WO1998031178A9

WO1998031178A9 - Method for establishing a connection in a mobile communication network

Info

Publication number: WO1998031178A9
Application number: PCT/FI1997/000813
Authority: WO
Inventors: Ilpo Paltemaa; Arto Harjula
Original assignee: Nokia Telecommunications Oy; Ilpo Paltemaa; Arto Harjula
Priority date: 1996-12-23
Filing date: 1997-12-19
Publication date: 1999-07-22
Also published as: EP0963671A1; CN1245622A; RU2201040C2; BR9713788A; FI108327B; FI965200A; CN1110222C; WO1998031178A1; FI965200A0; AU7995698A

Abstract

To detect a subscriber station duplicate in a mobile communication system a first counter (108) with a first value (EA) therein is maintained in the system, a second counter (308) is maintained in a network infrastructure, a value calculated by the second counter is stored separately as a second value (TA) for each subscriber station in a register, a call establishment request with the first value (EA) therein is sent from said subscriber station in the network infrastructure on a calling channel, the second value (TA) is retrieved from the register for said subscriber station, the first value is compared with the second value, and if the first value and the second value are identical, said call establishment is accepted, a random number is selected in the network infrastructure, the random number is sent to the subscriber station on a traffic channel and the first value and the second value are updated with the number. The invention further relates to a mobile communication system exchange, a mobile communication system subscriber station and a mobile communication system wherein the method of the invention can be utilized.

Description

METHOD FOR ESTABLISHING A CONNECTION IN A MOBILE COMMUNICATION NETWORK

FIELD OF THE INVENTION

The present invention relates to a method for establishing a con- nection in a mobile communication system which comprises at least one subscriber station including a first counter and a first value calculated by the counter, and a network infrastructure which comprises a second counter and a register including a second value calculated for the subscriber station by the second counter, which method comprises the following steps of: sending a call establishment request with the first value therein from said subscriber station to the network infrastructure on a calling channel, retrieving the second value for said subscriber station from the register in response to the first value included in said call establishment request, comparing said first value included in the call establishment request with said second value retrieved from the register and if the first value and the second value are identical as a result of said comparison, accepting said call establishment request.

The invention relates particularly to preventing misuses of non- encrypted subscriber station identifier information sent over the radio path in mobile communication systems utilizing analogue technology, but the inven- tion can also be applied in wireless data transmission systems utilizing other technologies if non-encrypted subscriber station identifier information and/or non-encrypted subscriber identifier information must be sent over the radio path and there is a danger of the identifier information misuse.

BACKGROUND OF THE INVENTION When a radio network does not support an encryption, or non- encrypted identifier information must be sent over the radio path for some other reason, there is a risk that someone copies this identifier information over the radio path and uses it for obtaining a subscriber station duplicate. Such duplicates can be used for calling on the account of the original sub- scriber station, the system not detecting the existence of the duplicate. The subscriber station is here taken to mean the actual apparatus and the entity of information and properties potentially added therein by means of for example a smart card, such as the SIM (Subscriber Identity Module) card in the GSM system. EP 163 358 describes a system where a check value is sent from the subscriber station to the network infrastructure in connection with the call establishment. If the check value found in the network infrastructure is identical with the check value sent by the subscriber station, the call establishment is accepted and both the subscriber station counter and the network infrastructure counter calculate a new check value from the check value by using the same formula, the new value being used in connection with the next call establishment. The problem with the solution is that the number used for the checking always changes in the same way and is by no means random. To prevent misuse of non-encrypted information sent over the radio path, a checking system has been developed for example for subscriber stations in the analogue NMT network (Nordic Mobile Telephone). The checking system requires that a separate circuit must be added to the subscriber station for performing a secret algorithm, and that the same complicated algorithm must be calculated both in the subscriber station and in the exchange during the checking phase by using a random number generated by the exchange as a given value. The subscriber station passes the checking if the result given by the algorithm and forwarded to the exchange by the subscriber station is identical with the result calculated by the exchange. Both the random number and the result from the subscriber station are sent as separate signalling. The problem then arises concerning checking complexity and a need for additional signalling. In addition, the solution does not in any way inform the operator of any attempts to misuse the subscriber station information. A similar method of performing a complicated algorithm and comparing results is used in networks utilizing digital technology.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention is to detect the existence of the subscriber station duplicate in a simple manner without additional signalling and to prevent the duplicate use if needed. This object is achieved by the method of the invention, characterized by maintaining in the network infrastructure means for selecting a random number, selecting the random number in response to the call establishment acceptance, sending said number to said subscriber station on a traffic channel, updating the first value with said number in said subscriber station, updating the second value with said number in the second counter, and replacing said second value in the register with the updated second value. A call establishment request is here taken to mean the method steps that are performed in connection with the call establishment originating from the subscriber station until the network infrastructure can unambiguously continue the routing, i.e. it knows the entire B number to be called. The B number is the phone number of a subscriber B. The subscriber B is the one to be called and a subscriber A is the caller. Subscriber station network registration is included in the call establishment request if needed.

