PATENT DOCUMENT

Publication Number: US-8687587-B2
Application Number: US-30446007-A
Country: US
Kind Code: B2

Title: Inter-subsystem transfers

Abstract:
In general, the present invention provides for a direct inter-subsystem transfer of an active communication session, such as a call, between a packet subsystem (PS) and a circuit-switched subsystem (CS) in an efficient and effective manner while maintaining service control and continuity. Further, the inter-subsystem transfer may take place between a PS of one generation and a CS of another generation.

Claims:
What is claimed is: 
     
       1. A method, comprising:
 anchoring, at a first function in a multimedia subsystem, session signaling for a communication session established between a user element and a remote endpoint; 
 providing, from the multimedia subsystem, centralized service control for the communication session, wherein the centralized service control is provided when the user element is served by a circuit-switched subsystem and when the user element is served by a packet subsystem; 
 during the communication session, effecting a radio layer handover of the user element from a transferring-out subsystem to a transferring-in subsystem, wherein the transferring-out subsystem is the packet subsystem, wherein the transferring-in subsystem is the circuit-switched subsystem, and wherein the centralized service control is maintained across the radio layer handover; and 
 wherein the radio layer handover includes representing a handover function as a target mobility management entity (MME) to the transferring-out subsystem and the handover function as a source mobile switching center (MSC) to the transferring-in subsystem. 
 
     
     
       2. The method of  claim 1  wherein the first function is a domain transfer function in the multimedia subsystem, wherein a session signaling path extends to the user element through a control channel in the circuit switched subsystem when the user element is served by the circuit-switched subsystem, and extends to the user element through the packet subsystem when the user element is served by the packet subsystem. 
     
     
       3. The method of  claim 2  wherein:
 a stateless remote user agent function is provided in the session signaling path in the circuit-switched subsystem and a stateful remote user agent function is provided in the session signaling path at the first function; 
 the stateless remote user agent and the stateful remote user agent represent the user element to the multimedia subsystem when the user element is served by the circuit-switched subsystem; and 
 the stateful remote user agent maintains service state information in the session signaling for the communication session across the radio layer handover. 
 
     
     
       4. The method of  claim 2  wherein a stateful remote user agent function is provided in the session signaling path at the first function and maintains service state information in the session signaling for the communication session across the radio layer handover. 
     
     
       5. The method of  claim 4  wherein the stateful remote user agent is the only remote user agent provided to represent the user element to the multimedia subsystem. 
     
     
       6. The method of  claim 1  wherein the first function is a circuit-switched control function in the multimedia subsystem. 
     
     
       7. The method of  claim 1  wherein the radio layer hand over of the user element from the transferring-out subsystem to the transferring-in subsystem is effected before a bearer path in the transferring-in subsystem is established to a target access cell in the transferring-in subsystem. 
     
     
       8. The method of  claim 1  further comprising delivering downlink traffic toward the user element over a first bearer path in the transferring-out subsystem and over a second bearer path in the transferring-in subsystem toward the user element in association with the radio layer handover, wherein the radio layer handover of the user element from the transferring-out subsystem to the transferring-in subsystem is effected after the second bearer path in the transferring-in subsystem is established to a target access cell in the transferring-in subsystem. 
     
     
       9. The method of  claim 8  wherein delivery of the downlink traffic toward the user element over the first bearer path is stopped upon receiving uplink traffic from the user element over the second bearer path. 
     
     
       10. The method of  claim 1  further comprising providing session updates toward the remote endpoint in association with the radio layer handover, wherein the session updates present communication information bearing on at least one of a group consisting of an address to which to direct bearer traffic and communication information associated with delivering the bearer traffic. 
     
     
       11. The method of  claim 1  wherein effecting the radio layer handover comprises providing control of the radio layer hand over from the multimedia subsystem. 
     
     
       12. The method of  claim 1  wherein the session signaling is anchored at a first function in the multimedia subsystem and a session signaling path extends to the user element in the circuit-switched subsystem when the user element is served by the circuit-switched subsystem, wherein the handover function is provided in the session signaling path. 
     
     
       13. The method of  claim 12  wherein a remote user agent representing the user element to the multimedia subsystem is provided in the session signaling path when the user element is served by the circuit-switched subsystem. 
     
     
       14. The method of  claim 12  wherein the session signaling path extends through a gateway interworking the multimedia subsystem with the packet subsystem, and further comprising providing a context update by the handover function to the gateway to inform the gateway of a transition of the communication session from the transferring-out subsystem to the transferring-in subsystem. 
     
     
       15. The method of  claim 1  further comprising:
 queuing session downlink signaling associated with a bearer path being established in the circuit-switched subsystem at the hand over function until the user element transitions to the transferring-in subsystem. 
 
     
     
       16. The method of  claim 1  wherein the radio layer hand over transfers wireless communication access from a source cell in the transferring-out subsystem to a target cell in the transferring-in subsystem, wherein the source cell supports wireless communications using one generation of wireless access technology and the target cell supports wireless communications using another generation of the wireless access technology. 
     
     
       17. The method of  claim 1  wherein the packet subsystem supports Fourth Generation (4G) wireless access technology and the circuit-switched subsystem supports one of First, Second, and Third Generation (1X, 2G, 3G) wireless access technologies. 
     
     
       18. A system, comprising:
 at least one communication interface; and 
 a control system associated with the at least one communication interface; 
 a handover node; 
 wherein the control system is configured to:
 anchor, at a first function in a multimedia subsystem, session signaling for a communication session established between a user element and a remote endpoint; 
 provide, from the multimedia subsystem, centralized service control for the communication session, wherein the centralized service control is provided when the user element is served by a circuit-switched subsystem and when the user element is served by a packet subsystem, wherein during the communication session, the centralized service control is maintained across a the radio layer handover of the user element from a transferring-out subsystem to a transferring-in subsystem, wherein the transferring-out subsystem is the packet subsystem, wherein the transferring-in subsystem is the circuit-switched subsystem; 
 
 wherein the handover node is configured to effect the radio layer handover including:
 representing a target mobility management entity (MME) to the transferring-out subsystem; and 
 representing a source mobile switching center (MSC) to the transferring-in subsystem. 
 
 
     
     
       19. The system of  claim 18  wherein the first function is a domain transfer function, wherein a session signaling path extends to the user element through a control channel in the circuit-switched subsystem when the user element is served by the circuit-switched subsystem and extends to the user element through the packet subsystem when the user element is served by the packet subsystem. 
     
     
       20. The system of  claim 19  wherein:
 a stateless remote user agent function is provided in the session signaling path in the circuit-switched subsystem and a stateful remote user agent function is provided in the session signaling path by the first function; 
 the stateless remote user agent and the stateful remote user agent represent the user element to the multimedia subsystem when the user element is served by the circuit-switched subsystem; and 
 the stateful remote user agent maintains service state information in the session signaling for the communication session across the radio layer handover. 
 
     
     
       21. The system of  claim 19  wherein a stateful remote user agent function is provided in the session signaling path by the first function and maintains service state information in the session signaling for the communication session across the radio layer hand
 over. 
 
     
     
       22. The system of  claim 18  wherein the first function is a circuit-switched control function in the multimedia subsystem. 
     
     
       23. The system of  claim 18  wherein the radio layer hand over of the user element from the transferring-out subsystem to the transferring-in subsystem is effected before a bearer path in the transferring-in subsystem is established to a target access cell in the transferring-in subsystem. 
     
     
       24. The system of  claim 18  further comprising a bi-cast node adapted to deliver downlink traffic toward the user element over a first bearer path in the transferring-out subsystem and over a second bearer path in the transferring-in subsystem toward the user element in association with the radio layer hand over, wherein the radio layer hand over of the user element from the transferring-out subsystem to the transferring-in subsystem is effected after the second bearer path in the transferring-in subsystem is established to a target access cell in the transferring-in subsystem. 
     
     
       25. The system of  claim 24  wherein delivery of the downlink traffic toward the user element over the first bearer path is stopped upon receiving uplink traffic from the user element over the second bearer path. 
     
     
       26. The system of  claim 18  further comprising a domain transfer function adapted to provide session updates toward the remote endpoint in association with the radio layer handover, wherein the session updates present communication information bearing on at least one of a group consisting of an address to which to direct bearer traffic and communication information associated with delivering the bearer traffic. 
     
