Abstract:
A method and apparatus for maintaining service continuity for User Equipment accessing an IP Multimedia Subsystem communication network. A routing identifier is established that identifies a Service Centralization and Continuity Application Server allocated to the User Equipment. The routing identifier is sent to the User Equipment. In the event of disruption to the service between the User Equipment and the Service Centralization and Continuity Application Server, a handover message is sent from the User Equipment via a Circuit Switched access network. The handover message includes the routing identifier, and is then forwarded to the identified Service Centralization and Continuity Application Server. This allows the same Service Centralization and Continuity Application Server to be used after the handover as was used before the handover, thereby providing service continuity.

Description:
TECHNICAL FIELD 
     The invention relates to the field of maintaining service continuity in an IP Multimedia Subsystem communication network. 
     BACKGROUND 
     The IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IP Multimedia services provide a dynamic combination of voice, video, messaging, data, etc. within the same session. As the number of basic applications, and the media which it is possible to combine, increases, so will the number of services offered to the end users, giving rise to a new generation of personalised, rich multimedia communication services. The IMS is defined in the 3GPP Specification 23.228. 
     The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly. 
     IMS relies on Internet Protocol (IP) as a transport technology. Using IP for voice communications, however, presents some challenges, especially in the mobile community where Voice Over IP (VoIP) enabled packet switched (PS) bearers may not always be available. To allow operators to start offering IMS-based services while voice enabled PS-bearers are being built out, the industry has developed solutions that use existing Circuit Switched (CS) networks to access IMS services. These solutions are referred to as IMS Centralized Services (ICS). ICS is also the name of the Work Item in 3GPP Release 8 addressing these matters (IP Multimedia System (IMS) centralized services (Release 8), 3GPP TS 23.292 V8.0.0). 
     A functional entity in an IMS network is a Service Centralization and Continuity Application Server (SCC AS), which is an application based in a user&#39;s home network IMS Application. The SCC AS provides functionality required to enable IMS Centralized Services. The SCC AS is inserted in a session path using originating and terminating Initial Filter Criteria (iFCs) and configured such that it is the first AS in an originating iFC chain and the last AS in a terminating iFC chain. 
     User Equipment (UE) typically communicates with the SCC AS for service control using the Gm reference point. In come circumstances, using a Gm reference point may not be possible (e.g. when using GERAN and the GERAN does not support DTM). In this case, a service control signalling path is handed over to I 1 . 3GPP TS 23.292, section 7.8 describes the procedure for signalling path handover from Gm to I 1  when Gm is lost or not available. The UE sends the handover request to the SCC AS over I 1  and the SCC AS is informed that it should now use I 1  instead of Gm for controlling the ongoing session with the UE. 
     When a handover from Gm to I 1  is required, the handover request should be routed from the UE over I 1  to the same SCC AS that was previously handling the session over Gm. Suppose, for example, that Unstructured Supplementary Service Data (USSD) is used to communicate over I 1 . Using current methods, a Home Location Register (HLR) can be statically configured with an E.164 number for the ICS service code for a range of subscribers. This E.164 number can represent the actual SCC AS or a Representative AS (see IP Multimedia Subsystem (IMS); Stage 2 (Release 8), 3GPP TS 23.228 V8.3.0) that performs dynamic allocation of users to an SCC AS. However, routing over Gm is not synchronized with routing over I 1 . No solution exists to allow the UE to route a request over I 1  to the same SCC AS over Gm. There is therefore no guarantee that in the event of a handover from Gm to I 1 , the handover request over I 1  will be routed to the same SCC AS that was previously handling the session over Gm, and so the handover may fail. This problem exists not only for the signalling path handover from Gm to I 1 , but also in the case of service continuity where a service transfer is made from a Packet Switched (PS) to a Circuit Switched (CS) access. 
     SUMMARY 
     The inventors have devised a method and apparatus for maintaining service continuity in the event of a handover from a Gm to an I 1  reference point or from a Packet Switched (PS) only to a Circuit Switched (CS) network, by identifying the Service Centralization and Continuity Application Server (SCC AS) in such a way that the same SCC AS can be used after handover. 
     According to a first aspect of the invention, there is provided a method of maintaining service continuity for User Equipment (UE) accessing an IP Multimedia Subsystem (IMS) communication network. A routing identifier identifying a SCC AS allocated to the UE is established, and sent to the UE. In the event of disruption to the service between the UE and the SCC AS, a handover message is sent from the UE via a CS access network. The handover message includes the routing identifier, and is then forwarded to the identified SCC AS. This allows the same SCC AS to be used after the handover as was used before the handover, thereby providing service continuity. 
