Abstract:
A method for performing the network direct system selection (NDSS) of a mobile station (MS) in a cellular communications system that includes the steps of: making a base station (BS) to include a dialed phone number designated for the NDSS in a CM SERVICE REQUEST message being transferred from the MS to a first MSC; making the first MSC to transmit a NDSS FEATURE REQUEST INVOKE message of the MS to the HLR upon confirming that the dialed phone number corresponds to the NDSS; making the first MSC to transmit a FEATURE REQUEST INVOKE CONFIRMATION message to the MS via the BS; making the HLR to transmit a QUALIFICATION DIRECTIVE message containing a service re-direction information to a first VLR coupled to the first MSC when the FEATURE REQUEST INVOKE message is accepted by the HLR; making the first VLR to confirm whether the QUALIFICATION DIRECTIVE message includes the service re-direction information; making the first VLR to transmit a SERVICE RE-DIRECTION message, consisting of the service re-direction information and a command to request the MS to report the return cause information when the NDSS failure occurs, to the MS; and, registering the location of the MS in a second VLR of a second MSC as directed by the SERVICE RE-DIRECTION message.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. Section 119 from an application for METHOD FOR MAKING NETWORK DIRECT SYSTEM SELECTION IN A CELLULAR COMMUNICATION SYSTEM filed earlier in the Korean Industrial Property Office on May 13, 1999 and there duly assigned Serial No. 17146/1999. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cellular mobile communications system, and more particularly, a method for directly connecting a mobile station (MS) in communication with a mobile switching center (MSC) from one network to an MSC in another network. 
     2. Description of the Related Art 
     In a cellular mobile communication system, a service area is divided into a plurality of cells, each with a base station system (BS) therein. The base stations are controlled by a mobile switching center (MSC) to enable the MS to make voice or data communication while roaming from one cell to another. With reference to FIG. 1, there is illustrated a conventional cellular communications system, which includes a mobile station (MS)  10  in communication with a base station (BS)  20 , and an MSC  30  coupled to a plurality of base stations, a PSTN Public Switched Telephone Network (PSTN)  60 , and another MSC  40 . The MSC coupled to the plurality of base stations (BSs) may be defined as one network. 
     The cellular network, as shown in FIG. 1, further includes a Home Location Register (HLR), which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The HLR  50  can service multiple MSCs, as shown in FIG. 1. A Visitor Location Register (VLR)  35  coupled to the MSC  30  is a database containing information about all MSs currently located within the MSC  30 /VLR  35  area. Thus, the MSC establishes a call connection upon receiving a call request from the user of an MS, by accessing the HLR  50  and VLR  35 / 45  for the requisite identification information of the MSs registered in the network. If an MS  20  roams into a new MSC  40 /VLR  45  area, the VLR  45  connected to the MSC  40  will request data about that MS  20  from the HLR  50  database. Accordingly, if the user of the MS  20  wishes to make a call, the local VLR  45  will have the requisite identification information. In the aforedescribed manner when the MS requests the call set-up or the location registration, the MS  30 / 40  first searches the VLR  35  or  45 , then the HLR  50  retrieves various subscriber information of the MS. 
     In such mobile communications network, the HLR  50  can selectively elect one of the MSCs  30  and  40  to be connected to the MS, depending on the type of service requested by the MS. This is known as the Network Direct System Selection (NDSS), used to provide a specific service that is requested by the MS. For example, if an MS in communication with an MSC requests the Internet service but the MSC is not equipped with the packet data service capability, the HLR having the requisite information can determine another MSC with the Internet service capability to be connected to the MS. The operation of NDSS involves transmitting NDSS signaling messages and processing the message through the network (BS, MSC, HLR, VLR, etc.) which enables the MS to roam between different networks to receive various services or the optimal route. This type of NDSS operation is usually performed when the MS requests the registration of an additional function (e.g., voice message service, Internet service, call forwarding service, etc.), the location registration, or the call transmission. 
