Abstract:
A method and apparatus for managing access domain selection for a user device accessing an IP Multimedia Subsystem (IMS) network. A Call Session Control Function (CSCF) in the IMS network stores an access domain indicator associated with a user s contact address. The access domain indicator is associated with the user s contact address when the user registers with the IMS network. The CSCF sends the access domain indicator to an Access Domain Selection (ADS) function in the IMS network, the access domain indicator to be used by the ADS function in selecting an access domain. This allows the ADS function to select the correct access domain to use when sending messages to the user device.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to the field of access domain selection in a communications network. 
       BACKGROUND 
       [0002]    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. 
         [0003]    The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals. The Session Description Protocol (SDP), carried by SIP signals, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, the IMS allows operators and service providers to control user access to services and to charge users accordingly. 
         [0004]      FIG. 1   a  illustrates schematically how the IMS fits into the mobile network architecture in the case of a General Packet Radio Service (GPRS) access network. As shown in  FIG. 1   a  control of communications occurs at three layers (or planes). The lowest layer is the Connectivity Layer  1 , also referred to as the bearer plane and through which signals are directed to/from user equipment, UE, accessing the network. The entities within the connectivity layer  1  that connect an IMS subscriber to IMS services form a network that is referred to as the IP-Connectivity Access Network, IP-CAN. The GPRS network includes various GPRS Support Nodes (GSNs). A gateway GPRS support node (GGSN)  2  acts as an interface between the GPRS backbone network and other networks (radio network and the IMS network). The middle layer is the Control Layer  4 , and at the top is the Application Layer  6 . 
         [0005]    The IMS  3  includes a core network  3   a,  which operates over the middle, Control Layer  4  and the Connectivity Layer  1 , and a Service Network  3   b.  The IMS core network  3   a  includes nodes that send/receive signals to/from the GPRS network via the GGSN  2   a  at the Connectivity Layer  1  and network nodes that include Call/Session Control Functions (CSCFs)  5 , which operate as SIP proxies within the IMS in the middle, Control Layer  4 . The 3GPP architecture defines three types of CSCFs: the Proxy CSCF (P-CSCF) which is the first point of contact within the IMS for a SIP terminal; the Serving CSCF (S-CSCF) which provides services to the user that the user is subscribed to; and the Interrogating CSCF (I-CSCF) whose role is to identify the correct S-CSCF and to forward to that S-CSCF a request received from a SIP terminal via a P-CSCF. The top, Application Layer  6  includes the IMS service network  3   b . Application Servers (ASs)  7  are provided for implementing IMS service functionality 
         [0006]    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. The Service Engine (service implementation) resides in IMS and the CS network is merely used as an access to the services in IMS. 
         [0007]    ICS-AS (IMS Application Server) allows signalling that is not supported over standard CS signalling (ISUP) to emulate an IMS terminal towards an IMS core network. In this way, an IMS terminal connected to an IMS network via a CS access network can use IMS services. Examples of such signalling include IMS registration, mid-call signalling, additional info for call set-up signalling (e.g. SIP URL). Unstructured Supplementary Service Data (USSD) is used to transport this additional signalling (termed ICCP, IMS CS Control Protocol) in a CS network. The session continuity aspect of a session is decoupled from the ICS-AS and resides in a Voice Call Continuity (VCC) Application Server (AS). VCC specifications define methods that allow a user with User Equipment (UE) to move between different access technologies without requiring different levels of service depending on the access technologies. VCC functionality includes, for example, support for allowing the UE to automatically select an access network. 
         [0008]    There are currently two models for ICS access that may be used. The first is to use an ICS-AS using the IP Multimedia Subsystem Control (ISC) interface: The ICS-AS behaves as an AS connected to an IMS Serving-Call Session Control Function (S-CSCF) via the ISC interface. The second is ICS-AS using an Mw interface: The ICS-AS emulates a Proxy-Call Session Control Function (P-CSCF) connected to an S-CSCF via an Mw interface. 
         [0009]      FIG. 2  illustrates ICS architecture for the first option of ICS-AS using an ISC interface. An ICS-AS  8  interfaces with an IMS Core network  9  using the ISC interface. The ICS-AS  8  is triggered and involved in an IMS session using initial Filter Criteria (iFC)s. When the ICS-AS  8  receives a SIP INVITE message, an Access Domain Selection (ADS) function, also known as Domain Selection Function (DSF) selects the access domain to reach the User Equipment (UE)  10 . 