     
       27. The system of  claim 18  wherein effecting the radio layer hand over comprises providing control of the radio layer hand over from the multimedia subsystem. 
     
     
       28. The system of  claim 18  wherein the session signaling is anchored at a first function in the multimedia subsystem and a session signaling path extends to the user element in the circuit-switched subsystem when the user element is served by the circuit-switched subsystem, wherein the handover function is provided in the session signaling path. 
     
     
       29. The system of  claim 28  wherein a remote user agent representing the user element to the multimedia subsystem is provided in the session signaling path when the user element is served by the circuit-switched subsystem. 
     
     
       30. The system of  claim 18  wherein the handover node is configured to queue session downlink signaling associated with a bearer path being established in the circuit-switched subsystem at the handover function until the user element transitions to the transferring-in subsystem. 
     
     
       31. The system of  claim 18  wherein the session signaling path extends through a gateway interworking the multimedia subsystem with the packet subsystem and the hand over node is further configured to provide a context update to the gateway to inform the gateway of a transition of the communication session from the transferring-out subsystem to the transferring-in subsystem. 
     
     
       32. The system of  claim 18  wherein the radio layer handover transfers wireless communication access from a source cell in the transferring-out subsystem to a target cell in the transferring-in subsystem, wherein the source cell supports wireless communications using one generation of wireless access technology and the target cell supports wireless communications using another generation of the wireless access technology. 
     
     
       33. The system of  claim 18  wherein the packet subsystem supports Fourth Generation (4G) wireless access technology and the circuit-switched subsystem supports one of First, Second, and Third Generation (1X, 2G, 3G) wireless access technologies. 
     
     
       34. The method of  claim 1 , wherein the representing includes:
 the handover function receiving a relocation request from a source MME; and 
 the handover function sending a prepare handover request to a target MSC. 
 
     
     
       35. The method of  claim 34 , wherein the source MME perceives the radio layer handover as a packet-subsystem-to-packet-subsystem handover. 
     