     The information regarding the current location of the MS is received in the CM SERVICE REQUEST message. Thus, when an MS in communication with the BS transmits the CM SERVICE REQUEST message with an NDSS code to request a certain feature/service, the VLR of the MSC relays this message to the HLR. The same message is relayed when the MS transmits the LOCATION UPDATE REQUEST message for registering the location of the MS, or when the MS transmits the CM SERVICE REQUEST message for requesting a call set-up as requested by the MS. Thereafter, the HLR confirms the NDSS code of the MS and determines the new MSC for providing the service as indicated in the NDSS code, then transmits the QUALIFICATION DIRECTIVE message or the REGISTRATION NOTIFICATION RETURN message back to the VLR of the MSC. Then, the MSC transmits the SERVICE RE-DIRECTION message to the MS via the BS to register the MS in the new MSC area or request the new MSC to generate a call. The address of the new MSC area is represented by the ID information included in the SERVICE RE-DIRECTION message. 
     However, in the above conventional NDSS operation, there are areas that need to be addressed to improve the communication efficiency. First, there is no means for the user of the MS to know whether the HLR has successfully registered the MS with a specific service as requested by the MS. For example, if the MS requests the Internet service or the voice mailing service, the user is not notified of whether the MS has been provided with the voice mailing service in the current the NDSS operation. Also, there is no protocol or set method for the VLR to transmit the QUALIFICATION DIRECTIVE message with the SERVICE RE-DIRECTION message therein from the HLR to the MS. Moreover, when performing the location registration through the NDSS operation, the procedures for transmitting the LOCATION UPDATE REQUEST from the MS to the MSC and for exchanging the REGISTRATION NOTIFICATION RETURN message between the MSC and the HLR are not defined by the TSB-80 of A-Interface in the present EIA/TIA North America CDMA Communications Standard. Furthermore, when the MS, which is already registered in the VLR, requires a call generation and transmits an NDSS code requesting a specific service, the VLR has no means to determine whether the MS is registered for the specific service. Hence, the VLR may establish a call-set up without consulting with the HLR and fails to provide the appropriate MSC that has the capability to provide the requested service. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for performing the Network Direct Select System (NDSS) in a communication system whenever the MS transmits an NDSS code requesting a specific feature/service from the system. 
     It is another object of the present invention to provide a method for performing the Network Direct Select System (NDSS) in a communication system whenever the MS executes the location registration. 
     It is a further object of the present invention to provide a method for performing the Network Direct Select System (NDSS) in a communication system whenever the MS attempts to establish a call set-up. 
     According to one embodiment of the present invention, a method for performing the NDSS operation of an MS in a cellular communications system includes steps of: inserting, by a BS, a dialed phone number with an NDSS code in a CM SERVICE REQUEST message to be transferred to a first MSC; transmitting, by the first MSC, an NDSS FEATURE REQUEST INVOKE message of the MS to the HLR after confirming the dialed phone number of the NDSS; transmitting, by the first MSC, a FEATURE REQUEST INVOKE CONFIRMATION message to the MS via the BS; upon receiving the FEATURE REQUEST INVOKE message from the first MSC, determining, by the HLR, whether a second MSC needs to be designated for providing the requested service; transmitting, by the HLR, a QUALIFICATION DIRECTIVE message to a first VLR of the first MSC and including a service re-direction information in the QUALIFICATION DIRECTIVE message if is determined to require the second MSC service; determining, by the first VLR, whether the QUALIFICATION DIRECTIVE message contains the service re-direction information; transmitting, by the first VLR, the service re-direction message including the service re-direction information and transmitting a command message to request the MS to report a connection failure to the second MSC; and, registering the location of the MS in a second VLR of the second MSC as directed by the service re-direction message. 
