Abstract:
The invention provides systems, including a methods and apparatus, for providing seamless telephone service across MSC&#39;s that support multiple or different protocols. In accordance with a preferred embodiment of the invention, the home mobile switching center, where the subscriber record is stored, is equipped to respond to a first inquiry signal from an originating mobile switching center by identifying itself to the originating mobile switching center. The incoming call may then be trunked to the home mobile switching center and forwarded to the visited mobile switching center according to the second protocol. According to another preferred embodiment, in response to the first inquiry signal, the home mobile switching center sends a second inquiry signal to the visited mobile switching center. The first inquiry signal represents a request for an identification number identifying, in the first protocol, the visited mobile switching center. The second inquiry signal represents a request for an identification number identifying in the second protocol, visited mobile switching center. In response to the second inquiry signal, the visited mobile switching center returns a first response signal to the home mobile switching center. In response to the first response signal, the home mobile switching center sends a second response signal to the originating mobile switching center, the second response signal representing, in the first protocol, a number identifying the visited mobile switching center. The call is then directly routed from the originating mobile switching center to the visited mobile switching center.

Description:
TECHNICAL FIELD 
     The present invention relates generally to cellular mobile communication systems. In particular, the present invention relates to a method and system for permitting seamless wireless telephone service across mobile switching centers that support multiple or different standards. 
     DESCRIPTION OF THE PRIOR ART 
     Cellular telephone service has been in widespread use for several years. Typical systems are characterized by radio coverage divided into small areas or “cells” using low power transmitters and coverage-restricted receivers. Such systems are described in U.S. Pat. Nos. 3,906,166 and 4,268,722. A cellular system is typically characterized by a pair of radio frequencies associated with each radio channel in each cell. Several voice channels and at least one signaling channel (also known as a control or access channel) are associated with each cell. The signaling channel processes requests for service to and from mobile and portable phones in order to permit the mobile or portable phone user to converse with another party. A series of Mobile Switching Centers (MSC&#39;s) associated with the cellular system provides the logical control features for efficient call routing and processing. 
     Numerous intersystem standards or protocols for U.S. cellular radiotelephone systems have been promulgated by the Electronics Industries Association/Telecommunications Industry Association (EIA/TIA) in order to ensure compatibility between MSC&#39;s manufactured by different vendors for use in the United States and its territories. For example, United States manufacturers currently support the ANSI-41 standard in their MSC equipment. For various reasons, such standards have not been adopted globally. Rather, the cellular networks in many foreign countries are characterized by different protocols. For example, the European Community favors a standard known as A Global System for Mobile Communication (GSM). Other nations, such as South Korea, have adopted their own internal standards, for example, a standard referred to as K-MAP, for cellular communications networks. Such standards differ in the particular content and format of the messaging involved over the datalink connecting MSC&#39;s in a cellular network. 
     In foreign countries, service providers are recognizing that superior technology and enhanced features sets are provided by MSC equipment manufactured in the U.S. and supporting standards such as ANSI-41. Unfortunately, for such foreign service providers who have implemented their networks according to different standards or protocols, adoption of the new standards is problematic and expensive. Because simultaneous replacement of the entire network of MSC equipment is prohibitively costly, such service providers have opted to migrate gradually to the new standards. During this transition, it is advantageous and necessary to provide MSC&#39;s which are capable of supporting both the old and new standards. However, a system for providing seamless service between an MSC supporting only the old standard to an MSC supporting the new standard has not heretofore been provided. 
     The aforementioned problems can be further illustrated with reference to a cellular system according to the prior art Korean K-MAP protocol. The messaging and call routing according to such a system is illustrated in FIG.  1 . As will be recognized by those of ordinary skill, the thin profile arrows in FIG. 1, and the subsequent FIGURES, represent messaging via the signaling channel and datalinks connecting the MSC&#39;s. The broader profile arrows depict voice channel communication that is routed over the trunk lines (not shown). Consistent with the same convention, a capital letter designation for message names represents an inquiry or invoke message, while lower case letters represent return results. 
