GB1584579A

GB1584579A - Telecommunications system

Info

Publication number: GB1584579A
Application number: GB54071/77A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1976-12-30
Filing date: 1977-12-29
Publication date: 1981-02-11
Also published as: JPS5384602A; NL7714545A; IT1089194B; DK585277A; CH625650A5; ATA938977A; BE862540A; NL176823C; DE2659622B1; AT360593B; FR2376574A1; DE2659622C2; NL176823B; JPS5826864B2; FR2376574B1; DK148049B

Abstract

In this radiotelephony method for a multiplicity of subscribers (Tn 1), the messages are transmitted with a specific subscriber-individual code by an encoding or encryption device (SG, M), in particular using digital signals. For this purpose, the subscriber number itself or, in addition, a special code signal is stored for each subscriber along with the specific individual code definition. A functional relationship known only to the higher-order exchange exists between the subscriber number or code signal and the individual code definition, this individual code being determined in the exchange, the encoding or encryption device (SG, M) being programmed therewith and the encoded transmission to the exchange (V) then being carried out and from there, if necessary following conversion to the code of the other party to the call, the messages being transmitted. <IMAGE>

Description

(54) A TELECOMMUNICATIONS SYSTEM (71) We, SIEMENS AKTIENGESELLSCHAFT, a German Company, of Berlin and Munich, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a telecommunications system and has particular application in radiotelephone systems where the number of subscribers are relatively large and items of information are to be transmitted in a coded or camouflaged form using a predetermined subscriber-assigned code, particularly by means of digital signals having, for example, a predetermined subscriber-assigned code sequence.

In radiotelephone systems in which mostly mobile stations are used (in a public or private sector ground radio system) it is desirable to protect the communications from eavesdropping and this protection is achieved by coding (or camouflaging) the signals which are transmitted on the radio paths. For this purpose, in a conventional system, the stations which communicate with one another are provided with a mutual code setting (or more than one such code setting where a degree of variability is required) by means of which the communications transmission can be effected in an agreed manner. For military and other specialised networks where the numbers of subscribers are relatively small code settings of this kind can be distributed and stored, by means of electronic components or by means of punched cards.However in a public radio network such as the ground radio networks mentioned above coding methods of this type meet with difficulties due to the large numbers of subscribers even within the relatively limited radio range of a relay point it is either unfeasible to protect the transmission against eavesdropping or, when the codes of the relay point are individually assigned, all the code settings of the subscribers (personal codes), including those operating at all the other relay points in the network, have to be stored. This requires substantial outlay since a network comprising a plurality of radio relay points normally serves several thousand subscribers. Also knowledge of the code settings would not only facilitate eavesdropping but also enables calls to be made under false subscriber numbers which leads to false charge accounts.

According to one aspect of this invention there is provided a telecommunications system including a plurality of subscriber stations each having an associated telecommunications link with a central exchange which can selectively interconnect the subscriber stations via said links, each subscriber station having coding means arranged for using a code setting in order to determine a code for coding or camouflaging the information sent via its associated link, said exchange having means for determining the code settings for use in the links comprising a connection between selected ones of the subscriber stations, respective different code settings being determined by the exchange in accordance with a predetermined algorithm from key signals respectively associated with the subscriber stations, said key signals being provided when necessary in the exchange as a result of signals from the subscriber stations.

The key signals may be stored in the respective subscriber stations which are arranged to send the key signals to the exchange when requesting connection with another subscriber station.

According to another aspect of this invention there is provided a radiotelephone system wherein information is transmitted in a coded or a camouflaged form in accordance with a predetermined subscriber-assigned code setting, each subscriber station storing a key signal, comprising the subscriber number itself or an additional code signal, and storing in addition the assigned code setting, a functional relationship for determining the code setting from the key signal being known only to a superordinate exchange for each subscriber station, this code setting being calculated in the exchange to enable operation of a coding or camouflage device whereupon the coded transmission to and from the exchange is effected.

Here it is advantageous for the code setting to be determined in the exchange by computer prior to the commencement of each communications transmission and to be erased at the end of the conversation.

Furthermore, in order to safeguard the transmitted data it is advantageous for the transmission of the key signal to be effected in redundant fashion before the start of each conversation.

An embodiment of this invention will now be described, by way of example, with reference to the accompanying drawings wherein: Figure 1 is a schematic block diagram of a subscriber station and exchange of a radiotelephone system embodying this invention; and Figure 2 is a schematic diagram of a part of the system components of which are shown in Figure 1.

Figure 1 shows a subscriber station Tnl, whose LF (low frequency) is converted in analogue-to-digital converter A/D into a digital clear text KT which is then coded to form secret text GT in a mixer M which effects coding using code text ST produced under the control of a code generator SG.

This secret text GT is then transmitted by a transmitting-receiving device S/E. The reverse procedure takes place at the receiving end.