     According to another embodiment of the present invention, a method for performing the NDSS operation of an MS in a cellular communications system includes the steps of: transmitting, by the MS via a base station, a LOCATION REVISION REQUEST message including the return cause information of a previous NDSS operation failure to a first VLR of a first MSC; storing, by the first VLR, the information of LOCATION REVISION REQUEST message from the MS and transmitting a LOCATION REGISTRATION NOTIFICATION message including the return cause information to the HLR; revising, by the HLR, the location information of the MS and transmitting a LOCATION REGISTRATION NOTIFICATION RETURN message including service re-direction information after determining whether the LOCATION REGISTRATION NOTIFICATION message includes the return cause information and whether the MS requires the assignment of a new MSC; if the location REGISTRATION NOTIFICATION RETURN message includes the service re-direction information, transmitting, by the first VLR, a SERVICE RE-DIRECTION message including the service re-direction information and a command message to request the MS to report when the connection to the new MSC fails; and, registering the location of the MS in the second VLR of a second MSC as directed by the service re-direction message. 
     According to another embodiment of the present invention, a method for performing the NDSS operation of an MS in a cellular communications system includes the steps of: transmitting a CM SERVICE REQUEST message including a phone number dialed by the MS to a first MSC via a BS; searching, by the first MSC, the subscriber information of the MS from a first VLR; transmitting a LOCATION REGISTRATION NOTIFICATION message including the NDSS feature request invoke information of the MS from the first VLR to the HLR, if the subscriber information of the MS is stored in the VLR, or if the MS is registered with the NDSS code; transmitting a location REGISTRATION NOTIFICATION RETURN message including the subscriber information with service re-direction information from the HLR to the first VLR if the MS requires a new MSC; determining, by the first MSC, whether the subscriber information stored in the first VLR includes the service re-direction information; if the location REGISTRATION NOTIFICATION RETURN message includes the service re-direction information, transmitting a service re-direction message including the service re-direction information and a command message to request the MS to report if the connection to the new MSC fails; terminating the connection with the first MSC with the MS; and, initiating, by the MS, the generation of a call to a second MSC as directed by the service re-direction message. 
     As explain above, there are three embodiments utilizing the NDSS operation to perform a call set-up between the originating MSC and the terminating MSC. The first embodiment includes the event that the MS transmits a specific feature request; the second embodiment includes the event that the MS executes the location registration; and, the third embodiment includes the event that the MS originates a call. To this end, the HLR transmits the service re-direction information including the ID of the new MSC to the VLR of the originating MSC when the MS requires the NDSS operation. The originating MSC transmits a SERVICE RE-DIRECTION message including the service re-direction information to the MS so as to request the new MSC to execute the location registration or call generation. 
     According to the embodiment of the present invention, the CM SERVICE REQUEST message, transmitted from the base station to the MSC, for call generation and the LOCATION REVISION REQUEST message, transmitted from the MS for location registration, include the return cause information. The return cause information includes information relating to the failure cause of the previous service re-direction attempt. The return cause information, as set forth under the MAP interface of IS-735 standard, includes information indicating that the new MSC is not detected and that the MS does not support the NDSS operation. Accordingly, the MSC and the HLR may take proper action, such as re-attempting the call request, upon detecting the failure of the MS for the NDSS via the return cause information from the MS. 
     In the embodiment, in order to receive the return cause information from the MS, the MSC includes a parameter for commanding the MS to report the cause of the NDSS failure to the new network in the service re-direction message that is transferred to the MS. Thereafter, the MS includes the return cause information in the CM SERVICE REQUEST message or the location revision request message that is transferred to the MSC so that the MSC can transfer the return cause information to the HLR. 
     In the embodiment of the present invention, the subscriber information of the MS further includes the information indicating whether it is registered or activated for the NDSS. Thus, upon detecting the subscriber information of the MS requesting a call generation which is also activated for the NDSS, the VLR requests the subscriber information from the HLR to determine whether the MS requires a new assignment of the MSC. Thus, the present invention provides the methods for checking the NDSS of the MS by the HLR in three embodiments. 
