Patent Publication Number: US-2010118861-A1

Title: Inter-Working Between a Packet-Switched Domain and a Circuit-Switched Domain

Description:
TECHNICAL FIELD 
     The present invention relates to a telecommunication solution for providing services via a packet-switched domain, such as IMS to a user with a circuit switched capable terminal being accessible via a circuit-switched domain, such as GSM. 
     BACKGROUND 
     Third Generation (3G) Networks such as UMTS (Universal Telecommunication Network) and CDMA 2000 provide high-speed wireless Internet access to mobile users based on a packet-switched technology. 
     A number of mobile communication networks, such as GSM, with a circuit-switched domain covering an access domain and a service domain are based on a circuit-switched technology. 
     Due to the fact that service providers shift their core network infrastructure from the circuit-switched domain to a consolidated common packet-switched infrastructure, the need occurs to enable a consistent provision of services to subscribers over a variety of accesses domains including circuit-switched CS and packet-switched PS accesses and different service domains. 
     Therefore common controlling platforms are introduced for providing enhanced services for the multitude of different access domains that may be available for an individual call. For the 3G networks the IP Multimedia Subsystem IMS, standardised by the Third Generation Partnership Project (3GPP), has been defined as a common controlling platform which supports voice and multimedia services, as examples of packet-switched services. The strength of IMS as an example of a packet-switched domain, is the provision of enhanced services over packet-switched technology, in particular over IP-network. The usage of IP-network as a single underlying standard allows an easy and fast service deployment. 
     A solution for consistently routing calls from a CS network access through the IMS network to the IMS service domain is currently investigated by the 3GPP. The work item is called IMS Centralized Services (ICS) and aims at moving all subscribers to the IMS for a harmonization of the service (3GPP TS 23.292 IP Multimedia System (IMS) centralized services (Release 8)). 
     The aim of the ICS is to be able to handle users accessing or being served by different service domains, like for example to handle a user having a subscription in a packet-switched domain and being for example in a coverage of a circuit-switched domain. The ICS concept requires an installation of the so called ICS client application, preferably in the user&#39;s terminal and a provision of a packet-switched adapter, like for example the IMS adapter IA being placed between the circuit-switched domain and the IMS packet-switched domain. Users with the ICS client are IMS subscribers, wherein the supplementary and value added services are provided by the IMS domain and merely a basic circuit-switched subscription is provided to the users. 
     In 3GPP TSG SA WG2 Architecture, 28-30 Mar., 2006 in Munich, “USSD based call flows” a mechanism for attaching an IMS user via a legacy network is mentioned. It describes to use a Voice Call Continuity VCC application, for combining circuit-switched and packet-switched functionality. The VCC application is an IMS functionality located in an IMS node in the IMS domain. Further the document describes to use Unstructured Supplementary Service Data (USSD) in the circuit-switched domain for attaching an end terminal to the VCC application. 
     However in some case the ICS concept does not fulfill user&#39;s expectations. For example, when the subscriber roams with the ICS terminal into a CS domain, where the hosting operator has blocked USSD operations, the user is not able to access the packet-switched services. The same problem occurs when for example the ICS terminal has a malfunction which forces the subscriber to swap the SIM card into a terminal without ICS client support. 
     These situations yield to loosing the any packet-switched services in the packet-switched domain so that for example terminating calls can not be carried out. 
     SUMMARY 
     It is an object to provide a technique that realises a unified service domain. In particular it is object of the present invention to provide a solution for providing packet-switched services for a user having a circuit-switched access in a circuit-switched network. 
     The invention is embodied in independent claims. Advantageous embodiments are described in the dependent claims. 
     According to a first aspect of the invention, a method is proposed for providing packet-switched services of a packet-switched domain to a user with a circuit-switched capable terminal being located in a circuit-switched domain and having a circuit-switched access to said circuit-switched domain. Said user is registered in a packet-switched domain via the circuit-switched domain through a packet-switched control channel using a packet-switched adapter to access the packet-switched domain. In this situation, the packet-switched adapter detects the loss of the packet-switched control channel in the circuit-switched domain. There might be different reasons for loosing the packet-switched control connection. The aim of the present invention is to keep the registration in the packet switched domain alive as long as the user is reachable in order to provide the packet switched services to said user. In order to keep the registration in the packet-switched domain alive, at first it is to be determined whether a user is reachable. For this purpose, a tracking procedure is activated. In particular it is proposed to base the tracking procedure on the reception of mobility event notifications related to the terminal. There might be any suitable kind of mobility event notifications, like for example periodical location updates or attach/detach notifications. On the basis of the mobility event notifications, user&#39;s registration status may be determined. 
