WO2003056855A1

WO2003056855A1 - Method for detection of accessability of a group of terminals located within a defined area

Info

Publication number: WO2003056855A1
Application number: PCT/FI2002/000950
Authority: WO
Inventors: Johannes Lindell; Lauri Isotalo
Original assignee: Oy Radiolinja Ab
Priority date: 2001-12-21
Filing date: 2002-11-26
Publication date: 2003-07-10
Also published as: FI20012613A; AU2002367211A1; FI20012613A0; FI113136B

Abstract

The invention concerns detection of accessibility of a group terminals in a mobile station system. The invention is especially related to detection of accessibility of a group of terminals located within a defined area of the mobile station system. According to the invention, the accessibility of a group of terminals is detected in such a way that a query message containing an identifier is first generated for a desired group of terminals. Then a target area is defined in the mobile station system and the query message is sent to the defined target area in such a way that it can be received by several terminals located within the said target area. In the receiving terminal the message is processed in order to conclude whether a reply message is to be sent from the terminal to the mobile station system. Thus, a check is made in practice to find out whether the terminal belongs to that group of terminals, whose accessibility is to be detected. The identifier included in the said query message is utilised in the conclusion. Finally, the accessibility of the desired terminals is detected based on the reply messages arrived in the system.

Description

METHOD FOR DETECTION OF ACCESSIBILITY OF A GROUP TERMI NALS ^' LOCATED WITHI N A DEFI NED AREA

Field of the invention

The invention relates to detection of the accessibility of a group of terminals in a mobile station system. The invention relates especially to detection of the accessibility of a group of terminals located within a defined area of the mobile station system.

Technical background As a consequence of the increasing popularity of mobile stations it has proved important to know the location of the mobile station with sufficient accuracy. Numerous different positioning methods have been developed to this end. One objective for the development was to meet the requirement made by the United States authorities for a positioning accuracy of 150 me- ters for terminal stations in mobile station networks within the next few years.

At the present time, the mobile station can be easily positioned with an accuracy of one cell. Positioning systems more accurate than this are usually based on such conditions that messages are sent to the terminal equipment from different base transceiver stations, whereupon the transit time or the signal strength of arrived messages is measured. The positioning often results in a sufficient accuracy, but it loads the network by causing traffic in the air interface. However, this has been considered acceptable e.g. in such cases where the authorities are doing the positioning. However, the positioning event is quite complicated and results are obtained slowly, espe- cially if there are several terminals to be positioned.

Due to the above-mentioned reasons, among others, it is not purposeful in many cases to position al! terminals belonging to a certain group. For example, a business enterprise or a public administration body may have tens or hundreds of employees. Positioning of these people will load the net- work's resources unnecessarily, which may be seen as high costs by the party desiring positioning.

Another essential question often raised in the positioning context is to do with debates concerning the privacy of the terminal's user. For example, if employees find that they can be positioned anywhere and at any time, such a situation can easily result where confidence in the employer will suffer. The reason behind the need for positioning is often quite local. With state-of-the-art solutions the party needing detection of accessibility has no possibility to receive quick answers to the question of what users or which terminals there are within the desired area. It would be possible to obtain in- formation even in quite great detail from the mobile station operator's data system, especially from some of its network elements, such as the VLR or Visitor Location Register, e.g. information about the user's location area within the network area. In practice, the Telecommunications Act prevents this by prohibiting the giving of information to outsiders. According to the pre- sent-time interpretation of the rules, it is not even possible to agree that only such information filtered by the operator would be used, from which the information of those remaining outside the target group would be filtered away.

Brief summary of the invention It is an objective of the invention to bring about a solution, by which the above-mentioned problems are eliminated. This is achieved by using the method or system according to the independent claims.

According to the invention, the accessibility of a group of terminals is detected in a mobile station system in such a way that a query message containing an identifier is first produced for the desired group of terminals. Then the target area is determined in the mobile station system and the query message is sent to the determined target area in such a way that it can be received by several terminals located in the concerned target area. In the receiving terminal equipment the message is processed in order to find out whether a reply message will be sent from the terminal equipment to the mobile station system. Thus, in practice a check is made to find out whether the terminal belongs to that group of terminals, whose accessibility is being detected. The said identifier included in the query message is utilised in this concluding. Finally, the accessibility of the desired terminals is detected based on the reply messages received by the system.