     The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more detailed description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, the emphasis instead placed upon illustrating the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     FIG. 1 is a schematic diagram for illustrating the typical structure of a cellular communications system; 
     FIG. 2 is a schematic diagram for illustrating the flow of performing the NDSS based on the feature request invoke message according to the present invention; 
     FIG. 3 is a schematic diagram for illustrating the flow of performing the NDSS based on the location registration according to the present invention; 
     FIG. 4 is a schematic diagram for illustrating the flow of performing the NDSS based on the call generation according to the present invention; 
     FIG. 5 is a flow chart for illustrating the process of performing the NDSS by the MSC at the request of a call generation according to the present invention; 
     FIG. 6 is a flow chart for illustrating the process of treating the service re-direction message according to the present invention; and, 
     FIG. 7 is a flow chart for illustrating the process of performing the NDSS by the MSC based on the location registration according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. For the purpose of clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
     According to the first embodiment of the present invention with reference to FIG. 2, the network direct system selection (NDSS) function may be initiated by the MS upon dialing a particular phone number or dialing a feature code defined for the NDSS. That is, each feature code represents various different services, i.e., the voice message service, the Internet service, the call forwarding service, etc. In step S 105 , when dialing the feature code through the MS  10 , the BS includes the feature code in the CM SERVICE REQUEST message to be transferred to the first MSC  30 , which is presently serving the area where the MS  10  is located. Upon confirming that the called phone number of the MS is intended for the NDSS to request a specific feature, the first MSC  30  transmits the FEATURE REQUEST INVOKE message to the HLR  40  in step S 110 . Upon storing the information of the FEATURE REQUEST INVOKE message, the HLR  50  transmits an acknowledgment message back to the first MSC  30  in step S 115 . Here, the HLR  50  determines whether the MS  10  is registered for the requested feature service and requires the NDSS to select a new MSC equipped to provide the requested feature service, then notifies the outcome of this determination to the first MSC. Then, the first MSC  30  sends a CONFIRMATION message to the MS  10  via the BS in step S 120 , so that the MS can generate a predetermined tone or visual message in the display unit of the MS to notify the user that the MS requires the NDSS. Thus, if the FEATURE REQUEST INVOKE message is successfully received by the HLR  50  and a determination is made to actuate the NDSS, the HLR  50  transmits the QUALIFICATION DIRECTIVE message to the first VLR  35  via first MSC  30 , to notify the revised information including the identification (ID) of the new MSC, which represents the service re-direction information, in step S 125 . 
     In step S 130 , the first VLR  35  determines whether the QUALIFICATION DIRECTIVE message contains the service re-direction information. To this end, the VLR  35  determines whether the ID of the MSC contained in the QUALIFICATION DIRECTIVE message is same as that of the first MSC  30 . If not the same, the QUALIFICATION DIRECTIVE message is determined to have the service re-direction information. 
     If the QUALIFICATION DIRECTIVE message contains the service re-direction information, the first VLR  35  transfers the SERVICE RE-DIRECTION message including the service re-direction information to the MS  10  via the BS and the command message for the MS to report the cause of failure in the event that the connection to the new MSC fails. However, if the QUALIFICATION DIRECTIVE message contains the service re-direction information and the MS  10  is presently in communication with the first MSC  30 , the first MSC  30  deletes the information of the MS  10  presently stored in the first VLR  35 . The disconnection is made so as to enable the MS  10  connect to the new MSC indicated by the service re-direction information. In an other embodiment, when the QUALIFICATION DIRECTIVE message contains the service re-direction information and the MS  10  in communication with the first MSC  30 , the first MSC  30  stores the service re-direction information into the first VLR  35  until the MS changes into the idle state. If the MS comes into the idle state, the first MSC  30  composes the service re-direction message using the service re-direction information stored in the first VLR  35 , then transfers the service re-direction message to the MS via the BS. 
     Upon receiving the service re-direction message, in step S 135 , the MS  10  requests the location registration from the second VLR  45  of the second MSC  40 , which is designated by the ID of the newly preferred MSC in the service re-direction message, so that the second VLR  45  notifies the HLR  50  of the location registration by the MS in step  140 . Thereafter, the HLR  50  deletes the location registration information of the MS  10  stored in the first VLR  35  of the first MSC  30  in steps S 145  and S 150 , then notifies the second VLR  45  the completion of the location registration in step S 155 . Finally, the second VLR  45  notifies the MS  10  of the acceptance of the location revision through the BS in step S 160 . 