An updating is here taken to mean the replacement of the old value with a new one, the new value depending on both the old value and the ran- domly chosen number.

The invention further relates to a mobile communication system exchange wherein the method of the invention can be applied. The exchange of the invention is characterized in that it further comprises selection means responsive to comparison means for selecting a random number in order to up- date the first value and a second value, a connection unit is arranged to send the number to the subscriber station on the traffic channel, the counter is arranged to update the second value with said number, and the controller is arranged to store said updated second value in the register.

The invention further relates to a mobile communication system subscriber station wherein the method of the invention can be applied. The subscriber station of the invention is characterized in that a transceiver is arranged to receive the number from the mobile communication system on the traffic channel, the number being used for updating the first value, that the counter further comprises calculation means for updating the first value with said number, and that said controller is arranged to control said memory means for storing the updated first value as said first value.

The invention further relates to a mobile communication system wherein the method, exchange and subscriber station of the invention can be applied. The mobile communication system is characterized in that it is ar- ranged to select a random number in response to the acceptance of the call establishment request, that the exchange is arranged to send the number to the subscriber station on the traffic channel, that the subscriber station is arranged to update the first value with said number, and that the second counter is arranged to update the second value with said number. The invention is based on the idea of using a randomly changing counter value for identifying the subscriber station in connection with a call set- up from the subscriber station. Thus, the most significant advantages of the invention are simple identification of the subscriber station, easy and reliable identification of the duplicate use, the use being detected at the latest when a call is made by the original subscriber station after the use of the duplicate, and short-time application of eavesdropped information, the information being applicable only until the next call. In addition, an advantage of the invention is that it makes eavesdropping difficult because the counter value to be checked is sent from the subscriber station on the calling channel and the random number incrementing the counter is sent to the subscriber station on the traffic channel. Using different channels complicates eavesdropping dramatically, and to make eavesdropping successful, complicated and expensive equipment is required, whereby the potential economical benefit achieved by the duplicate use cannot cover the eavesdropping expenses particularly since the duplicate use is detected at the latest when the original subscriber station de- sires to establish a connection to a public telecommunication network.

In a preferred embodiment of the invention, the subscriber station informs the network infrastructure of the activation of the counter, after which the counter is in use for this subscriber station in connection with each call made to the public telecommunication network. Such individual activation en- ables the property to be gradually introduced in the mobile communication system. It also enables the use of both old subscriber stations without a counter and new subscriber stations including a counter simultaneously in a mobile communication system without causing disturbances in the system operation. In addition, a counter easy to operate can be added to the subscriber station in connection with subscriber station maintenance.

In a preferred embodiment of the invention, the counter value on the network infrastructure side is not updated until the subscriber station has acknowledged the updating of the subscriber station counter, and if the acknowledgement is not received, the counter value is updated to such a value that the subscriber station counter value included in the next call establishment request is accepted whether it is the updated value or the previous value. This allows the effects of the operation disturbances to be minimized merely to a release of a connection establishment.

