Abstract:
The invention concerns a method of intercepting a telephone call in a telecommunications system using at least one switch ( 12, 13; 24 ), in which signals in an interception loop ( 16 ) in said switch ( 12; 24 ) and to which the pulse coded multiplex (PCM) containing the call to be intercepted has been diverted are sampled at high impedance before they are directed to the addressee ( 14 ). The signal sampled at high impedance is directed to interception equipment ( 110 ) which extracts said call to be intercepted from the PCM. The signals in said interception loop ( 16 ) can include voice data and signaling interception data or only voice data, interception data concerning said call to be intercepted being transmitted to said interception equipment over a dedicated line ( 112 ).

Description:
BACKGROUND OF THE INVENTION 
     The field of the invention is that of telecommunications. To be more precise, the invention concerns lawful telephone tapping systems, also known as call interception systems. 
     Telephone call interception systems have been used for many years in lawful telephone tapping systems (i.e. systems under the control of a defined authority, conventionally the state). 
     The oldest technique consists in applying a physical tap to the cables of a subscriber under surveillance. Unfortunately, this technique is impractical and easily detectable. Moreover, it is not compatible with digital communications (Integrated Services Digital Network—ISDN) or with mobile telephone systems. 
     Another technique consists in analyzing everything on a given line and then extracting therefrom the pertinent information. Clearly this solution is costly and complex, and ill-suited to universal telephone tapping, regardless of the communications system used. 
     A further technique that has been used is to create a conference call bridge within the exchange to which the subscriber to be intercepted is connected. The problem with this technique is that defects due to the various parties involved in the conference call bridge, and in particular the interceptor, are additive. It is therefore relatively easy to detect. Moreover, it is not possible to effect more than one interception of the same subscriber using this technique. 
     However, it is desirable for this to be possible in some situations. This is the case in particular when the same communications system covers geographical areas administered by different authorities. For example, a mobile radio system satellite can conventionally cover a plurality of states, each of which has its own law and its own requirements in terms of lawful telephone tapping. Consequently, various telephone tapping control centers must be provided (one per area). It must of course be possible for more than one of these centers to intercept the same subscriber, if necessary. 
     Generally speaking, the various systems that have been used in the past have at least some of the following drawbacks: 
     they are easily detectable by a subscriber, 
     they necessitate a great amount of processing to extract the wanted information from the signals intercepted, 
     they cannot be used interchangeably on all types of network (analog telephone network, digital network, cellular mobile radio network, satellite communication network, etc.), 
     they do not provide for selective interception as a function of different centers associated with separate geographical areas, 
     they are not completely effective against call rerouting and subscriber number changing techniques, 
     they cannot transmit intercepted data to more than one separate control center, 
     etc. 
     One objective of the invention is to overcome these various drawbacks of the prior art. 
     To be more precise, a first objective of the invention is to provide a method of intercepting calls sent and/or received by a subscriber, regardless of the communications systems employed, even if the subscriber uses particular means such as call rerouting, call transfer, number change, etc. 
     Another objective of the invention is to provide a method of the above kind which can intercept all types of call (voice, fax, data) and all required signaling information. 
     A further objective of the invention is to provide a method of the above kind which necessitates simple and effective processing of the data to be intercepted and which is totally undetectable by subscribers. 
     A further objective of the invention is to provide a method of the above kind providing for selective telephone tapping in different geographical areas and for transmitting an intercepted call to a plurality of interception control centers, corresponding to different beneficiaries (the courts, the police, etc.), for example. 
     A further objective of the invention is to provide a method of the above kind which provides for selective tapping of a group of lines (corresponding to all the lines used for an exhibition or all the lines of a company, for example). 
     The above objectives, and others that emerge more clearly hereinafter, are achieved by the invention by means of a method of intercepting a telephone call in a telecommunications system using at least one switch, in which method signals in an interception loop in said switch and to which the pulse coded multiplex (PCM) containing the call to be intercepted has been diverted are sampled at high impedance before they are directed to the addressee. 
     SUMMARY OF THE INVENTION 
     This makes it possible to perform lawful telephone tapping without this being detectable by the users. There is no restriction on the communications systems used (in particular digital and mobiles systems). 
     The signal sampled at high impedance is advantageously directed to interception equipment which extracts said call to be intercepted from the PCM. The interception equipment can be inside the switch or at a separate location. 
     In a first embodiment of the invention, the signals in said interception loop comprise voice data and signaling interception data. In this case, the interception equipment first extracts the interception data from the sampled signal and then performs the interception proper. 
     In a second embodiment, offering enhanced security, the signals in said interception loop comprise only voice data, interception data concerning said call to be intercepted being transmitted to said interception unit on a dedicated line by control means of the switch handling said call to be intercepted. 
     In this case, knowing only the content of the sampled signal is not sufficient to perform the interception. 
     In a third embodiment, part of the interception data from a call is transmitted together with the signals conveyed by said interception loop, and part via a dedicated line. 
     Said interception equipment is advantageously connected to an interception control center which in turn feeds one or more separate telephone tapping centers. In the invention, there is no intrinsic limitation on the number of telephone tapping centers it can feed. 
     Each of said telephone tapping centers preferably corresponds to a geographical interception area, such as a state or an administrative region. More than one telephone tapping center can of course be allocated to the same geographical area. 
     Each of said geographical interception areas is defined by at least one of the following, for example: 
     a list of identifiers of cells of a cellular network forming said geographical interception area, 
     a list of dialing prefixes allocated to said geographical interception area, 
     at least one item of information, supplied by way of a signaling parameter, defining the geographical interception area to which the call to be intercepted relates, and 
     at least one item of information, supplied by the switch, defining the geographical interception area to which the call to be intercepted relates. 
     This assures great flexibility, the invention requiring no correlation between the geographical interception areas and the coverage areas of the switch. 
     An advantageous feature of the invention is that said interception can cover a plurality of lines corresponding to the same source and/or the same destination (for example an exhibition, a company, etc.). 
     Said telecommunications system can be one of the following: 
     an integrated services digital communication network, 
     a cellular mobile radio network, or 
     a satellite radio network. 
     The invention also concerns the interception equipment used in a method as described hereinabove. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Other features and advantages of the invention will become more clearly apparent on reading the following description of a preferred embodiment of the invention and from the accompanying drawings, in which: 
     FIG. 1 shows the general principle of the interception method of the invention, and 
     FIG. 2 shows the complete architecture of a system according to FIG. 1 suitable for use with the Globalstar system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1—Glossary 
     To simplify the following description and the figures, the main abbreviations employed and their usual meaning are listed below: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 BSS 
                 base station; 
               