According to one embodiment of the invention, the said query message is sent as a Cell Broadcast message. According to another embodiment of the invention, the said reply message is a short message, a call request or a USSD message. In such a case where it is desirable to keep the information on the desired detection of accessibility inaccessible to outsiders, the query mes- sage may be encrypted using a key, which is known only to some of the terminals in the mobile station network, that is, to the group of terminals whose accessibility is to be detected within the defined area. Hereby the encryption of the received message is decrypted at the terminal equipment using the encryption key stored to be accessible for the terminal equipment. Various target groups, such as the police, rescuing and fire departments, may also have several decryption keys determined for joint use by the groups, besides the keys determined separately for the individual groups.

The identifier of the query message sent to an area in the mobile station network may refer to the quarter, whose accessibility is to be detected, or to the quarter attempting to detect the accessibility. The identifier may also refer to the encryption key used or to the type of call.

In addition, the detection of the accessibility state of a group of terminals can also be supplemented by positioning the mobile stations send- ing a message by using some positioning method of the mobile station system.

List of figures

In the following, the invention will be described in greater detail with the aid of the appended schematic figures given by way of example and of which

Figure 1 shows the architecture of a state-of-the-art mobile station network capable of sending Cell Broadcast messages and of positioning mobile stations,

Figure 2 shows implementation of the invention in a mobile station system similar to Figure 1, Figure 3 illustrates definition of a desired area in a mobile station network in order to send messages to a desired area in the network, Figure 4 shows in block diagram form the operation of the call system illustrated in Figure 2, Figure 5 depicts the encryption key generation algorithm in such a system according to the invention, which uses encryption of messages to be sent to the area, Figure 6 includes a block diagram of the processing taking place in the mobile station based on the received message, and Figure 7 is a view in greater detail of the operation of the subscriber identity module application at the mobile station in connection with reception of a message.

Detailed description of the invention

The inventive idea is to detect accessibility of a group of terminals in a mobile station system. Accessibility primarily means information about whether a terminal is accessible, that is, turned on so that the mobile station network can be in connection with the terminal when needed. State-of-the-art mobile station systems in general use, such as the

GSM system, the major network parts of which are shown in Figure 1, usually include various network elements together forming the operator's mobile station network 100. The users have terminals 101, 102 in the mobile station network, which terminals may be in connection with the network. The connection is kept up using base transceiver stations 111, 112 in such a way that the covering areas of the transceiver units of the base transceiver stations form cells, which for their part form location areas. The base transceiver stations are controlled with the aid of BSCs or Base Station Controllers 121, which contain information about the base transceiver stations in the areas under their control and about their location areas. The switching centre proper is the MSC or Mobile Switching Centre 131, which often also attends to duties of a VLR or Visitor Location Register. VLR contains information about the terminals located in the area and about the services ordered for these. For example, VLR contains information on which location area each such mobile station is located in, which is accessible for the network. The subscriber information proper of a GSM network is in a HLR or Home Location Register 132. HLR usually remains the same and it is located in the subscriber's home network, even if the subscriber would be staying in the area of another operator's network or in his own operator's network in a different geographical area. VLR, on the other hand, will change when needed. Usually the network also includes elements for attending to message traffic, such as e.g. a SMSC or Short Message Service Centre 141.