     
       36. A method, comprising:
 a user element communicating with a remote endpoint via a communication session, wherein session signaling for the communication session is anchored at a first function in a multimedia subsystem; 
 the user element participating in a handover between a circuit-switched subsystem and a packet subsystem, wherein centralized service control for the communication session is provided from the multimedia subsystem when the user element is served by the circuit-switched subsystem and when the user element is served by the packet subsystem; and 
 wherein the handover includes a transfer function being represented as a mobility management entity (MME) to the packet subsystem and the function being represented as a mobile switching center (MSC) to the circuit-switched subsystem.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to concurrently filed U.S. application Ser. No. 12/304,458, entitled METHOD FOR TRANSITIONING SUPPORT OF COMMUNICATION SESSIONS FOR A USER ELEMENT BETWEEN DIFFERENT TYPES OF SUBSYSTEMS OF DIFFERENT GENERATIONS. 
     This application is a 35 U.S.C. National Phase application based on PCT/IB2007/001549, which claims the benefit of U.S. provisional patent application Ser. No. 60/813,492 filed Jun. 14, 2006, U.S. provisional patent application Ser. No. 60/878,965 filed Jan. 5, 2007, U.S. provisional patent application Ser. No. 60/888,676 filed Feb. 7, 2007, and U.S. provisional patent application Ser. No. 60/893,253 filed Mar. 6, 2007, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to cellular communications, and in particular to facilitating transitions between access networks of different generations and residing in different types of subsystems. 
     BACKGROUND OF THE INVENTION 
     Wireless communications technology is rapidly evolving to address the ever-increasing need for additional bandwidth and services. With each generation of wireless communication standards, the available bandwidth and services that are made available to subscribers have dramatically increased. Unfortunately, each generation of wireless communication standards generally requires additional network infrastructure and compatible user elements. In many instances, the network infrastructure of a new generation does not support that of an earlier generation. Further, different networks and generations thereof handle voice and data in different ways. For example, second generation (2G) networks rely heavily on circuit-switched communications for voice and data, while many third generation (3G) networks provide circuit-switched subsystems as well as packet-based subsystems for voice and data, respectively. Upcoming fourth generation (4G) networks may use packet-based subsystems for voice and data with little or no reliance on a circuit-switched subsystem. 
     In many environments, different types of subsystems and different generations of networks are available to a user element. Many user elements are able to support services on these different subsystems and different generations of these subsystems. However, transitioning from a packet subsystem of one generation to a circuit-switched subsystem of another generation, and vice versa, has proven to be cumbersome. Accordingly, there is a need for technique to efficiently and effectively transition support of communication sessions for a user element between different types of subsystems of different generations in an effective and efficient manner. 
     SUMMARY OF THE INVENTION 
     In general, the present invention provides for a direct inter-subsystem transfer of an active communication session, such as a call, between a packet subsystem (PS) and a circuit-switched subsystem (CS) in an efficient and effective manner while maintaining service control and continuity. Further, the inter-subsystem transfer may take place between a PS of one generation and a CS of another generation. A user element is able to support communications via the PS and CS through PS access and CS access networks, respectively. Application layer service control for a communication session is anchored in a multimedia subsystem (MS), such as an Internet Protocol MS (IMS), regardless of whether the user element is being served by the PS or CS. When the user element transitions between the PS and CS, the state of the communication session is maintained in the MS across the inter-subsystem transfer. For an inter-subsystem transfer between the PS and CS, a radio layer handover supports the transition of radio access for the user element from one subsystem to another. To maintain service control across the transfer, the MS provides an application layer transfer to maintain a session signaling path for session signaling between the user element and a remote endpoint. The state of the communication session before the transfer is maintained after the transfer by the MS. 
     Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention. 
         FIG. 1  illustrates a communication environment according to a first embodiment, where a user element is supported through PS access. 
         FIG. 2  illustrates a communication environment according to the first embodiment, where the user element is supported through CS access. 
         FIG. 3  illustrates handover control signaling paths for the first embodiment of the present invention. 
         FIGS. 4A-4C  provide a communication flow for an inter-subsystem transfer from a PS access cell to a CS access cell according to the first embodiment of the present invention. 
         FIG. 5  illustrates a communication environment according to a second embodiment, where a user element is supported through PS access. 
         FIG. 6  illustrates a communication environment according to the second embodiment, where the user element is supported through CS access. 
         FIGS. 7A-7D  provide a communication flow for an inter-subsystem transfer from a PS access cell to a CS access cell according to the second embodiment of the present invention. 
         FIGS. 8A-8C  are a communication flow for an inter-subsystem transfer from the CS access cell to the PS access cell according to the first embodiment of the present invention. 
         FIG. 9  illustrates a communication environment according to a third embodiment, where a user element is supported through PS access. 
         FIG. 10  illustrates a communication environment according to the third embodiment, where the user element is supported through CS access 
         FIG. 11  illustrates handover control signaling paths for the third embodiment of the present invention. 
         FIGS. 12A-12D  provide a communication flow for an inter-subsystem transfer from a PS access cell to a CS access cell according to the third embodiment of the present invention. 
         FIG. 13  is a block representation of a service node according to one embodiment of the present invention. 
         FIG. 14  is a block representation of a user element according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
     In general, the present invention provides for a direct inter-subsystem transfer of an active communication session, such as a call, between a packet subsystem (PS) and a circuit-switched subsystem (CS) in an efficient and effective manner while maintaining service control and continuity. Further, the inter-subsystem transfer may take place between a PS of one generation and a CS of another generation. A user element is able to support communications via the PS and CS through PS access and CS access networks, respectively. Application layer service control for a communication session is anchored in a multimedia subsystem (MS), such as an Internet Protocol MS (IMS), regardless of whether the user element is being served by the PS or CS. When the user element transitions between the PS and CS, the state of the communication session is maintained in the MS across the inter-subsystem transfer. For an inter-subsystem transfer between the PS and CS, a radio layer handover supports the transition of radio access for the user element from one subsystem to another. To maintain service control across the transfer, the MS provides an application layer transfer to maintain a session signaling path for session signaling between the user element and a remote endpoint. The state of the communication session before the transfer is maintained after the transfer by the MS. Prior to delving into the details of the invention, an overview of the different bearer and session signaling paths for a user element being served by a PS and a CS, respectively, are provided according to one embodiment of the present invention. 
     With particular reference to  FIG. 1 , an exemplary communication environment  10  is provided wherein a user element  12  is served by PS and CS domains  14 . As illustrated, the user element  12  is engaged in a communication session with a remote endpoint  16 , and at least part of the session control for the communication session is provided by an IMS  18 . In this example, the PS and CS domains  14  are the currently serving, or visited, PS and CS domains, while the IMS  18  is the home IMS for the user element  12 . As depicted, the communication session in  FIG. 1  is supported in the PS, and as such, the PS bearer path between the user element  12  and the remote endpoint  16  extends through a PS access cell  20  and a System Architecture Evolution (SAE) or like gateway  22 . In this example, the PS access cell  20  is a fourth generation (4G) PS access cell that provides packet-based communications with the user element  12  via a given radio access link. Session signaling is facilitated using SIP, wherein a session signaling path extends from the user element  12  through the PS access cell  20  and the SAE gateway  22  into the home IMS  18 . From the SAE gateway  22 , the signaling path extends to a call/session control function (CSCF)  24 , to an IMS CS control function (ICCF)  26 , back to the CSCF  24 , to a domain transfer function (DTF)  28 , back to the CSCF  24 , and on toward the remote endpoint  16 . 
     With reference to  FIG. 2 , the communication session between the user element  12  and the remote endpoint  16  is supported by the CS. As such, the bearer path between the user element  12  and the remote endpoint  16  extends through a CS access cell  30 , a mobile switching center (MSC)  32  that supports the CS access cell  30 , and a media gateway (MGW)  34 . The CS access cell  30  is illustrated as being any one of a first, second, or third generation (1x, 2G, 3G) CS access cell that is provided in a first, second, or third generation access network. A CS bearer path extends between the user element  12  and the media gateway  34 , while a PS bearer path extends between the media gateway  34  and the remote endpoint  16 . The session signaling path has multiple segments. A first segment extends between the user element  12  and a CS Access Adaptation Function (CAAF)/remote user agent (RUA), which is referred to as a CMF/RUA  36 , through the CS access cell  30 , the MSC  32 , and a Mobility Management entity-Circuit Switched (MME-CS)  38 . This segment is referred to as an IMS CS control channel (ICCC), which may ride on an underlying CS signaling channel that extends along the same path. The second segment of the session signaling path extends between the CMF/RUA  36  to the SAE gateway  22 , while a third segment extends from the SAE gateway  22  toward the remote endpoint  16  through the CSCF  24 , ICCF  26 , and DTF  28 . Notably, the third segment of the session signaling path between the SAE gateway  22  and the remote endpoint  16  is the same as that illustrated in  FIG. 1 , wherein the communication session was supported in the PS. 
