Patent Publication Number: US-9854421-B2

Title: Transfer of emergency services session between disparate subsystems

Description:
This application is a continuation of U.S. application Ser. No. 13/675,788, entitled “TRANSFER OF EMERGENCY SERVICES SESSION BETWEEN DISPARATE SUBSYSTEMS”, filed Nov. 13, 2012, which is a continuation of U.S. application Ser. No. 11/760,199, of the same title, filed Jun. 8, 2007, now U.S. Pat. No. 8,331,961, which claims the benefit of U.S. Provisional Patent Application No. 60/812,814, entitled “VCC FOR IMS AND CS EMERGENCY CALLS”, filed Jun. 12, 2006, all of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     Field of the Invention 
     The present invention relates to communications, and in particular to providing a multimedia subsystem control for supporting emergency services sessions over circuit-switched subsystems and packet subsystems, as well as effecting transfers of established calls from one subsystem to another. 
     Description of the Related Art 
     Voice sessions or calls can be supported by circuit-switched and packet-based communications. In many communication environments, circuit-switched communications are provided by circuit-switched subsystems while packet communications are provided by packet subsystems. Traditionally, circuit-switched communications have been used for voice communications, and as such, access to emergency services for police, fire, and ambulance services has been provided by circuit-switched subsystems. 
     As voice services are being deployed over packet subsystems, there is a need to provide support for emergency services for packet subsystem users. Given the existing emergency services infrastructure in the circuit-switched subsystems, network providers are employing techniques to allow the emergency services provided in the circuit-switched subsystems to be accessed by the packet subsystem users. 
     In wireless communication environments, user elements are mobile, and as such, often move from one location to another. With movement, service for the user element may be transferred from one access point or base station to another within a given subsystem. Many user elements may be served by different types of subsystems, and based on the location of the user elements or a desired application, may transfer service from one type of subsystem to another. 
     For emergency services, it is important to maintain an emergency services session between an operator and the user element as the user element transfers from one access point or base station within a particular subsystem as well as between different subsystems. Although transfers from one access point or base station to another are handled within the given subsystem, there is a need to efficiently and effectively transfer existing emergency services sessions from one type of subsystem to another. 
     SUMMARY 
     The present invention provides session control for an emergency services session (E-session) in a multimedia subsystem (MS), such as the Internet Protocol (IP) Multimedia Subsystem (IMS) as defined by the Third Generation Partnership Project (3GPP). The MS resides in one or more packet based subsystems (PSs), and E-sessions may include emergency services calls, such as 911 calls, that are directed to a Public Services Access Point (PSAP) that supports emergency services. Session control of the E-sessions originating in a circuit-switched subsystem (CS) or the MS is provided by the visited MS and anchored at a domain transfer function (DTF), which is a service provided by a visited MS. The visited MS is the MS associated with the CS or PS that is currently serving the user element. As such, all call signaling for the E-session is passed through the DTF. The DTF enables efficient and effective subsystem transfers of the E-session between the CS and MS, while maintaining the E-session between the user element and the PSAP. The DTF may also provide to the PSAP location information bearing on the location of the user element when the E-session is established, as well as provide updated location information when transfers between the CS and MS occur. 
     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 DRAWINGS 
       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  is a communication environment illustrating multimedia subsystem access for a user element according to a one embodiment of the present invention. 
         FIG. 2  is a communication environment illustrating circuit-switched subsystem access for a user element according to one embodiment of the present invention. 
         FIGS. 3A-3C  depict a communication flow illustrating originating an emergency services session via a multimedia subsystem according to one embodiment of the present invention. 
         FIGS. 4A and 4B  depict a communication flow illustrating the transfer of the call established in  FIGS. 3A-3C  to the circuit-switched subsystem according to one embodiment of the present invention. 
         FIG. 5  is a communication flow illustrating the transfer of the call of  FIGS. 4A and 4B  back to the multimedia subsystem according to one embodiment of the present invention. 