     The term INCALL is used throughout this specification to refer to an incoming call which may arrive from the Public Switched Telephone Network (PSTN) or from another mobile phone. The term “V-MSC” will be used throughout this specification to refer to a Visited MSC—an MSC associated with the particular cell visited by the mobile. The term “H-MSC” will be used throughout this specification to refer to a Home MSC—an MSC where the subscriber record for the mobile resides. The term “O-MSC” will be used to refer to the Originating MSC—the MSC where the call to a mobile originates. 
     Referring to FIG. 1, when mobile  10  is powered-up or enters a cell, the mobile registers in that particular cell. When mobile  10  registers, the V-MSC sends a message, REGISTRATION(MSRN) to the H-MSC. REGISTRATION(MSRN) includes the Mobile Switch Routing Number (MSRN) which identifies the V-MSC. The MSRN is stored at the H-MSC for future reference. An MSRN is a static 14-digit number defined by the K-MAP standard that identifies a particular MSC and enables any other MSC in the system to address the MSC via datalink. In an MSRN, the first 7 digits identify the visited MSC and the last 7 digits identify the mobile unit. In response to the REGISTRATION(MSRN) message, the H-MSC returns a registration profile in the message registration(profile) which contains information about the particular services available to the mobile subscriber. 
     In accordance with prior art systems, both the O-MSC and V-MSC communicate with the H-MSC using the same protocol, for example, K-MAP protocol. When an INCALL arrives at the O-MSC, a K-MAP message, Routing Information Inquiry (MIN) (RII(MIN)) is transmitted to the H-MSC, requesting the Mobile Identification Number (MIN) of the mobile phone. In response, the H-MSC will return a K-MAP message, rii(msrn) including the MSRN of the V-MSC. Once the O-MSC has been provided with the MSRN of the V-MSC, the voice call is trunked from O-MSC to the V-MSC using the public network. The voice call is then transmitted via radio frequency from V-MSC to the mobile unit. 
     The aforementioned problems can be her illustrated by a hypothetical system where the V-MSC and O-MSC are connected to the H-MSC using different protocols, for example, where the O-MSC is an older unit adhering to the K-MAP protocol used in Korea and the V-MSC is a new unit adhering to the ANSI-41 protocol. The H-MSC is equipped to communicate in multiple protocols. However, under the K-MAP protocol, when an INCALL arrives at the O-MSC, the message RII(MIN), is sent to the H-MSC and the O-MSC “expects” a return response in the format of the message rii(msrn) containing the 14-digit MSRN of the V-MSC. However, the ANSI-41 protocol does not support a 14 digit static number such as the MSRN. Instead, ANSI-41 provides dynamic numbers associated with particular mobile calls. These numbers are known as Temporary Local Directory Numbers (TLDN&#39;s). As a result, the O-MSC adhering to the K-MAP standard would not “understand” a return result of a TLDN from an H-MSC operating under ANSI-41. Thus, the INCALL cannot be delivered to the mobile. 
     State-of-the-art MSC&#39;s support multiple protocols and may converse individually with neighboring MSC&#39;s in different standards. These MSC&#39;s permit the service provider to specify a given standard, i.e., K-MAP or ANSI-41 when provisioning datalinks between the state-of-the art MSC and other MSC&#39;s in the network that communicate according to only a single protocol. A state-of-the-art MSC is capable of routing a call from one K-MAP MSC to another K-MAP MSC or from one ANSI-41 MSC to another ANSI-41 MSC. That is, during any given time, they are able to “speak” either K-MAP or ANSI-41, but not both, while processing a particular call. Thus, state-of-the-art MSC&#39;s are not capable of providing seamless service from a K-MAP MSC to an ANSI-41 MSC. It would therefore be desirable to provide a system, including a method and an apparatus which permits seamless service between MSC&#39;s operating under different protocols. 