The preferred embodiments of the method, exchange and sub- scriber station are disclosed in the attached dependent claims. LIST OF FIGURES:

In the following, the invention will be described in more detail by means of a preferred embodiment of the invention with reference to the accompanying figures, of which Figure 1 illustrates subscriber station operation in a first preferred embodiment of the invention,

Figure 2 illustrates network infrastructure operation in a first preferred embodiment of the invention,

Figure 3 shows a block diagram of the mobile communication sys- tern exchange of the invention,

Figure 4 shows a block diagram of the mobile communication system subscriber station of the invention, and

Figure 5 shows a block diagram of the mobile communication system of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Figures 1 and 2 describe the method of the invention in a first preferred embodiment wherein it is assumed by way of example that the mobile communication system utilizing wireless data transmission is a trunking private land mobile radio system also providing a connection to fixed network, such as the PSTN network (Public Switched Telephone Network), a call made to the PSTN network henceforth referred to as a PSTN call. It is also assumed by way of example that equipment and the system are based on the analogue PMR system (Private Mobile Radio) standard defined in the publications "MPT 1327, January 1988, Revised and reprinted November 1991 , A Signalling Standard for Trunked Private Land Mobile Radio Systems, Radiocommunica- tions Agency" and "MPT 1343, January 1988, Revised and reprinted September 1991 , Performance Specification, Radiocommunications Agency" published by the British Department of Trade and Industry. It is assumed that a subscriber equipment is a radio unit RU according to the standard, an ex- change is a trunking system controller TSC and a data base, i.e. a register which contains basic information regarding the subscriber station and wherein subscriber stations are registered, is included in a node. Subscribers are defined by means of the register information. It is assumed by way of example that the register contains a second value calculated for the subscriber by a second counter. The second values could also be located elsewhere in the network, for example subscriber-specifically in their own register. It is also assumed by way of example that intra-network calls are allowed when also made by a duplicate, i.e. without a comparison between counter values. Only the PSTN calls to be billed separately are "controlled", i.e. the counter values are compared in connection with such calls. The comparison between the counters could also be performed in some other manner in predetermined call types, or correspondingly, not performing the comparison between the counters in connection with specific call types. The counter could also be used in connection with subscriber registration, but more events would then occur and signalling would take place entirely on the calling channel, i.e. at one frequency, whereby tracing the counter value would be easier.

Figure 1 shows a flow diagram of the subscriber station, i.e. the radio unit when the subscriber desires to make the PSTN call. The counter has been activated for the subscriber station. The subscriber station requests the PSTN call 10 in a normal manner on the calling channel and receives a long number request 11 from the network infrastructure on the calling channel, since a PSTN phone number, i.e. a B number to be called according to the standard in the example does not fit completely in the call request message, but the subscriber station sends it separately in a long number message. Since the subscriber station counter has been activated, the subscriber station adds a counter value automatically to the long number message 12, after which it sends the long number message 13 on the calling channel. Thus, a call establishment request according to the MPT 1327 standard whose signalling is according to the standard is sent, the long number message, however, containing invention-related information not included in the standard. Next, the subscriber station receives a security code request message 14 on a traffic channel, the message preferably containing a randomly selected number used for updating the counter value. The number can be sent as a separate message, but in that case, a new message and additional signalling are required. In response to the security code request, the subscriber station sends a security code 16 to the network infrastructure, updates the counter value 17 with the number and stores the updated value 18. The call establishment then continues in a normal manner 19. The updating 17 and storing 18 of the counter value can take place before sending the security code 16, or parallel with it. The counter value is preferably updated 17 by adding the received number to it by using modular arithmetic, the counter value staying within zero and a chosen limit, its value always fitting in the long number message, the value however depending on both the old value and the received number. The eavesdropper, i.e. the person who has made the duplicate must know a modulo number (i.e. a number below which the value always exists), the counter value and the received number. Other updating methods can also be used.

In Figure 2, a call request message 30 is received in the network infrastructure from the subscriber station on the calling channel. Since it is detected on the basis of the contents of the message that the subscriber sta- tion desires the PSTN call 31 , the subscriber station is requested to send the long number 32, i.e. the entire B number to be called on the calling channel. In response to the reception of the long number message 33 it is checked from the register R information if the subscriber station counter requesting a connection is activated 34. If not, the call establishment continues in a normal way 46. There is no need to check the activation of the counters in case all the subscriber stations in the network comprise a counter.

