Abstract:
An apparatus, system and method for call forwarding when roaming from a first type network, such as an ANSI-41 cellular network, to a second type network, such as a GSM cellular network. According to one embodiment of the present invention, a signaling message converter signals an originating MSC in a local communication system to redirect a call when a terminating foreign MSC cannot terminate the call due to predetermined circumstances.

Description:
PROVISIONAL APPLICATION 
     This application claims the benefit of the filing date of corresponding U.S. Provisional Patent Application No. 60/139,978, entitled “Interworking of Call Forwarding When Roaming From ANSI-41 to GSM,” filed Jun. 18, 1999. 
     RELATED APPLICATION 
     This application is related to commonly assigned and U.S. patent application Ser. No. 08/980,103 entitled “Methods and Systems for Late Call Forwarding,” filed Nov. 26, 1997, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to a method, apparatus and system for call forwarding when roaming from a first type network to a second type network in a communication system. More specifically, the present invention relates in general to cellular communication systems and in particular to a method and apparatus for completing call forwarding when a dial mode ANSI-41 mobile subscriber has roamed to a GSM network and a call to that mobile subscriber is not terminated. 
     BACKGROUND OF THE INVENTION 
     Cellular telephones have become increasingly prevalent in today&#39;s society. The cellular telephone makes use of a cellular network and traditional telephone networks to route calls using radio communication signals. Two types of cellular networks have evolved—Global Systems for Mobile communications (GSM) and American National Standards Institute-41 (ANSI-41) type networks. ANSI-41 is also sometimes referred to as IS 41 (TIA/EIA [Telecommunications Industry Association/Electronic Industries Association] Interim Standard 41). 
     The two types of cellular networks use message formats that are not completely compatible with one another as they are. Thus, if a mobile subscriber (MS) using cellular communication, roams from one type of network to another, the messages sent by the MS may not be understood by the network into which it roams unless some sort of conversion is performed. In order to allow roaming between “foreign” type networks, a signaling message converter, referred to herein as an IWU (InterWorking/interoperability Unit) operates to interconnect calling and called Mobile services Switching Centers (MSCs). The IWU passes signaling messages back and forth between network entities while converting the format of these messages such that the messages can be properly interpreted by the receiving MSC. 
     However, even with known signal message converters, GSM standards are inherently different from the ANSI-41 standards mentioned above. One of these differences is in the operation of “late call forwarding” or the action of the system when a call to a mobile subscriber (MS) cannot be terminated, i.e. connection completed, for any of several specific reasons like call collision, no page response of a page request sent to the called MS, and the like. A GSM system expects the terminating Mobile service Switching Center (MSC) to handle the call forwarding while an ANSI-41 system expects the originating or home MSC of the MS to handle the call forwarding. 
     As is known by those skilled in the art, call forwarding may occur when a call, to a given MS, cannot be terminated. The call forwarding can be “early” or “late”. Early call forwarding is defined as occurring when a contact with the MS has failed due to predetermined circumstances occurring while attempting to extend the call to another network. Late call forwarding is defined as occurring when call forwarding is initiated after the call has been extended to the last known “foreign” public land mobile network location and contact with the MS has failed due to predetermined circumstances. 
     In view of the above, it would be advantageous to have a converter mechanism that provides for late call forwarding from the originating MSC when a MS roams from an ANSI-41 network to a GSM network. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus, system and method for call forwarding when roaming from a first type network to a second type network. 
     According to one embodiment of the present invention, a signaling message converter signals an appropriate MSC in a local communication system to redirect a call when a terminating foreign MSC cannot terminate the call due to various circumstances. In a preferred embodiment, the calling party is located in a local communication system while the called party is in a foreign communication system. Once it is determined that the call cannot be terminated at the called party&#39;s communication device in the foreign communication system, a call redirect message is sent to an appropriate MSC of the local communication system. In response to receiving the call redirect message, a call forwarding number is identified. The communication connection between the local communication system and the foreign communication system is released and a communication connection to the communication device associated with the call forwarding number is thereafter attempted. 
     Thus, with the present invention, call forwarding from an originating MSC can be accomplished when the called party is located in a foreign communication system. Other features and advantages of the present invention will be described in, or will become apparent to those of ordinary skill in the art in view of, the following detailed description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein like numerals designate like elements, and wherein: 
     FIG. 1 is an exemplary block diagram of a communication system according to the present invention; 
     FIG. 2 is an exemplary block diagram of the interworking/interoperability unit according to the present invention; 
     FIG. 3A is an exemplary message flow diagram illustrating call forwarding according to known systems; 
     FIG. 3B is an exemplary message flow diagram illustrating call forwarding according to the present invention; and 
     FIG. 4 is an exemplary message flow diagram according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is an exemplary diagram illustrating a cellular communication system  100  according to the present invention. In FIG. 1 a GSM cellular network  10  in its basic configuration is shown along with a basic and similar ANSI-41 cellular network  12 . Also shown is an InterWorking/interoperability Unit (IWU)  14  and a Public Switched Telephone Network (PSTN)  16 . 