               
                   
                 IA 
                 interception area; 
               
               
                   
                 IE 
                 interception equipment; 
               
               
                   
                 IMC 
                 interception management center; 
               
               
                   
                 IRI 
                 interception related information; 
               
               
                   
                 LE 
                 local exchange; 
               
               
                   
                 LEA 
                 lawful enforcement agency; 
               
               
                   
                 LEMF 
                 lawful enforcement monitoring facilities; 
               
               
                   
                 MSC 
                 mobile switching center; 
               
               
                   
                 MSISDN 
                 international mobile station ISDN number; 
               
               
                   
                 PCM 
                 pulse coded multiplex; 
               
               
                   
                 RCP 
                 radio control point; 
               
               
                   
                 SSP 
                 service switching point. 
               
               
                   
                   
               
             
          
         
       
     
     2—General Principle of the Invention 
     FIG. 1 shows the general principle of the invention. As indicated above, the objective of the invention is to intercept telephone calls sent and/or received by a target telephone terminal  11 . 
     The target telephone terminal can be of any type (analog, digital, fixed, portable, etc.). It can also correspond to a set of identified terminals (corresponding to a set of lines of a company or the lines used for an exhibition, for example). 
     The target terminal  11  is conventionally connected to a telephone network via a local exchange (LE)  12 , in turn connected via the telephone network to the local exchange  13  from which the terminal  14  of the other party to the call depends. 
     According to the invention, the pulse coded multiplex (PCM)  15  carrying the call in the exchange  12  is diverted to a dedicated PCM loop  16  before it is passed in the conventional way to the remote exchange  13 . There is a high-impedance sampling point  18  on the PCM loop  16 . The signal sampled at the high impedance is transmitted (at 17) to interception equipment (IE)  110 , which can direct the intercepted calls to one or more interception management centers (IMC)  111   1  through  111   3 . 
     Interception therefore occurs at a time when the voice signal is easily decodable. The interception is undetectable and in particular imperceptible to the subscriber. 
     Two options are available: 
     either the signaling interception data and the voice signal are sampled on the PCM loop  16 , 
     or only the voice signal is sampled, on the basis of signaling interception information transmitted over a separate line  112  (which delivers the interception information PS and CIC, for example). 
     Note that the PCM loop  16  can be local (inside the exchange  12 ) or at any appropriate remote location. The same applies to the IE  110 . 
     Although only one loop  16  is shown, a plurality of similar loops could obviously be provided in an exchange. 
     3—Example: the Globalstar System 
     One particular embodiment of the invention suited to the Globalstar network is described below. In the context of this network, the same interception management center (IMC) may need to manage a plurality of different geographical interception areas (IA). The coverage of a single satellite may correspond to more than one state, each of which can have its own rules and requirements for lawful telephone tapping. 
     FIG. 2 shows the general architecture of the system employed. 
     The target mobile terminal  21  is linked to a base station (BSS)  22  via a satellite  23 . The DSS  22  is connected to a service switching point (SSP)  25  which is part of a mobile switching center (MSC)  24 . The PCM containing the call to be intercepted transits via the interception PCM loop  16  in the SSP  25  and is then directed to the remote exchange  13  to which the remote terminal  14  is connected. 
     The interception system of the invention is therefore based on the use of interception equipment (IE)  110  added to the SSP and an interception management center (IMC)  26 . The functions it offers include real time monitoring of call traffic in one or more geographical interception areas (IA) for particular subscribers. 
     The system can intercept the following information: 