         [0010]      FIG. 3  illustrates schematically ICS architecture for the second option described above, in which the ICS-AS  8  emulates a Proxy-Call Session Control Function (P-CSCF). The ICS-AS  8  interfaces with IMS Core  9  using the Mw interface and a SIP message is routed in the same way as a SIP message is routed from a S-CSCF  11  to a P-CSCF  12 . When a VCC AS  13  receives a SIP INVITE message, an ADS function selects the access domain at which the UE  10  should be reached. The ADS function may be deployed as an independent AS from the VCC AS  13 . Note that the ICS-AS  8  is only used when ADS selects CS access as the access domain, whereas the scenario in which the ICS-AS  8  uses an ISC interface, as described above, involves the ICS-AS  8  in both CS and PS access. 
         [0011]    According to 3GPP ReI7 VCC, a VCC user is not registered with an IMS network when he is using a CS access network. In this case, an unregistered trigger is used to provide IMS service, and a Telephony Application Server (TAS)  14  must implement an additional mechanism (e.g. using CAMEL) to ascertain whether a user is attached using a CS access network or not in order to provide registered IMS services to a user. One possible solution to avoid this is to support IMS registration from an ICS-AS  8  based on triggers received, for example from the UE  10  using the ICCP so that the TAS  14  can be informed from the S-CSCF  11  using a third party registration procedure. The S-CSCF  11  uses a Reg event package as specified in RFC 3680, “Session Initiation Protocol (SIP) Event Package for Registrations”, to ascertain whether a user is registered with an IMS network using a CS access network. 
         [0012]    Currently, an ADS function functions as follows: When a user access an IMS network using a CS access network, the ADS function changes a Request Uniform Resource Identifier (URI) from a sip or tel URI of the called party to a CSRN (CS Routing Number). The CSRN is used to route a call from the IMS network to the user in the CS domain. The CSRN may be a Mobile Station Roaming Number (MSRN) dynamically fetched from a Visited Mobile Switching Centre (VMSC), or may be obtained from a prefix to the called party&#39;s Mobile Subscriber ISDN Number (MSISDN), where the prefix is a routing number to the CS access domain. The S-CSCF  11  routes the INVITE request to the CS domain via a Breakout Gateway control Function (BGCF) and a Media Gateway Control Function (MGCF)  15 , as it recognizes that the CSRN does not belong to a user connected to the IMS network (using Telephone Number Mapping, ENUM). 
         [0013]    When a PS access network is selected, the ADS function returns the same Request URI unchanged which is a sip or tel URI of the called party. According to 3GPP ReI7 VCC, only the contacts using PS access are registered with the IMS network. The S-CSCF  11  therefore routes the INVITE request to the contact addresses registered in the IMS network as described in TS 24.229 subclause 5.4.3.3. Note that in 3GPP ReI7 VCC, a VCC AS  13  is the last AS invoked for terminating calls and the first AS invoked for originating calls in the ISC chain. 
         [0014]    In 3GPP ReI 8 ICS, a user is IMS registered even when he is using a CS access network. 
         [0015]    Where ICS-AS using an ISC interface is used, as described above, the ICS-AS is decoupled from the VCC AS  13 , and the ICS-AS  16  is the last AS invoked for terminating calls and the first AS invoked for originating calls in the ISC chain. The ICS-AS  16  also includes an ADS function. 
         [0016]    When a CS access network is selected, the ADS function in the ICS-AS  8  operates in the same way as described in 3GPP ReI7. The ADS function changes the Request URI from a sip or tel URI of the called party to a CSRN, and the S-CSCF  11  routes the INVITE request to the CS access domain. 
         [0017]    When PS access is selected, the ADS returns the same Request URI unchanged. The URI is a sip or tel URI of the called party. The S-CSCF  11  registers both the PS contact address and the CS contact address, and so the 3GPP ReI7 VCC mechanism as explained above does not work. The INVITE request may be routed to the CS contact address, which is associated with the ICS-AS  8 . If the request is routed to that contact address, the request will fail. A new mechanism is required in the ADS function and the S-CSCF  11  to exclude a CS contact addresses for INVITE request routing, and to route the request only to a PS access contact address. 
         [0018]    Where an ICS-AS  8  using a Mw interface is used, the ICS-AS  8  is decoupled from the VCC AS  13 , and the VCC AS  13  is the last AS invoked for terminating calls and the first AS invoked for originating calls. The VCC AS  13  includes an ADS function. 