         FIG. 6  is a communication environment illustrating multimedia subsystem access for a user element according to another embodiment of the present invention. 
         FIG. 7  is a communication environment illustrating circuit-switched subsystem access for a user element according to another embodiment of the present invention. 
         FIG. 8  is a block representation of a service node according to one embodiment of the present invention. 
         FIG. 9  is a block representation of a user element according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     The present invention provides session control for an emergency services session (E-session) in a multimedia subsystem (MS), such as the Internet Protocol (IP) Multimedia Subsystem (IMS) as defined by the Third Generation Partnership Project (3GPP). The MS resides in one or more packet based subsystems (PSs), and E-sessions may include emergency services calls, such as 911 calls, that are directed to a Public Services Access Point (PSAP) that supports emergency services. Session control for E-sessions originating in a circuit-switched subsystem (CS) or the MS is provided by the visited MS and is anchored at a domain transfer function (DTF), which is a service provided by a visited MS. The visited MS is the MS associated with the CS or PS that is currently serving the user element. As such, all call signaling for the E-session is passed through the DTF. The DTF enables efficient and effective subsystem transfers of the E-session between the CS and MS, while maintaining the E-session between the user element and the PSAP. The DTF may also provide to the PSAP location information bearing on the location of the user element when the E-session is established, as well as provide updated location information when transfers between the CS and MS occur. 
     As an E-session is being initiated, the DTF is inserted into the signaling path of the E-sessions by an emergency call/session control function (E-CSCF) in the visited MS. The E-CSCF functions as a specially configured serving CSCF (S-CSCF) of an IMS. In general, CSCFs are often Session Initial Protocol (SIP) proxies that provide various session control functions at various points in the session signaling path. The E-CSCF provides routing and translation functions for E-sessions, and notably, invokes services including the DTF when E-sessions are being established. To act as an anchor for the signaling path of an E-session, the DTF may employ a back-to-back user agent function. As such, when the user element originates an E-session, the DTF will terminate an access signaling leg from the user element and establish a remote signaling leg toward the PSAP. Subsequently, the DTF will coordinate call signaling between the access signaling leg and the remote signaling leg for the E-session. Notably, the user element may provide location information bearing on the location of the user element for delivery to the DTF, which will pass the location information to the PSAP. 
     Subsystem transfers enable the user element to move back and forth between the CS and the MS while maintaining an active E-session. Subsystem transfers associated with a given E-session, including initial and subsequent subsystem transfers, are executed and controlled in the visited MS by the DTF, generally upon a transfer-specific request or a new E-session received from the user element. Upon detecting conditions requiring a transfer from one subsystem to another, the user element will establish a new access signaling leg with the DTF using an appropriate address for the DTF or an emergency address, as if a new E-session were being established. The user element may provide updated location information to the DTF when establishing the new access leg. The new access signaling leg is established via the “transferring-in” subsystem and establishment of the new access signaling leg will trigger a transfer from the “transferring-out” subsystem to the transferring-in subsystem. The DTF will implement a subsystem transfer by replacing the old access signaling leg currently communicating with the remote signaling leg with the new access signaling leg established via the transferring-in subsystem. The DTF will also send an update toward the PSAP to provide information necessary to facilitate a transfer of the bearer path to the transferring-in subsystem. The DTF may also provide a location update for the user element to the PSAP, if a location update is available. The DTF will subsequently release the old access signaling leg that was established through the “transferring-out” subsystem. 
     The switch of the access signaling legs from the transferring-out subsystem to the transferring-in subsystem does not impact the remote signaling leg or the application services, including the DTF, in the remote signaling leg. Using the access signaling leg in the transferring-in subsystem and the remote signaling leg, the appropriate bearer path may be established to the user element via an appropriate CS client or MS client of the user element. Since all call signaling is provided through the DTF, additional services may be associated with the call through any number of transfers. 