     Further it is proposed to align the registration in said packet-switched domain with said derived registration status by means of performing a registration procedure on behalf of the user in the packet-switched domain. Thus, in case a user is still reachable via a circuit-switched domain, the packet-switched adapter emulates as if said user were still accessible via a packet-switched control connection. 
     Another aspect of the present invention proposes to provide a packet-switched adapter adapted to communicate between a packet-switched domain and a circuit-switched domain for providing packet-switched services to a user located in the circuit-switched domain with a circuit-switched capable terminal wherein the user is adapted to register via the circuit-switched domain through a packet-switched control channel in the packet-switched domain for receiving the packet-switched service, and is adapted to access the packet-switched adapter. It is proposed that the packet-switched adapter comprises a detection logic adapted to detect the loss of the packet-switched control channel in the circuit-switched domain. Further a tracking logic is proposed which is adapted to perform a tracking procedure. The tracking logic comprises a receiver being adapted to receive mobility event notifications related to the circuit-switched capable terminal. Further the packet switched adapter comprises a mapping logic adapted to map the received mobility event notifications to a registration status of the user in the packet-switched domain. The mapping logic is connected to an aligning logic adapted to align the registration in said packet-switched domain with said derived registration status. The alignment is carried out by means of processor which is adapted to perform a registration procedure on behalf of the user in the packet-switched domain. 
     Further the adapter node is adapted to perform all steps as claimed in connection with the method which are to be performed in said node. 
     The advantage of the present invention is the transparency for some packet-switched services, since for the packet-switched service domain it remains transparent, that the subscriber is roaming in a circuit-switched domain. The packet-switched adapter performs the registration procedure using the parameters of the packet-switched service domain on behalf of the user. 
     A further advantage is that some packet-switched services, like the speech service, can be provided to a user being transferred to a packet-switched service domain but being suddenly accessible only via a circuit-switched access. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating embodiments of network; and 
         FIG. 2  is a flowchart illustrating an embodiment of the method steps performed in a packet-switched adapter; and 
         FIG. 3  is a schematic illustration of a network system in which the embodiments of the present invention are applicable; and 
         FIG. 4  is a block diagram illustrating the embodiment of provision user&#39;s data; and 
         FIG. 5  is a block diagram illustrating the embodiment of attaching a user to a network; and 
         FIG. 6  is a block diagram illustrating the embodiment of registering a user in a packet-switched network over a circuit-switched access; and 
         FIGS. 7 to 9  are block diagrams illustrating the embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following preferred examples of the present invention shall be described in detail, in order to provide the skilled person with thorough and complete understanding of the invention, but these detailed embodiments only serve as examples of the invention and are not intended to be limiting. 
     It should be noted that the term “part”, “node”, or “logic” in the context of the present invention refers to any suitable combination of hardware and software for providing a predetermined functionality in the communication network. In this way, said terms generally refers to a logical entity that can be spread out over several physical entities of the network, but can also refer to a physical entity located in one physical location. 
     Preferably, the circuit-switched domain is a mobile communication network, e.g. a mobile communication network operating according to GSM, GPRS (General Packet-Switched Radio), UMTS (Universal Mobile Telephone System), LTE (Long Term Evolution) or any wireless system like for example EDGE, WCDMA, CDMA2000 or W-LAN. 
     Preferably, the packet-switched domain is an IMS domain, however this should not be seen as limitation; the present invention can also be practiced in other unified service domains different that encompassed by IMS. 
       FIG. 1  depicts an embodiment of the invention in view of the system structure. According to  FIG. 1  there is a user&#39;s terminal UE  101  located in a circuit-switched domain, CS-domain  100  and communicating with a packet-switched adapter IA  102  being further adapted to access the packet-switched domain, PS-domain  103 . 
     According to  FIG. 1  a user&#39;s terminal UE  101  has a circuit-switched functionality, CS  121  and a packet-switched functionality, PS  111 . In one embodiment it is proposed to have one terminal with a PS functionality and a separate terminal with CS functionality and a user&#39;s card which might be swapped between the PS terminal to the CS terminal. 