         [0019]    When a CS access network is selected, the ADS function needs to instruct the S-CSCF  11  to route the INVITE request only to the ICS-AS  8  and the CS access network. If a CSRN is used in the INVITE Request URI, the contact selection of a UE&#39;s Public User ID is not used. This is because a CSRN is different from a UE&#39;s Public User ID (i.e. call forwarding to different number) and is not a number registered with the IMS network. The SIP INVITE will not route to the ICS-AS  8  using the Mw interface in this case, but will instead route directly to the CS access domain via the BGCF/MGCF  15  without involving the ICS-AS  8 . If the same Request URI is returned unchanged, the S-CSCF  11  may route the INVITE message to a PS contact that is registered with the S-CSCF  11 , which is not desirable in this case. A new mechanism is required in the ADS and the S-CSCF  11  to route to only a CS access contact address and to the ICS-AS  8 . 
         [0020]    When a PS access network is selected, the same issue described above for the ICS-AS  8  using an ISC interface arises. When a PS access network is selected, the ADS returns the same Request URI unchanged, which is a sip or tel URI of the called party. The S-CSCF  11  registers both the PS contact address and the CS contact address, and so the 3GPP ReI7 VCC mechanism as explained above does not work. The INVITE request may be routed to the CS contact address, which is associated with the ICS-AS  8 . If the request is routed to that contact address, the request will fail. A new mechanism is required in the ADS function and the S-CSCF  11  to exclude CS contact addresses for INVITE request routing, and to route the request only to a PS access contact address. 
       SUMMARY 
       [0021]    The inventors have realised the problems associated with CS access to an IMS network, and have devised a method an apparatus to mitigate these problems. 
         [0022]    According to a first aspect of the invention, there is provided a method of managing access domain selection for a user device accessing an IP Multimedia Subsystem (IMS) network. A Call Session Control Function (CSCF) in the IMS network stores an access domain indicator associated with a user&#39;s contact address. The access domain indicator is associated with the user&#39;s contact address when the user registers with the IMS network. The CSCF sends the access domain indicator to an Access Domain Selection (ADS) function in the IMS network, the access domain indicator to be used by the ADS function in selecting an access domain. This allows the ADS function to select the correct access domain to use when sending messages to the user device. 
         [0023]    In an optional embodiment, the access domain indicator comprises a feature tag, the feature tag indicating that the access domain is a circuit switched access domain, and comprising an address identifying an IMS Centralized Services Application Server (ICS AS). As a further option, the ADS function selects a packet switched network to which a communication is to be sent. The CSCF determines if the Feature Tag is present in a Reject-Contact header of an INVITE message and, as a result of the determination, excludes the user&#39;s contact address associated with the Feature Tag, This prevents the invite message from being sent (either forked or forwarded) to the user device at the circuit switched access domain. As an alternative option, the ADS function selects a circuit switched network to which a communication is to be sent. The CSCF determine if the Feature Tag is present in an Accept-Contact header of an INVITE message. As a result of the determination, the CSCF amends an address in the INVITE message to the user&#39;s contact address associated with the feature tag, thereby preventing sending the INVITE message to the user device using a packet switched access domain. 
         [0024]    As an alternative option, the access domain indicator comprises a Globally Routable User Agent URL and an associated contact address. The Globally Routable User Agent URL and associated contact address identify, in the case that the user is accessing the IMS network using a packet switched access network, a user terminal, and in the case that the user is accessing the IMS network using a circuit switched network, an IMS Centralized Services Application Server. As a further option, the ADS function receives an INVITE message and selects a packet switched network to which the INVITE message is to be sent. The ADS function then amends a URI included in the INVITE message to the Globally Routable User Agent URL identifying the user terminal, and sends the amended INVITE message to a CSCF. As an alternative option, the ADS function receives an INVITE message and selects a circuit switched network to which the INVITE message is to be sent. The ADS function then amends a URI included in the INVITE message to the Globally Routable User Agent URL identifying the IMS Centralized Services Application Server, and sends the amended INVITE to a CSCF. 
         [0025]    Optionally, a CSCF in the IMS network stores a further access domain indicator associated with a user&#39;s further contact address, the further access domain indicator being associated with the user&#39;s further contact address when the user registers with the IMS network, and sends the further access domain indicator to an ADS function in the IMS network, the further access domain indicator to be used by the ADS function in selecting an access domain. This is useful where, for example, a user is attached to the IMS network using more than one device. 