For example, GSM networks and new terminals support Cell Broadcast message handling, and a description in greater detail of its opera- tion can be found in ETSI's (the European Telecommucations Standards Institute) specification TS 23.041. In practice, message handling is often im- plemented in such a way that a CBC or Cell Broadcast Center 122 contains information about the base transceiver stations of the network or its part, about the BSCs controlling these and possibly also geographical location information about the base transceiver stations. The Cell Broadcast message is sent to chosen cells through the BSC in such a way that it can be received by all the mobile stations located within the cell's audibility range. Mobile stations usually have different ways of determining the channels to be followed, whereby the criteria may be based e.g. on subject matters, which are numbered by channel, or on language definitions or subject matter areas of in- coming messages. Cell Broadcast messages contain no more than 15 pages, each one containing no more than 93 characters of information. Using the CBC system it is possible to send a message to the desired geographical area simply by defining the content and distribution area of the message. The system attends to sending of the messages according to the area definition through the needed BSC. The message transmission frequency can usually be controlled, in certain implementations the transmission interval is 4-1024 seconds.

A possible positioning system 161 connected to the network and mentioned in the foregoing is often located so that it can detect the VLR used by the subscriber by sending a query to the HLR. The positioning system is used by sending such a query to it, which contains the subscriber identity. Based on the query the positioning system can inquire further about the subscriber's location area from the VLR. Then various measuring messages are formed, whose form and purpose of use depend to a far extent on the posi- tioning method used. E.g. the E-OTD or Enhanced-Observed Time Difference method measures the differences of transit times of messages between different base transceiver stations and the terminal.

In order to detect the accessibility of a group of terminals a state- of-the-art GSM system may be supplemented with a call system 271 as shown in Figure 2. The call system includes a GIS or Geographical Information System 273. The GIS may also be implemented separately in the network or it may be located in connection with the CBC.

The call system is first used to define a group of subscriber identities of the mobile station system. The subscriber identities usually relate to those terminals, whose accessibility is to be detected, in such a way that the subscriber identity is included in the SIM or Subscriber Identity Module. The subscriber identity module for its part is related to the terminal equipment, which the subscriber is carrying with him. The defined subscriber identities are stored in a target database 272. The target database may include various subscriber identity groups of the mobile station system, and the groups may also be partly overlapping.

Then the geographical area is defined in accordance with the example shown in Figure 3. On the map shown in Figure 3 two restaurants are marked (RESTAURANT 1, RESTAURANT 2), as well as an office (OFFICE) and a residential home (RESIDENTIAL HOME). In the case shown as an ex- ample, a company providing security services has as its clients the mentioned two popular restaurants and an office doing brisk business. The company provides the guards in its employ with residential services in a residential home.

If guarding services are needed at night in the office, the problem forming the background of the invention is to reach such a group of guards, which is close to the office. There is no point in positioning all the guards, because it is known that there are many guards in the mentioned two restaurants.

According to the invention, such an area is defined on the GIS system's map, whose border area's coordinates (X1, Y1) and (X2, Y2) are delivered to the CBC system. The CBC system selects cells corresponding to the coordinates it has received from the base transceiver station network database 223 in such a way that they will cover the defined area with sufficient cover. In the example the cells in question are cells 1-8 (CELL NUMBER 1 - 8). The CBC system may also include cell identities (Cell ID) consisting of the CI or Cell Identity, the LAC or Location Area Code, the MNC or Mobile Network Code and the Mobile Country Code. Besides these, the CBC system usually also includes information on the BSC controlling each base transceiver station, and there are two such BSCs in the example for the cells cor- responding to the defined area, BSC ID = 5301 and BSC ID = 5302.

In the following, the operation of the system used for detecting accessibility of a group of terminals is examined in greater detail in connection with Figure 4. The group to be called is determined first. This may include e.g. in the case shown as an example in Figure 3 a client using the system, that is, the duty officer of the security company, who inputs the code "FIND SHIFT 74-52 OFFICE". The code may be entered e.g. with a terminal in the mobile station system and sent as a text message to the server. Hereby the message is routed from the SMSC to the system based on the FIND symbol located therein.

The system's user interface may also be implemented with client /server technique, whereby the client company using the system uses such software, which can be used for defining the group to be called. Hereby a choice can be made on the menu or the "FIND" alarm key may be pressed and more specific targets can be chosen separately. The system may also be implemented by using a web user interface, whereby the client company uses an Internet browser to set up a connection with the server, with which the system is used for detecting the accessibility of the group of terminals. The manner of implementing the user interface is not a central element from the invention's point of view.