     information carried by the voice channel (voice, fax, data), 
     interception related information (IRI), including in particular: 
     some of the use information, 
     the content of short messages, 
     additional data such as the date and time of the events concerned and the location of the mobile station. 
     Because interception is effected at high impedance on the PCM loop  16 , it is not detectable by the parties to the intercepted call and has no effect on any subscriber services. 
     One or more interception equipment units  110   l  through  110   n  can be associated with each SSP  25 . An interception management center  26  can supervise one or more MSC  24 . 
     According to the invention, the IMC  26  can manage more than one geographical interception area (IA). An IA is a subset of the geographical coverage area associated with a satellite  23  and is defined by a set of logical cells managed by an operator or a given service provider. The set of cells corresponds to a geographical area. The operator or service provider can define one or more IAs to cover all the geographical areas in which it provides a service. 
     Several IAs can cover all or part of the same geographical area, for example if that area is covered by more than one operator or service provider. 
     Interception of a given target subscriber in a given IA is initiated by a lawful enforcement agency (LEA). A lawful enforcement monitoring facilities (LEMF) center  28  manages interception-related information (IRI)  29  delivered by the IMC  26  and intercepted data  210  delivered by the IE  110   i . 
     A target subscriber is defined by an international mobile station ISDN number (MSISDN). The MSISDN to be intercepted and the addresses of the LEMF centers  28  to which that information must be sent are managed at the level of the IMC  26 . In the IMC, management of the interception is based on an approach at the IA level, subject to the following rules: 
     a target subscriber can be intercepted in one or more IAs, 
     a radio control point (RCP)  211  can cover more than one IA, and 
     an IA can be covered by more than one RCP  211 . 
     At the level of the interception equipment  110   i , the data concerning the target subscribers is also organized at IA level. Accordingly, an IMC  26  delivers a list of the IAs to all the IE  110   l  through  110   n  of a MSC  24  (name of each IA and information relating thereto). 
     When an interception has been defined: 
     the IMC  26  sends (at  212 ) the MSISDN of the target subscriber to the RCP  211  concerned, i.e. the one associated with the IA, and 
     the IMC  26  transmits (at  213 ) the MSISDN of the target subscriber and the identifier of the LEMF user of the LEA  27  to which the interception information must be sent to each IE  110   l  through  110   n  of the MSC  24  concerned. 
     When a target subscriber receives or makes a call, the call is directed by the SSP  25  to the PCM loop  16  before it is directed to its final destination  14 . 
     The interception equipment  110   i  performs the following operations in particular: 
     it determines the IA concerned from the identifier of the cells associated with the intercepted call, or from the MSISDN number of the intercepted subscriber, if their location is unknown, 
     it extracts signaling information and voice data from the channel corresponding to the intercepted call (identified by the interception data  215  supplied by the RCP  211 ), and 
     if the call concerns a subscriber to be intercepted: 
     the interception equipment  110   i  performs the interception using interception data included in the signaling associated with the call and/or on a dedicated data channel incorporated into the loop  16  and/or additional interception data received from the RCP  211 , 
     the IE  110   i  sends (at  216 ) the interception related information to the IMC  26 , and 
     the IE  110   i  sets up one or two ISDN digital calls (at  210 ) to the LEMF  288  concerned. 
     If two calls are set up at  210 , they respectively correspond to sending and receiving. If only one call is set up, sending and receiving are combined prior to transmission. 
     The IMC  26  transmits (at  29 ) interception reports to the LEMF  28 . A first report is supplied at the beginning of each call intercepted and a second report is transmitted at the end of the call. Each report includes an interception reference for cross-referencing to the information supplied (at  210 ) by the interception equipment. 
     The embodiment described above can easily be adapted to suit other communications systems without departing from the scope of the invention. 
     The operation of the various units downstream of the interception loop are not described in detail. They are not directly relevant to the invention and are described elsewhere, for example in the context of standardization.