To enable the connection between subscriber stations to be established, the coding device SG + M is firstly disconnected. At some time the subscriber station Tnl will establish a connection via its associated transmission link to local radio relay station V (i.e. a radio exchange) to which 'the subscriber station is assigned. The radio relay station interrogates the subscriber station in respect of its number and possibly in respect of the connection which it requires. The coding device SG + M could actually come into operation at this point of time, in the event of a call-back to check whether the subscriber station answers to its number.

The coding itself is based on the following basic principle. Normally all the code settings of all the subscribers would have to be stored in the exchange. However in a large network comprising a plurality of exchanges and possibly a very large number of subscribers, all such code settings would have to be stored in each exchange station which would involve a high technical and organisation outlay. In this embodiment a key signal, comprising the subscriber number of an additional code signal is assigned to each subscriber and between the key signal and the code setting there exists a functional relationship in the form of a mathematical function which is known only in a code determining device FC in the exchange.

This code determining device is informed of the key signal on the basis of which the code determining device determines the code setting of the calling subscriber by means of a calculating device in a very short length of time and stores this code in a device S-Sp/V for the duration of the connection. The exchange now calls back the subscriber station, already in coded form, and if the subscriber station Tnl answers correctly, following the connection of its code generator SG which receives the code setting from its code store S-Sp/F, the intelligibility of all further communications transmissions between the subscriber station and the radio relay station V is ensured.Otherwise the subscriber station will not respond to the call-back when it does not understand the call from the exchange V which uses a code setting different from that associated with the subscriber station as, for example, in a transmission using a false number. Thus false charge metering is prevented. Neither can the subscriber artificially produce a false charge accounting by manipulating his code generator or store, since he does not know the functional relationship between his key signal and code setting. This relationship is known only to the radio relay stations V where it is stored in a computer.

In the event that the Tnl desires to establish a connection e.g. with subscriber Tn3, the exchange V will now attempt to reach the subscriber Tn3 (see Figure 2) and if he is in fact present or attainable via'his local exchange the same procedure for linking the subscriber station to the exchange commences at the called subscriber end by means of the code setting assigned to the subscriber Tn3.

At the latest, when communications transmission actually takes place between subscribers Tnl and Tn3, a recoding of the transmitted signals is carried out by means of the coder device SC of the exchange i.e.

the received information signals from the two subscribers are decoded and throughconnected in the exchange on this level (clear text). In this way it is possible to ensure a safeguarded communications transmission since the individual code settings are in each case known only to the associated subscriber.

WHAT WE CLAIM IS: 1. A telecommunications system including a plurality of subscriber stations each having an associated telecommunications link with a central exchange which can selectively interconnect the subscriber stations via said links, each subscriber station having coding means arranged for using a code setting in order to determine a code for coding or camouflaging the information sent via its associated link, said exchange having means for determining the code settings for use in the links comprising a connection between selected ones of the subscriber stations, respective different code settings being determined by the exchange in accordance with a predetermined algorithm from key signals respectively associated with the subscriber stations, said key signals being provided when necessary in the exchange as a result of signals from the subscriber stations.

2. A system according to Claim 1 wherein the subscriber stations store their respective associated key signals and are arranged to send the key signals to the exchange when requesting connection with another subscriber station.

3. A system according to Claim 2 wherein the subscriber stations are arranged to send their associated key signals to the exchange when a connection involving the relevant subscriber stations is to be established, each link of the connection being assigned the respective code setting corresponding to the key signal associated with the subscriber station associated with the relevant link.

4. A system as claimed in any one of the preceding Claims wherein the code settings are determined in the exchange by a computer prior to the commencement of each communications transmission and is erased at the end of the conversation.

5. A system as claimed in any one of the preceding Claims wherein the transmission of the key signal is effected by the subscriber station in redundant fashion before the start of each conversation.

6. A system according to any one of the preceding Claims wherein said transmission links provide transmission of digital signals and said coding is effected on the digital signals.

7. A system according to any one of the preceding Claims wherein said transmission links are radio links.

8. A radiotelephone system wherein information is transmitted in a coded or camouflaged form in accordance with a predetermined subscriber-assigned code setting, each subscriber-station storing a key signal, comprising the subscriber number itself or an additional code signal, and storing in addition the assigned code setting, a functional relationship for determining the code setting from the key signal being known only to a superordinate exchange for each subscriber station, this code setting being calculated in the exchange to enable operation of a coding or camouflage device whereupon the code transmission to and from the exchange is effected.

9. A system according to Claim 7 or Claim 8 wherein said exchange is a radio relay station a plurality of which are provided in the system.

10. A telecommunications system substantially as described herein with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. are in each case known only to the associated subscriber. WHAT WE CLAIM IS:

1. A telecommunications system including a plurality of subscriber stations each having an associated telecommunications link with a central exchange which can selectively interconnect the subscriber stations via said links, each subscriber station having coding means arranged for using a code setting in order to determine a code for coding or camouflaging the information sent via its associated link, said exchange having means for determining the code settings for use in the links comprising a connection between selected ones of the subscriber stations, respective different code settings being determined by the exchange in accordance with a predetermined algorithm from key signals respectively associated with the subscriber stations, said key signals being provided when necessary in the exchange as a result of signals from the subscriber stations.