         [0026]    According to a second aspect of the invention, there is provided a CSCF node for use in an IMS network. The CSCF node has a receiver for receiving an access domain indicator, which is associated with a user&#39;s contact address. A memory is provided for storing the access domain indicator, and a transmitter is provided for sending the access domain indicator to an ADS function. 
         [0027]    The CSCF node optionally further comprises a second receiver for receiving an INVITE message from an ADS function, and a processor for determining if an access domain indicator comprising a Feature Tag is present in a Reject-Contact header of the INVITE message. The processor is arranged to, as a result of the determination, exclude the user&#39;s contact address associated with the feature tag, thereby preventing sending the INVITE message to the user at the excluded user&#39;s contact address. A second transmitter is provided for sending the INVITE message to a user device. 
         [0028]    As an alternative option, the CSCF node is provided with a second receiver for receiving an INVITE message from an ADS function and a processor for determining if an access domain indicator comprising a Feature Tag is present in an Accept-Contact header of the INVITE message. The processor is further arranged to, as a result of the determination, amend an address in the INVITE message to the user&#39;s contact address associated with the feature tag, thereby preventing sending the INVITE message to the user using a packet switched access domain. A second transmitter is provided for forwarding the invite message to the user&#39;s contact address. 
         [0029]    According to a third aspect of the invention, there is provided an ADS function node. The ADS function node is provided with a first receiver for receiving from a CSCF in an IMS network an access domain indicator associated with a user&#39;s contact address. The access domain indicator was associated with the user&#39;s contact address when the user registered with the IMS network. A memory is provided for storing the received access domain indicator. A second receiver is arranged to receive an INVITE message addressed to the user. A processor is provided for amending an address in the INVITE message to a target address on the basis of the stored access domain indicator, and a transmitter is provided for sending the amended INVITE to the CSCF for forwarding to the amended address. In this way, the INVITE is not sent to an incorrect address. 
         [0030]    As an option, the ADS function node is located in one of an IMS Centralized Services Application Server node and a Voice Call Continuity Application Server. 
         [0031]    According to a fourth aspect of the invention, there is provided an ICS AS that is provided with a receiver for receiving a registration request from a User Equipment via a circuit switched access domain. A processor is arranged to associate a feature tag indicating that the access domain is a circuit switched access domain with a user address, and to include an address identifying an ICS AS in the request, and a transmitter is provided for sending the registration request to a CSCF. 
         [0032]    According to a fifth aspect of the invention, there is provided a user device for accessing an IMS network. The user device is provided with a processor for determining an access domain via which the user device can connect to the IMS network, and a transmitter for sending a message requesting registration with the IMS network, the message including an access domain indicator associated with a user&#39;s contact address. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  illustrates schematically the integration of an IP Multimedia Subsystem into a 3G mobile communications system; 
           [0034]      FIG. 2  illustrates schematically IMS Centralized Services architecture where an IMS CS Control Function interfaces with an IMS core network using an IMS Subsystem Control interface, and emulates an Application Server; 
           [0035]      FIG. 3  illustrates schematically IMS Centralized Services architecture in the case where which an IMS CS Control Function emulates a Proxy-Call Session Control Function (P-CSCF); 
           [0036]      FIG. 4  illustrates schematically an example of registration status in the S-CSCF according to a first embodiment of the invention; 
           [0037]      FIG. 5  illustrates schematically SIP INVITE routing to a Packet Switched access network according to a first embodiment of the invention; 
           [0038]      FIG. 6  illustrates schematically SIP INVITE routing to a Circuit Switched access network according to a first embodiment of the invention; 
           [0039]      FIG. 7  illustrates schematically an example of registration status in the S-CSCF according to a second embodiment of the invention; 
           [0040]      FIG. 8  illustrates schematically routing a SIP INVITE message to a Packet Switched access network according to the second embodiment of the invention; 
           [0041]      FIG. 9  is a flow diagram illustrating the steps of an embodiment of the invention; 
           [0042]      FIG. 10  illustrates schematically in a block diagram a Call Session Control Function node according to an embodiment of the invention; 
           [0043]      FIG. 11  illustrates schematically in a block diagram an Access Domain Selection function node according to an embodiment of the invention; 
           [0044]      FIG. 12  illustrates schematically in a block diagram an IMS Centralized Services Application Server according to an embodiment of the invention; and 
           [0045]      FIG. 13  illustrates schematically in a block diagram a User device according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    According to a first embodiment of the invention, a “feature tag” is introduced to indicate at IMS registration of a user that the user is reachable via a CS access network. In the case where an ADS function selects a PS network, a caller preference mechanism (RFC 3841 “Caller Preferences for the Session Initiation Protocol”) is applied to the user&#39;s registration to instruct the S-CSCF to exclude the user&#39;s contacts in a CS access network that match the feature tag, and to select the contacts in a PS access network instead. In the case where a CS access network is selected, and the ADS function is disposed at a Voice Call Continuity Application Server, the feature tag provides information to instruct the S-CSCF to route a call only to the contact(s) associated with the feature tag. 