When the person on duty at the security company mentioned in the example has sent the message FIND SHIFT, the system detects at step 402 which subscriber identities of the mobile station network (MSISDN) the chosen SHIFT group contains. This information may be input in the system e.g. with the aid of the user interface of such a system for detecting the accessibility of a group of terminals, which has an Internet connection, to which the entire staff's telephone numbers are first supplied and then the work shifts of each guard are entered.

Having defined the accessibility of the group to be called, the party desiring detection defines the area, from which it desires access to the group. In the example described above, which refers to Figure 3, the coordinates were given already in connection with the call given in the SMS format. 74-52 refers to a coordinate system identified by a client and by the accessibility detection system. There are numerous possible ways of expressing the coordinates, the simplest ones being, for example, numbers referring to a map page or square in the telephone directory. More advanced systems may also include map information, which can be found in the GIS system, whereby the calling quarter may e.g. choose an area from which detection of the accessibility is desired. However, the system identifies the desired area definition at step 404, and it sends to the CBC system a request to choose the cells of the chosen area. This is done in step 406. An identifier is attached to messages to be sent. In principle, the identifier to be attached may be simple, e.g. the text FIND, which refers to all mobile stations receiving the message. In practice, however, a group-specific key is defined for the group to be called. This may preferably be implemented in connection with the definition of groups. In the present case, the security company-client may be given GUARD as identifier. The system chooses the identifier and attaches it to the message in step 408. Then the message to be sent is generated in step 410, which may include e.g. encryption of the GUARD identifier to be sent with the encryption key to be used. Next, the messages are sent.

In one embodiment of the invention the group definition (step 402), the definition of the geographical area (step 404), the choice of identifiers to use (step 408) are carried out before generation of the messages, that is, before step 410. Only after this a request is sent to the CBC system, which attends to the choice of the area in the mobile station network (step 406) and to the actual sending from the mobile station network to the chosen area. Having sent the request for detection of accessibility to the CBC system, the system remains waiting for a reply (step 414). If replies have not arrived within the scope of a defined timer or until use of the service is discontinued, the process moves on to the end (step 424). Hereby there obviously are not any terminals of the sought group of terminals within the de- fined area. In this situation, the quarter which sent the request for detection of accessibility may e.g. be told that no replies have arrived.

If replies do arrive, a check is made (step 416) to find out whether the replying terminals need to be positioned in order to add to the accessibility information. This may be deducted e.g. in such a way that if the call codes nothing more than the group and area (POSITION SHIFT 74-52), positioning need not be carried out. If the call code also contains a possible target (OFFICE), then the replying terminals will be positioned. In one embodiment of the invention the OFFICE definition in the user code of the service can be changed e.g. so that it refers to the mobile station network or into a geo- graphical coordinate point using a GIS system connected to the system.

Replying terminals are positioned in step 418. In the practical example, the security company may have a sequence list relating to positioning, that is, e.g. those of shortest service age are positioned first. Alternatively, all replying terminals could be positioned, or e.g. a certain number of those terminals which replied first. In practice the positioning is carried out in such a way that such a positioning request is sent or such positioning re- quests are sent to the mobile station system's positioning centre, which contain at least one subscriber identity of the mobile station network. The positioning system returns the location of terminals e.g. referring to the mobile station network or as a geographical coordinate point. A target choice facility (step 420), could also be included in the system, whereby the subscriber to be called is chosen in such a way that the distance of the terminal to be called from the target (OFFICE) is the shortest of the distances to the target calculated for the replying subscribers. At this point of the system it is possible to include an intelligent GIS system in such a way that the distance is calculated e.g. along a road, whereby the water obstacle in the case given as an example is taken into account. Correspondingly, it is sufficient for the norm used in the distance calculation to comply with a suitable metric. For example, the distance can be calculated as the average driving time to the target taking into account the momentary average speed distribution of road network segments.

