Abstract:
The present invention relates to a radio communication system and destination portable terminal time identification method capable of identifying the time of a destination portable telephone terminal in advance by an originating portable telephone terminal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a radio communication system and destination portable terminal time identification method capable of identifying the time of a destination portable telephone terminal in advance by an originating portable telephone terminal. 
     In wire telephone terminals and telephone networks, a telephone number containing a toll number is assigned in association with an area where a telephone terminal is installed. For international direct distance dialing, the preassigned country code of a destination must be dialed. In wire telephone terminals, their installation locations are fixed, and telephone numbers are assigned in correspondence with the installation locations. In calling a destination telephone terminal, the area and time zone (time difference) where the destination telephone terminal exists must be identified from the destination telephone number containing a toll number and country code. 
     In a so-called portable telephone system in which a telephone terminal and base station are connected by radio and communicate with each other, a portable telephone terminal can move geographically. For this reason, an area where the portable telephone terminal is located cannot be identified from a telephone number assigned to the portable telephone terminal. In calling a portable telephone terminal usable over a plurality of countries by so-called international roaming or the like, the originating side cannot determine the origination time in consideration of the time difference from the terminating portable telephone terminal in advance. The same problem occurs in a country such as USA which has a plurality of time zones. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a radio communication system and destination portable terminal time identification method capable of identifying by an originating radio terminal the current time of a terminating portable telephone terminal which moves over a plurality of time zones. 
     To achieve the above object, according to the present invention, there is provided a radio communication system comprising first and second portable terminals each of which is assigned a unique identifier and has a display for displaying time information and time difference information and a timepiece for measuring current time, and a first switching network having first and second base stations which are assigned unique identifiers and perform radio communication with the first and second portable terminals, a first switching unit connected to the first and second base stations, a first database which stores positions of the first and second portable terminals in association with positions of the first and second base stations, and a second database which stores pieces of time difference information of the first and second base stations, wherein the first portable terminal comprises time difference acquisition request means for adding the identifier of the second portable terminal and issuing a time difference acquisition request for the second portable terminal to the first base station when the second portable terminal connected to the second base station is to be called via the first base station, and current time calculation means for calculating current time of the second portable terminal on the basis of time difference information sent from the first base station in response to the time difference acquisition request from the time difference acquisition request means and time information from the timepiece, and outputting the current time of the second portable terminal to the display, the first base station adds the identifier of the first base station to the time difference acquisition request containing a terminal identifier received from the first portable terminal and transmits the time difference acquisition request to the first switching unit, and the first switching unit comprises first time difference acquisition means for acquiring the time difference information of the first base station from the second database on the basis of the identifier of the first base station contained in the time difference acquisition request received from the first base station, position information acquisition means for acquiring position information of the second base station containing a base station identifier from the first database on the basis of the identifier of the second portable terminal contained in the time difference acquisition request received from the first base station, second time difference acquisition means for acquiring the time difference information of the second base station from the second database on the basis of the base station identifier contained in the position information of the second base station output from the position information acquisition means, and time difference calculation means for calculating a time difference between the first and second base stations by using pieces of time difference information output from the first and second time difference acquisition means, and transmitting a calculation result to the first portable terminal via the first base station. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a portable telephone system according to an embodiment of the present invention; 
         FIG. 2  is a block diagram showing a portable telephone terminal in  FIG. 1 ; 
         FIG. 3A  is a table showing the registration status of various pieces of information in a position information DB in  FIG. 1 ; 
         FIG. 3B  is a table showing the registration status of various pieces of information in a base station information DB in  FIG. 1 ; 
         FIG. 3C  is a view showing identifier information stored in a nonvolatile memory in  FIG. 2 ; 
         FIG. 4  is a flow chart showing destination time difference inquiry operation by the portable telephone system in  FIG. 1 ; 
         FIG. 5  is a sequence chart showing position registration operation by the portable telephone system in  FIG. 1 ; 
         FIG. 6  is a sequence chart showing position registration cancellation operation by the portable telephone system in  FIG. 1 ; 
         FIG. 7  is a sequence chart showing time difference inquiry operation (successful) by the portable telephone system in FIG.  1 ;. 