     SUMMARY OF THE INVENTION 
     The invention provides systems, including methods and apparatus, for providing seamless telephone service across MSC&#39;s that support multiple or different protocols. In accordance with a preferred embodiment of the invention, the H-MSC, in response to a first inquiry signal from the O-MSC requesting the identity of the V-MSC, sends a first response signal identifying itself to the O-MSC. The first response signal identifies the H-MSC according to the first protocol, for example, by forwarding its own MSRN as defined by the K-MAP standard. In essence, the H-MSC “fools” the O-MSC into “thinking” that the H-MSC is the V-MSC. The INCALL is then routed to the H-MSC. Once the INCALL is routed to the H-MSC, it can then be routed to the V-MSC via messaging in the second protocol on the datalink connecting the H-MSC and V-MSC. For example, the H-MSC sends a second inquiry signal to the V-MSC according to the second protocol by requesting a Temporary Local Directory Number (TLDN) as defined by the ANSI-41 standard. The second inquiry signal represents a request for an identification number identifying, in the second protocol, the V-MSC. In response to the second inquiry signal, the V-MSC returns a second response signal to the H-MSC. The call is then routed from the H-MSC to the V-MSC and sent to the mobile unit from the V-MSC via radio link. Thus, the INCALL is seamlessly routed from the O-MSC to the V-MSC. 
     In accordance with another preferred embodiment of the invention, in response to a first inquiry signal from the O-MSC, the H-MSC sends a second inquiry signal to the V-MSC. The first inquiry signal represents a request for an identification number identifying, in the O-MSC&#39;s protocol, the V-MSC. The second inquiry signal represents a request for an identification number identifying, in the V-MSC&#39;s protocol, the V-MSC. In response to the second inquiry signal, the V-MSC returns a first response signal to the home mobile switching center, identifying the V-MSC in the V-MSC&#39;s protocol. In response to the first response signal, the H-MSC sends a second response signal to the O-MSC, the second response signal representing, in the O-MSC&#39;s protocol, a number identifying the V-MSC. Once the O-MSC is provided with the identifier of the V-MSC, the call is directly routed from the O-MSC to the V-MSC. 
     A primary advantage of the present invention is that it provides a system for seamlessly providing mobile telephone service across mobile switching centers that adhere to different messaging protocols without requiring changes to the message processing features of those mobile switching centers. That is, neither the V-MSC nor the O-MSC requires any changes to their message processing control logic. 
     Other objects, advantages novel features, and the further scope of applicability of the present invention will be set forth in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings, in which like numbers refer to like parts throughout: 
     FIG. 1, as described above, is a schematic depicting the messaging and call routing flow of a prior art call routing system; 
     FIG. 2A is a schematic depicting the messaging and call routing according to a preferred embodiment of the present invention in which a visited mobile switching center communicates via ANSI-41 standard with a home mobile switching center and an originating mobile switching center communicates via the Korean K-MAP standard with the home mobile switching center; 
     FIG. 2B is a schematic depicting the messaging and call routing according to another preferred embodiment of the present invention in which a visited mobile switching center communicates via ANSI-41 standard with a home mobile switching center and an originating mobile switching center communicates via the Korean K-MAP standard with the home mobile switching center; 
     FIG. 2C is a schematic depicting the architecture of a mobile switching center according to the embodiment depicted in FIG. 2A; 
     FIG. 2D is a schematic depicting the architecture of a mobile switching center according to the embodiment depicted in FIG. 2B; 
     FIGS. 3A-3C depict the logic flow of a system according to the preferred embodiment depicted in FIG. 2A; 
     FIG. 4 is a schematic depicting the messaging and call routing according to a preferred embodiment of the present invention in which a visited mobile switching center communicates via Korean K-MAP standard with a home mobile switching center and an originating mobile switching center communicates via ANSI-41 standard with the home mobile switching center; 
     FIG. 4A is a schematic depicting the architecture of a mobile switching center according to the embodiment depicted in FIG. 4; 
     FIG. 5 is a schematic depicting the messaging and call routing according to another preferred embodiment of the present invention in which a visited mobile switching center communicates via ANSI-41 standard with a home mobile switching center and an originating mobile switching center communicates via the Korean K-MAP standard with the home mobile switching center; 
     FIG. 5A is a schematic depicting the architecture of a mobile switching center according to the embodiment depicted in FIG. 5; and 
     FIGS. 6A-6C depict the logic flow of a system according to the preferred embodiment depicted in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Data messages on the control channel are provided first to the cell, and then through a datalink to the MSC, which establishes a voice connection between mobile radiotelephones and the main telephone network via a voice channel. 