     As an option, the method can be used when the disruption is caused by handover of signalling between the UE and the SCC AS from a Gm reference point to an I 1  reference point, or alternatively, when the disruption is caused by handover from a PS access network to the CS access network. 
     Optionally, the routing identifier is sent to the UE in one of a Session Initiation Protocol 200 OK message and a Session Initiation Protocol INVITE message, depending on whether the message originates at the UE or terminates at the UE. 
     As an option, the handover message is sent from the CS access network to a Representative Application Server, and the Representative Application Server selects a SCC AS on the basis of the routing identifier prior to forwarding the handover message to the identified SCC AS. 
     The handover message is optionally sent in an Unstructured Supplementary Service Data format. 
     According to a second aspect of the invention, there is provided a UE for use in an IMS network. The UE is provided with a receiver for receiving a routing identifier identifying a SCC AS allocated to the UE. A memory is provided for storing the received routing identifier, and a processor is provided for detecting disruption to an ongoing service between the UE and the SCC AS. A transmitter is also provided for, in the event that disruption is detected, sending a handover message to an access network node, the handover message including the routing identifier. This allows the UE to request the same SCC AS after the handover as was used before the handover. 
     The processor is optionally arranged to detect a loss of Gm availability between the UE and the SCC AS. Alternatively, or additionally, the processor is optionally arranged to detect a handover from a PS access network to a CS access network. 
     According to a third aspect of the invention, there is provided a HLR for use in a communication network. The HLR is provided with a processor for allocating a routing identifier to a SCC AS allocated to a UE, and a memory for storing the routing identifier and an associated E.164 number for the SCC AS. A transmitter is provided for sending the routing identifier to the UE. 
     As an option, the HLR is provided with a receiver for receiving a handover request from the UE, the handover request including the routing identifier. A second processor is provided for using the routing identifier to determine the E.164 number of the SCC AS, and a second transmitter is provided for forwarding the handover request to the SCC AS. In this case, the HLR optionally is provided with a third processor for removing the routing identifier from the received handover request prior to forwarding the handover request to the SCC AS. 
     According to a fourth aspect of the invention, there is provided a SCC AS for use in an IMS communication network. The SCC AS is provided with a processor for allocating a routing identifier to the SCC AS, a memory for storing the routing identifier, and a transmitter for sending the routing identifier to the UE. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a signalling diagram according to an embodiment of the invention; 
         FIG. 2  is a signalling diagram illustrating routing identifier assignment according to an embodiment of the invention; 
         FIG. 3  is a signalling diagram illustrating handover signalling according to an embodiment of the invention; 
         FIG. 4  is a signalling diagram illustrating signalling required for handover where a Representative AS  7  is used according to an embodiment of the invention; 
         FIG. 5  illustrates schematically in a block diagram a User Equipment according to an embodiment of the invention; 
         FIG. 6  illustrates schematically in a block diagram a Home Location Register according to an embodiment of the invention; and 
         FIG. 7  illustrates schematically in a block diagram a Service Centralization and Continuity Application Server according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1  herein, there is illustrated a UE  1 , a Circuit Switched network  2 , an IMS network  3  and an Application Server  4  such as an SCC AS or a representative AS. Each SCC AS  4  is allocated a routing identifier, which can be carried in USSD signalling. The allocation of the routing identifier can be performed in any suitable node, such as a Home Location Register (HLR). 
     The following steps are performed: 
     S 1 . When the UE  1  initiates a session over a Gm reference point, the SCC AS  4  is allocated to the UE  1  for handling the session using standard IMS procedures. A routing identifier is created according to standard USSD procedures. The routing identifier is known to the SCC AS  4 , and can be used by a HLR to find the SCC AS  4 , for example by mapping the identifier to an E.164 number which points to the SCC AS  4 . The routing identifier of the SCC AS  4  is then sent back to the UE  1  over Gm, for example in the response of the first INVITE send by the UE  1  or in a terminating INVITE.
 