       Registration 
       [0047]    When a UE registers with an IMS network using ICCP via an ICS-AS, the ICS-AS registers the CS contact of the UE using a feature tag (herein termed CS-FT). The feature tag indicates that a CS access network is being used by the UE, and further includes a contact address associated with the ICS-AS via which the UE can be contacted. 
         [0048]    The ADS function must be aware of the type of access network through which the UE is registered. Where the ICS-AS using an ISC interface model is used, the ADS function is included in the ICS-AS, and so is already aware that the user is registered via a CS access network (because it has triggered registration via the CS access network). Where the ICS-AS using a Mw interface is used, the ADS function is not included in the ICS-AS but instead included in a VCC AS. The fact that the user is accessing the IMS network via a CS access network must be communicated to the VCC AS by a third party registration. 
         [0049]    The S-CSCF stores the CS contact address and the CS-FT.  FIG. 4  illustrates schematically an example of an IMS registration status of a user in the S-CSCF. In this example, the UE  10  is dual access (which means it can access via a CS access network  16  and an IP-CAN 1 /PS access network  17 ), and registers both a CS contact address (contact 1 )  18  and a PS contact address (contact 2 )  19  in the S-CSCF  11 . The CS contact  18  is associated with a CS-FT. An IP Multimedia Public Identity (IMPU) is also registered from a personal computer  20  via access network IP-CAN 2   21  using a different IP-CAN PS contact (contact 3 )  22 . 
       Access Domain Selection 
       [0050]    Where an ICS-AS  8  using an ISC interfaced is used, the ADS function is located in the ICS-AS  8 . When the ICS-AS  8  receives a SIP INVITE request from the S-CSCF  11 , triggered by an iFC, the ADS function is invoked for access domain selection. Where CS access is selected, access domain selection works in the same way as for 3GPP ReI7 VCC. The ADS function changes the Request URI of the SIP INVITE to a CSRN in order that the S-CSCF  11  can route the request to the CS access domain. Contact information associated with the CS access domain is not used for routing in this case. However, when a PS access network is selected, the ADS function does not map the Request URI to a CSRN, but does alter the Request URI to add a Reject-Contact header including the CS-FT. 
         [0051]      FIG. 5  illustrates the case described above where a PS access network is selected for routing a SIP INVITE request. When the S-CSCF  11  in the IMS network receives a SIP INVITE request with an IMPU in the Request URI, it matches the three contacts (this procedure is as defined in step 9 of sub-clause 5.4.3.3 of TS 24.229), one of the contacts being to a CS network  16  and the other two to IP networks  17 ,  21 . However, since the Reject-Contact header contains the CS-FT, Contact 1   18  is excluded from the target sets for further routing as per the caller preference mechanism. Contact 1   18  is rejected because it uses a CS access network  16 . Dependent on local policy or q-values indicated by the UE  10  and the personal computer  20  the SIP INVITE request is forked to UE PS contact 2   19  and/or PC PS contact 3   22  and not to the CS contact 1   18 . 
         [0052]    Where an ICS-AS  8  using the Mw interface is used, the ADS function is located in a VCC AS  13 . When a VCC AS  13  receives a SIP INVITE request from the S-CSCF  11 , triggered by an iFC, the ADS function at the VCC AS  13  is invoked for access domain selection. 
         [0053]    When a PS access network is selected, the same mechanism as described above for  FIG. 5  is applied. The Request-URI is substantially unchanged but a Reject-Contact header includes a CS-FT to exclude CS access network contacts  18 . 