     For an inter-subsystem transfer from the PS to the CS, the bearer and session signaling paths illustrated in  FIG. 1  will transition to the bearer and session signaling paths of  FIG. 2 . Similarly, for an inter-subsystem transfer from the CS to the PS, the bearer and session signaling paths of  FIG. 2  will transition to the bearer and session signaling paths of  FIG. 1 . For an inter-subsystem transfer in either direction, a radio layer handover between the PS and CS access cells  20 ,  30  is initially provided. 
     In one embodiment, the present invention employs the MME-CS  38  to facilitate a radio layer handover between the PS and CS access cells  20 ,  30  of the respective subsystems. The MME-CS  38  is provided between a standard MME  40  associated with the PS access cell  20  of the PS and the MSC  32 , which supports the CS access cell  30  of the CS. For a radio layer handover from the PS to the CS ( FIG. 1  to  FIG. 2 ), the MME-CS  38  acts as a target MME, wherein a radio layer handover is provided from the standard MME  40  toward the MME-CS  38 . The MME-CS  38  acts as a source MSC to relay the radio layer handover toward the MSC  32  for the given CS access cell  30 . As a result, the radio access is transitioned from the PS access cell  20  of the PS to the CS access cell  30  of the CS. For a radio layer handover from the CS to the PS ( FIG. 2  to  FIG. 1 ), the MME-CS  38  acts as a target MSC, wherein a radio layer handover is provided from the MSC  32  toward the MME-CS  38 . The MME-CS  38  acts as a source MME to relay the radio layer handover toward the standard MME  40  for the given PS access cell  20 . The handover (HO) control signaling paths are illustrated in  FIG. 3 . Notably, the MME  40  may need to work with the SAE gateway  22  in conjunction with the PS access cell  20  to facilitate inter-subsystem transfers into and out of the PS. 
     The MME-CS  38  is also associated with the CAAF/RUA  36 . When the user element  12  is in the CS, the CAAF/RUA  36  acts a user agent toward the IMS  18  for service control on behalf of the user element  12 . As such, the CAAF/RUA  36  acts a liaison between the user element  12  and the IMS  18  when the user element  12  is served by the CS. The CAAF/RUA  36  and the user element  12  communicate over the ICCC, which extends through the MME-CS  38 , MSC  32 , and the CS access cell  30 . The CMF/RUA  36  may communicate with the IMS  18  using a session control protocol, such as the Session Initiation Protocol (SIP), directly or via the SAE gateway  22 . The CAAF/RUA  36  provides interworking between signaling over the ICCC and a SIP signaling path that extends into the IMS  18 . 
     Session signaling for a communication session may be anchored in the IMS  18  at the DTF  28  and the ICCF  26 . The DTF  28  and ICCF  26  may be provided by the same or different entities. The DTF  28  facilitates the application layer transfers such that service control moves from one subsystem to another while the ICCF  26  maintains the service state for a communication session across inter-subsystem transfers. The ICCF  26  may also act as another RUA on behalf of the user element  12 , and may cooperate with the CAAF/RUA  36  to maintain service control and service state, when the user element  12  is served by the CS. As noted above, session signaling from the PS and CS is routed through the CSCF  24 , which functions to place the ICCF  26  and the DTF  28  in the session signaling path. 
     An exemplary PS-to-CS inter-subsystem transfer is illustrated with the communication flow provided in  FIGS. 4A through 4C , according to one embodiment of the present invention. Initially, assume that a PS bearer path is established between the user element  12  and the remote endpoint  16  through the PS access cell  20  and the SAE gateway  22  (step  100 ). Further assume that a session signaling path is established between the user element  12  and the remote endpoint  16 , and extends through the PS access cell  20 , SAE gateway  22 , CSCF  24 , ICCF  26 , and DTF  28 , wherein the CSCF  24  invokes both the ICCF  26  and the DTF  28  (step  102 ). At this point, the session signaling path is a SIP signaling path from the user element  12  to the remote endpoint  16 . As such, a communication session is provided between the user element  12  and the remote endpoint  16 , wherein the user element  12  is served by the PS access cell  20  in the PS. 
     At some point, the user element  12  will determine that there is a need to transfer from the currently serving PS access cell  20  to the CS access cell  30 . In this example, the PS access cell  20  is a fourth generation (4G) PS access cell, wherein the CS access cell  30  is a third generation (3G) CS access cell. While the PS access cell  20  is serving the user element  12 , signal strength measurements associated with the serving cell are compared to those associated with adjacent cells in the same or different subsystem (step  104 ). The adjacent cells are potential target cells, and in this example, will include the CS access cell  30 . When conditions dictate a handover from the PS access cell  20  to the CS access cell  30 , the PS access cell  20  will initiate a handover (step  106 ) and send a Handover Required message toward the MME  40 , which is designated as a source MME (S-MME) (step  108 ). The target for the Handover Required message corresponds to the MME-CS  38 , which is designated as a target MME-CS (T-MME-CS). From the perspective of the S-MME  40 , the T-MME-CS  38  appears as a target MME within the PS, and as such, the S-MME  40  will initiate what it believes is a PS-to-PS handover (step  110 ) by sending a Relocation Request toward the T-MME-CS  38  (step  112 ). The T-MME-CS  38  will send a Prepare Handover Request to the MSC  32  that supports the CS access cell  30  (step  114 ). The MSC  32  will act to establish radio resources in the CS access cell  30  by sending a Handover Request to the CS access cell  30  (step  116 ), which will establish resources for the handover (step  118 ) and provide an Acknowledgment (ACK) back to the MSC  32  (step  120 ). 
     Upon receipt of the acknowledgement, the MSC  32  will send a Prepare Handover Response to the T-MME-CS  38  to indicate that the radio resources have been established in the CS access cell  30  (step  122 ). The T-MME-CS  38  will send an Integrated Services User Part (ISUP) Initial Address Message (IAM) to the MSC  32  (step  124 ) to set up a circuit-switched bearer path between the media gateway  34 , which is associated with the T-MME-CS  38 , and the MSC  32 . The MSC  32  will respond to the IAM with an ISUP Address Complete Message (ACM) (step  126 ) to indicate that the CS bearer path is being established. The T-MME-CS  38  will then update the signaling context for the session at the SAE gateway  22  to indicate that the session signaling is being transferred from the user element  12  to the CAAF/RUA  36  (step  128 ). At this point, the portion of the ICCC between the user element  12  and the T-MME-CS  38  is not available. As such, the T-MME-CS  38  will queue the downlink session signaling (step  130 ) until the ICCC can be established to the user element  12 . The T-MME-CS  38  will then initiate preparation of the remote end for the handover (step  132 ) by sending a message to the CAAF/RUA  36  to prepare the remote end for the handover (step  134 ). At this point, the CAAF/RUA  36  may register with the CSCF  24  on behalf of the user element  12  by sending a Register message to the CSCF  24  (step  136 ). The CSCF  24  will respond by providing a 200 OK message (step  138 ). 
     At this point, the T-MME-CS  38  will send a Relocation Response to the S-MME  40  to instruct the user element  12  to effect the radio layer handover (step  140 ). As such, the S-MME  40  will send a Handover Required message to the PS access cell  20  (step  142 ), which will send a Handover Command to the user element  12  (step  144 ). The user element  12  will then retune itself to effectively change from a channel within the PS access cell  20  to a channel in the CS access cell  30  (step  146 ). During this process, the CS access cell  30  will detect the presence of the user element  12  (step  148 ) and send a Handover Detect message toward the MSC  32  to indicate that the user element  12  has switched to the CS access cell  30  (step  150 ). The MSC  32  will send the Handover Detect message to the T-MME-CS  38  (step  152 ), which will send the Handover Detect message to the CMF/RUA  36  to indicate that the user element  12  is now being served by the CS access cell  30  (step  154 ). 
     Acting on behalf of the user element  12  and upon recognizing that the user element  12  has switched to the CS access cell  30 , the CAAF/RUA  36  will initiate a domain transfer procedure to transfer session signaling from the user element  12  to the CAAF/RUA  36  by sending an Invite message to the CSCF  24  (step  156 ). Notably, the CMF/RUA  36  in this embodiment will not have access to the state information for the communication session prior to the inter-subsystem transfer. As such, the Invite will not provide any pre-existing state information for the communication session. However, the Invite may include CS communication information associated with the media gateway  34  to facilitate a transfer of the bearer path that was running through the SAE gateway  22  to the media gateway  34 . 