     GSM cellular network  10  includes a Mobile services Switching Center (MSC)  18  connected to a Visitor Location Register (VLR)  20  and to a Home Location Register (HLR)  22 . An additional direct connection  38  is illustrated between VLR  20  and HLR  22 . The MSC  18  is connected to a Base Station Controller (BSC)  24  which has a connection to a Base Transceiver Station (BTS)  30  within cell  26 . Mobile Subscriber (MS)  34  communicates with BTS  30  within cell  26 . The HLR  22  is connected via a message channel  40  to the IWU  14 . A voice channel  42  connects the MSC  18  to a PSTN  16 . 
     ANSI-41 cellular network  12  includes a Mobile Telecommunications Switching Office (MTSO)  50  connected to a VLR  52  and also to an HLR  54 . A MTSO is the ANSI-41 counterpart of an MSC in a GSM cellular network and may in some cases herein be referred as an MSC. An additional connection  53  is shown for supplying messages directly between the VLR  52  and the HLR  54 . A BSC  56  is connected to MTSO  50 . The BSC  56  is further connected to a BTS  62  within a cell  58 . A further BTS  64  within a cell  60  is also connected to BSC  56 . A message link  68  is shown interconnecting IWU  14  and VLR  52 . A voice channel  78  is shown interconnecting PSTN  16  and MTSO  50 . Within cell  60  there is shown a MS  66 . Within cell  58  is another MS  70 . 
     For the purposes of the following description, mobile subscriber  34  will be considered a dual mode transceiver unit which is designed to operate and locally communicate in either a GSM or an ANSI-41 type network. For illustrative purposes and in conjunction with following figures, mobile subscriber  34  will be also designated as being operated by a user, customer “B”. Mobile subscriber  70  is further designated as being used by a customer “C” such that calls from mobile subscriber  66  (customer “A”) that cannot be terminated with mobile subscriber  34  (customer “B”) are to be forwarded to mobile subscriber  70  (customer “C”). 
     The Visitor Location Registers (VLRs)  20  and  52  are network entities responsible for storing roaming information related to a mobile subscriber. The roaming information includes information related to the identity of the mobile subscriber as well as address information to be used for contacting the roaming mobile subscriber. 
     The Home Location Registers (HLRs)  22  and  54  are network entities responsible for storing subscription information related to mobile subscribers for which the respective networks  10  and  12  are the mobile subscribers&#39; home networks. Such information may include, for example, identities of the mobile subscribers, address information for use when calling the mobile subscribers, call forwarding information, and the like. 
     The BTS  30 ,  62  and  64  send and receive cellular communications to and from the mobile subscribers  34 ,  66  and  70 . The BSCs  24  and  56  route cellular communications to the appropriate cell  58 ,  60  or  26 , based on routing information supplied by the MSC  18  or MTSO  50 , and coordinates communications between BTSs (e.g., handoff between BTSs). The MSC  18  and MTSO  50  provide cellular communication services based on information obtained from the HLRs and VLRs of the respective networks  10  and  12 . The MSC  18  and MTSO  50  provide routing information to the BSCs  56  and  24  for use in routing the cellular communication to an appropriate cell in which the receiving mobile subscriber is located. The BSC is a base station controller which coordinates calls between BTSs (e.g., handoff between BTSs). 
     The MSC, VLR and HLR entities may or may not be co-located. If they are not co-located, the MSC is responsible for transferring information to and from the VLR and HLR whenever applicable. 
     In the present invention, an originating MSC refers to a switch which provides an interface between a local network and an external or foreign network (i.e. GSM or ANSI-41). This originating MSC is in charge of 1) fetching location information for MSs visiting external networks and 2) routing calls toward the switch (MSC) in the external network through which the subscriber can obtain service at that instant. 
     Although as shown in the accompanying drawings, the originating MSC and the home MSC, of the roaming MS are the same unit for ease of illustration, these two terms may also refer to two different MSCs. It should also be noted that although the originating caller is shown as an ANSI-41 MS  66  and the phone to which the call is forwarded is shown as a ANSI-41 MS  70 , the MS  70  may be a land line phone, GSM MS, and/or a phone of a completely different type of network from the GSM and ANSI-41 networks shown. 