         FIG. 8  is a sequence chart showing time difference inquiry operation (failed) by the portable telephone system in  FIG. 1 ; 
         FIG. 9  is a sequence chart showing position registration operation (roaming destination switching network) by the portable telephone system in  FIG. 1 ; 
         FIG. 10  is a sequence chart showing position registration cancellation operation (roaming destination switching network) by the portable telephone system in  FIG. 1 ; 
         FIG. 11  is a sequence chart showing time difference inquiry operation (in destination terminal roaming) by the portable telephone system in  FIG. 1 ; 
         FIG. 12A  is a functional block diagram showing a controller in  FIG. 2 ; and 
         FIG. 12B  is a functional block diagram showing a switching unit in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will be described in detail below with reference to the accompanying drawings. 
     As shown in  FIG. 1 , a portable telephone system according to an embodiment is constituted by a plurality of portable telephone terminals  1 A to  1 C, and a plurality of switching networks  2 A and  2 B which perform telephone switching between the portable telephone terminals  1 A to  1 C. 
     The switching network  2 A is comprised of base stations  3 A and  3 B which perform radio communication between the portable telephone terminals  1 A and  1 B belonging to them, a switching unit  4 A which is connected to the base stations  3 A and  3 B and switches between the portable telephone terminals  1 A and  1 B, a position information database (to be referred to as a position information DB hereinafter)  6 A which stores information representing which base station is connected to the portable telephone terminals  1 A and  1 B, a base station information database (to be referred to as a base station information DB hereinafter)  7 A which stores time zone information about locations where the base stations  3 A and  3 B are installed, and an international gateway  5 A. In this case, the users of the portable telephone terminals  1 A and  1 B have made a contract with a carrier who provides the switching network  2 A for service offering. 
     The switching network  2 B is comprised of a base station  3 C which performs radio communication with the portable telephone terminal  1 C belonging to it, a switching unit  4 B, a position information DB  6 B which stores position information of the portable telephone terminal  1 C, the switching unit  4 B which is connected to the base station  3 C and switches the portable telephone terminal  1 C, a base station information DB  7 B which stores time zone information about a location where the base station  3 C is installed, and an international gateway  5 B. The different switching networks  2 A and  2 B are connected by connecting the international gateways  5 A and  5 B. Similarly, a plurality of switching networks including the switching networks  2 A and  2 B are arbitrarily connected by connecting the international gateways of the respective switching networks. 
     As shown in  FIG. 2 , the portable telephone terminal  1 A comprises a controller  11  which controls the whole operation of the portable telephone terminal  1 A, a display  12  which displays various pieces of information, a timepiece  13  which maintains and measures the time, an operation unit  14  which receives user&#39;s operation, a voice input/output unit  15  which inputs/outputs a voice signal, a radio unit  16  which performs radio communication with the base station, and a nonvolatile memory  17  which stores the identifier (telephone number) of the portable telephone terminal  1 A and the identifier of the switching network  2 A to which the portable telephone terminal  1 A belongs. The portable telephone terminals  1 B to  1 D also have the same arrangement as that of the portable telephone terminal  1 A. 
     As shown in  FIG. 3A , the position information DB  6  stores pieces of position information of the portable telephone terminals  1 A to  1 D for respective terminal identifiers assigned to the portable telephone terminals  1 A to  1 D in correspondence with each other. As pieces of position information of the portable telephone terminals  1 A to  1 D stored in the position information DB  6 , the identifiers of the base stations  3 A and  3 B are stored while the portable telephone terminals  1 A and  1 B are connected to the base stations  3 A and  3 B, and dead space information is stored while no portable telephone terminal is connected to the base stations  3 A to  3 C. While a portable telephone terminal is connected to the switching network  2 B, the identifier of the connected switching network  2 B is stored. 
     If information about a portable telephone terminal connected to the switching network  2 B is stored, time zone information of an area where the portable telephone terminal  1 C exists at present is stored in the position information DB  6  on the basis of the time difference from the international standard time. The position information DB  6  allows information retrieval using the terminal identifier as a key. 