     FIGS. 2A and 2B illustrate a preferred embodiment of the present invention in which an O-MSC  40  communicates via datalink  35  with an H-MSC  30  according to a first protocol, for example, Korean K-MAP protocol. The H-MSC  30  and the V-MSC communicate via datalink  25  according to a second protocol, for example, ANSI-41. It will be understood that the designations “O-MSC” and “V-MSC” derive from the particular function being assumed by corresponding MSC&#39;s. That is, the O-MSC is designated as such because the INCALL originates there. Likewise, the V-MSC is defined as the MSC where a mobile unit has registered as a visiting mobile unit. On the other hand, the H-MSC, via a Home Location Register (HLR) is permanently associated with a particular mobile unit as it permanently stores subscriber information associated with that particular mobile unit. 
     FIG. 2A illustrates the datalink messaging and call routing according to a preferred embodiment of the present invention. When mobile unit  10  enters the cell associated with V-MSC  20 , through a known process, the mobile unit  10  registers with the V-MSC  20 . At the V-MSC, a determination is typically made as to whether the mobile is an authorized user or subscriber by looking up the mobile&#39;s telephone number, electronic serial number and other information provided by the phone to determine if there is an entry in a database, usually referred to as the Visiting Location Register (VLR), stored at the V-MSC  20 . Part of the normal function of an MSC is to validate the Electronic Serial Number (ESN) and Mobile Identification Number (MIN) received from the mobile and to determine if the mobile unit is “home.” If the mobile is not “home”, the V-MSC  20 , according to the ANSI-41 protocol, sends a message REGISTRATION (MSRN)  210  to the H-MSC  30 . In response to that message, the H-MSC  30  returns a message registration (profile)  212  which represents a particular service profile associated with the mobile unit  10 . 
     Under the K-MAP protocol, when an INCALL  208  originates at O-MSC, the O-MSC sends across datalink  35  an inquiry message  214  requesting a Mobile Subscriber Routing Number MSRN, which uniquely identifies the MSC where the mobile has last registered. According to the K-MAP protocol, the inquiry message  214  is called a Routing Information Inquiry RII(MSRN)  214  to H-MSC  30 . Thus, RII(MSRN) represents a first inquiry signal from the O-MSC representing a request, according to the K-MAP protocol, for identification (i.e., the Mobile Switch Routing Number) of the V-MSC  20 . According to K-MAP, the O-MSC will “expect” a return result containing a Mobile Subscriber Routing Number (MSRN) which is a 14-digit number, as defined by the K-MAP protocol. 
     According to the present invention, the H-MSC  30  is equipped to recognize the particular protocol governing communications with both O-MSC  40  and V-MSC  20 . The H-MSC is provided with global look-up tables which associate a particular protocol with each MSC in the network. The tables may be stored in a stand-alone Home Location Register (HLR), in an HLR integrated into an MSC, or in the VLR of an MSC. Thus, the H-MSC  30 , upon receiving the first inquiry signal, for example RII(MSRN), is able to determine that the O-MSC communicates according to a first protocol, for example, K-MAP protocol. Similarly, the H-MSC  30 , upon registration of mobile unit  10  is able to determine that the V-MSC communicates with the H-MSC  30  according to a second protocol, for example, ANSI-41 protocol. 
