Abstract:
A method in a 3G.IP network of routing a call from an originating subscriber in an originating network to a mobile terminating subscriber roaming in a visited network, the mobile terminating subscriber having a home network that maintains location information for the mobile terminating subscriber. When the originating subscriber sends a call origination message to a gatekeeper in the originating network, the gatekeeper obtains location information from the home network for the mobile terminating subscriber. An IP address of a media gateway in the visited network (MGW1) is then obtained and provided to a media gateway in the originating network (MGW2). The IP address of MGW2 is then sent to the originating subscriber. Thereafter, the call is routed from the originating subscriber through MGW2 in the originating network directly to MGW1 in the visited network. An MSC Server in the visited network then routes the call to the mobile terminating subscriber.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     This invention relates to telecommunication systems and, more particularly, to a method of routing calls in third generation Internet Protocol (3G.IP) networks. 
     2. Description of Related Art 
     In second generation. (2G) circuit-switched networks today, calls to a mobile subscriber roaming in a visited network are forced by the characteristics of 2G networks to follow a voice transmission path which is not efficient. Calls to the mobile subscriber must be delivered first to the subscriber&#39;s home network, and then to the visited network, regardless of where the subscriber is roaming. This procedure can sometimes lead to very inefficient call routing. 
     For example, if a subscriber from Montreal roams to Dallas, and someone in Dallas calls him utilizing his Montreal telephone number, the call is routed to the subscriber&#39;s home network in Montreal and then back to the subscriber in Dallas. This occurs because number analysis in the caller&#39;s switch in Dallas recognizes the 514 prefix for Montreal, and directs the switch to deliver the call as a Public Switched Telephone Network (PSTN) call towards Montreal. The call is received in Montreal by a Gateway Mobile Switching Center (G-MSC) that interrogates the Montreal Home Location Register (HLR) and gets back a routing number such as a Temporary Location Directory Number (TLDN) allocated by the Dallas MSC currently serving the roamer. Finally, the G-MSC routes the call back to Dallas based on the TLDN analysis. The result of this routing is that instead of being routed as a local Dallas call, the call involves two long-distance trunks between Dallas and Montreal. 
     As noted above, there are characteristics (or constraints) of the 2G networks which force the call to follow an inefficient voice transmission path. The first constraint is related to the circuit-switched transport of the voice information and signaling. A voice trunk is seized between the caller and the called party, and remains dedicated to carrying the voice information for the duration of the call. The call control signaling, even though it does not follow the same physical path as the voice, is also bound to the configuration of the circuit-switched network. For example, even though the Integrated Services User Part/Signaling System 7 (ISUP/SS7) signaling is delivered over the SS7 signaling infrastructure rather than the voice trunk, the signaling still has to reach the same switch where the voice trunk terminates. For example, if a call between Subscriber-A and Subscriber-B must cross a transit switch because of a particular circuit-switched configuration, then the ISUP/SS7 signaling must also cross the same transit switch. 
     The second constraint of 2G networks that causes inefficient routing is the North American number allocation process which does not make a distinction between landline and wireless subscribers. Because of this, in order to keep the switch routing tables throughout the network at a manageable size, the distinction between a landline subscriber and a wireless subscriber is made only in the home network of each particular subscriber. The G-MSC function discriminates a wireless subscriber number from a landline number and performs the HLR interrogation. Therefore the G-MSC functionality must always be performed in the home network. 
     This routing procedure has also been proposed for third generation all IP (3G.IP) networks. The introduction of 3G.IP networks will make one of the constraints disappear: the new 3G networks will replace the circuit-switched infrastructure with packet switched technology providing multiple connectionless transmission paths between the origin and the destination points. The subscriber number allocation constraint, however, is maintained. So even in 3G.IP networks, the subscriber&#39;s home network will be the only one that knows where the subscriber is roaming and what services are activated for the subscriber. 
     In order to overcome the disadvantage of existing solutions, it would be advantageous to have a method of efficiently routing calls in 3G.IP networks which, while continuing to perform the location interrogation by a Gateway functionality located in the home network, chooses an optimal path for the payload transport. The present invention provides such a method. 
     SUMMARY OF THE INVENTION 
     The present invention is an improved call routing method applicable to 3G.IP subscribers capable of performing voice and/or video calls, and requesting real-time characteristics such as multimedia sessions. For doing so, the subscriber makes use of the H.323 signaling protocol from the International Telecommunications Union (ITU) or the Session Initiation Protocol (SIP) signaling protocol from the Internet Engineering Task Force (IETF). The call routing method of the present invention enables 3G.IP network infrastructures to choose an optimal path for the payload transport while continuing to perform the location interrogation by a Gateway functionality located in the home network. 
