GB2196816A - Communications link and method of increasing the security thereof - Google Patents

Abstract

The invention provides a method of and an apparatus for enhancing the privacy of a communications link. For random short intervals, the audio frequencies of the link are alternatively inverted and non- inverted. The random intervals are generated by each station of the link in turn, a first station initiating an interval and the second station terminating that interval and initiating a next interval.

Description

SPECIFICATION Communications link and method of increasing the security thereof This invention relates to communication links in which information is transferred between stations, and to a method of increasing the security thereof.

For example, present generation telephones which are analogue instruments, are reasonably secure against unauthorised listening, primarily due to the very limited points of access. However, cordless telephones, which are now legal and in fairly common usage are not at all secure. The market is growing rapidly in these products. Only 8 channels are allocated to cordless phones, so it will soon be obvious to the users that others can easily, accidentally or otherwise listen in to their conversations. This is likely to produce a customer resistance to the product well before the channel availability approaches saturation.

Some means of ensuring privacy, which is very easy to use, and which will prevent other parties from listening in, without leading to prohibitive costs, is required.

The requirement is privacy rather than encryption; if the system can only be tapped by fairly sophisticated means, this will be seen as adequate. Against this, the means of coding must be flexible enough not to require key or tokenised responses by the users, i.e. it must be transparent to the users.

It is an object of the present invention to provide a method of rendering more secure a communications link, such as a cordless telephone, and, accordingly, a more secure communications link.

According to the present invention, there is provided a method of rendering more secure a communications link transferring information having a predetermined frequency range between first and second stations, comprising the steps of modifying the information so that the information frequencies within the range are inverted, and alternately transferring the modified and the unmodified information for random intervals of time.

The invention also provides a communications link comprising first and second stations for transferring information therebetween, information modifying means in one of the stations for inverting the frequencies of the information, within a range of frequencies occupied by the information, timing means for generating random intervals and alternately transferring modified and unmodified information, and reception means in the other of the stations for receiving the modified and unmodified information, and for reinverting the frequencies of the modified information.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a graph of the range of frequencies appropriate to the transmission of speech over a communications link; Figure 2 is a graph of the frequencies obtained when speech is mixed with a carrier wave; Figure 3 is a graph of the frequencies of a lower side-band of the mixed signal of Figure 2; Figure 4 is a diagrammatic representation of a communications link incorporating the present invention; and Figure 5 is a block diagram of one station of a pair of stations incorporating a communications link in accordance with the present invention.

The invention consists of two basic elements; these are the modifying method and the means of transfer, constituting the communication link.

For analogue transmissions, such as a cordless telephone link, a simple coding method consists of frequency inversion of the signal.

In Figure 1, a baseband voice signal is shown diagrammatically. The range in telephony systems is 300Hz to 3.3KHz. Figure 2 shows the result of mixing this with a carrier waveform, at, for example, 4 KHz. The frequency of the carrier wave should be at least equal to the highest frequency of the baseband voice signal. The mixed signal then comprises a lower sideband in which the frequencies are inverted (and may be slightly offset) and an upper sideband in which the frequencies are uninverted but offset by the frequency of the carrier wave from the original baseband voice signal.

After low-pass filtering, the signal (Figure 3) then occupies substantially the same range as the baseband, but is inverted in the frequency domain. Such a signal would be unintelligible to a conventional receiver, but would be acceptable to a receiver with a similar inverting circuit available.

In the link, this inversion, alternating with unmodified plain speech, forms the basis of coding; the change over period is short enough not to be noticable to a user. The point of changeover is signalled by one of several means such as a tone on the signal, a high frequency tone, a subaudible tone or by detection of the signal inversion itself.

As described so far, the link is still not very secure; the security comes from the changeover command.

Considering a cordless telephone as shown in Figures 4 and 5 the link comprises a cordless handset 10 and a base 12. Here, one station, for example, the base 12, can be designated as the "master" and the other station, the "slave".

Each station comprises, as shown in Figure 5, a receiver 14 and a transmitter 16, an audio output stage 18 and an audio input stage 20. Such elements are provided in a conventional cordless telephone apparatus. In addi tion, each station comprises also an oscillator 22 to produce the carrier wave for frequency inversion, for example, a 4 KHz carrier wave.

Element 24 is a mixer and filter. Audio frequency input from the element 20 or inverted audio frequency input from the receiver 14 is mixed with the 4 KHz carrier wave and then filtered to provide the lower sideband which constitutes an audio frequency signal in which the frequency components are inverted relative to the input thereto. For transmission, this lower sideband signal is sent to the transmitter 1 6 for modulating the appropriate carrier wave frequency. In reception, the new noninverted audio frequency signal-is sent to the audio output stage 18. The station also comprises a random interval timer 26 set to provide a random output interval between, for example, 20% and 150% of the average duration of a spoken syllable. An upper limit of 300% of the average duration may, however, be employed.The interval generated by the timer 26 is initiated by the receipt of a changeover signal detected by a detector/generator 28. The detector/generator 28 controls the mixer/filter element 24 to terminate mixing and filtering or to initiate mixing and filtering at the end of the random interval, and to generate a changeover signal. Thereafter, the audio frequency sent by or received by the station is used without inversion for an interval timed by a similar timer in the other station.

On initiation of a call (originated or received by the telephone), the master transmits and the slave receives normal speech. After a short time interval which is randomised by a timer circuit in the master, the code for a changeover and the changeover itself is transmitted. Now both stations transfer inverted speech. Most importantly, control is passed to the "slave" Again, after a randomised timer interval, set now by the slave, the speech is returned to normal at both ends of the link, with a corresponding shift of control.