If the subscriber station counter is activated, the first counter value 35 and the B number is separated from the message. Then, the second subscriber station counter value is retrieved from the register R 36, and the values are compared 37. If the values are identical 38, the call establishment request is accepted 39, transfer is made onto the traffic channel and a number is preferably selected randomly 40 from the set of numbers stored in the network infrastructure and the number is added to the security code request message 41. The number can also be selected on other grounds, for example by always taking the following number from the set of numbers. It is not necessary to store the set of numbers either, since the set of numbers can be obtained as a result of a calculation algorithm performed in the network infrastructure. The calculation algorithm can also produce only a number, whereby the set of numbers is not needed. The number can also be obtained from a random number generator. The security code request message is sent 42 to the subscriber station on the traffic channel. Thus, the first counter value and the number used for its updating are transferred over the radio path on different channels. The subscriber station acknowledges the reception of the number to be used for the updating by sending the security code. At the same time it also acknowledges the updating of the first counter value. If the security code is obtained 43, the second counter value is updated 44 in the second counter in the network infrastructure by using the same updating method as used in the subscriber station. The updated second value is stored in the register 45 as the new second value. Thus, the first value of the subscriber station and the second value of the register R are identical. Then, the call establishment con- tinues in a normal manner 46.

If the security code is not received from the subscriber station 43, it is then not known whether the subscriber station has received the message including the updating number and whether the first value is thus updated. For this reason, a numeric character is preferably stored as the second value for the subscriber station counter in the register 47 and the acceptance of the call establishment request is cancelled 48. The numeric character ensures that the first counter value, it being any value, sent by the subscriber station is accepted in connection with the next PSTN call. In response to the next PSTN call request, the second value of the register R is replaced with the value ob- tained by updating the first counter value received from the subscriber station with a randomly selected number.

A more secure solution, yet taking up more memory capacity and making the comparison more complicated than before, is to store the updated second value as a third value in the register R already containing the second value, and in connection with the next PSTN call, to compare the first counter value received from the subscriber station with the second value and the third value and accept the call establishment request only if the first value is identical with the second value or the third value. In response to the next PSTN call being accepted, the second value and the third value are replaced with the value obtained by updating the first counter value received from the subscriber station with a randomly selected number.

If the first counter value and the second counter value differ from each other 38, a message is sent to the operator 49 about the existence of the duplicate, after which the procedure is continued according to the operator's decision 50. The operator decides upon the contents of the message, its destination and the way of delivery. The message can be for example an alarm. The operator also decides upon the way the call establishment is continued and the way the second counter value is updated. The first counter value is not updated. Regarding the desired call, the operator is given the options of continuing the call establishment in a normal manner, cancelling the acceptance of the call establishment request and marking the subscriber station as stolen, cancelling the acceptance of the call establishment request and restricting PSTN rights by for example allowing only local calls, and cancelling the acceptance of the call establishment request. Regarding the updating of the second value in the register, the operator is given the options of: not up- dating the second value, updating the numeric character as the second value, storing the first counter value received from the subscriber station as the second value, and storing the first counter value received from the subscriber station as a third value in the register R.

If the counter is activated, but the message including the B number does not contain a counter value, the procedure is the same as in the case where the first value deviates from the second value.

Figure 3 shows a block diagram of the mobile communication system exchange of the invention. The exchange comprises a connection unit 301 , a controller 302 and memory means 303 for controlling the system. The controller 302 receives and sends messages through the connection unit 301. The exchange has a connection with the radio path either through an antenna included in the connection unit 301 , through a base station (one or several) or through a base station subsystem. The connection unit is connected to the base stations by a fixed connection accomplished by cables or a directional radio link. The exchange can be connected to other exchanges and networks through the connection unit. In addition, the exchange of the invention comprises comparison means 304 for comparing the first value received from the mobile communication system subscriber station with the second value of the subscriber, the second value being retrieved from a register including the sec- ond values of the mobile communication system subscriber. In addition, the exchange of the invention comprises retrieval means 305 for retrieving the second value from the register. The exchange of the invention further comprises message means 306 for sending a message to mobile communication system network management about the detection of a duplicate if the first value deviates from the second value.