Finally, the party making the request for detection of accessibility can be provided with the possibility to contact the replying terminals (422). This can be implemented very simply in the SMS user interface case of the example by sending identifiers of the replying terminals to the caller's termi- nal. Depending on the type of terminal in use, the caller can hereby make contact manually by choosing the "pick a number" facility, whereupon the caller chooses the right number and makes a call to the chosen number. This procedure can be followed e.g. to gain access to the guard who is closest to the office and who was indicated to the caller by the system. On account of practical requirements and the former police radio phenomenon, requests for accessibility must usually be encrypted. For this purpose keys are generated or chosen for the group to be used (step 502) in connection with the definition of the group (Figure 5, step 500). This may also be implemented in such a way that several groups are defined. Groups could be, for example, "the authorities", "the fire and rescue department", "the police", "the drug department of the police" and "the criminal investigation department". Hereby the key generated for the group "the authorities" would be combined with all subscriber identities belonging to the group. Correspondingly, the group's "drug department of the police" key would be combined with the subscriber identities belonging to the "police" group, but not with those belonging to the "drug department of the police" group. In addition to bigger groups, it is also possible to define groups of one person as well as empty test groups.

The generated keys are stored so that they can be reached by the SIM application arranged in connection with the mobile stations (step 504). This can be carried out in several ways. The simplest although maybe clumsiest way is to store the keys at the place of work of the client or party providing the service. Hereby an application connected with the SIM card reader will store the file generated by the key generation application on the SIM card at a defined place. When the first keys have been generated, it is also possi- ble to define a key transfer mechanism utilising the air interface. Methods not using the air interface are somewhat more reliable as regards bugging.

The generated keys are also stored in the call system (step 506), in practice in a target database 272 within the reach of the call system 271.

The key concept was presented above as quite a general concept. There are numerous enciphering and encryption methods. It must be observed that a public key enciphering method has become quite wide-spread recently. The key needed to decrypt the public key encryption is located at the terminal equipment. As terminals are developed that alternative cannot either be regarded as impossible, that the key for decrypting encryption, which the user's electronic identifier card contains, would also be available to the terminal equipment. However, in state-of-the-art solutions the encryption key is stored in the subscriber identity module.

When the terminal equipment receives a query arriving as a Cell Broadcast message, the SIM application tool kit notices this (Figure 6, step 602). If encryption has been done in the system, the SIM application will then run at least a part of the received message through the encryption decryption filter, whereby that secret key is used to decrypt the encryption, which is stored available to the terminal equipment. In case the decryption fails, which is noticed in step 606, the message was obviously not intended for the sub- scriber in question and the performance ends (step 614). If the decryption succeeded, or if the message was not encrypted, then the inquiry can be analysed (step 608).

If the inquiry contains an identifier, which in the example was GUARD, corresponding with the identifier stored available to the SIM applica- tion, it can be concluded in step 610, that the subscriber is the correct recipi- ent. Hereby a reply can be given in a predetermined manner to the message in step 612.

One manner of replying is by sending an automatic reply message in the form of a short message or a USSD message to the number stored in the SIM application, e.g. to the number of the person on duty. Alternatively, the terminal may send a call request to such a service number reserved for detection of the accessibility state, which will position the terminal attempting the call request and will deliver the positioning result to the quarter making the request for detection of the accessibility state. The inquiry may also contain an identifier indicating the quarter making the request for detection of the accessibility state. Hereby the SIM application receiving the request for detection of the accessibility state can be controlled to reply e.g. to an employee, who when needing help has made a request for detection of the accessibility state to his colleagues located nearby. For this purpose of use explicitly the actual request for detection need not contain any definition of an area in the mobile station network, e.g. a map square, because this can also be obtained, if the call system sends to the mobile station system's positioning system a request for detection of the location of the party making the detection request and then defines the area relating to the mobile station network for a query message with the aid of a possible GIS system.

The service according to the invention may also be offered to the mobile station network's consumer-client. The query message may then be simply in the form "MATES", which is sent e.g. as a SMS message to the network's service centre through the SMSC. When sending, the user sends a short message in the form mentioned, e.g. to number 17700, which corresponds with the identifier of vaiue-added services implemented with text messages. The system is able to route the messages further to the system for detection of accessibility. In the previous case, the terminal of the consumer-client is positioned and an area is chosen from the mobile station network based on the positioning result. The system then generates a query message, which is to be sent to the chosen area. The message may contain as identifier e.g. the consumer-client's subscriber identity, which can also be delivered in a pro- tected message, or a signature chosen by the consumer-client for himself, which signature is arranged to be known by the consumer-client circle of acquaintances.