     The CSCF  24  will forward the Invite to the ICCF  26  (step  158 ), which has access to the state information for the communication information because the session signaling is anchored at the ICCF  26 . As such, the ICCF  26  can update the Invite with any necessary service state information (step  160 ) and forward the Invite back to the CSCF  24  for further processing (step  162 ). The CSCF  24  will then forward the Invite to the DTF  28 , which also provides an anchor for the session signaling (step  164 ). The DTF  28  will initiate a PS-to-CS bearer and session signaling transfer toward the remote endpoint  16  (step  166 ). Accordingly, the DTF  28  will generate a Re-Invite to provide the remote endpoint  16  or entities operating on behalf of the remote endpoint  16  with the necessary information to support a new bearer path that runs through the media gateway  34  instead of through the SAE gateway  22 . Accordingly, the CSCF  24  will receive the Re-Invite (step  168 ) and forward the Re-Invite toward the remote endpoint  16  (step  170 ). To facilitate communications with the media gateway  34 , the Re-Invite will include the necessary CS communication information, which may include the address, port and Session Data Protocol (SDP) information for the media gateway  34 . With this information, the remote endpoint  16  can send bearer traffic to the media gateway  34 . Although not illustrated, the remote endpoint  16  will respond to the Re-Invite with a 200 OK message or the like, which will include the PS communication information necessary to allow the media gateway  34  to send bearer traffic toward the remote endpoint  16 . The 200 OK message is propagated along the session signaling path to the CAAF/RUA  36 , and if necessary, passed on toward the user element  12 . For the domain transfer initiation, the 200 OK message will stop at the CAAF/RUA  36 . 
     In the meantime, the CS access cell  30  may generate a Handover Complete message, which is sent to the MSC  32  to indicate that the handover is fully transitioned to the CS access cell  30  from the perspective of the user element  12  (step  172 ). The MSC  32  will send the Handover Complete message to the T-MME-CS  38  (step  174 ), and will subsequently send an ISUP Answer message to instruct the T-MME-CS  38  to complete the bearer path between the media gateway  34  and the remote endpoint  16 , as well as to the user element  12  via the MSC  32  (step  176 ). As such, the bearer path of the user element  12  as served by the CS extends between the user element  12  and the remote endpoint  16  through the CS access cell  30 , MSC  32 , and media gateway  34  (step  178 ). The session signaling (step  180 ) comprises two primary segments. The first segment is the ICCC, which extends between the CAAF/RUA  36  and the user element  12  through the CS access cell  30 , the MSC  32 , and the T-MME-CS  38 . The second segment is a SIP segment that extends between the CMF/RUA  36  and the remote endpoint  16  through the SAE gateway  22 , CSCF  24 , ICCF  26 , and DTF  28 , wherein the CSCF  24  invokes the ICCF  26  and the DTF  28 . 
     Notably, the above embodiment injects a significant delay in the bearer path as the bearer path is being transitioned from the SAE gateway  22  to the media gateway  34 . In the following embodiment, a media proxy  42  is provided in the IMS  18  and is closely associated with the DTF  28 . As illustrated in  FIG. 5 , when the user element  12  is served by the PS, the bearer path between the user element  12  and the remote endpoint  16  extends through the PS access cell  20 , the SAE gateway  22 , and the media proxy  42 . The session signaling path remains the same as that of the above embodiment, and as such, extends between the user element  12  and the remote endpoint  16  through the PS access cell  20 , SAE gateway  22 , CSCF  24 , ICCF  26 , and DTF  28 , wherein the CSCF  24  invoke the ICCF  26  and the DTF  28 . When the user element  12  is supported in the CS, the bearer path again moves from the SAE gateway  22  to the media gateway  34 ; however, as illustrated in  FIG. 6 , the bearer path remains anchored in the media proxy  42 . As such, the bearer path between the user element  12  and the remote endpoint  16  extends through the CS access cell  30 , the MSC  32 , the media gateway  34 , and the media proxy  42 . The session signaling path between the user element  12  and the remote endpoint  16  remains the same as that in the above embodiment, and as such, the session signaling path between the user element  12  and the remote endpoint  16  extends through the CS access cell  30 , MSC  32 , MME-CS  38 , CAAF/RUA  36 , SAE gateway  22 , CSCF  24 , ICCF  26 , and DTF  28 , wherein the ICCF  26  and the DTF  28  are invoked by the CSCF  24 . 
     For inter-subsystem transfers, the MME  40 , MME-CS  38 , and CAAF/RUA  36  operate as described above to provide the radio layer handover. For the application layer transfer, the DTF  28  controls the media proxy  42  in a manner configured to reduce the length of the interruption of the bearer path as the inter-subsystem transfers occur. In particular, the bearer path in the transferring-out subsystem remains intact as a new bearer path is being established in the transferring-in subsystem. For an inter-subsystem transfer from the PS to the CS, the bearer path to the user element  12  through the PS access cell  20  will remain intact while a new bearer path is being established through the CS access cell  30 , and in fact, will remain intact after the new bearer path through the CS access cell  30  is established. For this embodiment, the DTF  28  will instruct the media proxy  42  to send downlink traffic toward the user element  12  over the bearer path through the PS access cell  20 , as well as over the bearer path through the CS access cell  30 . As such, downlink traffic is immediately available in the bearer path provided through the CS access cell  30  when the user element  12  completes its radio layer handover from the PS access cell  20  to the CS access cell  30 . 
     With the previous embodiment, the user element  12  transitions to the CS access cell  30  well before the bearer path is transitioned from the SAE gateway  22  to the media gateway  34 . As such, downlink traffic is not available to the media gateway  34  for delivery to the user element  12  until the remote endpoint  16  is instructed to deliver downlink traffic to the media gateway  34  instead of to the SAE gateway  22 . With the current embodiment, the remote endpoint  16  will always send downlink traffic to the media proxy  42 , which will provide the downlink traffic over the old and new bearer paths to the user element  12 , in order to reduce any interruption in the bearer path during an inter-subsystem transfer. Further, the user element  12  may deliver uplink traffic over the new bearer path established in the transferring-in subsystem toward the remote endpoint  16  through the media proxy  42 . When the uplink traffic is received at the media proxy  42  over the bearer path through the transferring-in subsystem, the media proxy  42  can stop sending downlink traffic over the old bearer path through the transferring-out subsystem and update the uplink traffic toward the remote endpoint  16  to use the bearer path through the transferring-in subsystem. 
     With reference to  FIGS. 7A through 7D , a communication flow is provided to illustrate a PS-to-CS inter-subsystem transfer according to an embodiment of the present invention that employs the media proxy  42  to anchor the bearer path. Initially, assume that a PS bearer path is established between the user element  12  and the remote endpoint  16  through the PS access cell  20 , the SAE gateway  22 , and the media proxy  42  (step  200 ). Further assume that a session signaling path is established between the user element  12  and the remote endpoint  16 , and extends through the PS access cell  20 , SAE gateway  22 , CSCF  24 , ICCF  26 , and DTF  28 , wherein the CSCF  24  invokes both the ICCF  26  and the DTF  28  (step  202 ). At this point, the session signaling path is a SIP signaling path from the user element  12  to the remote endpoint  16 . As such, a communication session is provided between the user element  12  and the remote endpoint  16 , wherein the user element  12  is served by the PS access cell  20  in the PS. 
     At some point, the user element  12  will determine that there is a need to transfer from the currently serving PS access cell  20  to the CS access cell  30 . In this example, the PS access cell  20  is a fourth generation (4G) PS access cell, wherein the CS access cell  30  is a third generation (3G) CS access cell. While the PS access cell  20  is serving the user element  12 , signal strength measurements associated with the serving cell are compared to those associated with adjacent cells in the same or different subsystem (step  204 ). The adjacent cells are potential target cells, and in this example, will include the CS access cell  30 . When conditions dictate a handover from the PS access cell  20  to the CS access cell  30 , the PS access cell  20  will initiate a handover (step  206 ) and send a Handover Required message toward the MME  40 , which is designated as a source MME (S-MME) (step  208 ). The target for the Handover Required message corresponds to the MME-CS  38 , which is designated as a target MME-CS (T-MME-CS). From the perspective of the S-MME  40 , the T-MME-CS  38  appears as a target MME within the PS, and as such, the S-MME  40  will initiate what it believes is a PS-to-PS handover (step  210 ) by sending a Relocation Request toward the T-MME-CS  38  (step  212 ). The T-MME-CS  38  will send a Prepare Handover Request to the MSC  32  that supports the CS access cell  30  (step  214 ). The MSC  32  will act to establish radio resources in the CS access cell  30  by sending a Handover Request to the CS access cell  30  (step  216 ), which will establish resources for the handover (step  218 ) and provide an Acknowledgment (ACK) back to the MSC  32  (step  220 ). 
     Upon receipt of the Acknowledgement, the MSC  32  will send a Prepare Handover Response to the T-MME-CS  38  to indicate that the radio resources have been established in the CS access cell  30  (step  222 ). The T-MME-CS  38  will send an Integrated Services User Part (ISUP) Initial Address Message (IAM) to the MSC  32  (step  224 ) to set up a circuit-switched bearer path between the media gateway  34 , which is associated with the T-MME-CS  38 , and the MSC  32 . The MSC  32  will respond to the IAM with an ISUP Address Complete Message (ACM) (step  226 ) to indicate that the CS bearer path is being established. The T-MME-CS  38  will then update the signaling context for the session at the SAE gateway  22  to indicate that the session signaling is being transferred from the user element  12  to the CAAF/RUA  36  (step  228 ). At this point, the portion of the ICCC between the user element  12  and the T-MME-CS  38  is not available. As such, the T-MME-CS  38  will queue the downlink session signaling (step  230 ) until the ICCC can be established to the user element  12 . The T-MME-CS  38  will then initiate preparation of the remote end for the handover (step  232 ) by sending a message to the CMF/RUA  36  to prepare the remote end for the handover (step  234 ). At this point, the CMF/RUA  36  may register with the CSCF  24  on behalf of the user element  12  by sending a register message to the CSCF  24  (step  236 ). The CSCF  24  will respond by providing a 200 OK message (step  238 ). 