Depending on the code used for a changeover signal, there may be a brief intermittent audible tone on the line. For most links this could be filtered out. If a neighbour has a cordless phone 30 (Figure 4) on the same channel, the speech received by him consists alternately of intelligible and unintelligible speech. If the random interval is centred on the average length of a syllable, the none of the speech can be understood. It is preferred that the duration of the interval between changeover signals should be randomised between not less than 20% and not greater than 300% of the duration of an average syllable.

Should a neighbour's instrument provide such a strong signal that it attempts to become part of the link by exchanging control with the master unit, this will become obvious to the intentional users by the loss of intelligibility, and they would presumably hang up. However, this can only occur at the channel saturation point. If a would-be intentional eavesdropper, endeavours to listen in, existing cordless telephones present no difficulties. Cordless telephones operate on two frequencies for duplex transferrance of information. These are 1.7 MHz and 49 MHz. Receivers capable of receiving transmissions at these frequencies are readily available. Present cordless telephones links are totally insecure.However, if the link utilised a more secure transfer, according to the invention, it would be necessary for the intentional eavesdropper to add a decoder circuit and to provide automatic switching means between the decoder and the normal reception means in order to listen in.

Such an arrangement would constitute a major deterrent.

The invention may also be applied to the communication links for cellular radio. The operation is essentially as above, with the exception that the base transmitter is exchanged as the mobile goes frcm one cell to the next.

The signalling could be carried over in the normal "hand-off" procedure, or could be inherent in the remote mobile equipments.

The invention is also applicabie to the normal telephone network whether fixed or mobile and the random intervalised inversion/noninversion method of the invention would provide for a high degree of security at little extra cost or inconvenience. Again, the possibility of tapping the phone line is very much reduced; connection of an illicit instrument according to the invention would lead to incorrect handover to control and hence los of intelligibility. When this occurred, the users would become aware of the tap and could take appropriate action.

It is not necessary for either end of the communication link to transmit a changeover signal. The inversion to non-inversion and vice-versa changeover may itself be sensed by the other of the stations. A more complicated detection arrangement would be required as also would be a "fall-back" arrangement to some form of code transmission in the event of an long silence.

The invention has been described above with reference to analogue speech transmission. However, it may also be applied to digital transmission in which, for example, bits were inverted over similar randomised intervals. Again, any attempt at unauthorised access would lead to loss of intelligibility or would require sophisticated and therefore very expensive detection equipment.

Claims (19)

1. A method of rendering more secure a communications link transferring information having a predetermined frequency range between first and second stations, comprising the steps of modifying the information so that the information frequencies within the range are inverted, and alternately transferring the modified and the unmodified information for random intervals of time.

2. A method as claimed in claim 1 wherein the duration of the random intervals are alternately controlled by the first and second stations.

3. A method as claimed in claim 1 wherein the duration of the interval for unmodified information transference is controlled by the first station and the duration of the interval for modified information transference is controlled by the second station.

4. A method as claimed in claim 1, 2 or 3 wherein the information transferred is speech and the intervals are random within a range of duration centred on the average length of a syllable.

5. A method as claimed in claim 4 wherein the range of duration Is not less than 20% and not greater than 300% of the average length of a syllable.

6. A method as claimed in any preceding claim wherein, in order to modify the information, the information is mixed with a carrier wave of frequency at least equal to the maximum frequency of the predetermined range, the lower sideband of the mixed signal being used as the modified information.

7. A method as claimed in any preceding claim wherein a random interval is initiated by the transmission of a changeover signal alternately by the first and second stations.

8. A method as claimed in claim 7 wherein the changeover signal comprises a signal of frequency above the predetermined range.

9. A method of rendering more secure a communication link, substantially as hereinbefore described.

10. A communications link comprising first and second stations for transferring information therebetween, information modifying means in one of the stations for inverting the frequencies of the information, within a range of frequencies occupied by the information, timing means for generating random intervals and alternately transferring modified and unmodified information, and reception means in the other of the stations for receiving the modified and unmodified information, and for reinverting the frequencies of the modified infomation.

11. A link as claimed in claim 10 wherein each station has the modifying means, the timing means and the reception means.

12. A link as claimed in claim 11 wherein the timing means in the first station controls the duration of intervals of modified information transference and the timing means of the second station controls the duration of intervals of unmodified information transference.

13. A link as claimed in any of claims 10 to 12 wherein each of the first and second stations has a means for generating a changeover signal for terminating a random interval and for initiating a next random interval.

14. A link as claimed in any of claims 10 to 13 wherein the modifying means comprises a means for generating a carrier wave of frequency at least equal to the highest frequency of the range of frequencies of the information, means for mixing the information with the carrier wave, and filter means for retrieving the lower side band of the mixed signal as the modified information.

15. A link as claimed in any of claims 10 to 14 wherein the information is transferred between the two stations by radio frequency transmission.

16. A link as claimed in any of claims 10 to 14 wherein the information is transferred between the two stations by a hard-wired link.

17. A communications link substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

18. A cordless telephone comprising a communication link as claimed in any of claims 10 to 15 or claim 17, wherein the handset comprises the first station and the base station comprises the second station.

19. A cellular comprising a communications link as claimed in any of claims 10 to 15 or claim 17, wherein the mobile station comprises the first station and the base station comprises the second station.