     According to the second embodiment of the present invention with reference to FIG. 3, when the NDSS operation is actuated based on the location registration process, the MS periodically notifies its location to the MSC, which is in communication with the MS, for the location registration purposes, so that the MSC can properly select the proper BS in response to the location of the MS. In the event that the MS fails to roam into a new MSC through the NDSS, the MS transmits the return cause information and re-attempts to register its location to the previous MSC. Namely, the MS  10  re-transmits the LOCATION REVISION REQUEST message containing the return cause information to the first VLR  35  of the first MSC  30  via BS, in step S 205 . The step of re-transmitting the LOCATION REVISION REQUEST message back to the previous MSC is triggered because the MSC is designed to command the MS to report if the NDSS connection to the new MSC fails, and, if unsuccessful, command the MS to re-attempt the location registration process. Hence, the return cause information represents whether the network roaming of the MS has been successfully achieved in the previous attempt in response to the prior service re-direction attempt. 
     In step S 210 , the first MSC  30  stores the information of the MS  10  into the first VLR  35 , transmits the LOCATION REGISTRATION NOTIFICATION message to the HLR  50 . Here, the LOCATION REGISTRATION NOTIFICATION message includes the information on the return cause information from the prior attempt from the MS  10 . Upon confirming that the LOCATION REGISTRATION NOTIFICATION message includes the return cause information from the previous attempt, the HLR  50  commands the first MSC  30  to accept the location registration request of the MS  10 , instead of the second MSC  40 . In another embodiment, at the request of the operator, the HLR  50  can command the first MSC to treat the location registration request from the MS  10  as failure in step S 205  in the event that the LOCATION REGISTRATION NOTIFICATION message includes the prior return cause information. 
     The HLR  50  revises the location information of the MS and determines whether the MS requires the NDSS. If it requires the NDSS, the HLR  50  includes the service re-direction information in the LOCATION REGISTRATION NOTIFICATION RETURN message to be transmitted back to the VLR  35  in step S 215 . The service re-direction information includes the ID of the new MSC  40  to which the MS  10  is to attempt the location registration. Thus, in step S 220 , the first VLR  35  transfers the SERVICE RE-DIRECTION, including the service re-direction information, to request the new MSC for the location registration and commands the MS to report the connection failure to the new MSC to the MS  10  via the BS. 
     In step S 225 , upon detecting the ID of the MSC included in the service re-direction message, the MS  10  transmits the location registration request to the second VLR  45  of the second MSC  40 . Then, the second VLR  45  notifies the HLR  50  of the location registration in step S 230 . Upon deleting the location registration information of the MS  10  that is previously stored in the first VLR of the first MSC, the HLR  50  notifies the second MSC  40  the completion of the location registration in step S 235 . Then, in step S 240 , the second MSC  40  stores the information of the MS  10  into the second VLR  45  and notifies the MS  10  that the location revision is accepted by the second MSC  40  through the BS. 
     According to the third embodiment of the present invention with reference to FIG. 4, when the NDSS operation is initiated based on the call generation by the MS, the BS transmits the CM SERVICE REQUEST message including the phone number dialed by the MS  10  to the first MSC  30 , in step S 305 . The CM SERVICE REQUEST message may contain the return cause information relating the cause of the NDSS failure from the previous service re-direction attempt. In step S 310 , the first MSC  30  requests the subscriber information of the MS  10  from the first VLR  35 . In the prior art, if the first VLR  35  already includes the subscriber information of the MS  10 , it may proceed with the call generation without requesting the HLR  50  for further subscriber information. However, this makes it impossible for the HLR  40  to determine whether the MS  10  requires the NDSS, thus the HLR can not actuate the NDSS when required. In order to prevent this, the subscriber information of the MS according to the present invention includes the NDSS active information representing whether the MS is registered for various features. 