     In the following description, the user&#39;s terminal is a circuit-switched capable terminal having circuit-switched functionality and being adapted to provide a communication between the terminal and the circuit-switched domain. Said terminal might have additionally also packet-switched functionality, the so-called ICS client. The terminal uses the packet-switched functionality, ICS client, to communicate with the packet-switched adapter, IA, in particular in order to register the user in the packet-switched domain. In a preferred embodiment the registration is performed over a circuit-switched domain, wherein for this purpose a packet-switched control connection, like an ICS connection, over a circuit-switched bearer, like for example the USSD bearer is used. In other embodiment the communication is based on a packet-switched protocol, like for example the IMS SIP protocol. 
     Further it is proposed to have a user of the terminal, wherein said user has a packet-switched subscription for the provision of packet-switched services. The packet-switched services might be any multimedia services, including audio and video services, further any supplementary and value added services as defined or feasible in IMS serve as examples but also pure voice services. 
     In one embodiment the information about the packet-switched services, the user is registered thereto, might be stored in a user&#39;s personal card, like for example in a SIM-card. Such user&#39;s card might be inserted into a terminal enhanced with a packet-switched functionality, ICS client. It might however occur that the user&#39;s personal card is swapped to a terminal only with a circuit-switched functionality. Thus the user is still registered in the packet-switched domain, although the terminal is not able to communicate over a packet-switched control connection with the packet-switched domain. 
     The present invention comes into play, when said packet-switched functionality, the ICS client, in a circuit-switched terminal is either currently not available or not usable, because of the network conditions although the user is subscribed to and registered in a packet-switched domain, IMS. In the following the term circuit-switched capable terminal will be used for a terminal with only circuit-switched functionality and for a terminal having both, the packet-switched and circuit-switched capability. 
     Returning to  FIG. 1 , as becomes apparent, it comprises also a packet-switched adapter, IA  102 . The packet-switched adapter might be either located in a circuit-switched domain or in the packet-switched service domain or on the boarder between both domains. The packet-switched adapter  102  comprises a detection logic, Dect  112  being adapted to perform the detection step for detecting the availability of a packet-switched control channel. The tracking logic, Track  122 , is adapted to perform the tracking of the user&#39;s reachability. In particular the receiver Recv  162  is adapted to receive messages from the CS domain, like the mobility event notifications being used for the tracking purpose. The mapping logic, Map  132 , is adapted to map the mobility event notification to the registration status of the user in the packet-switched domain. The registration logic, Reg.  142 , is adapted to align the registration in the packet-switched domain to the derived registration status by means of the processor, Proc  152 , being adapted to perform a registration procedure. The components of the IA  102  are connected with each other in a way to be able to perform the method steps according to  FIG. 2 . 
     The packet-switched adapter, IA  102  is further adapted to perform an adaptation between the circuit-switched domain and the packet-switched domain. One of the tasks is to perform protocol conversion between both domains. Further it comprises a user agent, which is some kind of a decision logic adapted to decide on behalf of the user. For example the user agent might collect information regarding the terminal capabilities in order to perform a suitable registration of said user in the packet-switched domain. Further the packet-switched adapter might have logic for administrating different addresses needed for routing calls to and/or from the user. It might include a gateway logic needed for accessing different domains. Further it is proposed that the packet-switched adapter is adapted to communicate with user&#39;s databases, for example in order to receive the user&#39;s correlated information. 
       FIG. 2  depicts the method steps according to one of the embodiments of the present invention. Preferably, the method is to be carried out in the packet-switched adapter, IA  202  and  102 . In the first step,  212 , it is detected that the packet-switched control channel in the circuit-switched domain does not exist anymore. 
     According to an aspect of the invention it is proposed that the registration in the packet-switched domain via a packet-switched control connection might be realised in any preferable and suitable way providing the adapter with the content necessary for the registration. For example it might be a SIP IMS registration message. According to the present invention said message might be defined in a packet-switched control protocol, like the ICS protocol which is to be packed in a circuit-switched transport bearer such as USSD having the advantage that the complete information is provided to the domain adapter. 
     According to an aspect of the invention it is proposed that the detecting step whether the user is accessible via the circuit-switched terminal over the circuit-switched domain might be realized in any preferable and suitable way. In one embodiment it is proposed to check a reception of a re-registration message from the user. A user which is registered in a packet-switched domain sends periodically so-called re-registration messages to the packet-switched domain, in order to inform said domain, that the user is still available and ready for receiving packet-switched services. In a preferred embodiment the packet-switched adapter IA has a running registration timer and if said timer expires before receiving a re-registration message, it is assumed that the user is not reachable via the packet-switched control connection. 