     Thus, the present invention is a method in a 3G.IP network of routing a call from an originating subscriber in an originating network to a mobile terminating subscriber roaming in a visited network, the mobile terminating subscriber having a home network that maintains location information for the mobile terminating subscriber. The method begins when the originating subscriber sends a call origination message to a gatekeeper in the originating network. The gatekeeper then obtains from the home network, location information for the mobile terminating subscriber in the visited network. This is followed by sending an IP address for a node in the visited network to the originating network, and routing the call directly from the originating network to the visited network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and its numerous objects and advantages will become more apparent to those skilled in the art by reference to the following drawings, in conjunction with the accompanying specification, in which: 
     FIGS. 1A and 1B (Prior Art) are two parts of a signaling diagram illustrating the flow of messages when routing a call to a roaming mobile subscriber using the existing procedures; and 
     FIGS. 2A and 2B are two parts of a signaling diagram illustrating the more efficient flow of messages when routing a call to a roaming mobile subscriber using the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The invention is an improved call routing method applicable to 3G.IP subscribers. A 3G.IP subscriber is a user who can perform voice and/or video calls, requesting real-time characteristics such as multimedia sessions. For doing so, the subscriber makes use of the H.323 or SIP signaling protocols. The call routing method of the present invention enables 3G.IP network infrastructures to choose the most optimal path for the payload transport while continuing to perform the location interrogation by a Gateway functionality located in the home network. 
     In 3G.IP networks, the HLR functionality is replaced by an enhanced functionality called a Home Subscriber Server (HSS). In addition, the call control for multimedia calls is no longer performed by an MSC but by a Call State Control Function (CSCF). The CSCF is, in fact, a generic term for either an H.323 Gatekeeper or a SIP server. For purposes of the exemplary embodiment described herein, the detailed description below describes the improved routing method utilizing the H.323 protocol. 
     The improved call routing method is explained using a call scenario in which an H.323 fixed subscriber calls a 3G.IP mobile terminating subscriber. This call scenario involves three networks: (1) an originating network where the H.323 fixed subscriber is located; (2) a home network of the called 3G.IP mobile subscriber; and (3) a visited network where the called 3G.IP mobile subscriber is currently roaming. To illustrate the merits of the invention, the inefficient call routing method inherited from the 2G legacy network is first described in FIGS. 1A and 1B, and then the efficient routing method of the present invention is described in FIGS. 2A and 2B. 
     FIGS. 1A and 1B are two parts of a signaling diagram illustrating the flow of messages when routing a call to a roaming mobile subscriber using the existing procedures. An Originating Network  11  includes a Gatekeeper (GK)  12  and a Location Server—Domain Name Server (DNS)  13 . A Home Network  14  includes a Home Subscriber Server (HSS)  15 , a Home Call State Control Function (CSCF)  16 , a Home Media Gateway Control Function (H-MGCF)  17 , and a Home Media Gateway (MGW1)  18 . A Visited Network  19  includes a Mobile Switching Center (MSC) Server  21 , a Visited Media Gateway (MGW2)  22 , a Location Server DNS  23 , and a Radio Network Server (RNS)  24 . 
     An originating subscriber  25  initiates a call by sending an Admission Request (ARQ) message  26  to the Gatekeeper  12  which returns an Admission Confirm (ACF) message  27  to the originating subscriber. The originating subscriber then sends a Fast Setup message  28  to the Gatekeeper. The Fast Setup message includes an identifier for the destination user (Destination Alias) and a destination media address (H.245 Address). At  29 , the Gatekeeper sends a Query of the Destination Alias to the Location Server  13  in the Originating Network. This Query results in a Response to the Gatekeeper at  31  which indicates the address of the Home CSCF  16 . 
     Since the address of the Home CSCF  16  was returned to the Gatekeeper  12 , the Gatekeeper forwards the call setup to the Home CSCF in a Fast Setup message  32 . At  33 , the Home CSCF queries the HSS  15  to obtain location information for the called (terminating) subscriber  34 . Therefore, the HSS sends a Routing Request (RouteReq) message  35  to the MSC Server  21  in the Visited Network  19 . The MSC Server requests the RNS  24  to page for the terminating subscriber at  36 . The RNS returns a response at  37  indicating whether or not the terminating subscriber is available. If the terminating subscriber is available, the MSC Server sends a RouteReq Return Result message  38 , including a routing number such as a TLDN, to the HSS. At  39 , the HSS sends the TLDN to the Home CSCF. 