     As shown in  FIG. 3B , the base station information DB  7  stores pieces of time zone information for respective identifiers unique to the base stations  3 A to  3 C in correspondence with each other. As time zone information stored in the base station information DB  7 A, pieces of time zone information of areas where the base stations  3 A to  3 C are installed are stored based on time differences from the international standard time. The base station information DB  7  allows information retrieval using the base station identifier as a key. 
     &lt;Destination Terminal Time Difference Inquiry Operation&gt; 
     Time difference inquiry operation for a destination portable telephone terminal by the portable telephone terminal  1 A will be explained with reference to  FIG. 4 . Operations are controlled by the controller  11  of the portable telephone terminal  1 A. 
     If the user of the portable telephone terminal  1 A manipulates the operation unit  14  to input the telephone number of a destination portable telephone terminal (to be referred to as a destination terminal hereinafter), the controller  11  transmits a time difference acquisition request to the base station  3 A (step S 1 ). At this time, the time difference acquisition request contains the telephone number of the destination terminal input by the user. 
     The portable telephone terminal  1 A receives time difference information transmitted from the base station  3 A in response to the time difference acquisition request (step S 2 ). If the base station  3 A which has received the time difference acquisition request finds the position of the destination terminal, the base station  3 A transmits time difference information of the destination terminal to the portable telephone terminal  1 A. If the base station  3 A cannot find the position of the destination terminal, the base station  3 A notifies the requesting portable telephone terminal  1 A of a failure in acquiring time difference information. 
     The controller  11  of the portable telephone terminal  1 A which has received the time difference information checks based on the information transmitted from the base station  3 A whether the position of the destination terminal is within the service area or in the dead space (step S 3 ). If the position of the destination terminal is within the service area, the controller  11  reads out the current time from the timepiece  13  (step S 4 ), and adds the readout current time and the time difference from the destination terminal that has been acquired from the base station  3 A, thereby obtaining the current time of the destination terminal (step S 5 ). The controller  11  then displays the current time of the destination terminal on the display  12  (step S 6 ). If the position of the destination terminal is in the dead space in step S 3 , the controller  11  displays a time difference unacquirable display on the display  12  (step S 7 ). The time difference unacquirable display is, e.g., an error message that acquisition of the time difference fails. 
     &lt;Position Registration Operation&gt; 
     Position registration operation when the portable telephone terminal  1 A is connected to the base station  3 A of the switching network  2 A will be described with reference to  FIG. 5 . 
     If the portable telephone terminal  1 A detects that the base station  3 A is in a radio communicable state, the portable telephone terminal  1 A transmits a position registration request to the base station  3 A (step S 11 ). The position registration request contains a terminal identifier assigned to the portable telephone terminal  1 A. As shown in  FIG. 3C , the terminal identifier is made up of two identifiers: a terminal-specific identifier ID 1  which is assigned to each portable telephone terminal and stored in the nonvolatile memory  17 , and a switching network identifier ID 2  assigned to each switching network. The terminal-specific identifier ID 1  is a telephone number itself or a value convertible into a telephone number by a switching unit. 
     Upon reception of the position registration request transmitted from the portable telephone terminal  1 A, the base station  3 A transmits to the switching unit  4 A a position registration request obtained by adding the identifier of the base station  3 A to position registration information (step S 12 ). The identifier of the base station  3  is a unique identifier assigned to each base station. Upon reception of the position registration request from the base station  3 A, the switching unit  4 A transmits the contents of the received position registration request as a position registration request to the position information DB  6 A (step S 13 ). Upon reception of the position registration request from the switching unit  4 A, the position information DB  6 A registers the received terminal identifier and base station identifier in association with each other. 
     &lt;Position Registration Cancellation Operation&gt; 
     Position registration cancellation operation when the portable telephone terminal  1 A is to be disconnected from the connected base station  3  will be explained with reference to  FIG. 6 . 
     When degradation in radio status with the connected base station  3 A is detected or operation of, e.g., turning off the power supply is to be performed, the portable telephone terminal  1 A transmits a position registration cancellation request to the base station  3 A (step S 15 ). The position registration cancellation request contains the terminal identifier of the portable telephone terminal  1 A. 