     After first inquiry signal, RII(MSRN)  214  is sent, the O-MSC  40  is “expecting” the MSRN of the V-MSC. In accordance with the invention, the H-MSC is provided with appropriate processing features to return it&#39;s own MSRN to the O-MSC, thereby identifying, in K-MAP, itself to the O-MSC  40 . Thus, the message rii(home msrn)  218  represents a first response signal to the first inquiry signal RII (MSRN)  214  identifying H-MSC  30 . In response, INCALL  208  is routed over the public network, or “trunked” to the H-MSC  30 . 
     Once the INCALL  208  arrives at H-MSC  30 , the INCALL  208  is trunked over to the V-MSC using the ANSI-41 protocol. To accomplish this, a message ROUTEREQ(TLDN)  220  is sent to the V-MSC  20  as a second inquiry signal requesting a number, here the TLDN, identifying the V-MSC  20  in the ANSI-41 protocol. TLDN&#39;s are dynamically allocated numbers that are mapped to a particular MIN identifying mobile units registered at the V-MSC  20 . In response to the second inquiry signal ROUTEREQ(TLDN)  220 , the message routereq(tldn)  222  is returned, identifying the V-MSC  20  to the H-MSC  30  under the ANSI-41 protocol. The message routereq(tldn) thus represents a second response signal identifying the V-MSC in the second protocol, for example, ANSI-41. The call is then trunked over from H-MSC  30  to V-MSC  20  as represented by arrow  224  and transmitted via radio link to the mobile unit  10 . 
     FIG. 2B represents the messaging and call routing of a system according to another preferred embodiment of the present invention. This system permits direct routing of the INCALL  208  from the O-MSC  40  to the V-MSC  20 . As described above with respect to FIG. 2A, the mobile unit  10  registers at V-MSC. Then, in response to an INCALL, the O-MSC sends a first inquiry signal, in the form of message RII(MSRN)  214  to H-MSC to request a number identifying, in the first protocol, K-MAP, the V-MSC. According to this embodiment, in response to the first inquiry RII(MSRN), H-MSC sends a second inquiry signal, ROUTEREQ(TLDN)  220  in the second protocol, ANSI-41, to V-MSC requesting a TLDN. As discussed above, TLDN is a number assigned to the mobile unit  10  by the V-MSC. The TLDN is returned in message routereq(tldn)  222  to H-MSC  30 . Using the global look-up tables described above, the H-MSC recognizes that the TLDN returned in message routereq(tldn) must be encoded into destination digits which identify the V-MSC to the O-MSC. Thus, as a result of this first response signal being received by the H-MSC, the H-MSC generates a second response signal, message rii(destination digits)  226  which is sent to the O-MSC. Since the V-MSC has been identified to the O-MSC, the INCALL  208  may be directly routed from the O-MSC to the V-MSC over the public access network (or by whatever means the O-MSC chooses) and delivered via radio link to the mobile unit  10 . 
     FIG. 2C illustrates the architecture of an H-MSC configured to accomplish the messaging and call routing illustrated in FIG.  2 A. Call processor  240 , which may comprise a central processing unit, including memory for storing preprogrammed instructions, provides the control logic to process calls and messages received by H-MSC  30 . Call processor  240  reads data representing a mobile subscriber profile from the HLR  242 . HLR  242  is defined in global memory within the H-MSC  30 . Call processor  240  also reads data representing the O-MSC (or any neighboring MSC) protocol support from system configuration manager  244 . Call processor  240  writes local variables—variables which are local to the H-MSC—to a call register  246  defined in memory. Similarly, data representing an MSRN is written to memory as represented by block  248 . The MSRN is a number represented in memory which identifies a particular call being processed by call processor  240 . 