     For routing of emergency calls originating via circuit-switched subsystem and for subsystem transfers, the DTF is addressable from the different subsystems using different service identities (SI). In the CS, a subsystem transfer directory number (STN) associated with the DTF may be used for routing call signaling messages from within the CS toward the DTF in the MS. In the MS, a subsystem transfer uniform resource location (STU) associated with the DTF is used for routing call signaling messages along the access signaling leg toward the DTF. In one embodiment, the STN and the STU are provided to the user element when the E-session is being established for the first time. In the following description, 3GPP TS 24.008 (DTAP) is used in the CS, while the Session Initiation Protocol (SIP) is used in the MS to effect origination, termination, and transfer of calls. Those skilled in the art will recognize other applicable and useful protocols as substitutes for DTAP and SIP. 
     With reference to  FIG. 1 , a wireless communication environment is illustrated with a visited MS  10 , and a CS-based PSAP domain  12 . As depicted, a user element (UE)  14  has established an E-session via the visited MS  10  into the PSAP domain  12 . The signaling path for the E-session includes an access signaling leg and a PSAP signaling leg. The access signaling leg extends from the user element  14  through a proxy-CSCF (P-CSCF)  16 , an E-CSCF  18 , and to a DTF  20 , where the session signaling is anchored. The PSAP signaling leg extends from the DTF  20  through the E-CSCF  18 , to a voice over IP (VoIP) positioning center (VPC)  22 , back through the E-CSCF  18 , through a PSAP media gateway controller (MGC)  24 , to a PSAP  26 , which is in the PSAP domain  12 . Notably, the DTF  20  and the VPC  22  are applications that are closely associated with the E-CSCF  18 . The bearer path for the E-session extends from the user element  14  to a PSAP media gateway (MG)  28 , which is associated with the PSAP MGC  24 , to the PSAP  26 . 
     The P-CSCF  16  is typically the signaling entry point for the visited MS  10 , and as such, facilitates routing of an E-session to an appropriate E-CSCF  18  in the visited MS  10 . When an E-session is initiated, the E-CSCF  18  will invoke the DTF  20 , effectively insert the DTF  20  in the session signaling path, and then cooperate with the DTF  20  to route the E-session toward the PSAP  26 . During this routing, the E-CSCF  18  may also invoke the VPC  22 , such that both the DTF  20  and the VPC  22  are inserted into the signaling path. Since the PSAP  26  is in a CS-based PSAP domain  12  in this example, the PSAP MGC  24  is employed to route the call to the PSAP  26  and establish a bearer path via the PSAP media gateway  28 . 
     The VPC  22  helps the E-CSCF  18  to determine an appropriate PSAP  26 , assuming there are multiple PSAPs  26  available. Preferably, the PSAP  26  selected for a given E-session is determined based on the location of the user element  14 . Since the user element  14  may send a location reference with the initiation of an E-session, the VPC  22  will access the location reference, identify a location of the user element  14  based on the location reference, and locate an appropriate PSAP  26  to use for the E-session. 
     In one embodiment, the VPC  22  will determine an emergency services routing number (ESRN) for the selected PSAP  26 , and a routing key associated with the E-session. The E-CSCF  18  will use the ESRN to route the E-session toward the selected PSAP  26 . The routing key is used as a reference for the E-session and any contextual information associated with the E-session. The routing key is helpful in associating additional signaling or session requests associated with the E-session. Notably, a location information service (LIS)  30  may be accessible by the VPC  22  to obtain PSAP and ESRN information based on location information associated with the user element  14 . 
     For the present invention, the DTF  20  is a new function that is closely associated with, if not integrated with, the E-CSCF  18 . The DTF  20  is inserted into the signaling path when an E-session is established, such that the DTF  20  can control subsequent subsystem transfers between a CS and the visited MS  10 , and in particular, a PS supporting the visited MS  10 . Once an E-session is established, the DTF  20  will recognize that an E-session is active for a given user element  14 . When a request for a new E-session comes in from a user element  14  that is already engaged in an active E-session with the DTF  20 , the DTF  20  will effect a transfer from a transferring-out subsystem to a transferring-in subsystem from which the new E-session is being initiated. The DTF  20  will effectively associate the PSAP signaling leg, which remains intact during the transfer, with a new access signaling leg established via the transferring-in subsystem. Additionally, the DTF  20  will send an update toward the PSAP  26  to provide information regarding the new bearer path through the transferring-in subsystem, as well as updated location information associated with the user element  14 . 