     At the request of the first MSC  30 , the first VLR  35  searches the subscriber information of the MS  10  to determine whether the MS is registered for the NDSS. If there were the subscriber information and the NDSS information were not activated, the VLR transfers the subscriber information to the MSC  30  to establish the call set-up with the MS  10 . However, if the first VLR  35  does not store the subscriber information of the MS or that the NDSS of the MS  10  is not activated, the first VLR  35  transmits the LOCATION REGISTRATION NOTIFICATION message to the HLR  50  to request the subscriber information of the MS  10 . Meanwhile, if the return cause information relating the prior NDSS attempt is received from the MS  10 , the first VLR  35  includes the return cause information in the LOCATION REGISTRATION NOTIFICATION message. Then, upon confirming that the LOCATION REGISTRATION NOTIFICATION message contains the return cause information, the HLR  50  commands the first MSC  30  to process the call generation to set-up the call. Alternatively, at the request of the operator, the HLR  50  can treat the call generation request of the MS  10  as failure. 
     However, if the location registration notification message does not contain the return cause information, the HLR  50  proceeds with the normal NDSS. In order to proceed a with the normal NDSS, the HLR  50  determines whether the MS  10  requires the NDSS. That is, the HLR  50  determines whether the MS needs to be relocated to a different MSC which can fully provide various features, such as the Internet service, the voice message service, and other services registered to the MS. If the MS  10  requires its location to be registered in the second MSC  40  through the NDSS process, the HLR  50  transmits, in step S 320 , the subscriber information of the MS including the service re-direction information to the first VLR  35  through the LOCATION REGISTRATION NOTIFICATION RETURN message. Here, the service re-direction information includes the ID of the new MSC. 
     In step S 325 , the first VLR  35  notifies the first MSC  30  of the subscriber information received from the HLR  50 . Upon detecting that the ID of the MSC contained in the subscriber information is not same as its own ID, the first MSC  30  determines that the subscriber information includes the service re-direction information. If the subscriber information does not contain the service re-direction information, the first MSC  30  establishes the call set-up to the MS  10 . However, if the subscriber information includes the service re-direction information, the first MSC  30  transfers, in step S 330 , the service re-direction message to the MS  10  via the BS and requests the MS to report if the connection to the new MSC fails. 
     In step S 335 , upon detecting the ID of the new MSC contained in the service re-direction message, the MS  10  sends the CM SERVICE REQUEST message to the second MSC  40  indicated by the ID information in the service re-direction message. Then, the second MSC  40  requests the subscriber information from the second VLR  45  in step S 340 , transmits the LOCATION REGISTRATION NOTIFICATION message to the HLR  40  to notify the subscriber information of the MS in step S 345 . Upon deleting the location registration information of the MS  10  from the first VLR of the first MSC, the HLR  50  transmits, in step S 350 , the location REGISTRATION NOTIFICATION RETURN message containing the subscriber information to the second VLR  45 . Thereafter, in step S 355 , the second MSC  40  treats the call from the MS  10  according to the subscriber information registered in the second VLR  45 . 
     In the following description, a method for performing the NDSS according to the CM SERVICE REQUEST message of the first and the third embodiments is described with reference to FIG.  5 . When an MS dials a phone number requesting a call set-up or requesting a specific feature from the system, the BS in communication with the MS includes the phone number in the CM SERVICE REQUEST message to be transferred to the serving MSC. Upon receiving the CM SERVICE REQUEST message from the MS via the BS in step S 402 , the serving MSC translates the phone number contained in the CM SERVICE REQUEST message, in step S 404 , to determine in step S 406  whether the phone number represents a request for an ordinary call set-up or a specific feature requiring the NDSS. 