     In response to message RII(MSRN)  214 , message generator  250  returns message rii (home MSRN)  216  identifying the H-MSC to the O-MSC. In response, the call is trunked to the H-MSC as represented by broad arrow  218 . Call processor  240  reads local-variable information from the call register  246 . Similarly, data representing the neighboring MSC protocol support is read from the system configuration manager  244 . Call processor  240  then instructs message generator to generate a ROUTEREQ(TLDN) signal  220  to obtain the TLDN of the V-MSC in the ANSI-41 protocol. In response a return result routereq(tldn)  222  is sent back to the call processor  240  of H-MSC  30  and the call is trunked at  224 . 
     FIG. 2D illustrates the architecture of an H-MSC configured to accomplish the messaging and call routing illustrated in FIG.  2 B. Call processor  240  reads data representing a subscriber profile from HLR  242 . This data permits the H-MSC to determine the location of the V-MSC at which the mobile unit is registered. Call processor  240  also reads data representing a profile of a neighboring MSC from system configuration manager  244 . Call processor  240  receives an RII (MSRN) message  214  from O-MSC  40  and generates a message ROUTEREQ(TLDN)  220  to V-MSC  20 . In response, V-MSC returns a result message routereq(tldn)  222  to call processor  240 . In response to the return result  222 , call processor  240  provides a signal to O-MSC representing a return result  226  containing the destination digits of V-MSC in the K-MAP protocol. The INCALL is then directly trunked from the O-MSC to the V-MSC. 
     FIGS. 3A-3C represent the control logic implemented at the O-MSC, H-MSC and V-MSC, respectively, according to the preferred embodiment of the present invention described above with respect to FIG.  2 A. Referring to FIG. 3A, INCALL is received at the O-MSC at state  310 . At state  312 , the MSRN of the H-MSC for the mobile unit is determined. The Directory Number (DN) is included as the called party number in the INCALL message that is delivered to the closest MSC. The DN is then used as a key to a table that indicates the MSC Identifier (MSCID) of the MSC where the subscriber record is stored and updated. The table dictates the datalink to be used for sending the RII or Message Location Request and permits a determination of the protocol to use and the actual message, for example, the RII under the K-MAP protocol or the Location Request under the ANSI-41 protocol. The message RII(MSRN) is sent from the O-MSC at state  314 . 
     Referring to FIG. 3B the message RII(MSRN) is received at the H-MSC at state  316 . At state  318 , a determination is made as to the standard or protocol used by the last known visited MSC according to the Home Location Register (HLR). When a message is received, the information on what standard is to be used is determined from the look-up tables described above which associate a particular protocol with each MSC in the network. If the last known visited MSC adheres to the K-MAP standard, the logic proceeds to step  320  where the MSRN of the last visited MSC is returned as a result of the RII(MSRN) invoke. The call is then trunked to the identified V-MSC according to the conventional K-MAP protocol. On the other hand, if at state  318 , it is determined that the last known visited MSC adhered to the ANSI-41 standard, the MSRN of the H-MSC is returned at state  322  in the message rii(home msrn). 
     Referring again to FIG. 3A, the return result rii(home msrn) is received by the O-MSC at state  323  and the INCALL is trunked to the H-MSC identified by home msrn at state  324 . It will be recognized by those of ordinary skill that the invention does not require modified logic at the O-MSC; the logic steps performed by the O-MSC are the same as they would be if the O-MSC and H-MSC were linked via K-MAP protocol. While the logic performed at O-MSC is unchanged, the data communicated from the H-MSC to the O-MSC is different than in the purely K-MAP context. The first response signal represents the identity of the H-MSC to the O-MSC. Thus, in accordance with the invention, the messaging sent from H-MSC results in the O-MSC “thinking” it is communicating with the visited MSC. 
     Referring again to FIG. 3B, INCALL is received at the H-MSC at state  325 . The H-MSC logic then makes a determination at state  326  as to whether the H-MSC is the MSC being visited by the mobile unit  10 . In this case, the H-MSC would be the V-MSC and the call would require no further routing before being sent via radio link to mobile unit  10 . If the H-MSC is not associated with the geographical region including the mobile unit  10 , the H-MSC logic proceeds to state  329  where an ANSI- 41  message ROUTEREQ(TLDN) is sent via datalink to the V-MSC. 