     Upon reception of the position registration cancellation request from the portable telephone terminal  1 A, the base station  3 A transmits the contents of the received position registration cancellation request as a position registration cancellation request to the switching unit  4 A (step S 16 ). Upon reception of the position registration cancellation request from the base station  3 A, the switching unit  4 A transmits the contents of the received position registration cancellation request as a position registration cancellation request to the position information DB  6 A (step S 17 ). Upon reception of the position registration cancellation request from the switching unit  4 A, the position information DB  6 A searches the database using, as a key, the identifier ID 1  of the portable telephone terminal  1 A contained in the received position registration cancellation request, and updates position information of the portable telephone terminal  1 A to “dead space”. 
     &lt;Time Difference Inquiry Operation (Successful)&gt; 
     Inquiry operation of the portable telephone terminal  1 B for time difference information of the portable telephone terminal  1 A belonging to the same switching network will be explained with reference to  FIG. 7 . Inquiry operation will exemplify a case wherein the portable telephone terminal  1 A is connected to any base station within the switching network  2 A to which the portable telephone terminal  1 B belongs. 
     If the user of the portable telephone terminal  1 B executes time difference inquiry operation (step S 21 ), the portable telephone terminal  1 B transmits a time difference acquisition request to the connected base station  3 B (step S 22 ). The terminal identifier (telephone number) of the destination portable telephone terminal  1 A whose time difference is to be obtained is added to the time difference acquisition request. 
     The base station  3 B transmits to the switching unit  4 A a time difference acquisition request obtained by adding the identifier of the base station  3 B to the contents of the time difference acquisition request received from the portable telephone terminal  1 B (step S 32 ). Upon reception of the time difference acquisition request, the switching unit  4 A transmits to the position information DB  6 A a position acquisition request which contains the identifier of the terminal  1 A extracted from the time difference acquisition request (step S 24 ). The position information DB  6 A searches the database using, as a key, the identifier of the terminal  1 A contained in the position acquisition request, adds to the searched position information the base station identifier of the base station  3 A connected to the portable telephone terminal  1 A, and transmits the position information to the switching unit  4 A (step S 25 ). 
     The switching unit  4 A extracts the identifier of the base station  3 A from the received position information, adds the extracted identifier to a TZ acquisition request, and transmits the TZ acquisition request to the base station information DB  7 A (step S 26 ). The base station information DB  7 A searches the database using, as a key, the base station identifier contained in the TZ acquisition request received from the switching unit  4 A, adds time zone information of the base station  3 A to TZ information, and transmits the TZ information to the switching unit  4 A (step S 27 ). The switching unit  4 A which has received the time zone information of the base station  3 A transmits to the base station information DB  7 A a TZ acquisition request to which the identifier of the base station  3 B is added (step S 28 ). The base station information DB  7 A which has received the TZ acquisition request transmits to the switching unit  4 A the TZ information to which the time zone information of the base station  3 B is added (step S 29 ). 
     The switching unit  4 A which has acquired the two pieces of time zone information of the base stations  3 A and  3 B calculates the time difference between the base stations  3 A and  3 B by an equation (the time zone of the base station  3 A—the time zone of the base station  3 B) (step S 30 ). The calculation result is added to time difference information, and transmitted to the base station  3 B (step S 31 ). The base station  3 B transmits the contents of the time difference information received from the switching unit  4 A as time difference information to the portable telephone terminal  1 B (step S 32 ). Upon reception of the time difference information, the portable telephone terminal  1 B calculates the time of the portable telephone terminal  1 A from the time difference contained in the received time difference information and the current time obtained from the timepiece  13 , and displays the time of the portable telephone terminal  1 A as a destination time on the display  12  (step S 33 ). Accordingly, the user of the portable telephone terminal  1 B can recognize the current time of the portable telephone terminal  1 A. 
     &lt;Time Difference Inquiry Operation (Failed)&gt; 
     Operation when position information is indefinite because the destination portable telephone terminal  1 A whose time difference information is to be inquired by the portable telephone terminal  1 B is in the dead space or the power supply is OFF will be explained with reference to  FIG. 8 . 