     In addition to the VLRs and HLRs of the respective ANSI-41 and GSM networks  10  and  12 , the IWU  14  also maintains a VLR functionality  210  and HLR Mobility Application Protocol (MAP) functionality  220  as shown in FIG.  2 . MAP is a protocol generally known in the art. The IWU VLR  210  and HLR MAP  220  functionalities are used for converting messages received in ANSI-41 network  12  format into GSM network  10  format, as shown in FIG. 1, and vice versa. 
     As shown in FIG. 2, the IWU  14  maintains a VLR functionality  210  for the roaming ANSI-41 MS  34  and a HLR MAP functionality  220  for the GSM network  10 . The IWU  14  may be a separate unit as shown in FIGS. 1 and 2 or may be incorporated into one or more of the GSM HLR  22 , GSM VLR  20 , ANSI HLR  54  or VLR  52 . Thus, any combination of the IWU  14  with these elements  20 , 22 ,  52  and  54  may be utilized without departing from the spirit and scope of the present invention. For purposes of clarity of the following description of the preferred embodiments, a separate IWU unit  14  is assumed. 
     When a subscriber to an ANSI-41 network  12 , for example, roams from the ANSI-41 network into the GSM network  10 , the IWU  14  acts as an ANSI-41 VLR for the ANSI-41 network and a GSM HLR for the GSM network. Messages related to call termination sent from the ANSI-41 network  12  to the GSM network  10  are first routed using ANSI-41 messaging to the IWU  14 . The IWU  14  determines routing information in the GSM network  10  by using subscriber B registration information in IWU  14 . 
     The IWU  14  receives the message from the ANSI-41 network  12  and, using the ANSI-41 IWU VLR functionality  210  and the GSM MAP IWU HLR functionality  220 , converts the message into a GSM format. Thereafter, GSM messaging may be performed with GSM VLR to determine appropriate routing information to the roaming MS  34 . 
     The present invention is directed to the situation when a call from a calling MS  66  to a roaming MS, such as MS  34  in the GSM network  10 , cannot be terminated, i.e. completed to the called MS  34 . In such a situation, late call forwarding may be permitted, using the present invention, such that the call is forwarded by the originating MSC to another destination, such as MS  70 , and terminated at that location. This situation, and the differences between the prior art and the present invention, will be described with reference again to FIG. 1, and with further reference to the message flow diagrams of FIGS. 3A and 3B. 
     For purposes of explanation it will be assumed that the MTSO  50  is an originating MSC, in addition to being a home location MSC for MS  34 , and that both mobile subscribers  66  and  70 , representing customers “A” and “C”, respectively, are under control of originating MSC  50 . Further, it is assumed in this example that both mobile subscribers  66  and  70  use the same VLR  52  and the same HLR  54 . It will also be assumed that the GSM and ANSI-41 networks  10  and  12  are configured for Optimal Routing. Within the standards of Optimal Routing, a routing of a call to a roaming MS can be prepared for as follows. In ANSI-41 systems, the address of the ANSI-41 originating MSC, i.e. MTSO  50 , is included in the location request message sent to the HLR  54 . This information is also provided as part of the route request message that is sent to the IWU  14  (also called a gateway) when calling a roaming ANSI-41 subscriber, such as MS  34 , presently located in the environment of the GSM network  10 . Thus the HLR  54  and IWU  14  are provided the address information needed to contact the ANSI-41 gateway MSC  50  when necessary. 
     FIG. 3A is an exemplary diagram illustrating the prior art method of call forwarding when a call cannot be terminated at the called MS. As shown in FIG. 3A, a call originates from mobile subscriber “A” and is to be terminated at the mobile subscriber “B”. However, if the call cannot be terminated with mobile subscriber “B”, the call is forwarded by the terminating MSC, i.e. the MSC associated with the network in which mobile subscriber “B” is located, to a mobile subscriber “C”. Thus, late call forwarding is performed from the terminating MSC. 
     FIG. 3B is an exemplary diagram illustrating the method of call forwarding according to the present invention. As shown in FIG. 3B, a call originates from mobile subscriber “A” and is to be terminated at the mobile subscriber “B”. However, if the call cannot be terminated with mobile subscriber “B”, the call is forwarded by the originating MSC, i.e. the MSC associated with the network in which mobile subscriber “A” is located, to a mobile subscriber “C”. Thus, late call forwarding is performed from the originating MSC. 