S 2 . The UE  1  loses Gm capability.
 
S 3 . The UE sends a request for signalling path handover due to loss of Gm capability, and uses the routing identifier over I 1 .
 
S 4 . A HLR in the CS network  2 , when it receives this message, maps the routing identifier directly to the SCC AS  4  that was handling the service over Gm, and forwards the signalling path handover message to the SCC AS  4 . Alternatively, the HRL forwards the signalling path handover message to the representative AS which in turn uses the routing identifier to select the correct SCC AS  4 .
 
     Referring to  FIG. 2  herein, assignment and delivery of the routing identifier is illustrated. For sessions originating from the served user UE, (outgoing calls), a normal SIP INVITE procedure is performed, illustrated by steps S 5  to S 8 . Then the SCC AS  4  replies to the IMS network  3  with a SIP 200 OK message (step S 9 ), the routing identifier is included in that message, and forwarded (S 10 ) to the UE  1  from the IMS network  3 . As mentioned above, the routing identifier is determined by which SCC AS  4  has been assigned to the user, and is therefore also know to the SCC AS  4  when adding it to the SIP 200 OK. 
     For terminating requests (incoming calls), illustrated in steps S 11  to S 16 , the routing identifier is added to the incoming INVITE to the UE  1 , as shown in step S 12 . 
     There are several ways in which the routing identifier can be included in the SIP message (either the 200 OK or the INVITE). Example include:
         A separate header in the SIP message, dedicated for the routing identifier, i.e., “P-Routing-Identifier: xyzabc”;   As part of an existing header in the SIP message, such as the ‘via’ header, or ‘contact’ header.   As part of Session Description Protocol, in the media line related to the CS media.
           m=audio-PSTN-   c=PSTN E164+1234567   a=route-identifier:xyzabc   
               

       FIG. 3  illustrates signalling path handover using the routing identifier. The following numbering corresponds to the numbering in  FIG. 3 : 
     S 17 . The UE  1  detects that it has lost Gm. There are several possible ways in which this detection can be made, for example by detecting that handover to GERAN has been performed and the UE does not support DTM, or detecting that a packet bearer (PDP context) has been lost.
 
S 18 . The UE  1  initiates a signalling path handover by sending a handover message over I 1  to the CS network  2 . In addition, the UE  1  includes the routing identifier in the handover message.
 
S 19 . The message is forwarded through the CS network  2  all the way to the HLR  6 .
 
S 20 . The HLR  6  uses the routing identifier to identify the SCC AS  4  that should be used for the UE  1  and its handover message.
 
S 21 . The signalling path handover message is forwarded to correct SCC AS  4 . The HLR may remove the routing identifier from the message before forwarding to the SCC AS  4 , since such information is of no use to the SCC AS  4 , although it is not essential to do so.
 
S 22 . An answer is sent from the SCC AS  4  to the HLR  6 .
 
S 23 . The answer to the signalling path handover message is forwarded back through the CS network  2 .
 
S 24 . The answer to the signalling path handover message is forwarded back to the UE  1 .
 