It is possible to store subscriber identities and/or pseudonyms of consumer-clients in the application. The users may themselves perform the storing on their terminals, or this may be done e.g. by a group transmission of text messages in such a way that the user defines his mates (possibly in the form name + telephone number) by a web interface in the data system. He then activates the accessibility detection service by sending e.g. the list of mates he has defined to the call system, possibly by way of the Internet. The call system may attend to the distribution of the user's identifier to the subscriber identity number on the defined list, whereby the information is stored on the SIM cards of the subscribers on the defined list. When using pseudonyms it is possible to arrange a duplicate prevention function, whereby the system will make sure that users cannot choose as their own pseudonyms those already in use.

When the application receives a query message and the identifier in the call is found in the terminal, the terminal will reply as also in the previous case. It will then be sufficient that the user making the query is told the name and/or telephone number e.g. of the five first replying mates. As a non- exclusive alternative these mates may also be informed about the inquiring person's stay within the defined area.

In pricing the service described above, the sender of the query message will as a main rule pay for the detection of the accessibility state. It is hereby possible to carry out a different pricing of the reply messages relat- ing to the service.

In case of consumer-clients, it is very important that the service can also be disconnected in a flexible manner. This may be implemented in such a way that even if identifiers of mates are stored on the subscriber's own SIM card, the stored identities can be arranged in orders of importance. Hereby queries arriving e.g. from entries in the VIP category are always answered, except when the telephone is mute. In other words, various categories may have a dependency on the mode of the telephone, and in addition replying may be connected on and off by category when needed. Useable categories may be, for example, "date company", "drinking buddies" and "of- ficial people", which describe very well why it can be desirable to avoid disclosing one's own accessibility state to a certain group. The query may also contain an identifier, which identifies the encryption key used. Especially when receiving longer public key enciphering keys, which may be of a length of e.g. 256 and 512 bits, the terminal's limited calculation capacity may prove to be a limiting factor. Hereby the query may describe the key used in such a way that the SIM application tool kit can begin immediately decrypting the message with the correct key.

Such an identifier may also be included in the query, which describes the location area or the cell identifier. Hereby the application can be asked to compare the information stored in the subscriber identity module or in the telephone's memory location with the identifier in the query and to act if the identifiers are the same. The message may hereby be sent to a larger area in the network, but in such a way that a reply is requested only from terminals located within a certain partial area, whereby the definition of query area is facilitated between the call system, the GIS system and the CBC sys- terns.

It is also possible to include in the message such information, which is to be presented on the terminal's display. He may hereby be told e.g. the occurrence of an accessibility state query, the purpose of the query or other corresponding information, for example, information about the iden- tity or contact information of the party making the query.

In connection with a presentation of Figure 7 the operation of the SIM application when a message arrives at the terminal will be studied in greater detail. When a message arrives at the terminal, the terminal copies it in a block reserved for messages in the subscriber identity module. When the message arrives in this block (step 702), the SIM application notices the message. If the terminal and the SIM do not support the Cell Broadcast Data Download feature, the terminal will function in accordance with the ETSI TS 23.041 specification. It is said in this specification that if the message identifier of an incoming Cell Broadcast message is not found, then the terminal must conclude in accordance with the TS 23.041 and TS 11.11 specifications whether the message is shown or not. In other words, the terminal checks if the message in question is on the reception list. Showing of an arriving query message is not a problem in practice, if some subscriber wishes to look on the display of his mobile station at query messages possibly arriving in an encrypted mode. This is especially true for older terminals, which are located within the mobile station network's area. From the viewpoint of the invention this would correspond to the situation that the subscriber identity module of a subscriber connected to the system according to the invention has ended up in an old terminal for some reason.