     After registration, the CMF/RUA  36  will send an Invite into the IMS  18  toward the CSCF  24  to initiate a session signaling transfer from the user element  12  to the CMF/RUA  36  (step  240 ). The Invite will include the CS communication information for the media gateway  34 . In this embodiment, the SAE gateway  22  will remain in the session signaling path after the inter-subsystem transfer; however, the CAAF/RUA  36  will act as an RUA on behalf of the user element  12  with respect to the IMS  18 . The DTF  28  will then initiate a transfer of the session signaling from the PS to the CS by directing session signaling toward the CMF/RUA  36  instead of toward the user element  12 . The CSCF  24  will forward the Invite to the ICCF  26  (step  242 ), which will have access to the state information for the communication information because the session signaling is anchored at the ICCF  26 . As such, the ICCF  26  can update the Invite with any necessary service state information (step  244 ) and forward the Invite back to the CSCF  24  for further processing (step  246 ). The CSCF  24  will then forward the Invite to the DTF  28 , which also provides an anchor for the session signaling (step  248 ). The DTF  28  will initiate a PS-to-CS bearer and session signaling transfer toward the remote endpoint  16  (step  250 ). 
     In this embodiment, there is no need for the DTF  28  to instruct the remote endpoint  16  to redirect traffic from the SAE gateway  22  to the media gateway  34 . Instead, the remote endpoint  16  will continue to deliver downlink traffic toward the media proxy  42  and receive uplink traffic from the media proxy  42 . As such, the remote endpoint  16  need not be aware of the change in the bearer path across the inter-subsystem transfer. Accordingly, the DTF  28  will send a message to initiate a bi-east of downlink traffic over the old bearer path to the user element  12  through the SAE gateway  22 , as well as over a new bearer path through the media gateway  34  (step  252 ). The DTF  28  may obtain the CS communication information necessary to deliver downlink traffic to the media gateway  34  from the Invite. In response, the media proxy  42  will provide a downlink traffic bi-cast, wherein downlink traffic is delivered over the old (transferring-out) and new (transferring-in) bearer paths toward the user element  12  (step  254 ). The media proxy  42  will also begin monitoring for uplink traffic via the new CS bearer path. 
     At this point, the new bearer path extends between the CS access cell  30  and the remote endpoint  16  through the MSC  32 , media gateway  34 , and media proxy  42  (step  256 ). Notably, the new bearer path is only carrying downlink traffic toward the CS access cell  30  at this point. However, the old bearer path facilitates bidirectional communications between the user element  12  and the remote endpoint  16  via the media proxy  42 . Further, the portion of the bearer path between the media proxy  42  and the remote endpoint  16  is common to the old and new bearer paths. 
     Meanwhile, the radio layer handover is taking place. At this point, the T-MME-CS  38  will send a Relocation Response to the S-MME  40  to instruct the user element  12  to effect the radio layer handover (step  258 ). As such, the S-MME  40  will send a Handover Required message to the PS access cell  20  (step  260 ), which will send a Handover Command to the user element  12  (step  262 ). The user element  12  will then retune itself to effectively change from a channel within the PS access cell  20  to a channel for the CS access cell  30  (step  264 ). During this process, the CS access cell  30  will detect the presence of the user element  12  (step  266 ) and send a Handover Detect message toward the MSC  32  to indicate that the user element  12  has switched to the CS access cell  30  (step  268 ). The MSC  32  will send the Handover Detect message to the T-MME-CS  38  (step  270 ), which will send the Handover Detect message to the CMF/RUA  36  to indicate that the user element  12  is now being served by the CS access cell  30  (step  272 ). 
     In the meantime, the CS access cell  30  may generate a Handover Complete message, which is sent to the MSC  32  to indicate that the handover is fully transitioned to the CS access cell  30  from the perspective of the user element  12  (step  274 ). The MSC  32  will send the Handover Complete message to the T-MME-CS  38  (step  276 ), and subsequently send an ISUP Answer Message to instruct the T-MME-CS  38  (step  278 ) to complete the bearer path between the media gateway  34  and the remote endpoint  16 , as well as to the user element  12  via the MSC  32 . 
     At this point, the new (transferring-in) bearer path is established in the CS between the user element  12  and the remote endpoint  16  through the CS access cell  30 , MSC  32 , media gateway  34 , and media proxy  42  (step  280 ). When the user element  12  is capable of sending uplink traffic over the new bearer path toward the remote endpoint  16 , it will do so. Upon receiving the first uplink traffic, the media proxy  42  will switch completely from the old (PS) bearer path to the new (CS) bearer path (step  282 ). The DTF  28  may be informed of the switch by the media proxy  42  and take the necessary steps to tear down the old bearer path and the old session signaling path. As in the first embodiment, the session signaling (step  284 ) comprises two primary segments. The first segment is the ICCC, which extends between the CAAF/RUA  36  and the user element  12  through the CS access cell  30 , the MSC  32 , and the T-MME-CS  38 . The second segment is a SIP segment that extends between the CAAF/RUA  36  and the remote endpoint  16  through the SAE gateway  22 , CSCF  24 , ICCF  26 , and DTF  28 , wherein the CSCF  24  invokes the ICCF  26  and the DTF  28 . 
     With the above embodiment, the use of the media proxy  42  reduces any interruption in the bearer path associated with transferring between the CS and the PS. Notably, the media proxy  42  does not need to be invoked in the bearer path at all times. The DTF  28  may selectively invoke the media proxy  42  when inter-subsystem transfers are likely. Notably, the media proxy  42  may be invoked and left in the bearer path for the remainder of the communication session, or may be selectively invoked as is deemed appropriate, for this alternative embodiment. 
     With reference to  FIGS. 8A through 8C , a communication flow is provided to illustrate an inter-subsystem transfer from the CS back to the PS, and in particular from the CS access cell  30  to the PS access cell  20 . This inter-subsystem handover corresponds to the embodiment of  FIGS. 1 through 3 , wherein session signaling is routed through the SAE gateway  22  for PS and CS access. Initially, assume the user element  12  is being served by the CS access cell  30  and is engaged in a communication session with the remote endpoint  16 . As such, the bearer path of the user element  12  as served by the CS extends between the user element  12  and the remote endpoint  16  through the CS access cell  30 , MSC  32 , and media gateway  34  (step  300 ). The session signaling (step  302 ) comprises two primary segments. The first segment is the ICCC, which extends between the CAAF/RUA  36  and the user element  12  through the CS access cell  30 , the MSC  32 , and the T-MME-CS  38 . The second segment is a SIP segment that extends between the CAAF/RUA  36  and the remote endpoint  16  through the SAE gateway  22 , CSCF  24 , ICCF  26 , and DTF  28 , wherein the CSCF  24  invokes the ICCF  26  and the DTF  28 . 
     While the CS access cell  20  is serving the user element  12 , signal strength measurements associated with the serving cell are compared to those associated with adjacent cells in the same or different subsystem (step  304 ). The adjacent cells are potential target cells, and in this example, will include the PS access cell  20 . When conditions dictate a handover from the CS access cell  30  to the PS access cell  20 , the PS access cell  20  will initiate a handover (step  306 ) and send a Handover Required message to the associated MSC  32  (step  308 ). In this example, the CS access cell  30  is the source access cell, and the PS access cell  20  is the target access cell. Accordingly, the MME  40  that is associated with the PS access cell  20  will be referred to as a target MME (T-MME)  40 . Similarly, the MME-CS  38  that is associated with the CS access cell  30  is a source MME-CS (S-MME-CS)  38 . 
     Once the MSC  32  receives the Handover Required message from the CS access cell  30 , a Prepare Handover Request is sent to the S-MME-CS  38  (step  310 ), which will send a Relocation Request to the T-MME  40  (step  312 ). Again, the S-MME-CS  38  appears as a source MME to the T-MME  40 . Further, the S-MME-CS  38  may appear as a target MSC to the MSC  32 . Accordingly, the T-MME  40  will send a Handover Request to the PS access cell  20  to establish resources in the PS access cell  20  for the handover (step  314 ). The PS access cell  20  will then establish resources for the handover (step  316 ) and provide an Acknowledgement to the Handover Request back to the T-MME  40  (step  318 ). The T-MME  40  will then update the signaling context at the SAE gateway  22  to indicate that a handover is taking place into the PS access cell  20  through the T-MME  40  (step  320 ). The T-MME  40  will also send a Relocation Response back to the S-MME-CS  38  to indicate that resources for the handover have been established in the PS access cell  20  (step  322 ). The S-MME-CS  38  will provide a Prepare Handover Response back to the MSC  32  (step  324 ). Once the MSC  32  recognizes that the resources for the handover are available in the PS access cell  20 , a Handover Command is sent to the CS access cell  30  (step  326 ), which will send a Handover Command to the user element  12  (step  328 ). The Handover Command provides an instruction for the user element  12  to change from a channel in the CS access cell  30  to a channel in the PS access cell  20  to effect a transition from the CS to the PS (step  330 ). 