     If the user of the portable telephone terminal  1 B executes time difference inquiry operation (step S 41 ), the portable telephone terminal  1 B transmits a time difference acquisition request to the connected base station  3 B (step S 42 ). The terminal identifier (telephone number) of the destination portable telephone terminal  1 A whose time difference is to be obtained is added to the time difference acquisition request. 
     Upon reception of the time difference acquisition request from the portable telephone terminal  1 B, the base station  3 B transmits to the switching unit  4 A in step S 43  a time difference acquisition request obtained by adding the identifier of the base station  3 B to the contents of the time difference acquisition request. Upon reception of the time difference acquisition request, the switching unit  4 A transmits to the position information DB  6 A a position acquisition request which contains the identifier of the destination terminal  1 A extracted from the time difference acquisition request (step S 44 ). The position information DB  6 A searches the database using, as a key, the identifier of the destination terminal  1 A contained in the received position acquisition request. At this time, if the portable telephone terminal  1 A is not connected to any base station, “dead space” is recorded as position information in the position DB  6 A. Thus, the position DB  6 A adds “dead space” to position information, and transmits the position information to the switching unit  4 A (step S 45 ). 
     Upon reception of the position information, the switching unit  4 A adds “dead space” to the time difference information on the basis of the received position information “dead space”, and transmits the time difference information to the base station  3 B (step S 46 ). The base station  3 B transmits the contents of the received time difference information as time difference information to the portable telephone terminal  1 B (step S 47 ). Since the received time difference information represents “dead space”, the portable telephone terminal  1 B displays a failure in acquiring the time difference of the destination on the display  12 , and notifies the user of a message to this effect (step S 48 ). 
     &lt;Position Registration Operation (Roaming Destination Switching Network)&gt; 
     Operation when the portable telephone terminal  1 C belonging to the switching network  2 A is to be connected to the base station  3 C of the switching network  2 B will be described with reference to  FIG. 9 . This is concerned with so-called roaming operation, and is control of allowing the use of a portable telephone terminal in a switching network different from a switching network provided by a contracted carrier. 
     If the portable telephone terminal  1 C detects a radio communicable state with the base station  3 C, the portable telephone terminal  1 C transmits a position registration request to the base station  3 C (step S 51 ). The position registration request contains a terminal identifier assigned to the portable telephone terminal  1 C. Since the portable telephone terminal  1 C belongs to the switching network  2 A, the switching network identifier contained in the identifier of the destination terminal  1 C represents the switching network  2 A. The base station  3 C transmits to the switching unit  4 B a position registration request obtained by adding the identifier of the base station  3 C to the position registration request received from the portable telephone terminal  1 C (step S 52 ). The identifier of the base station  3 C is a unique identifier assigned to each base station. 
     Upon reception of the position registration request from the base station  3 C, the switching unit  4 B transmits the contents of the received position registration request as a position registration request to the position information DB  6 B (step S 53 ). The position information DB  6 B creates a database record for the portable telephone terminal  1 C, and registers a pair of the received identifiers of the terminal  1 C and base station  3 C in the database. This database record must be newly created because the position information database record of a portable telephone terminal belonging to another switching network does not generally exist in the roaming destination switching network. 
     The switching unit  4 B reads out the switching network identifier from the identifier of the destination terminal  1 C, if the readout identifier is different from the identifier of the switching network  2 B, transmits a TZ acquisition request to the base station information DB  7 B (step S 54 ), and receives TZ information from the base station information DB  7 B (step S 55 ). The TZ information contains time zone information of the base station  3 C. 
     The switching unit  4 B adds the time zone information of the base station  3 C to a position registration notification together with the identifier of the destination terminal  1 C, and transmits the position registration notification to the international gateway  5 B (step S 56 ). Upon reception of the position registration notification, the international gateway  5 B transmits to the international gateway  5 A of the switching network  2 A a position registration information obtained by adding the identifier of the switching network  2 B to the received position registration notification (step S 57 ). 