     The Home CSCF  16  then sends a Fast Setup message  41 , including the TLDN and the H.245 Address, to the H-MGCF  17 . The H-MGCF responds by sending an Add Connection message  42  to the MGW1  18 . The Add Connection message includes a first Context (C1) associating two media terminations, a first Real-time Transport Protocol Termination (RTP-T1), and a second RTP Termination (RTP-T2). These parameters are fully described in the H.248 standards. MGW1 then returns an Acknowledgment  43  with the IP address that it has selected to use for this particular session. The process then moves to FIG. 1B, where the Home Network  14  then begins to route the call to the Visited Network  19 . 
     At  44 , the H-MGCF  17  sends a call setup message such as an ISUP Initial Address Message (IAM), including the Destination Alias and the TLDN for the mobile terminating subscriber  34 , to the MSC Server  21  in the Visited Network  19 . The MSC Server responds by sending an Add Connection message  45  to the MGW2  22 . The Add Connection message includes a second Context (C2), and a third and fourth RTP Termination (RTP-T3 and RTP-T4). MGW2 then returns an Acknowledgment  46  with the IP address selected for use with this session. The MSC Server then sends an Application Transport Message (APM)  47 , including the IP address of MGW2  22 , to the H-MGCF  17  in the Home Network  14 . The H-MGCF sends a Call Proceeding message  48  to the Gatekeeper  12  with the IP address of MGW1  18 . The H-MGCF also sends a Modify Connection message  49  to MGW1 with C1 and the IP address of MGW2. At  51 , MGW1 acknowledges the Modify Connection message. At  52 , the Gatekeeper sends a Call Proceeding message to the originating subscriber  25  with the IP address of MGW1. 
     This process results in the dissemination of IP addresses such that a call can be established from the originating subscriber  25  to the Home Network  14 , and from there to the Visited Network  19 . Thus, an H.245 Establishment message  53  is sent from the originating subscriber to MGW1  18  in the Home Network, and an Establishment Acknowledgment message  54  is returned to the originating subscriber. At  55 , MGW1 sends an H.245 Establishment message to MGW2  22  in the Visited Network, and an Establishment Acknowledgment message  56  is returned to MGW1. At this point, MGW2 sends a Notify message  57  to the MSC Server  21  indicating that a bearer has been established. 
     The MSC Server  21  then sends a ring order  58  to the mobile terminating subscriber  34 , and sends an ISUP Address Complete (ACM) message  59  to the H-MGCF  17  in the Home Network  14 . The H-MGCF sends an Alert signal  61  to the Gatekeeper  12  which forwards the Alert signal at  62  to the originating subscriber  25 . When the mobile terminating subscriber answers the ring order, an Answer signal  63  is sent to the MSC Server. The MSC Server sends an ISUP Answer Message (ANM)  64  to the H-MGCF. The H-MGCF then sends a Connected signal  65  to the Gatekeeper which forwards the Connected signal to the originating subscriber at  66 . 
     Thus, in FIGS. 1A and 1B, it can be seen that the gateway function is performed by the H-CSCF  16 , and the process results in inefficient call routing. By way of example, if the originating subscriber was located in Vancouver, while the terminating subscriber&#39;s home network was in Montreal, and the terminating subscriber was roaming in Seattle, the payload would be carried from the originating&#39;subscriber&#39;s fixed H.323 terminal in Vancouver to MGW1 in Montreal, and finally through MGW2 in Seattle to the mobile terminating subscriber. So instead of sending the voice directly from Vancouver to Seattle, it would go from Vancouver to Montreal, and then to Seattle. 
     FIGS. 2A and 2B are two parts of a signaling diagram illustrating the more efficient flow of messages when routing a call to a roaming mobile subscriber using the method of the present invention. Once the location of the roaming subscriber is known, the present invention returns the IP address of the node where he is roaming, and then routes the call directly. An Originating Network  71  includes a Gatekeeper (GK)  72 , a Location Server DNS  73 , an Originating Media Gateway Control Function (O-MGCF)  74 , and an Originating Media Gateway (MGW2)  75 . A Home Network  76  includes a Home Subscriber Server (HSS)  77  and a Home Call State Control Function (CSCF)  78 . A Visited Network  79  includes a Visited Media Gateway (MGW1)  81 , a Mobile Switching Center (MSC) Server  82 , a Location Server DNS  83 , and a Radio Network Server (RNS)  84 . 
     An originating subscriber  85  sends an ARQ message  86  to the Gatekeeper  72  which returns an ACF message  87  to the originating subscriber. The originating subscriber then sends a Fast Setup message  88 , including the Destination Alias and the H.245 Address, to the Gatekeeper. At  89 , the Gatekeeper sends a Query of the Destination Alias to the Location Server  73  in the Originating Network. This Query results in a Response to the Gatekeeper at  91  which indicates the address of the Home CSCF  78 . 
