Patent GB-1448272-A (GB)
Title: Data transmission systems
IPC: H04K1/02
Filed: 1973-09-03
Abstract: 1448272 Monitoring and synchronizing secret communication systems SIEMENS AG 3 Sept 1973 [23 Nov 1972] 41274/73 Heading H4L A secret communication system in which a transmitting station is associated with one or more receiving stations includes, in each of the latter, a device for detecting faulty deciphering and operative, on occurrence of a criterion signifying faulty deciphering, to cause the receiving station cipher generator to be reset in accordance with a setting-up signal in the received signal. As described, the system employs a radio relay link, or the phase modulator of a radio link, or the line equipment section of a transmission line 21 to act as a link unit 17 in which a binary ciphered signal GTS is converted into a form suitable for transmission in terms of amplitude or phase, but the unit 17 may be replaced by a direct connection between the transmitter and each receiver. For transmission of telephone conversations, input channels K1 ... Kn are connected to a plain-language unit 15 utilizing pulse code modulation and operating on the time-division multiplex principle. In unit 15, channels K1 ... Kn pass to A-D converter 1 supplied with a bit train T from pulse generator 2 and frame synchronizing signals from a frame pulse generator 3. The binary plain-language signal KTS passes to an enciphering unit 16 where it is converted, in encoder 6 to the ciphered signal GTS, e.g. by modulo-2 addition with a binary cipher signal from cipher generator 5. The ciphered signal GTS passes to one input terminal of a changeover switch 7 to whose other input terminal an output carrying synchronizing information from a synchronizing device 6 is supplied. The switch 7 normally connects the encoder output directly to the link unit 17. but is moved, at intervals, to connect the synchronizing signals from device 6 to the unit 17. The device 6 receives, from the cipher generator 5, its particular setting information so that when switch 7 is in its last-mentioned state, the setting information is transmitted. This is preferably preceded by a short pulse train in Barker code derived from device 6. At the receiver a link unit 18 (corresponding to transmitter link unit 17) recovers the bit train T (from its section 18<SP>1</SP>) and passes the received enciphered binary signal GTE to decoder 8, e.g. modulo-2 adder receiving a binary cipher signal from cipher generator 9. Plain-language binary text KTE is supplied from the decoder 8 to D-A converter 10 which distributes the signal between output channels K1<SP>1</SP> ... Kn<SP>1</SP>. The frame synchronizing signal required for the D-A conversion and resolution of the TDM-frame is supplied from a unit 11, the frame synchronizing signal from unit 11 also being compared (in a synchronism monitor section of unit 11) with the binary plain-language signal KTE. If the frame synchronizing signal in the received signal KTE corresponds with that generated locally in unit 11, correct deciphering is assumed; but, if there is a lack of correspondence a change-over switch 13 is operated to connect an analyser unit 12 to the output of link unit 18. The unit 12 includes a circuit which responds to reception of the Barker code signal to prepare the unit for the reception of the transmitted setting information. When the latter is received, unit 12 relays it directly to cipher generator 9 to reset the same whereby synchronism is restored and switch 13 returned to its normal position. The switch 7 at the transmitter may be operated to pass setting-up signals from the synchronizing device 6 at regular intervals, e.g. after each second or third pulse frame or, alternatively, only at those instants when, as a function of the plain-language signal being transmitted, there are gaps in the information flow. It is also possible for a return line 14 to be provided between the receiver and transmitter whereby demand information, calling for transmission of the setting-up signal, can be sent to the transmitter.