         [0054]    When a CS access network  16  is selected, a mechanism is required to exclusively select CS access contacts  18  to ensure that the session is routed to the ICS-AS  8  rather than to any of the PS access contacts  19 ,  22 , as illustrated in  FIG. 6 . In this case, the ADS function  23  , disposed at the VCC AS  13 , does not substantially alter the Request URI but adds the Accept-Contact header to the Request URI. The Accept-Contact header includes a CS-FT having a “require” and “explicit” parameter. As per the caller preference mechanism, the S-CSCF  11  only routes the call to the contact associated with CS-FT (Contact 1   18  in  FIG. 6 ). In this way, the SIP INVITE is not routed to any of the contacts  19 ,  22  associated with a PS access network. 
         [0055]    According to a second embodiment of the invention, a Globally Routable User agent URL (GRUU) is allocated dynamically by the S-CSCF  11  to each of a user&#39;s contacts in both CS  16  and PS  17 ,  21  access networks. The GRUU is provided to an ADS function  24  using a registration event package from the S-CSCF. The ADS function  24  uses the GRUU in a Request URI to inform the ADS function which access may be used. Whilst this embodiment has the disadvantage that the UE  10  and the IMS core must support the GRUU, it has the advantage that the ADS function  24  can control forking to different access networks without needing to involve the S-CSCF  11 . 
       Registration 
       [0056]    When a UE  10  registers with an IMS network  9  using a CS access network  16  using ICCP via an ICS-AS  8 , the S-CSCF  11  assigns a GRUU to the CS contact, and the ICS-AS  8  registers the CS contact  18  and the assigned GRUU. In the same way, when a UE  10  registers with an IMS network using a PS access network  17 , the UE  10  assigns a GRUU to its contact  19 . An example of registration status in a S-CSCF  11  using a GRUU is illustrated in  FIG. 7 . 
         [0057]    The ADS function  24  subscribes to a registration event package, and receives the binding information on an Address of Record (AoR), Contact address, and its GRUU. As described previously, the ADS function  24  is located in the ICS-AS  8  in the case of an ICS-AS  8  using an  1 SC interface, and as part of a VCC AS  13  for ICS-AS using a Mw interface. 
         [0058]    In the example of  FIG. 7 , the UE  10  is dual access (which means it can access via a CS access network  16  and an IP-CAN 1 /PS access network  17 ), and registers both a CS contact address (contact 1 )  18  and a PS contact address (contact 2 )  19  in the S-CSCF  11 . The CS contact (contact 1 )  18  is associated with GRUU 1 , and the UE  10  PS contact is associated with GRUU 2 . The same IMPU is also registered from a personal computer via access network IP-CAN 2   21 , and has a PS contact (contact 3 )  22 , which is associated with GRUU 3 . 
         [0059]    The ADS function  24  is aware if a particular GRUU is associated with a CS access or not by checking if a contact address matches that of a pre-configured ICS-AS address. 
       Access Domain Selection 
       [0060]    Where an ICS-AS  8  using an ISC interface is used, the ADS function  24  is located at the ICS-AS  8 . When an ICS-AS receives a SIP INVITE request from the S-CSCF triggered by an iFC, the ADS function  24  is invoked for access domain selection. 
         [0061]    In the case where a CS access is selected, the access domain is selected in the same way as described in 3GPP ReI7 VCC. The ADS function changes the Request URI of the SIP INVITE to a CSRN so that S-CSCF can route the SIP INVITE to the CS access network  16 . Contact information associated with the CS access is not used for routing in this case. 
         [0062]    In the case where a PS access is selected, the ADS function  24  uses GRUU 2  or GRUU 3  associated with the user&#39;s PS contacts.  FIG. 8  illustrates the case when a SIP INVITE request is routed to a PS access (GRUU 2 ). In this way the SIP INVITE is routed to a PS access network  17  and not to a CS access network  16 . 
         [0063]    Where an ICS-AS  8  using a Mw interface is used, the ADS function  24  is located at the VCC AS  13 . When the VCC AS  13  receives a SIP INVITE request from the S-CSCF  11 , triggered by an iFC, the ADS function  24  is invoked for access domain selection. When PS access is selected, the same mechanism described above for an ICS-AS  8  using an ISC interface is used, the Request-URI is changed to a GRUU associated with one or more of the PS contacts. Similarly, when CS access is selected, the Request-URI is changed to a GRUU associated with one or more of the CS contacts  18 . 