     The PS access cell  20  will detect the presence of the user element  12  in the PS (step  332 ), and will send a Handover Complete message to the T-MME  40  (step  334 ). The T-MME  40  will send a Relocation Complete message to the S-MME-CS  38  to indicate that the user element  12  has transitioned from the CS to the PS (step  336 ). The S-MME-CS  38  will send an Update Context message to inform the SAE gateway  22  that the handover is complete, and in particular, to provide information assisting the SAE gateway  22  in delivering the bearer and session signaling to the user element  12  instead of to the media gateway  34  and the CAAF/RUA  36 , respectively (step  338 ). The S-MME-CS  38  will send a Relocation Complete Acknowledgement back to the T-MME  40  (step  340 ), as well as send a Handover Complete message to the MSC  32  (step  342 ). The Handover Complete message informs the MSC  32  that the handover is complete. The MSC  32  will then take the necessary steps to release the bearer path extending to the user element  12  through the CS access cell  30  upon receiving a Release instruction from the S-MME-CS  38  (step  344 ). 
     When the user element  12  transitions to the PS access cell  20 , the user element  12  will register with the IMS  18 , perhaps by sending a Register message to the CSCF  24  (step  346 ). The Register message may provide updated contact or address information for the user element  12  as the user element  12  is being served in the PS. Prior to the handover, the CAAF/RUA  36  was registered on behalf of the user element  12 . As such, the user element  12  is registered directly with the IMS  18  instead of indirectly via the CMF/RUA  36 . The CSCF  24  will send a 200 OK message back to the user element  12  in response to the Register message (step  348 ). 
     Next, the user element  12  will initiate a communication session from the CMF/RUA  36  to the user element  12  by sending an Invite indicating the same into the IMS  18 . The Invite will also include the PS communication information necessary to deliver packet traffic to the user element  12 . The Invite is received at the CSCF  24  in the IMS  18  (step  350 ), and is routed through the ICCF  26  (step  352 ) to update the service state (step  354 ). Notably, the user element  12  may not be aware of the service state, because the CAAF/RUA  36  was acting on behalf of the user element  12  prior to the handover into the PS. Since the ICCF  26  maintains service state information and is an anchor point for session signaling, the service state may be maintained at the ICCF  26  and updated as necessary after an inter-subsystem transfer. 
     After the service state update, the ICCF  26  will send an Invite with updated service state information back to the CSCF  24  (step  356 ), which will forward the Invite to the DTF  28  (step  358 ). The DTF  28  will initiate the CS-to-PS handover (step  360 ), and in particular will send information to the remote endpoint  16  to indicate the transfer of the communication session from the CAAF/RUA to the user element  12 . Accordingly, the DTF  28  will send a Re-Invite including the PS communication information for the user element  12  toward the remote endpoint  16  through the CSCF  24  (steps  362  and  364 ). Although not illustrated, a 200 OK message in response to the Re-Invite may be passed along the session signaling path to the user element  12 . The 200 OK message may include PS communication information for the remote endpoint  16  or a remote user agent acting on behalf of the remote endpoint  16 . At this point, the bearer path is established between the user element  12  and the remote endpoint  16  through the PS access cell  20  and the SAE gateway  22  (step  366 ). The session signaling path is established between the user element  12  and the remote endpoint  16 , and extends through the PS access cell  20 , SAE gateway  22 , CSCF  24 , ICCF  26 , and DTF  28 , wherein the CSCF  24  invokes both the ICCF  26  and the DTF  28  (step  368 ). 
     In the above embodiment, the CAAF/RUA  36  provides a stateless RUA on behalf of the user element  12  to the IMS  18 . Further, the ICCF  26  provides a stateful RUA on behalf of the user element  12  to the IMS  18 . The stateful and stateless RUAs are presented to the IMS  18  when the user element  12  is served by the CS. Notably, a stateless RUA does not have access to the service state for the communication session after an inter-subsystem transfer, whereas a stateful RUA will have service state information after an inter-subsystem transfer. As such, the CAAF/RUA  36  and the ICCF  26  effectively provide-a bifurcated RUA, wherein one portion resides in the CS and the other resides in the IMS  18 . 
     With reference to  FIG. 9 , in another embodiment of the present invention the functionality of the CAAF/RUA  36  is integrated into the functionality of the ICCF  26 . As such, there is not a bifurcated RUA, and the MME-CS  38  is configured to extend the ICCC to the ICCF  26  via the SAE gateway  22  and the CSCF  24 . Additionally, the media gateway  34  that was associated with the MME-CS  38  is eliminated and replaced with a media gateway  44 , which resides in the IMS  18 . The media gateway  44  is controlled by a media gateway control function (MGCF)  46  that resides in the IMS  18  and provides interworking between the CS and IMS  18 . Notably, the ICCF  26  may also interact with the MGCF  46  to facilitate bearer control through the media gateway  44 . Further, a media gateway  48  is provided in close association with the DTF  28 , and will act in a similar fashion to the media proxy  42 , which was described above. During an inter-subsystem transfer, downlink traffic may be bi-east over the transferring-in and transferring-out bearer paths to minimize bearer interruption as the user element  12  transitions from the transferring-out subsystem to the transferring-in subsystem. 
     As depicted in  FIG. 9 , when the user element  12  is served by the PS, the bearer path is a PS bearer path, and extends between the user element  12  and the remote endpoint  16  through the PS access cell  20 , SAE gateway  22 , and media gateway  48 . The session signaling path between the user element  12  and the remote endpoint  16  extends through the PS access cell  20 , SAE gateway  22 , and CSCF  24 , which will invoke the ICCF  26  and DTF  28 . 
     With reference to  FIG. 10 , the bearer and session signaling paths are illustrated for a scenario where the user element  12  is supported by the CS. The bearer path between the user element  12  and the remote endpoint  16  extends through the CS access cell  30 , the MSC  32 , the media gateway  44 , and the media gateway  48 , which is associated with the DTF  28 . The bearer path between the user element  12  and the media gateway  44  is circuit-switched, while the bearer path between the media gateway  44  and the remote endpoint  16  may be packet-based. The session signaling between the user element  12  and the remote endpoint  16  extends through the CS access cell  30 , the MSC  32 , the MME-CS  38 , the SAE gateway  22 , and the CSCF  24 , which will invoke the ICCF  26  and the DTF  28 . CS call control may be provided between the ICCF  26  and the MGCF  46  through the CSCF  24 . Further, the MGCF  46  may interwork with the MSC  32  or other entities in the CS to facilitate bearer control. Operational details are provided further below. 
       FIG. 11  illustrates the various handover control signaling paths. Notably, the MME-CS  38  is able to communicate effectively with the ICCF  26 , preferably through the SAE gateway  22  and the CSCF  24 . As such, the ICCF  26  may provide handover control in the CS from the IMS  18 . 
     With reference to  FIGS. 12A through 12D , a communication flow is provided to illustrate an inter-subsystem transfer from the PS access cell  20  to the CS access cell  30 . The bearer and session signaling paths are depicted as described immediately above (steps  400  and  402 ). Notably, the media gateway  48 , which is associated with the DTF  28 , resides in the bearer path between the user element  12  and the remote endpoint  16 . While the PS access cell  20  is serving the user element  12 , signal strength measurements associated with the serving cell are compared to those associated with adjacent cells in the same or different subsystem (step  404 ). The adjacent cells are potential target cells, and in this example, will include the CS access cell  30 . When conditions dictate a handover from the PS access cell  20  to the CS access cell  30 , the PS access cell  20  will initiate a handover (step  406 ) and send a Handover Required message toward the MME  40 , which is designated as a source MME (S-MME) (step  408 ). The target for the Handover Required message corresponds to the MME-CS  38 , which is designated as a target MME-CS (T-MME-CS). From the perspective of the S-MME  40 , the T-MME-CS  38  appears as a target MME within the PS, and as such, the S-MME  40  will initiate what it believes is a PS-to-PS handover (step  410 ) by sending a Relocation Request toward the T-MME-CS  38  (step  412 ). The T-MME-CS  38  will send a Prepare Handover Request to the MSC  32  that supports the CS access cell  30  (step  414 ). The MSC  32  will act to establish radio resources in the CS access cell  30  by sending a Handover Request to the CS access cell  30  (step  416 ), which will establish resources for the handover (step  418 ) and provide an Acknowledgment back to the MSC  32  (step  420 ). 
     Once the MSC  32  receives an indication that resources have been established for the handover at the CS access cell  30 , the MSC  32  will send a Prepare Handover Response back to the T-MME-CS  38  (step  422 ). Notably, the Prepare Handover Response will include a handover number that is associated with the MSC  32 . The T-MME-CS  38  will use the handover number to trigger the ICCF  26  to establish a circuit-switched connection for a portion of the CS bearer path between the media gateway  44  and the MSC  32 . To accomplish this, the T-MME-CS  38  may send an Invite that may include a Handover Request with the handover number toward the ICCF  26  through the CSCF  24  (steps  424  and  426 ). The ICCF  26  may update service state information (step  428 ) and proceed to initiate the portion of the bearer path between the media gateway  44  and the MSC  32 . As such, an Invite with the handover number is generated and sent toward the MGCF  46 , which is associated with the media gateway  44  (step  430 ). The MGCF  46  will initiate an ISUP IAM toward the handover number, and thus towards the MSC  32  (step  432 ). The MSC  32  will recognize that an incoming call from the media gateway  34  is being received, and will begin establishing a connection with the media gateway  44 . Further, the MSC  32  will send an ISUP ACM to the MGCF  46  (step  434 ) to indicate that the call is progressing. The MGCF  46  will send a Ringing message back toward the ICCF  26  (step  436 ). Notably, communications between the MGCF  46  and the ICCF  26  may be direct or indirect via the CSCF  24 . Although a handover number and an Invite are disclosed, those skilled in the art will recognize other techniques for initiating the circuit-switched connections. 