The exchange 300 of the invention can further comprise selection means 307 for selecting a number preferably randomly from a set of numbers stored in the mobile communication system for updating the first value and the second value and, in addition, a counter 308 for calculating and updating the second values, the controller 302 being arranged to store the second subscriber station values in the register or replace them with values calculated by the counter 308. The selection means 307 can also select the number in some other manner, for example as described in connection with Figure 2. The update method of the second value of the exchange 300 can be chosen according to the way the acknowledgement of the first value update is received from the subscriber station.

The connection unit 301 of the exchange 300 can be arranged to affix said number to the security code request message and separate the first value from the message including the B number. The exchange controller 302 can be arranged to interpret that the security code received from the sub- scriber station is the acknowledgement of the first value.

The exchange 300 of the invention can further comprise checking means 309 for checking from the register if the subscriber has activated the counter. The exchange controller 302 is then arranged to control the exchange operation is such a way that the other means (304, 305, 306, 307 and 308) used in the comparison of the exchange values are activated in response to the activation of the subscriber station counter.

Figure 4 shows a block diagram of the subscriber station 100 of the invention. The subscriber station comprises a transceiver 101 arranged to send and receive messages, memory means 103 and a controller 102 for controlling the subscriber station operation. An antenna A is connected to the transceiver, the antenna through which the subscriber station has a connection to the radio path. The subscriber station of the invention also comprises a counter 108 containing a first value EA and transmitter means 111 for sending the first value EA to the mobile communication system network infrastructure. In addition, the counter 108 in the subscriber station 100 of the invention can comprise calculation means 118 for updating the first value with a number received from the network infrastructure. The updating can comprise for example adding a number to the first counter value EA. The controller 102 is then arranged to control the memory means 103 for storing the updated first value in the counter 108 as the first value EA.

The transceiver 101 in the subscriber station 100 of the invention can be arranged to inform that the counter 108 is put into use, i.e. the counter is activated. The counter cannot be deactivated from the subscriber station 100. The subscriber station according to the first preferred embodiment of the invention puts the counter into use, i.e. activates the counter in connection with its network registration. After this, the subscriber station must give the counter value in connection with the B number every time it makes a PSTN call.

The transceiver 101 in the subscriber station 100 according to the first preferred embodiment of the invention can be arranged to affix the first value EA together with the B number to the same message and separate the number used in the updating from the security code request message.

Figure 5 shows a block diagram of the mobile communication system 500 of the invention. The mobile communication system 500 comprises at least one subscriber station 100 having a connection with the radio path through an antenna. The system can comprise several subscriber stations, but for the sake of clarity, only one subscriber station is drawn in the figure. The subscriber station 100 can comprise the first counter 108 comprising a first value EA. In addition, the mobile communication system 500 comprises at least one exchange 300 that can have a connection with other exchanges and other networks, such as public switched telephone network PSTN. The exchange has a connection with the radio path by way of example through a base station BS connected to the exchange by a cable. In addition, the mobile communication system 500 comprises a register R wherein a second value TA is stored for each subscriber station 100, the second value being calculated by a second counter 308 in the mobile communication system 500. The register R is preferably the register wherein the basic information of the subscriber station 100 is stored, wherein the subscriber station is registered and whereto the subscriber is defined. The mobile communication system 500 of the invention utilizes the first value EA stored in each subscriber station 100 and the corre- sponding second value TA stored in the register R in the call establishment originating from the subscriber station 100 in the manner shown earlier in Figures 1 and 2 for detecting the duplicate of the subscriber station 100 by comparing the first value EA with the second value TA.

The mobile communication system 500 of the invention can store information in the register R regarding the activation of the counter for the subscriber station 100. After that, no calls can be made from the subscriber station 100 without comparing the first value with the second value. The activation information cannot be deleted from the subscriber station.