When the terminal and the subscriber identity module support the Cell Broadcast Data Download feature, this is downloaded in the application in step 706. When an arriving query has been downloaded in its entirety in the application, it will be started (step 708). The application will decrypt any encryption in the manner described above, it analyses its contents and forms e.g. a short message (step 710) and sends it (step 712). The performance of the application then stops (step 730). Correspondingly, it would be possible for the application also to form a USSD message and send it, or to make the terminal form a call and then to cut it off.

Although the invention was presented above with the aid of simple application examples by using a GSM network as the place for embodying the invention, this is not intended to be understood as a fact limiting the invention. The invention should be understood in the spirit of the appended independent claims. For a professional in the art it is trivial to change the embodiment of the invention presented in a GSM system to correspond e.g. to a packet-switched GPRS network, whereby the location area may also corre- spond to the routing areas. It is equally simple to implement the invention in some other cellular radio network, such as W-CDMA or UMTS. The inventive idea can also be achieved by using IP based networks, such as a WLAN network, whereby the defined network areas for sending call messages must of course be understood based on the application object.

Claims

1. Method for detection of accessibility of a group of terminals in a mobile station system, characterised in that

- for the said group a query message is generated, which contains an identifier common to the group to be called,

- a target area is defined in the mobile station system,

- a query message is sent to the defined target area in such a way that it can be received on several terminals of the mobile station system located in the said target area, - the received message is processed in the individual terminal in order to conclude whether a reply message is to be sent from the terminal to the mobile station system, in which conclusion the said identifier is utilised, and

- the accessibility of the terminals is detected based on the reply messages sent by the terminals.

2. Method according to claim 1 , characterised in that the said query message is sent as a Cell Broadcast message.

3. Method according to claim 1 , characterised in that the said reply message is a short message.

4. Method according to claim 1 , characterised in that the said reply message is a call request.

5. Method according to claim 1 , characterised in that the said reply message is a USSD message.

6. Method according to claim 1 , characterised in that - the said query message is encrypted using a key, which is known only to some of the terminals of the mobile station network, and

- the encryption of the received message is decrypted using an encryption key stored to be available to the terminal.

7. Method according to claim 1 , characterised in that the said identifier refers to the quarter, whose accessibility is to be detected.

8. Method according to claim 1, characterised in that the said identifier refers to the quarter trying to detect accessibility.

9. Method according to claim 6, characterised in that the said identifier refers to the encryption key used.

10. Method according to claim 1 , characterised in that - the terminal's accessibility is detected with more accuracy by positioning the mobile stations sending reply messages by using a positioning method of the mobile station system.

11. System for detection of accessibility of a group of terminals in a mobile station system, characterised in that the system includes

- means for generating for the said group a query message including an identifier common to the group to be called,

- means for defining a target area in the mobile station system,

- means for sending a query message to the defined target area in such a way that it can be received by several terminals of the mobile station system,

- means for receiving reply messages arriving from terminals based on the sending of the query message,

- means for detecting the accessibility of the terminal based on the arriving reply messages.

12. System according to claim 11 , characterised in that the said sending means are adapted to send the query message as a Cell Broadcast message.

13. System according to claim 11, characterised in that the re- ceiving means are adapted to receive the reply message as a short message.

14. System according to claim 11 , characterised in that the receiving means are adapted to receive the reply message as a call request.

15. System according to claim 11, characterised in that the re- ceiving means are adapted to receive the reply message as a USSD message.

16. System according to claim 11 , characterised in that the system also includes means for encrypting the query message by using a key known only to some of the terminals in the mobile station network.

17. System according to claim 11 , characterised in that the said identifier refers to the quarter, whose accessibility is to be detected.

18. System according to claim 11 , characterised in that the said identifier refers to the quarter trying to detect accessibility.

19. System according to claim 16, characterised in that the said identifier refers to the encryption key used.

20. System according to claim 11 , characterised in that the means for detecting the terminal's accessibility also include such means dependent on the means for receiving reply messages, which are adapted to send a positioning request to the positioning system of the mobile station system in order to position the terminal sending the reply message.