     The Gatekeeper  72  then sends a Location Request (LRQ) message  92  to the Home CSCF  78  rather than sending a Fast Setup as was previously known. In the present invention, the Gatekeeper establishes a Fast Setup only when the destination is in its own domain. Otherwise, the Gatekeeper sends an LRQ message. At  93 , the Home CSCF queries the HSS  77  to obtain location information for the called (terminating) subscriber  94 . Therefore, the HSS sends a Routing Request (RouteReq) message  95  to the MSC Server  82  in the Visited Network  79 . The MSC Server requests the RNS  84  to page for the mobile terminating subscriber at  96 . The RNS returns a response at  97  indicating whether or not the mobile terminating subscriber is available. If the terminating subscriber is available, the MSC Server sends a routing number such as a TLDN to the HSS at  98 . At  99 , the HSS sends the TLDN to the Home CSCF. 
     The Home CSCF  78  then sends the TLDN to the Originating Network Gatekeeper  72  in a Location Confirm (LCF) message  101 . The Gatekeeper then sends the TLDN to the Location Server DNS  73  in a Routing message  102 . In response, the Location Server DNS returns the address of the O-MGCF  74 . The Gatekeeper then sends a Fast Setup message  104  to the O-MGCF with an H.245 Address. The O-MGCF responds by sending an Add Connection message  105  to MGW2  75  and includes the H.248 parameters C1, RTP-T1, and RTP-T2. MGW2 then returns an Acknowledgment  106  with the IP address selected for use with this session. The process then moves to FIG. 2B, where the Originating Network  71  then begins to route the call directly to the Visited Network  79 . 
     At  107 , the O-MGCF  74  in the Originating Network  71  sends a call setup message such as an ISUP IAM message to the MSC Server  82  in the Visited Network  79 . The IAM message includes the Destination Alias and the TLDN for the mobile terminating subscriber  94 . The MSC Server responds by sending an Add Connection message  108  to MGW1  81 , and includes the C2, RTP-T3, and RTP-T4 parameters. MGW1 then returns an Acknowledgment  109  with the IP address selected for use with this session. The MSC Server then sends an Application Transport Message (APM)  111  to the O-MGCF in the Originating Network  71  and includes the IP address of MGW1  81 . The O-MGCF sends a Call Proceeding message  112  to the Gatekeeper  72  with the IP address of MGW2  75 . The O-MGCF also sends a Modify Connection message  113 , including C1 and the IP address of MGW1, to MGW2. At  114 , MGW2 acknowledges the Modify Connection message. At  115 , the Gatekeeper sends a Call Proceeding message, including the IP address of MGW2, to the originating subscriber  85 . 
     This process results in the dissemination of IP addresses such that a call can be established from the originating subscriber  85  to the Originating Network  71 , and from there directly to the Visited Network  79 . Thus, an H.245 Establishment message  116  is sent from the originating subscriber to MGW2  75  in the Originating Network, and an Establishment Acknowledgment message  117  is returned to the originating subscriber. At  118 , MGW2 sends an H.245 Establishment message to MGW1  81  in the Visited Network, and an Establishment Acknowledgment message  119  is returned to MGW2. At this point, MGW1 sends a Notify message  121  to the MSC Server  82  indicating that a bearer has been established. 
     The MSC Server  82  then sends a ring order  122  to the mobile terminating subscriber  94 , and sends an ISUP ACM message  123  to the O-MGCF  74  in the Originating Network  71 . The O-MGCF sends an Alert signal  124  to the Gatekeeper  72  which forwards the Alert signal to the originating subscriber at  125 . When the mobile terminating subscriber answers the ring order, an Answer signal  126  is sent to the MSC Server in the Visited Network  79 . The MSC Server sends an ISUP ANM message  127  to the O-MGCF. The O-MGCF then sends a Connected signal  128  to the Gatekeeper which forwards the Connected signal to the originating subscriber at  129 . 
     Thus, in the case where the originating subscriber was located in Vancouver, the terminating subscriber&#39;s home network was in Montreal, and the terminating subscriber was roaming in Seattle, FIGS. 2A and 2B illustrate a methodology in which the gateway functionality remains in the home network in Montreal, but the payload follows an optimized path. This path originates in the originating subscriber&#39;s H.323 fixed terminal, crosses MGW2 in Vancouver to reach MGW1 in Seattle, and finally to the mobile terminating subscriber&#39;s 3G.IP terminal. Therefore the voice information goes directly from Vancouver to Seattle. 
     It is thus believed that the operation and construction of the present invention will be apparent from the foregoing description. While the method shown and described has been characterized as being preferred, it will be readily apparent that various changes and modifications could be made therein without departing from the scope of the invention as defined in the following claims.