     In an alternative embodiment of the invention, a Representative AS represents the SCC AS.  FIG. 4  is a signalling diagram illustrating signalling required for handover where a Representative AS  7  is used, with the following numbering corresponding to the numbering of  FIG. 4 : 
     S 25 . The UE  1  detects that it has lost Gm by some means. 
     S 26 . The UE  1  initiates a signalling path handover by sending a handover message over I 1  to the CS network  2 . In addition, the UE  1  includes the routing identifier in the message. 
     S 27 . The message is forwarded through the CS network  2  to the HLR  6 . 
     S 28 . The HLR  6  identifies the Representative AS  7  that handles all SCC ASs  4 , and forwards the message to the Representative AS  7 . 
     S 29 . The Representative AS uses the routing identifier to identify the SCC AS  4  that should be used for the UE  1  and the handover message 
     S 30 . The signalling path handover message is forwarded to correct SCC AS  4 . Before forwarding the message, the Representative AS  7  may remove the routing identifier from the message, although this is not essential. 
     S 31 . An answer to the signalling path handover message is sent back from the SCC AS  4  to the Representative AS  7 . 
     S 32 . The answer to the signalling path handover message is forwarded back to the HLR  6 . 
     S 33 . The answer to the signalling path handover message is forwarded back through the CS network  2 . 
     S 34 . The answer to the signalling path handover message is forwarded back to the UE  1 . 
     According to a further embodiment of the invention, the routing identifier is used during service transfer from PS to CS, when performing service continuity (see IP Multimedia System (IMS) Service Continuity (Release 8), 3GPP TS 23.237 V8.0.0). The same problem is addressed, namely how the UE can reach the same SCC AS over I 1  after a service transfer from PS to CS. Using the routing identifier ensures that signalling over I 1  ends up in the same SCC AS that previously handled the session over Gm before the service transfer. In this case, the routing identifier can be allocated by the SCC AS when the SCC AS contacts the UE over I 1  first (since the SCC AS would have the IMSI of the UE) after the service transfer. 
     Referring now to  FIG. 5 , there is illustrated a UE  1  according to an embodiment of the invention. The UE  1  is provided with a receiver  8  that receives the routing identifier, and a memory  9  for storing the routing identifier. A processor  10  is used to detect a loss of signalling between the UE  1  and the SCC AS  4 , as described above, which may be either because of loss of availability of the Gm reference point or a handover from a PS to a CS access network. In the event of loss of signalling, a transmitter  11  is provided for sending a handover message that includes the routing identifier to the CS access network. 
       FIG. 6  illustrates schematically a HLR  6  according to an embodiment of the invention. The HLR  6  is provided with a processor  12  that allocates a routing identifier to the SCC AS  4  that is allocated to the UE  1 . A memory  13  stores the routing identifier and an associated E.164 number for the SCC AS  4 , and a transmitter  14  sends the routing identifier to the UE  1 . A receiver  15  is arranged to receive a handover request from the UE  1 . A second processor  16  uses the routing identifier received in the handover request to determine the E.164 number of the SCC AS  4 , and a second transmitter  17  is arranged to for forward the handover request to the SCC AS  4 . If required, a third processor  18  is arranged to remove the routing identifier from the received handover request prior to forwarding the handover request to the SCC AS  4 . The three processors  12 ,  16 ,  18  are all embodiment in a single processor in the example of  FIG. 7 , although it will be appreciated that they may also be one or more separate processors. 
     Referring now to  FIG. 7 , there is illustrated schematically the SCC AS  4 . The SCC AS  4  is provided with a processor  19  for allocating a routing identifier to the SCC AS  4 . A memory  20  is provided for storing the routing identifier, and a transmitter  21  is provided for sending the routing identifier to the UE  1 . A receiver  22  may also be provided for receiving signalling from the UE  1 . 
     The invention ensures that the same SCC AS is selected when changing the signalling between Gm and I 1 , and also after performing service transfer from PS to CS. This improves the user experience of the service. 
     It will be appreciated by the person of skill in the art that various modifications may be made to the above-described embodiments without departing from the scope of the present invention.