     Referring to FIG. 3C, the message ROUTEREQ(TLDN) is received by the V-MSC at state  330 . At state  332 , a paging signal may be sent via radio link to the mobile unit. When the paging signal has been acknowledged, and thus the presence of the mobile unit in the geographical region associated with the V-MSC established, a return result routereq(tldn) is messaged from the V-MSC to the H-MSC at state  334 . The TLDN is assigned to each mobile unit currently registered at the V-MSC. The TLDN is assigned from a pool of numbers designated by the service provider that are used to route a call from another MSC in the network to the V-MSC. This pool is created on a per switch basis where the service provider indicates the starting number and how many of these numbers to make available. When one is assigned the TLDN is stored in a table with the actual MIN of the mobile unit  10 . This TLDN is dynamically allocated. Once the TLDN is used to route the INCALL into the V-MSC, the TLDN is removed from the table and can be assigned to another mobile unit to route another call. 
     Referring again to FIG. 3B, the message routereq(tldn) is received by the H-MSC at state  336  and the call is trunked to the V-MSC via the public access network at state  338 . Referring again to FIG. 3C, the INCALL is received at the V-MSC at state  340  and delivered to the mobile unit via radio frequency link at state  342 . 
     FIGS. 4 and 5 depict the messaging and call routing according to a preferred embodiment of the invention for the situation where the V-MSC communicates with the H-MSC via the Korean K-MAP protocol and the O-MSC communicates with the H-MSC via the ANSI-41 standard. Registration of mobile unit  10  occurs at V-MSC in a known manner. According to the K-MAP standard, a registration notification message REGNOT (MIN/MSRN)  410  is sent to H-MSC via datalink  25 . A return result regnot(profile)  420  is returned to V-MSC identifying a plurality of features that the mobile unit user is entitled to use. When an INCALL  208  is received at O-MSC, according to the ANSI-41 standard, a message LOCREQ(MIN)  422  is sent from the O-MSC to the H-MSC. LOCREQ(MIN)  422  represents a first inquiry signal representing a request for a number, i.e., MIN, identifying the MSC at which the mobile unit  10  has registered. According to the invention, in response to the inquiry signal, the return result locreq(home tldn)  424  is sent from the H-MSC to the O-MSC. Thus, the return result locreq(home tldn) is a first response signal representing a number identifying the H-MSC. As a result of the response signal, the INCALL is trunked from the O-MSC to the H-MSC as represented by arrow  426 . H-MSC then sends a second inquiry signal, message RII(MSRN)  428 , under the K-MAP protocol to V-MSC. A second response signal, rii(msrn)  430  is returned and the INCALL is trunked to V-MSC as represented by arrow  432 . 
     FIG. 4A illustrates the architecture of an H-MSC configured to accomplish the messaging and call routing illustrated in FIG.  4 . Call processor  240  writes to call register  246  and to TLDN memory location which is represented at block  440 . In response to receipt of message LOCREQ (MIN)  422 , message generator returns a result locreq(tloln)  424  indicating the TLDN of H-MSC. Message generator  250  then returns a return result  424  to the O-MSC. In response, the call is trunked to the H-MSC as represented by broad arrow  426 . Once the call is trunked to the H-MSC, a second inquiry signal RII(MSRN)  428  is sent to the V-MSC. Call processor  240  recreates an MSRN at block  248  by examining the subscriber profile in the HLR. At registration, the V-MSC passed the MSRN to the H-MSC. The V-MSC then returns a result rii(msrn) representing the MSRN of the V-MSC and the call is trunked to the V-MSC at  224 . 