         [0064]    Both embodiments of the invention described above allow a network operator to control Access Domain Selection from an Application Server, while also allowing a user&#39;s CS contact to be registered in an IMS network. The key steps of both described embodiments of the invention are summarized in the flow chart of  FIG. 9 . When a user registers S 1  with an IMS network, an access domain indicator (which may be a FT or a GRUU) is associated S 2  with the user&#39;s access and provided S 3  to the S-CSCF. The access domain indicator is provided S 4  to an Access Domain Selection function when an access domain is to be selected, and used S 5  by the Access Domain Selection function to select an access domain as described above. 
         [0065]    Referring to  FIG. 10 , there is shown schematically an S-CSCF  11 . The S-CSCF  11  is provided with a receiver for receiving an access domain indicator associated with a user&#39;s contact address, which may be a feature tag or a GRUU as described above. A memory  26  is provided for storing the access domain indicator, and a transmitter  27  is also provided for sending the access domain indicator to an ADS function  24 . A second receiver for  28  may be provided receiving an invite message from the ADS  24 . A processor  29  is used to determine if an access domain indicator comprising a Feature Tag is present in a Reject-Contact header of the invite message or an Accept-Contact header, and, as a result of the determination, amend a target address in the invite message to the correct user&#39;s contact address. A second transmitter  30  is provided for forwarding the invite message to the user&#39;s contact address. 
         [0066]      FIG. 11  illustrates schematically an ADS  24  according to an embodiment of the invention. The ADS  24  is provided with a first receiver  31  for receiving an access domain indicator associated with a user&#39;s contact address from the S-CSCF  11 . A memory  32  is provided for storing the received access domain indicator. A second receiver  33  is provided for receiving an invite message addressed to the user, and a processor  34  is used to amend the address in the invite message to a target address on the basis of the stored access domain indicator. A transmitter  35  is used for sending the amended invite to the S-CSCF  11  for to the amended address. 
         [0067]      FIG. 12  illustrates schematically an ICS-AS  8  according to an embodiment of the invention in which a receiver  36  is provided for receiving a registration request from a UE via a circuit switched access domain, and a processor  37  is provided for associating a feature tag indicating that the access domain is a circuit switched access domain with a user address, and for including an address identifying an IP Multimedia Subsystem Centralized Services Application Server in the request. A transmitter  38  is provided for sending the registration request to a Call Session Control Function. 
         [0068]      FIG. 13  illustrates a User device  10  according to an embodiment of the invention. The user device  10  is provide with a processor  39  arranged to determine an access domain via which the user device  10  can connect to the IMS network, and a transmitter  40  for sending a message requesting registration with the IMS network, the message including an access domain indicator associated with a user&#39;s contact address. 
         [0069]    The following abbreviations have been used in this document: 
         [0070]    3GPP: 3rd Generation Partnership Project 
         [0071]    ADS: Access Domain Selection 
         [0072]    AoR: Address of Record 
         [0073]    AS: Application Server 
         [0074]    BGCF: Breakout Gateway Control Function 
         [0075]    CAMEL: Customised Application for Mobile network Enhanced Logic 
         [0076]    CS: Circuit Switched 
         [0077]    CSCF: Call Session Control Function 
         [0078]    CSRN: CS Routing Number 
         [0079]    DSF: Domain Selection Function 
         [0080]    ENUM: E.164 Numbering 
         [0081]    FT: Feature Tag 
         [0082]    GRUU: Globally Routable User agent URL 
         [0083]    ICS: IMS Centralized Services 
         [0084]    ICS-AS: ICS Application Server 
         [0085]    ICCP: IMS CS Control Protocol 
         [0086]    iFC: Initial Filter Criteria 
         [0087]    IMPU: IP Multimedia Public Identity 
         [0088]    IMS: IP Multimedia Subsystem 
         [0089]    IP-CAN: IP Connectivity Access Network 
         [0090]    ISC: IP Multimedia Subsystem Control 
         [0091]    ISUP: ISDN User Part 
         [0092]    MGCF: Media Gateway Control Function 
         [0093]    MSISDN: Mobile Subscriber ISDN Number 
         [0094]    PS: Packet Switched 
         [0095]    SIP: Session Initiation Protocol 
         [0096]    TAS: Telephony Application Server 
         [0097]    UE: User Equipment 
         [0098]    URL: Uniform Resource Locator 
         [0099]    USSD: Unstructured Service Supplementary Data 
         [0100]    VCC: Voice Call Continuity 
         [0101]    VMSC: Visited Mobile Switching Centre 
         [0102]    It will be appreciated by a 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.