     Upon reception of the position registration notification from the international gateway  5 B, the international gateway  5 A transmits the contents of the received position registration notification as a position registration notification to the switching unit  4 A of the switching network  2 A (step S 58 ). Upon reception of the position registration notification, the switching unit  4 A extracts the identifier of the destination terminal  1 C, the identifier of the switching network  2 B, and the time zone of the base station  3 C that are contained in the position registration notification, adds the pieces of extracted information to a position registration request, and transmits the position registration request to the position information DB  6 A (step S 59 ). The position information DB  6 A records the switching network  2 B as position information and the time zone of the base station  3 C as a time zone in the database record corresponding to the portable telephone terminal  1 C. 
     &lt;Position Registration Cancellation Operation (Roaming Destination Switching Network)&gt; 
     Operation when the portable telephone terminal is to be disconnected from the roaming destination switching network will be explained with reference to  FIG. 10 . 
     When degradation in radio status with the connected base station  3 C is detected or operation of, e.g., turning off the power supply is to be performed, the portable telephone terminal  1 C transmits a position registration cancellation request to the base station  3 C (step S 61 ). The position registration cancellation request contains the terminal identifier of the portable telephone terminal  1 C. Upon reception of the position registration cancellation request, the base station  3 C transmits the contents of the position registration cancellation request as a position registration cancellation request to the switching unit  4 B (step S 62 ). The switching unit  4 B transmits the contents of the received position registration cancellation request as a position registration cancellation request to the position information DB  6 B (step S 63 ). 
     Upon reception of the position registration cancellation request, the position information DB  6 B searches the database using, as a key, the ID of the portable telephone terminal  1 C (identifier of the terminal  1 C) contained in the received position registration delete request, and deletes the target record. Subsequent to transmission of the position registration cancellation request to the position information DB  6 B, the switching unit  4 B transmits a position registration cancellation request to the international gateway  5 B (step S 64 ). Upon reception of the position registration cancellation request, the international gateway  5 B extracts a switching network identifier from the identifier of the destination terminal  1 C contained in the received position registration cancellation request, and transmits a position registration delete notification to the international gateway  5 A of the switching network  2 A represented by the switching network identifier (step S 65 ). 
     Upon reception of the position registration delete notification, the international gateway  5 A of the switching network  2 A transmits the contents of the received position registration delete notification as a position registration delete notification to the switching unit  4 A of the switching network  2 A (step S 66 ). Upon reception of the position registration delete notification from the international gateway  5 A, the switching unit  4 A transmits a position registration delete request based on the position registration delete notification to the position information DB  6 A (step S 67 ). The position information DB  6 A searches the database on the basis of the identifier of the designated destination terminal  1 C, and rewrites position information in the database record to “dead space”. 
     &lt;Time Difference Inquiry Operation (In Destination Terminal Roaming)&gt; 
     Time difference inquiry operation of the portable telephone terminal  1 B while the destination portable telephone terminal  1 C is connected (roams) to another switching network will be explained with reference to  FIG. 11 . 
     If the user of the portable telephone terminal  1 B executes time difference inquiry operation (step S 71 ), the portable telephone terminal  1 B transmits a time difference acquisition request to the connected base station  3 B (step S 72 ). The time difference acquisition request contains the terminal identifier (telephone number) of the destination terminal  1 C whose time difference is to be obtained. 
     Upon reception of the time difference acquisition request from the portable telephone terminal  1 B, the base station  3 B transmits to the switching unit  4 A a time difference acquisition request obtained by adding the identifier of the base station  3 B to the contents of the time difference acquisition request (step S 73 ). Upon reception of the time difference acquisition request from the base station  3 B, the switching unit  4 A transmits to the position information DB  6 A a position acquisition request which contains the identifier of the destination terminal  1 C extracted from the time difference acquisition request (step S 74 ). 
     The position information DB  6 A searches the database using, as a key, the identifier of the destination terminal  1 C contained in the position acquisition request. Since position information of the portable telephone terminal  1 C is “switching network  2 B”, the position information DB  6 A adds the stored time zone information of the portable telephone terminal  1 C to the position information, and transmits the position information to the switching unit  4 A (step S 75 ). The switching unit  4 A which has received the time zone information of the portable telephone terminal  1 C transmits to the base station information DB  7 A a TZ acquisition request obtained by adding the identifier of the base station  3 B (step S 76 ). Upon reception of the TZ acquisition request, the base station information DB  7 A transmits TZ information which contains the time zone information of the base station  3 B to the switching unit  4 A (step S 77 ). 