     Thus, the present invention provides a mechanism by which late call forwarding can be accomplished by the originating MSC. The details of the messaging used to accomplish the features of the present invention will now be described with reference to the message flow diagram of FIG.  4  and again with reference to the elements in FIG.  1 . 
     As shown in FIG. 1, and with further reference to FIG. 4, when mobile subscriber  66  attempts to call mobile subscriber  34 , a call origination message is transmitted to the originating MSC  50 , i.e. MTSO  50 . The call origination procedure uses the address of mobile subscriber  34  as a subscriber address. As shown, the home MTSO  50  of MS  34  is also an originating MSC  50  and thus, the routing address of the home MTSO  50  and the originating MSC  50  are identical. Assuming that home location of mobile subscriber  34  is recorded in the HLR  54  at the time of subscriber registration, in accordance with known methods, a location request message is sent from the originating MSC  50  to the HLR  54  to obtain routing information. The HLR  54  sends a route request message to the IWU  14  for the purpose of obtaining a roaming number. If the last operational contact of mobile subscriber  34  was within GSM network  10 , its location at that time is recorded in the IWU  14  as a part of an update location function. If IWU  14  finds mobile subscriber  34  in its database, IWU  14  relays a provide roaming number message to the GSM VLR  20 . 
     The GSM VLR  20  then returns a roaming number acknowledgement message with a roaming number to be used as a Mobile Subscribing Routing Number (MSRN). The IWU  14  changes the format of the MSRN to a Temporary Location Directory Number (TLDN) format, used by the ANSI-41 network  12 , and provides this number in an acknowledgement message to the HLR  54 . The HLR  54  relays this information within a location request acknowledgement message to gateway MSC  50 . 
     The originating MSC  50  starts a call setup procedure set of messages using voice channels  42  and  78  and using the provided subscriber address of mobile subscriber  34  within GSM network  10 . If MSC  18  cannot terminate the call for reasons such as call collision, MS  34  being unavailable, no answer after alerting, or no response, a “send information for incoming call message” is returned from MSC  18  to VLR  20 . The VLR  20  responds with a “send information for incoming call” acknowledgement message that includes information pertaining to the called roaming MS  34 . 
     In response to receiving the acknowledgement message from the GSM VLR  20 , the GSM MSC  18  sends a “resume call forwarding” message to the IWU  14 . The “resume call forwarding” message is a message that instructs the IWU  14  to perform call forwarding. This message also includes information identifying the cause of the inability to terminate the call with MS  34 . This information may later be used, for example, to determine which call forwarding number to use when forwarding the call to another destination location. 
     When the “resume call forwarding” message is received by the IWU  14  subsequent to the failure of a call being terminated in the GSM network  10 , the IWU  14  of the present invention sends a redirection request (RED REQ) to the ANSI-41 originating MSC  50 . The redirection request message is a message requesting information for use in redirecting the call to another termination point. 
     The ANSI-41 gateway MSC  50  then sends a request (TRANUM REQ) for a call forwarding number to the ANSI-41 HLR  54 . If the call forwarding number is available, the call forwarding number is sent from the ANSI-41 HLR  54  to the ANSI-41 originating MSC  50 . This call forwarding number may be determined based on, for example, information stored in the HLR  54  as well as the information identifying the cause of the inability to terminate the call. 
     The ANSI-41 originating MSC  50  then responds to the IWU  14  with a “redirection request” response (red req) message. Upon receipt of the “redirection request” response message, the IWU  14  sends a “resume call forwarding” acknowledgement message to the GSM terminating MSC  18  and at the same time releases the connection between the ANSI-41 originating MSC  50  and GSM terminating MSC and  18 . Thereafter, if the call forwarding number is available, the ANSI-41 originating MSC  50  can then attempt to establish a connection using the call forwarding number as routing information to the MS  70 . 
     Thus, the present invention provides a system, apparatus and method by which call forwarding may be performed when a mobile subscriber roams into a foreign network. In this way, calls that are unable to be terminated at the roaming mobile subscriber may be redirected to another number, such as another telephone number, a voice mail number, or the like, where the calling party may have an opportunity to contact the called party, leave a message for the called party, or speak with someone who may be able to provide information to the calling party. The present invention has a primary difference between the prior art of performing late call forwarding from the originating MSC rather than the terminating MSC. This allows late call forwarding to be performed in accordance with the expectations of the ANSI-41 network architecture even when a mobile subscriber roams into a GSM network. 
     It is important to note that while the present invention has been described in the context of a fully functioning data processing system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media such a floppy disc, a hard disk drive, a RAM, and CD-ROMs and transmission-type media such as digital and analog communications links. 
     The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.