In the following, the operation of the invention is described with ref- erence to Figure 5 in situations where a subscriber station duplicate (not shown in the figure) is made by eavesdropping the radio path in the first pre- ferred embodiment of the invention. In connection with eavesdropping, even the first value EA of the subscriber station 100 has been detected, the first value in the present example being 12. The desire is to call a PSTN call by using the duplicate and the first value EA = 12 is sent to the exchange 300 in the message that includes the B number. The exchange 300 detects that the duplicate is the subscriber station 100, checks from the register R that the counter is activated, separates the first value EA 12 from the message and compares it with the second value TA received from the register, the second value in the present example being 12. Since the values are identical, the ex- change 300 selects a number L randomly for example from numbers 4 to 7 and sends it to the duplicate. The randomly selected number L in the present example is 5. Since the duplicate updates the new value as its counter value by using modular arithmetic, the duplicate maker must know the modulo number N being used. Knowing the modulo number is very difficult even if only one N value were used in the network in accordance with this example, since the modulo number is not transmitted via the radio path and, thus, it cannot be directly eavesdropped. It is of course possible to use an individual N number for each subscriber station, but this requires additional information to be stored in the register R. In the present example, the duplicate maker knows that the updated counter value is calculated by using a modulo number 16, the updated first value then being (12+5) mod 16 = 1. The duplicate stores this value as the counter value EA and then sends the security code it has detected by eavesdropping to the exchange 300 which upon receiving the security code updates the value 1 as the second value ((12+5)mod 16) = 17 mod 16 = 1 ) in the present example, the value being stored in the register R as the second value TA for the next PSTN call. The duplicate is now able to make the PSTN call on the account of the subscriber station 100. After this call, the original subscriber station 100 desires to make a PSTN call and sends the first counter value EA, which is 12, in a message that includes the B number. The ex- change identifies the subscriber station 100, checks from the register R that the counter is activated, separates the first value EA 12 from the message and compares it with the second value TA received from the register, the second value updated by the duplicate PSTN call being now 1. The exchange 300 detects the difference between the values and sends a message to the op- erator about the existence of the duplicate. The present example shows that the use of the duplicate made by however skilful eavesdropping is detected and that the application time of the eavesdropped information is short.

The way the comparison and updating of counter values described above by means of the figures is performed in other wireless data transmission systems and/or other types of calls, such as intra-network calls, is obvious to those skilled in the art.

It is to be understood that the above description and the related figures are only intended to illustrate the present invention. A variety of modifications and variations will be apparent to those skilled in the art without deviating from the scope and spirit of the invention disclosed in the appended claims.

Claims

CLAIMS:

1. A method for establishing a connection in a mobile communication system which comprises at least one subscriber station including a first counter and a first value calculated by the counter, and a network infrastruc- ture which comprises a second counter and a register including a second value calculated for the subscriber station by the second counter, which method comprises the following steps of: sending a call establishment request with the first value therein from said subscriber station to the network infrastructure on a calling channel (33), retrieving the second value (36) to said subscriber station from the register in response to the first value included in said call establishment request, comparing said first value included in the call establishment request with said second value (37) retrieved from the register and as a result of said comparison if the first and the second values are identical (38), accepting said call establishment request (39), c h a r a c t e r i z e d in that the method comprises the following steps of: maintaining in the network infrastructure means for selecting a random number, selecting the random number (40) in response to the call establishment acceptance, sending said number to said subscriber station on a traffic channel, updating the first value (17) with said number in said subscriber station, updating the second value (44) with said number in the second counter, and replacing said second value in the register with the updated second value.

2. A method as claimed in claim ^ c h a ra c t e r i z e d by: sending an acknowledgement of the first value updating from said subscriber station to the network infrastructure, updating the second value (44) with said second counter number in response to the acknowledgement, and replacing the second value in the register with the updated second value.

3. A method as claimed in claim 2, c h a r a c t e r i z e d in that if said acknowledgement from the subscriber station is not received in the network infrastructure, the method further comprises the following steps of: cancelling said acceptance of the call establishment request, replacing the second value in the register with a numeric character

(47), and in response to the next call establishment request of said subscriber station accepting any first value included in said next call establishment request of said subscriber station in said comparison to be performed (37), and replacing said numeric character in the register with a new second value received by updating the first value included in said call establishment request of said subscriber station in the second counter with said number.