     With reference to  FIG. 2 , the signaling and bearer paths are illustrated for an E-session that was initiated from the user element  14  in a visited CS  22 . The access signaling leg extends from the user element  14  to the DTF  20  via a visited mobile switching center (VMSC)  34  that serves the user element  14 , a CS MGC  36  that is associated with a CS media gateway (MG)  38 , an interrogating CSCF (I-CSCF)  40 , and a CS adaptation function (CSAF)  42 . The bearer path extends from the user element  14  through the visited CS  32  to the CS MG  38 . The bearer path is provided through the visited MS  10  between the CS MG  38  and the PSAP MG  28 . The final section of the bearer path through the CS-based PSAP domain  12  is provided between the PSAP MG  28  and the PSAP  26 . 
     The CS MGC  36  and the CS MG  38  cooperate to provide access between the visited CS  32  and the visited MS  10 . The I-CSCF  40  initially receives the E-session and selects the CSAF  42  to use for representing the user element  14  to a selected E-CSCF  18 . The CSAF  42  effectively emulates the behavior of the user element  14 , if the user element  14  were served by the visited MS  10 . Accordingly, the CSAF  42  provides a remote user agent function on behalf of the user element  14 , when the user element  14  is served by the visited CS  32 . The E-CSCF  18  will invoke the DTF  20  to anchor the access signaling and PSAP signaling legs for the E-session. The E-CSCF  18  will also invoke the VPC  22 , as described above, to help select an appropriate PSAP  26  and obtain an ESRN or other routing information necessary to allow the E-CSCF  18  to route the E-session toward the PSAP  26 . In this embodiment, the E-CSCF  18  need not know whether the user element  14  is supported by the visited CS  32  or the visited MS  10 , because of the CSAF  42 . Notably, a home subscriber service (HSS)  44  is provided to provide routing information for the I-CSCF  40  and perhaps for the VMSC  34 . In this embodiment, the I-CSCF  40  may access the HSS  44  to determine a CSAF  42  to which the E-session should be routed. 
     With reference to  FIGS. 3A through 3C , a communication flow is provided to illustrate origination and establishment of an E-session that is originated from a user element  14  that is served by the visited MS  10  (and thus, is not being served by the visited CS  32 ). Assuming SIP is used for session signaling within the visited MS  10 , the user element  14  may initiate an emergency Invite message (E-Invite), which is received by the P-CSCF  16  (step  100 ). The E-Invite may include an indication that it is an emergency session request, such as by including an emergency services number or address ( 911 ) and indicate that it is from the user element  14 . The user element  14  may also include a location reference, which provides information bearing on the actual location of the user element  14 . The location reference may relate to a base station, access point, network ID, or the like from which the location of the user element  14  may be derived. If the user element  14  is equipped with positioning capability, the actual position of the user element  14  may be provided as the location reference, alone or in association with the above-mentioned location indicia. 