     If the phone number is for the NDSS, the MSC transmits the FEATURE REQUEST INVOKE message to the HLR in step S 408 . The MSC, in step S 412 , determines that the feature request is successfully transmitted to the HLR if an acknowledging message ACK is received, in step S 410 , from the HLR. The ACK message can include the information representing that the MS is registered in the NDSS network, and requires the NDSS operation. However, if the attempt to transmit the feature request message fails, the MSC considers the NDSS for the MS as unsuccessful in step S 414  and notifies the user of the MS of such failure by sending a predetermined tone or message to MS, then terminates the call. 
     If the feature request message is successfully transmitted, the MSC connects a call set-up with the MS in step S 416  and notifies the user of the acceptance of the NDSS by sending a predetermined tone or message. 
     As described above, the MSC generates the tone/message at the moment of succeeding the transmission of the feature request message, instead of being connected to a new MSC so that the subscriber may be informed of the possible NDSS before the actual roaming to the new MSC. The reason for generating the tone/message after the successful transmission of the feature request message is that, if the service re-direction information contained in the QUALIFICATION DIRECTIVE message delivered from the HLR to the VLR includes the command for the MS to report the possible failure of the service re-direction, the MSC need to not take any subsequent action after the failure of the NDSS other than having the MS to request the location registration again to the MSC. 
     Thus, after receiving the acknowledgment message that the MS is registered in the NDSS network, in step S 418 , the MSC terminates the call with the MS to connect to the new MSC and notifies the VLR that the MS is recovered into an idle state. Then, in step S 420 , the VLR determines whether the HLR has sent the QUALIFICATION DIRECTIVE message containing the service re-direction information for the NDSS. Upon detecting the QUALIFICATION DIRECTIVE message, in step S 422 , the VLR composes the service re-direction message based on the service re-direction information contained in the QUALIFICATION DIRECTIVE message, and then transmits the service re-direction message to the MS via the BS in step S 424 . The VLR also includes information in the service re-direction message commanding the MS to report if it fails to be connected to the new MSC. Thereafter, the MS requests the location registration from the new MSC as designated by the service re-direction message received therein. However, if the HLR transfers the QUALIFICATION DIRECTIVE message to the VLR before the original MSC terminates the call connection with the MS, the VLR stores the QUALIFICATION DIRECTIVE message until the MS changes into an idle state. That is, the original MSC has to be disconnected with the MS before being connected to the new MSC assigned by HLR, and the transition period after the disconnection with the original MSC and before the connection to the new MSC is considered an idle state. 
     Herein, a more detailed description of the steps (S 422  and S 424 ) for processing the service re-direction message is explained with reference to FIG.  6 . When the HLR transfers in step S 510  the QUALIFICATION DIRECTIVE message to the VLR before the original MSC terminates the call with the MS, the VLR determines in step S 520  whether the MS is in the idle state. If the MS is still in communication with the original MSC, the VLR stores in step S 530  the service re-direction information of the QUALIFICATION DIRECTIVE message so that it can be transferred later when the MS changes to an idle state. However, if the MS is in the idle state, the VLR composes in step S 422  the service re-direction message based on the service re-direction information contained in the QUALIFICATION DIRECTIVE message, then transfers the service re-direction message to the MS through the BS, in step S 424 . Hence, the MS can request the call generation from the new MSC as indicated in the service re-direction message. 
     Meanwhile, if the received phone number is not for the NDSS but intended as an ordinary call set-up in step S 406 , the MSC treats the call as an ordinary call. To this end, the MSC searches the subscriber information of the MS from the VLR. In step S 428 , the VLR searches the subscriber information to determine whether the MS is activated or registered for the NDSS. If there were subscriber information and the NDSS information were not activated, the VLR transfers in step S 430  the subscriber information to the MSC to establish the call set-up with the MS. However, if there is no subscriber information, or if the NDSS information is activated with the subscriber information, the VLR composes in step S 432  the LOCATION REGISTRATION NOTIFICATION message to be transferred to the HLR in step S 434 . Here, the VLR includes the NDSS feature request information from the MS in the LOCATION REGISTRATION NOTIFICAITON message. 