     Returning to  FIG. 2 , in the next step,  222 , it is proposed to perform a tracking procedure in order to determine whether the user is still reachable. The tracking procedure comprises the reception of mobility event notifications, which are foreseen to report periodically about user&#39;s reachability and to notify about changes in respect of the user&#39;s availability. 
     The mobility events notification might be realised in any preferable and suitable way. In a preferred embodiment the CAMEL approach is taken as an example however it has no restriction to the present invention. In particular in networks without the CAMEL functionality, it is proposed to implement any suitable trigger message. 
     CAMEL is a network feature which allows the network operator to provide the mobile subscribers with the operator specific services even when mobile subscribers are roaming outside the home network. In the frame of CAMEL, a so called Trigger Detection Point (TDP) is defined, which specifies a point of time in transaction handling. The Mobility Trigger Point is an example of the TDP informing about mobility events and changes, for example a Mobility Trigger might have as parameters an attach, detach, location update or periodic location update event. 
     In one embodiment the provision of the of mobility event notifications is activated per default, which results that all mobility events notification are sent automatically to the packet-switched adapter. By this the packet-switched adapter can keep track on the reachability state of the user. The advantage of this approach is that the packet-switched adapter does not need to be active in order to get the information since it is informed per default about the user&#39;s status. 
     In another embodiment the provision of the mobility triggers notification is not activated per default, but it has to be activated. For this purpose at first a request for checking reachability of the user is sent out in order to check the first status of the user and in order to activate in a further step the provisioning of the notifications. The advantage of this embodiment is that the network resources are saved since the pooling of the user for the status is done when it is detected that the user is not reachable via the packet-switched control connection. 
     In one embodiment it is proposed to send a standardised circuit-switched request towards a user&#39;s database for retrieving a user&#39;s state in order to check the reachability of the user. The standardised circuit-switched request might be for example a MAP Any Time Interrogation message. In a telecommunication network, service related messages are transported over different communication links via a corresponding protocol. For example in GSM between the terminal and the MSC the Direct Transfer Application Part (DTAP) transport protocol is defined. Further between the MSC and HLR a so called Mobile Application Part (MAP) protocol is standardized and this protocol might be also used for transporting specific service commands like the Any Time Interrogation message from the packet-switched adapter, IA to the HLR to query the HLR for position information of a user. The HLR responds by providing the adequate information. 
     Since the packet-switched adapter is a new concept, there are no standardized interfaces, therefore it is proposed in another embodiment to use a proprietary implemented user&#39;s state query for getting the user&#39;s state. This might for example be a service interface based on a PARLEY mechanism, like the OSA/Parley/Parley-X User Status interface provided by the circuit-switched domain to determine the registration state of a subscriber. 
     In case the checking step yields that the user is reachable, the packet-switched adapter IA activates the provisioning of mobility trigger notifications in order to be periodically updated or when the status changes. In a preferred embodiment a MAP Any Time Modification is sent by the packet-switched adapter IA to the HLR to activate the provisioning of mobility trigger notifications for the subscriber. 
     In case the result of the checking procedure is that the user&#39;s state is not-reachable, it is proposed that the packet-switched adapter IA periodically re-checks the user&#39;s state. It might namely happen that the user is in a not-reachable branch when the re-registration timer expires, but after checking again the result might be that the user is available again. 
     Returning to  FIG. 2 , in step  232 , it is proposed to map the received mobility event notifications to a registration status of the user in the packet-switched domain. In a preferred embodiment a MSC reports any detach, attach or location update event (including periodic location updates) by means of a Mobility Trigger Notification to the packet-switched adapter, IA, which is adapted to map these events to an IMS registration status of the user. The registration status might be that the user is already registered and a re-registration is necessary, or that the user is to be registered or de-registered. 
     In a subsequent step  242  of  FIG. 2 , it is proposed to align the registration status in the packet-switched domain on behalf of the user depending on the registration status in said packet-switched domain. The appropriate registration procedure might be re-registration if a periodic location update or a normal location update notification is received, which implies that the user is still reachable, or it might be registration if an attach notification is received which means that the user is available again, or it might be a de-registration procedure if a detach notification is received, which means that the user is not reachable anymore. 
     Further it is proposed that the packet-switched adapter abandons the performing of the registration procedure on behalf of the user when it detects that the user is accessible via a packet-switched based connection. This might for example happen when the user swaps back the SIM card into a terminal with a packet-based functionality. 
     In particular in case when the reporting of the mobility event notifications has been actively initiated, it is proposed that the packet-switched adapter requests termination of provisioning of mobility event notifications related to the user&#39;s terminal and deactivates the procedure for receiving said notifications. This might be realized in any preferable and suitable way. One way might be to send a MAP Any Time Modification to the user&#39;s database for deactivation of the Mobility Event Notifications, like in this case to the HLR. 