     The P-CSCF  16  will select an E-CSCF  18  and send the E-Invite toward the E-CSCF  18  (step  102 ), which will forward an Invite to the DTF  20  (step  104 ). Forwarding the Invite to the DTF  20  will effectively invoke the DTF  20  as an anchor point for the signaling path for the E-session. As such, the DTF  20  will establish itself as an anchor and take the necessary steps to complete establishment of the E-session (step  106 ). The DTF  20  will send an Invite back to the E-CSCF  18  (step  108 ), which will invoke the VPC  22  by sending an Invite to the VPC  22  (step  110 ). The VPC  22  will identify a location of the user element  14  based on the location reference provided in the Invite (step  112 ), and will select a PSAP  26  based on the location of the user element  14  (step  114 ). As noted above, the VPC  22  may query the LIS  30  to identify the location of the user element  14  and select an appropriate PSAP  26 . The VPC  22  will create a context for the call, and will provide an associated emergency services routing key (ESK) associated with the context (step  116 ). The context maintains a reference to the E-session and any services or information related to the user element  14 , a subscriber associated with the user element  14 , or services associated with the E-session. Finally, the VPC  22  will identify an emergency services routing number (ESRN) for the selected PSAP  26  (step  118 ). 
     The VPC  22  may then send to the E-CSCF  18  a Re-Direct message, which is configured to route the E-session toward the PSAP  26  (step  120 ). The Re-Direct message may include the emergency services routing number, the emergency services routing key, and the location reference or location derived therefrom. In response, the E-CSCF  18  will send an Invite toward the PSAP  26 . In this instance, the Invite is directed to the PSAP MGC  24 , which is associated with the ESRN and the PSAP  26  (step  122 ). In response to receiving the Invite, the PSAP MGC  24  will send an Integrated Services User Part (ISUP) Initial Address Message (IAM) toward the PSAP  26  through the CS in which the PSAP  26  is located (step  124 ). The IAM will include the location reference, such that the PSAP  26  can identify the location of the user element  14 . 
     While the E-session is being presented to the PSAP  26 , the PSAP MGC  24  will send a 183 Session Progress message back toward the E-CSCF  18  to indicate that the call is being presented to the PSAP  26  (step  126 ). Since the session signaling path may extend through the VPC  22  and the DTF  20 , the E-CSCF  18  will send the 183 Session Progress message to the VPC  22  (step  128 ), which will send the 183 Session Progress message back to the E-CSCF  18  (step  130 ). Notably, other embodiments will not keep the VPC  22  in the signaling path; however, the VPC  22  is maintained in the signaling path for the described embodiment. The E-CSCF  18  will then route the 183 Session Progress message through the DTF  20  (steps  132  and  134 ) and then toward the P-CSCF  16  (step  136 ). The P-CSCF  16  will send the 183 Session Progress message to the user element  14  (step  138 ), wherein the user element  14  may provide an alert to the user that the call is being presented to emergency services. During this time, assume that the E-session is processed as indicated by the providing of an ISUP Address Complete Message (ACM) back toward the PSAP MGC  24  (step  140 ). A corresponding SIP message may be routed back toward the user element  14  over the signaling path (not shown). 
     Once the E-session is answered, the PSAP  26  will send an ISUP Answer Message (ANM) toward the PSAP MGC  24  (step  142 ). In response to the ANM, the PSAP MGC  24  will send a 200 OK message to the E-CSCF  18  (step  144 ), which will route the 200 OK message through the VPC  22  (steps  146  and  148 ), and then to the DTF  20  (step  150 ). Since the E-session is anchored at the DTF  20 , and the DTF  20  will control subsequent subsystem transfers, an STN for MS-to-CS transfers and an STU for CS-to-MS transfers may be provided for the user element  14  (step  152 ). Again, the STN is a directory number that the user element  14  may use to initiate a new access signaling leg toward the DTF  20  from within the visited CS  32 . The STU is a uniform resource location (URL) that the user element  14  may use to initiate a new access signaling leg toward the DTF  20  from within the visited MS  10 , or in other words, from a PS served by the visited MS  10 . The DTF  20  will then forward the 200 OK message with the STN and STU to the E-CSCF  18  (step  154 ), which will send the 200 OK message toward the user element  14  via the P-CSCF  16  (steps  156  and  158 ). The user element  14  will recognize that the call has been answered upon receipt of the 200 OK message, and will extract and store the STN and the STU provided in the 200 OK message (step  160 ). Notably, the initial Invite may include the Session Data Protocol (SDP) information of the user element  14  to allow the PSAP MG  28  to deliver bearer traffic to the user element  14 . Similarly, the 200 OK message provided by the PSAP MG  28  will provide SDP information to allow the user element  14  to deliver bearer traffic to the PSAP MG  28  to facilitate a bearer path (step  162 ). Notably, the bearer path between the user element  14  and the PSAP MG  28  is packet-based and supported by the underlying PS, wherein the CS-based portion of the bearer path between the PSAP MG  28  and the PSAP  26  may be a circuit-switched time division multiplexed (TDM) connection. 