     The switching unit  4 A which has acquired the two pieces of time zone information of the portable telephone terminal  1 C and base station  3 B calculates the time difference between the portable telephone terminal  1 C and the base station  3 B by an equation (the time zone of the portable telephone terminal  1 C—the time zone of the base station  3 B) (step S 78 ), adds the calculation result to time difference information, and transmits the time difference information to the base station  3 B (step S 79 ). The base station  3 B transmits the contents of the time difference information received from the switching unit  4 A as time difference information to the portable telephone terminal  1 B (step S 80 ). Upon reception of the time difference information, the portable telephone terminal  1 B calculates the current time of the portable telephone terminal  1 C on the basis of the time difference acquired from the received time difference information and the current time obtained from the timepiece  13 , and displays the calculation result on the display  12  (step S 81 ). Hence, the user is notified of the time of the destination. 
     Note that when the portable telephone terminal  1 C issues a position registration cancellation request as described above and position information corresponding to the portable telephone terminal  1 C in the position information DB  6 A is rewritten to “dead space”, “dead space” is transmitted as position information from the position information DB  6 A. In this case, upon reception of the position information “dead space”, the switching unit  4 A adds “dead space” to time difference information, and transmits the time difference information to the base station  3 B. Upon reception of the time difference information, the base station  3 B transmits the contents of the time difference information as time difference information to the portable telephone terminal  1 B without any change. Upon reception of the time difference information “dead space”, the portable telephone terminal  1 B displays a failure in acquiring the time difference of the destination on the display  12 , and notifies the user of a message of this effect. 
       FIGS. 12A and 12B  show the functional blocks of the controller  11  of the portable telephone terminal  1 A and the functional blocks of the switching unit  4 A of the switching network  2 A. As shown in  FIG. 12A , the controller  11  comprises a time difference acquisition request unit  101  which executes steps S 1  ( FIG. 4 ) and S 21  ( FIG. 7 ), and a current time calculation unit  102  which executes steps S 5  ( FIG. 4 ) and S 33  ( FIG. 7 ). The switching unit  4 A comprises a first time difference acquisition unit  103  which executes steps S 28  ( FIG. 7 ) and S 29  ( FIG. 7 ), a second time difference acquisition unit  104  which executes steps S 24  ( FIG. 7 ) and S 25  ( FIG. 7 ), a position information acquisition unit  105  which executes steps S 26  ( FIG. 7 ) and S 27  ( FIG. 7 ), a time difference calculation unit  106  which executes step S 30  ( FIG. 7 ), and a registration unit  107  which executes steps S 59  ( FIG. 9 ) and S 67  ( FIG. 10 ). 
     In this manner, according to the above embodiment, the user of a portable telephone terminal can be notified of the current time of a destination terminal before origination regardless of the area to which the destination terminal moves and becomes connectable to a base station. In addition, the user can recognize that the destination terminal is in an incommunicable state, e.g., the dead space without calling the destination terminal. Even while the destination terminal is connected to a switching network other than one provided by a contracted carrier, the user of the originating portable telephone terminal can be notified of the current time of the destination terminal before origination. 
     The above embodiment has exemplified a speech communication portable telephone terminal, but the present invention can also be applied to a data communication portable terminal. 
     As has been described above, according to the present invention, an originating terminal can acquire the time difference from a destination terminal before origination regardless of the area to which the destination terminal moves as long as the destination terminal is connected by radio to the base station of this area. The originating terminal calculates the current time of the destination terminal from the acquired time difference and the current time of the timepiece, and displays the current time of the destination terminal on the display, thereby notifying the user of the current time of the destination terminal in advance. 
     Even while the destination terminal is connected to a switching network provided by a carrier different from a carrier with which the originating portable terminal contracts, the user can be notified of the current time of the destination terminal in advance.