4. A method as claimed in claim 2, c h a r a c t e r i z e d in that if the acknowledgement is not received, it further comprises the following steps of: cancelling said acceptence of the call establishment request, calculating a third value by the second counter, the third value obtained by updating the second value with said number, and storing the third value in the register together with the value already existing there, and in response to the next call establishment request of said subscriber station accepting said next call establishment request in said comparison to be performed (37) if the first value included in said next call establishment request of said subscriber station is identical with the second or the third value, and replacing the second value and the third value in the register with a new value obtained by updating the first value included in said call establishment request of said subscriber station in the second counter with said number.

5. A method as claimed in claim 2, 3, or 4, c h a r a c t e r i z e d by sending the first value together with the B number to be called in the same message (13), sending said number to the subscriber station in a message requesting a security code (42), and updating the second value (44) in response to the security code

(43).

6. A method as claimed in claim 1 , c h a r a c t e r i z e d in that it further comprises the following steps of: activating the first counter for the subscriber station on the basis of the message received by the subscriber station in such a way that the counter that is once activated cannot be deactivated from the subscriber station, storing the information regarding the first counter activation in the register, before the comparison between the first value and the second value, checking from the register whether the subscriber station has activated the first counter (34), and if the first counter is activated, performing other steps in the method, or if the first counter is not activated, continuing the call establishment (46) by accepting the call establishment and ignoring other steps in the method.

7. A method as claimed in claim 1 , c h a r a c t e r i z e d by performing the comparison between the first value and the second value in specific predetermined call types.

8. A method as claimed in claim 7, c h a r a c t e r i z e d by performing the comparison for the calls from the subscriber network to another network.

9. A mobile communication system exchange (300) comprising a connection unit (301 ) arranged to send and receive messages, a controller (302) for controlling the exchange operation, a counter (308) for calculating a second value for a mobile communication system subscriber station, the controller (302) being arranged to store the second value in the mobile communication system register, retrieval means (305) for retrieving the second value from the register in response to the first value received from the subscriber station, and comparison means (304) for comparing the first value with the second value, characterized in that the exchange further comprises selection means (307) responsive to the comparison means (304) for selecting a random number in order to update the first value and the second value, the connection unit ( 301) is arranged to send the number to the subscriber station on a traffic channel, the counter (308) is arranged to update the second value with said number, and the controller (302) is arranged to store said updated second value in the register.

10. An exchange as claimed in claim 9, characterized in that it further comprises checking means (309) for checking from the register whether the subscriber station has activated the counter.

11. A mobile communication system subscriber station (100) comprising a transceiver (101) arranged to send and receive messages, memory means (103), a controller (102) for controlling subscriber station operation, a counter (102) containing a first value (EA), and transmitter means (111) for sending the first value (EA) to the mobile communication system in a call establishment request on a calling channel, characterized in that the transceiver (101) is arranged to receive the number used for updating the first value from the mobile communication system on a traffic channel, the counter (108) further comprises calculation means (118) for up- dating the first value (EA) with said number, and said controller (102) is arranged to control said memory means (103) for storing the updated first value as said first value (EA).

12. A subscriber station (100) as claimed in claim 11, characterized in that the transceiver (101 ) is arranged to inform of the activation of the counter (108).

13. A mobile communication system (500) comprising at least one subscriber station (100) for establishing a call, the subscriber station comprising a first counter (108) comprising a first value (EA), an exchange (300) for connecting calls, a register (R), and a second counter (308), and the mobile communication system (500) is arranged to utilize the first value (EA) in the subscriber station (100) and the second value (TA) calculated by the second counter (308) and stored in the register (R) in the es- tablishment of a subscriber station originating call for detecting a duplicate of the subscriber station (100) by comparing the first value (EA) sent to the exchange (300) by the subscriber station (100) in a call establishment request on a calling channel with the second value (TA) retrieved from the register (R) of the exchange (300), characterized in that the mobile communication system (500) is arranged to select a random number in response to the acceptance of the call establishment request, the exchange (300) is arranged to send the number to the subscriber station on a traffic channel, the subscriber station (100) is arranged to update the first value with said number, and the second counter (308) is arranged to update the second value with said number.

14. A mobile communication system as claimed in claim 13, characterized in that the mobile communication system (500) is ar- ranged to perform said comparison in connection with specific predetermined calls (PSTN).

15. A system as claimed in claim 14, characterized in that said comparison is performed when a call is made from the subscriber network to an external network (PSTN).