     With reference to  FIGS. 4A and 4B , a communication flow illustrating a subsystem transfer from the visited MS  10  to the visited CS  32  is provided. Initially, assume the user element  14  determines a need to transfer from the visited MS  10  to the visited CS  32  (step  200 ). Notably, the E-session established via the communication flow of  FIGS. 3A through 3C  is active and anchored in the DTF  20 . To initiate the subsystem transfer, the user element  14  may send an emergency origination message (E-Origination) to the VMSC  34 , which is serving the user element  14  (step  202 ). Notably, the E-Origination may include the STN and an updated location reference in case the location of the user element  14  has changed from the time the E-session was originally initiated. In response to the E-Origination message the VMSC  34  discovers the STN associated with the E-origination if one is not included by the UE and will send an ISUP IAM toward the CS MGC  36  that is associated with the STN (step  204 ). Notably, the ISUP IAM will include the updated location reference. The CS MGC  36  will generate an Invite, which is delivered into the visited MS  10  and received by the I-CSCF  40  (step  206 ). The Invite will include the STN, the updated location reference, and the SDP information associated with the CS MG  38 . 
     The I-CSCF  40  may access the HSS  44  to identify an CSAF  42  to which the session request should be routed in light of the STN (step  208 ). Once the CSAF  42  has been identified, the I-CSCF  40  will forward the Invite to the CSAF  42 , which is associated with the E-CSCF  18  (step  210 ). The CSAF  42  will act as a remote user agent on behalf of the user element  14  (for the CS access signaling leg) and forward the Invite to the E-CSCF  18  (step  212 ). The E-CSCF  18  will forward the Invite to the DTF  20  (step  214 ), which will initiate an MS-to-CS transfer and provide a location update for the PSAP  26  (step  216 ). 
     Accordingly, the DTF  20  will send a Re-Invite toward the PSAP  26  to update the communication information for the bearer path and provide the updated location reference, which was provided in the Invite or derived therefrom (step  218 ). The CS communication information may include the SDP of the media gateway, and any other information necessary for support of a bearer path between the PSAP MG  28  and the CS MG  38 . The E-CSCF  18  will route the Re-Invite through the VPC  22  (steps  220  and  222 ) and on toward the PSAP MGC  24  (step  224 ). The PSAP MGC  24  will process the CS communication information to enable delivery of information from the PSAP  26  over a new bearer path to the CS MG  38  as well as provide information as to the updated location reference to the PSAP  26  (step  226 ). The call will be processed, and a 200 OK message, which is not illustrated, is propagated back toward the user element  14  along the signaling path, and will provide the SDP information of the PSAP MG  28  to the CS MGC  36  for processing by the CS MG  38 . As s result, a bearer path is established between the user element  14  and the PSAP  26  via the CS MG  38  and the PSAP MG  28  (step  228 ). The bearer path is packet-based between the CS MG  38  and the PSAP MG  28 . A TDM-based circuit-switched connection is provided between the PSAP MG  28  and the PSAP  26 , as well as between the CS MG  38  and the user element  14  via the VMSC  34 . 