     At this point, the ICCF  26  may respond to the original Invite (of step  426 ) by forwarding the Invite back to the CSCF  24  (step  438 ), which will forward the Invite to the DTF  28  (step  440 ). As described above, the DTF  28  will initiate a PS-to-CS transfer of the session signaling (step  442 ), and initiate a bi-cast over the transferring-out and transferring-in bearer paths toward the user element  12  via the PS and CS, respectively. As such, the DTF  28  may send a message to initiate the bi-cast to the media gateway  48 , which is associated with the DTF  28  (step  444 ). The media gateway  48  will provide a downlink traffic bi-cast over the transferring-out and transferring-in bearer paths, and then begin to monitor for uplink traffic via the CS bearer path (step  446 ). The media gateway  48  may acknowledge initiation of the bi-cast (step  448 ), wherein the DTF  28  may initiate a 200 OK message back toward the T-MME-CS  38 . The 200 OK message is received by the CSCF  24  (step  450 ), which will route the 200 OK message through the ICCF  26  and then toward the T-MME-CS  38  (steps  452 - 456 ). 
     Next, the T-MME-CS  38  may update the signaling context at the SAE gateway  22  to indicate that the session signaling is moving from the user element  12  to the T-MME-CS  38  (step  458 ). Next, the T-MME-CS  38  will being queuing the downlink session signaling until such signaling can be delivered to the user element  12  over the ICCC (step  460 ). At this point, the original bearer path (of step  400 ) is capable of bidirectional communications between the user element  12  and the remote endpoint  16 . This bearer path is the transferring-out bearer path. The transferring-in bearer path has been established based on the above signaling between the CS access cell  30  and the remote endpoint  16  through the MSC  32 , media gateway  44 , and media gateway  48  (step  462 ). Notably, the portion of the bearer path between the media gateway  48  and the remote endpoint  16  supports both the transferring-out and the transferring-in bearer paths. Further, only downlink traffic is provided from the media gateway  48  to the CS access cell  30  over the transferring-in bearer path until the user element  12  is able to successfully transition, at the radio layer, to the CS access cell  30  from the PS access cell  20 . 
     At this point, the T-MME-CS  38  will respond to the Relocation Request (of step  412 ) with a Relocation Response directed toward the S-MME  40  (step  464 ). The S-MME  40  will send a Handover Required Acknowledgement to the PS access cell  20  (step  466 ), which will generate a Handover Command and send it toward the user element  12  (step  468 ). The user element  12  will then change channels to effect a radio layer handover from the PS access cell  20  to the CS access cell  30  (step  470 ). The CS access cell  30  may be able to detect the presence of the user element  12  (step  472 ), and will send a Handover Complete message to the MSC  32  (step  474 ). The MSC  32  will inform the T-MME-CS  38  that the handover is complete (step  476 ), and send an Answer message back to the MGCF  46  to complete the ISUP signaling for the bearer path established between the media gateway  44  and the MSC  32  (step  478 ). The MGCF  46  will then send a 200 OK message back toward the ICCF  26  in response to the Invite of step  430  (step  480 ). 
     When the media gateway  48  detects uplink traffic from the user element  12  via the transferring-in bearer path in the CS, the bi-cast will end, such that downlink traffic is only delivered over the transferring-in bearer path. The DTF  28  may take the necessary steps to end any session signaling legs or bearer portions extending toward the user element  12  via the PS. The resultant transferring-in bearer path extends between the user element  12  and the remote endpoint  16  via the CS access cell  30 , MSC  32 , media gateway  44 , and media gateway  48  (step  482 ). The session signaling extends between the user element  12  and the remote endpoint  16  through the CS access cell  30 , the MSC  32 , the T-MME-CS  38 , the SAE gateway  22 , and the CSCF  24 , which will invoke the ICCF  26  and the DTF  28  (step  484 ). Notably, the ICCC portion of the session signaling path extends between the user element  12  and the ICCF  26 . The T-MME-CS  38  may provide interworking between session messages of the CS and SIP or like messages of the IMS  18 . Again, the ICCF  26  provides a remote user agent for the user element  12  into the IMS  18 . 
     From the above, those skilled in the art will recognize various alternatives to the specific call flows provided to implement the concepts of the present invention. Further, the various functions may be provided in the same or separate service nodes in the various subsystems. For example, the ICCF  26  and DTF  28  may be provided in the same or different service nodes. Further, the CMF/RUA  36  and the MME-CS  38  may be provided in the same or different service nodes, and may also be associated with an imbedded media gateway control function to facilitate control of the associated media gateway  34 . Also, the entities and functions illustrated may be supported by other networks and network nodes that are capable of handling and providing various messaging processing therebetween. The communication flows are logical in nature, and will vary from one implementation to another. Although the scenarios provided above relate to inter-subsystem transfers between a 4G PS access network and a 3G CS access network, other generations of networks and corresponding access cells are supported by the present invention. 
     The ICCF  26  and DTF  28  may employ back-to-back user agents as well as third party call control functionality to anchor session signaling and allow signaling access legs toward the user element  12  to move from one subsystem to another without impacting a remote access signaling leg toward the remote endpoint  16 . The ICCF  26  and DTF  28  may be addressable using public service identities (PSI) from the CS and PS domains  14 . In the CS, a directory number associated with the respective functions may be used for routing signaling messages within the CS. In the PS or the IMS  18 , a uniform resource locator (URL) associated with the particular function may be used for routing signaling messages. For additional information relating to session continuity and alternative handover techniques, reference is made to U.S. patent application Ser. No. 11/378,776 filed Mar. 17, 2006; U.S. patent application Ser. No. 11/440,165 filed May 24, 2006; U.S. patent application Ser. No. 11/452,069 filed Jun. 12, 2006; U.S. patent application Ser. No. 11/451,722 filed Jun. 13, 2006; U.S. patent application Ser. No. 11/466,115 filed Aug. 22, 2006; and U.S. patent application Ser. No. 11/554,930 filed Oct. 31, 2006; and International Application serial number PCT/IB2007/001555 entitled METHOD FOR TRANSITIONING SUPPORT OF COMMUNICATION SESSIONS FOR A USER ELEMENT BETWEEN DIFFERENT TYPES OF SUBSYSTEMS OF DIFFERENT GENERATIONS and filed concurrently herewith, which are incorporated herein by reference in their entireties. 
     As noted, the present invention supports different generations of access technology. Each generation is an evolutionary generation of wireless communication infrastructures and communication standards. Second generation (2G), third generation (3G), and fourth generation (4G) communication systems are referenced herein. 2G standards are digital in nature and rely primarily on a CS domain for voice and data. Select 2G systems include but are not limited to Global System for Mobile Communications (GSM), Digital Advanced Mobile Phone System (D-AMPS), Interim Standard 95 (IS-95) (Code Division Multiple Access—CDMA), General Packet Radio Service (GPRS), and CDMA2000 (1xRTT/IS-2000). 3G standards are digital in nature and employ a PS domain in parallel with a CS domain to provided increased data rates over 2G systems. Select 3G systems include but are not limited to Enhanced Data Rates for GSM Evolution (EDGE), Enhanced GPRS (EGPRS), Wideband CDMA (W-CDMA), and Universal Mobile Telecommunications System (UMTS) (Third Generation GSM-3GSM), 1x Evolution-Data Only (1x-DO)/IS-856, and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). 4G standards are digital in nature and will generally rely on a PS domain for voice and data. In most systems, no CS domain is necessary. 4G systems include, but are not limited to Worldwide Interoperability for Microwave Access (WiMax), Wireless Metropolitan Area Network (WirelessMAN), IEEE802.16, and the proposed Third Generation Partnership Project (3GPP) Long Term Evolution work-in-progress technologies, such as enhanced UMTS and W-CDMA. 
     With reference to  FIG. 13 , a service node  50  is provided according to one embodiment of the present invention. The service node  50  may reside in the home IMS  18  or visited PS and CS domains  14  and includes a control system  52  and associated memory  54  to provide the functionality for any one or a combination of the following: the MME-CS  38 , MME  40 , CAAF/RUA  36 , CSCF  24 , ICCF  26 , and DTF,  28 . The control system  48  will also be associated with a communication interface  56  to facilitate communications as described above based on the functions being implemented. 
     With reference to  FIG. 14 , a block representation of a user element  12  is provided. The user element  12  may include a control system  58  having sufficient memory  60  to support operation of a CS client  62  and an MS client  64 , which support CS and PS access and communications, respectively. The control system  58  will cooperate closely with a communication interface  66  to allow the CS client  62  and the MS client  64  to facilitate communications over a CS or the PS (IMS) as described above. The control system  58  may also be associated with a user interface  68 , which will facilitate interaction with the user. The user interface  68  may include a microphone and speaker to facilitate voice communications with the user, as well as a keypad and display to allow the user to input and view information to support media sessions and control of the user element  12 . 
     Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.

Metadata:
Filing Date: 20070608
Publication Date: 20140401
Grant Date: 20140401
Priority Date: 20060614
Inventors: MAHDI KANIZ
STOJANOVSKI SASO
STEGALL MARK
Assignee: APPLE INC
CPC Classifications: [{"code": "H04W36/00224", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L12/66", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L12/66", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L65/1086", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04L65/1086", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04W36/00224", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 38832160