     FIG. 5 illustrates messaging and call routing according to another preferred embodiment of the present invention. Like the system depicted in FIG. 4, this system is applicable to the situation where the V-MSC communicates via K-MAP with the H-MSC and the H-MSC communicates via ANSI-41 with the O-MSC. After the registration messaging occurs as described above with reference to FIG. 4, a first inquiry signal, message LOCREQ(MIN)  522 , is sent from the O-MSC in response to INCALL  208 . When the message LOCREQ(MIN)  522  is received at the H-MSC, a return response locreq(routing #=msrn)  524  is returned which contains the 14-digit MSRN of V-MSC. Thus, the H-MSC creates the MSRN of the visited system and returns it in a destination parameter of the LOCREQ return response. Once the MSRN of the V-MSC is returned to the O-MSC, the INCALL can be directly routed to the V-MSC as represented by arrow  526 . 
     The MSRN is a 14-digit number where the first seven digits indicate the MSC which contains the subscriber record. Of these first seven digits, the first three digits indicate the routing prefix, the next three digits identify a particular MSC and the last digit identifies the cellular provider. The last seven digits of the MSRN indicate the mobile number (3-digits are an office code). 
     FIG. 5A shows the architecture of an H-MSC configured to accomplish the messaging and call routing depicted in FIG.  5 . In response to message LOCREQ (MIN)  522  call processor  240  instructs message generator  250  to return a result with the MSRN of the mobile unit. The H-MSC simply returns the entire MSRN, message locreq (routing #=MSRN)  524 . The H-MSC “knows” the MSRN where the mobile registered. 
     FIGS. 6A-6C depict the logic flow occurring at the O-MSC, H-MSC and V-MSC, respectively. Referring to FIG. 6A, INCALL arrives at state  610 . At state  612 , the home MSC corresponding to the MIN of the number associated with INCALL is determined. A message LOCREQ (MIN) is sent at state  614  to the H-MSC from the O-MSC. Referring to FIG. 6B, the LOCREQ(MIN) message is received at the H-MSC at state  616  and the V-MSC at which the mobile unit was last registered is determined at state  618 . At state  620 , the standard associated with the V-MSC is determined from the look-up tables described above. If the V-MSC standard is ANSI-4 1 , the same standard for communication between the O-MSC and the H-MSC, the logic proceeds to state  622  in which the INCALL is routed to the V-MSC using the ANSI-41 standard. If, however, the standard of the V-MSC is a K-MAP standard, a determination is made at state  624  that an MSRN should be returned to the O-MSC. If an MSRN is to be returned to the O-MSC, the logic proceeds to state  626  in which the MSRN of the V-MSC is returned and the call is trunked directly to the V-MSC. This sequence corresponds to the messaging depicted in FIG.  5 . If, however, at state  624 , it is determined that a TLDN should be returned to the O-MSC, the 10-digit TLDN of the H-MSC is returned in the message locreq(home tldn) at state  628 . 
     Referring again to FIG. 6A, the  10 -digit TLDN is received by the O-MSC at state  630 . In response, the INCALL is routed to the H-MSC at state  632 . Referring again to FIG. 6B, INCALL is received at the H-MSC at state  634 . The visited MSC is determined at state  636  and at decision  638 , a determination is made as to whether the V-MSC is the H-MSC, in which case the call need not be further routed and may be delivered to the mobile unit  10  via radio link. If the H-MSC is not the V-MSC, the logic proceeds to decision  642  where a determination is made as to whether the V-MSC is a K-MAP MSC. If not, the call is trunked to the V-MSC using ANSI-41 standards at state  644 . If the V-MSC is a K-MAP standard, an MSRN is created for the visited MSC and the call is trunked to the V-MSC using the created MSRN. The 14-digit MSRN is outpulsed onto the selected trunk along with other data including the calling party number. The trunk terminates at an entity in the network and the MSRN is converted into a DN or MIN. The process to accomplish this is to remove the first seven digits and to prepend the area code for the given area to the last seven digits of the mobile subscriber. 
     The foregoing merely illustrates the principles of the present invention. Those skilled in the art will be able to devise numerous alternative arrangements which, although not explicitly shown or described herein, embody those principles and are thus within the spirit and scope of the invention as defined in the appended claims.