     With reference to  FIG. 5 , a communication flow is provided for transferring the E-session from the visited CS  32  back to the visited MS  10 . Initially, the user element  14  will determine a need to transfer from the visited CS  32  to the visited MS  10  (step  300 ). The user element  14  will initiate an E-Invite toward the DTF  20 , potentially using the STU. The E-Invite will include an updated location reference in case the user element  14  has moved since the last transfer. The P-CSCF  16  will receive the Invite from the user element  14  (step  302 ), and forward the E-Invite to the E-CSCF  18  (step  304 ). The E-CSCF  18  will forward an Invite to the DTF  20  (step  306 ), which will initiate a subsystem transfer from the visited CS  32  to the visited MS  10 , and provide a location update for the PSAP  26  (step  308 ). Accordingly, the DTF  20  will send a Re-Invite with communication information for the visited MS  10  and the updated location reference to the E-CSCF  18  (step  310 ). The communication information for the visited MS  10  may include the SDP information for the user element  14 . The E-CSCF  18  will then route the Re-Invite through the VPC  22  (step  312  and  314 ), and then to the PSAP MGC  24  (step  316 ). The PSAP MGC  24  may send an Information message providing the updated location reference to the PSAP  26  (step  318 ). At this point, the PSAP MGC  24  will have access to the SDP information of the user element  14 , and will ultimately provide a 200 OK message (not shown) in response to the Re-Invite back toward the user element  14  over the signaling path. In the 200 OK message, the SDP information of the PSAP MG  28  is provided to the user element  14 . With the exchange of the SDP information between the user element  14  and the PSAP MG  28 , the packet portion of a bearer path may be established between the user element  14  and the PSAP MG  28  (step  320 ). The TDM-based CS connection between the PSAP MG  28  and the PSAP  26  will remain intact and will be interworked with the packet portion of the bearer path via the PSAP MG  28 . 
     From the above, the DTF  20  may recognize that an E-session is already established with the user element  14  in one domain when a new request for an E-session is received from the user element  14  in another domain. In response, the DTF  20  will effect a transfer to the subsystem from which the new session is being requested. In effect, the remote access leg is maintained while a new access signaling leg is established through the subsystem from which the new E-session is requested. The DTF  20  will associate the new access signaling leg with the remote access signaling leg, and release the old access signaling leg to effect the transfer from one subsystem to another. During this process, any updated location information provided by the user element  14  or associated with the user element  14  may be forwarded toward the PSAP  26  by the DTF  20 . 
     In the above embodiments, assume that the PSAP domain  12  and the PSAP  26  therein are served by a CS. As illustrated in  FIGS. 6 and 7 , the PSAP domain  12 , like the visited MS  10 , may be served by a PS. As such, there is no need for a media gateway, such as the PSAP MG  28 , to provide interworking between the PS that supports the visited MS  10  and a CS supporting the PSAP domain  12 . When the PSAP domain  12  is supported by a PS, packet-based communications may take place directly or indirectly between the E-CSCF  18  and the PSAP  26 . Similarly, the bearer path may be completely packet based from the user element  14  to the PSAP  26 , as illustrated in  FIG. 6 , or between the CS MG  38  and the PSAP  26 , as illustrated in  FIG. 7 . 
     With reference to  FIG. 8 , a service node  46  is provided according to one embodiment of the present invention. The service node  46  may reside in the visited MS  10  and include a control system  48  and associated memory  50  to provide the functionality for any one or a combination of the following: the P-CSCF  16 , the I-CSCF  40 , the E-CSCF  18 , the CSAF  42 , the DTF  20 , the VPC  22 , or any combination thereof. The control system  48  will also be associated with a communication interface  52  to facilitate communications with any entity affiliated with the visited MS  10  or appropriately associated networks. 
     With reference to  FIG. 9 , a block representation of a user element  14  is provided. The user element  14  may include a control system  54  having sufficient memory  56  to support operation of a CS client  58  and an MS client  60 , which support CS and PS communications, respectively. The control system  54  will cooperate closely with a communication interface  62  to allow the CS client  58  and the MS client  60  to facilitate communications over a CS or the PS (MS) as described above. The control system  54  may also be associated with a user interface  64 , which will facilitate interaction with the user. The user interface  64  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  14 . 
     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.