field 18 which is, at that moment, the inverse of the encoding field 8 at the particular premises equipment. The true data bits are therefore fed to a central station indicator 19, which indicates the state of the data which have been requested from the premises equipment.

Each data transmission may comprise 24 bit intervals, with the data being included twice for validation purposes.

The decoded signals received from the premises equipment may be fed into a store, and each new received signal may be compared with that previously stored. If there is a disparity between the signals, the detector 15 can cause the generator 11 to return immediately to the beginning of its scan. This will ensure that, if the disparity is due to an illicit equipment having been connected to the line, and such equipment had been set fortuitously to correspond correctly with the scanning at the central station, the scan sequence would be immediately put out of correspondence.

The scanning of the interrogation tones could be at completely random intervals, instead of pseudorandomly as described above. The tones can be transmitted in a periodic order, instead of in a set sequence which is continuously repeated.

Figure 3 shows an example of a set of signals produced in the system. All of the transmitted signals are contained within a telephone channel speech bandwidth indicated by a dotted line 22.

Interrogation tones 20 are contained in a band 23 of approximately 950Hz to 2,200Hz, i.e. above the frequency band 24 which is normally occupied by the speech and noise signals.

The wide-band spectrum 21 including the data and redundant signal tones occurs in a band 25, from approximately 2,200Hz to approximately 3,400Hz.

Claims (14)

1. A method of signalling between premises at which a condition is to be monitored and a station remote therefrom, comprising feeding data representing the monitored condition to a first transmitter associated with the premises; transmitting over a line from a second transmitter at the remote station to the first transmitter randomly or pseudorandomly varying signals including an interrogation to which the first transmitter is uniquely responsive, thereby causing the first transmitter to transmit the data over the line to the remote station, the data being represented by a.c. signals within a transmitted spectrum which also includes a substantial proportion of signals which are not necessary for data recovery; and recovering the data from the transmitted signals at the remote station.

2. A method as claimed in Claim 1, wherein the transmitted signals comprise a plurality of tones having intervals the durations of which are varied randomly or pseudo-randomly.

3. A method as claimed in Claim 1 or Claim 2, wherein the transmitted signals comprise a plurality of tones the durations of which vary randomly or pseudo-randomly.

4. A method as claimed in any preceding claim, wherein the transmited signal spectrum contains a number of frequencies or combinations of frequencies each carrying the data; and wherein the recovery of the data signals includes checking that a predetermined one or combination of said frequencies is received.

5. A method as claimed in any preceding claim, wherein the transmitted data are encoded in accordance with an arbitrarily-selected one of a plurality of available codes; and wherein recovery of the data includes decoding the received data only if the received data are found to be correctly encoded according to the selected code.

6. A system for signalling to a remote station a condition occurring at premises, the system comprising means to monitor the condition; a first transmitter associated with the premises and coupled to the monitoring means; a second transmitter at the remote station and operative to transmit to the first transmitter, via a line, randomly or pseudorandomly varying signals including an interrogation; means coupled to the first transmitter and uniquely responsive to the interrogation to cause the first transmitter to transmit via the line to the remote station data representing the monitored condition, the data being represented by a.c. signals within a transmitted spectrum which also includes a substantial proportion of signals which are not necessary for data recovery; and means at the remote station to recover the data from the transmitted signals.

7. A system as claimed in Claim 6, wherein the means coupled to the first transmitter includes means to generate a plurality of tone signals to form the transmitted spectrum, intervals between the tone signals having durations which vary randomly or pseudo-randomly.

8. A system as claimed in Claim 6 or Claim 7, wherein the means coupled to the first transmitter includes means to generate a plurality of tone signals to form the transmitted spectrum, the durations of said tone signals varying randomly or pseudo-randomly.

9. A system as claimed in Claim 6, Claim 7 or Claim 8, wherein the means coupled to the first transmitter includes means to generate a number of frequencies or combinations of frequencies each carrying the data; and wherein the recovery means at the remote station includes means to check that a predetermined one or combination of said frequencies is received.

10. A system as claimed in any one of Claims 6-9, wherein the means coupled to the first transmitter includes means to encode the data in accordance with an arbitrarily-selected one of a plurality of available codes; and wherein the recovery means at the remote station includes means to decode the data only if the data received thereby are found to be correctly encoded in accordance with the selected code.

11. A system as claimed in any one of Claims 6-10, wherein the transmitted signals are contained within the speech bandwidth of a telephone transmission line.

12. A system as claimed in any one of Claims 6-11, wherein the first transmitter is operative to transmit the spectrum of a.c. signals by modulation of a carrier wave.

13. A method as claimed in Claim 1 and substan tial ly as hereinbefore described with reference to the accompanying drawings.

14. A system as claimed in Claim 6 and substan tally as hereinbefore described with reference to the accompanying drawings.