     Upon receiving the LOCATION REGISTRATION NOTIFICAITON message, the HLR determines whether the MS requires the NDSS in order to select a new MSC for the NDSS. If the MS requires the NDSS, the HLR includes the ID of the new MSC in the location REGISTRATION NOTIFICATION RETURN message, which is an acknowledging message of the REGISTRATION NOTIFICATION message to be transferred to the VLR. If the MS does not require the NDSS, the HLR includes the ID of the presently serving MSC in the REGISTRATION NOTIFICATION RETURN message. Upon receiving the LOCATION REGISTRATION NOTIFICATION RETURN message from the HLR in step S 436 , the VLR determines in step S 438  whether the MSC ID of the service re-direction received therein contains the same ID as that of the presently serving MSC. If so, the MSC registers the subscriber information of the MS in the VLR in step S 440  because the LOCATION REGISTRATION RETURN message does not contain the service re-direction information. 
     Alternatively, if the ID of the MSC in the service re-direction is different from that of the presently serving MSC in step S 422 , the VLR composes the service re-direction message using the service re-direction information (ID of the new MSC) contained in the REGISTRATION NOTIFICATION RETURN message to be transferred to the MS through the BS in step S 424 . In addition, the VLR includes the command for the MS to report the failure source to connect to the new MSC in the service re-direction message. Thereafter, the MS requests the call generation from the new MSC as directed in the service re-direction message. 
     Herein, a detailed description of performing the NDSS according to the second embodiment is explained with reference to FIG.  7 . When the MS roams into a certain MSC, it transfers the LOCATION REVISION REQUEST message to the VLR through the BS to inform its location. In this case, the LOCATION REVISION REQUEST message may include the return cause information from a previous service re-direction attempt. Upon receiving the LOCATION REVISION message in step S 610 , the VLR composes the location registration notification message in step S 620  to be transferred to the HLR in step S 630 . Based on the return cause information contained in the LOCATION REGISTRATION NOTIFICATION message, the HLR determines whether the previous service re-direction attempt was a failure. If so, the HLR commands the presently serving MSC to register the location of the MS. Alternatively, at the control of the operator, the HLR can treat the LOCATION REVISION REQUEST of the MS as failure. However, if the previous service re-direction was successful, the HLR transfers the LOCATION REGISTRATION NOTIFICATION RETURN message with the ID of the new MSC for the NDSS to the VLR. 
     Upon receiving the LOCATION REGISTRATION NOTIFICATION RETURN message in step S 640 , the VLR determines in step S 650  whether the ID of the new MSC contained in the service re-direction information is same as the presently serving MSC. If the new MSC ID is same as the present MSC, the VLR considers the LOCATION REGISTRATION NOTIFICATION RETURN message as not including the service re-direction information for the NDSS, then stores the information of the MS in step S 660  and thereafter notifies the MS of the acceptance of the location revision request. However, if the new MSC ID is not same as the present MSC in step S 650 , the VLR composes in step S 670  the service re-direction message based on the service re-direction information of the LOCATION REGISTRATION NOTIFICATION RETURN message to be transferred to the MS via the BS, in step S 680 . Thus, the MS transfers again the LOCATION REVISION REQUEST message to the new MSC as directed by the MSC ID in the service re-direction message. It should be noted that the present invention can be applied to a situation when the MS, in communication with a PCS network, attempts to connect to the IMT2000 network. In such a case, there should be an appropriate interface defined between the networks as these networks have their respective HLRs. 
     In sum, if there is a need for the MS to connect with a more suitable MSC, the present invention makes the HLR to provide the MS with the information of the suitable MSC using the LOCATION REGISTRATION NOTIFICATION RETURN message and the QUALIFICATION DIRECTIVE message, thus facilitating the communication services. For example, when the MS requests packet data services from a MSC that is not equipped to provide the packet data service, the HLR may command the MS to perform the NDSS to connecte to another MSC that has the packet data service capability. 
     While the present invention has been described in connection with specific embodiments accompanied by the attached drawings, it will be readily apparent to those skilled in the art that various changes and modifications may be made thereto without departing the gist of the present invention.