     Preferably, the Session Initiation Protocol SIP can be used as an embodiment of a protocol for transporting a registration message in the IMS. SIP is a signaling protocol used between the IMS user&#39;s equipment UE and the IMS as well as between the components within the IMS. The IMS uses SIP also to complete voice and multimedia calls. 
     In the following presented embodiments GSM or UMTS are examples for the circuit-switched domain, the IMS is taken as an example for a packet-switched domain. The centric approach of IMS (ICS) is taken as an example for packet-switched control access, wherein a packet-switched functionality in a terminal is realized by means of an ICS client implemented in the terminal. IMS adapter IA might serve as an embodiment for a packet-switched adapter. The IA might be a stand-alone node or it might be integrated in any suitable node and be located in any suitable domain. 
     The network system of  FIG. 3  shows a terminal  301 , UE, comprising an IMS client  311 . MSC-S  305  and Media Gateway (MGW)  304  are additionally included in the system and are responsible for providing signaling and the corresponding payload connectivity. Further there is an IMS adapter IA  302  performing registration procedure on behalf of the terminal  301  in the IMS core network  303 . The ICS client includes two main functional components reflecting the layered architecture of the network system of  FIG. 3 . On a control layer, the ICS client includes a functional block “C” for session control handling. On the payload layer, the ICS client includes a functional block “M” for media connectivity handling. The individual functional blocks of the ICS client  301  communicate with the corresponding functional blocks in the IA  302  node. Communication on the control layer involves an appropriate control protocol, which in  FIG. 3  is the ICS protocol over USSD. Further example might be also a SIP protocol, or any other appropriate control protocol. The communication on the payload layer involves protocols according to the 3GPP technical specification 24.008 with protocols like ISUP/BICC/SIP-I. 
     As shown in  FIG. 3 , the IMS  303  includes IMS core as well as IMS application Servers (such as Multimedia Telephony (MMTEL) Server or Voice Call Continuity (ICS CC) and Domain Selection Server (DS)). The IMS core and the IMS application services communicate via an ISC interface. 
     The network according to  FIG. 3  might be configured to perform present invention and in particular the adapter node IA  302  might be configured to perform the method embodiment according to  FIG. 2 . 
       FIG. 4  depicts the provisioning of subscriber data. The O&amp;M is a central entity; it might be for example an operation and maintenance centre of an operator, which provides the data to the different databases. According to  FIG. 3  the HLR and the HSS are the databases which receive subscriber data, preferably in advance, before the user starts using the network connections. 
     There are different user data which are to be distributed in the network. It is for example proposed that the HLR contains a basic CS subscription related to the speech service. Additional subscriptions may be added depending if these shall be executed in the CS domain on behalf of an IMS Service Engine. Further the HLR might include CAMEL data that causes that all originating calls are routed to the IA. However CAMEL is only one example for fetching a roaming number from the IA, further example might be to use USSD for that purpose. In the following the CAMEL approach is used as a preferred embodiment. 
     It is further proposed that the HLR comprises CAMEL data for providing CAMEL mobility triggers as an embodiment of mobility event notification. According to the embodiments of the invention, said mobility event notifications might be marked either as activated or as de-activated per default. The preferred option might depend on the assumptions of an operator. For example it is to be expected that the operator when using ICS approach provides also ICS capable terminals to the users having consequently an ICS subscription. In this case the default setting of the CAMEL mobility triggers can be de-activated, since the ICS client in the terminal takes care of IMS registration handling whenever a re-registration event from the user is received. However, if the operator allows using non-ICS terminals, then the default setting of the CAMEL mobility triggers might be preferably set to activated in order to always inform the IA about changes of the user&#39;s reachability. 
     In one embodiment it is further proposed that also the HSR located in the IMS contains subscriber data. According to  FIG. 4  the O&amp;M entity provides said data to the HSR, wherein the user data depends on the implemented solution. In one embodiment it is proposed to provide IMS related subscription information, which might be for example requested by the packet-switched adapter during a registration procedure. 
     In the following the registration of a user in a packet-switched domain over a packet-switched control channel is described in more details in respect to  FIG. 5 . 
     Initially, when a user switches on the terminal an attach message is sent out through the network. The attach procedure is depicted in  FIG. 5  showing a terminal  501  and an MSC  502  and a HLR  503 . The attach message  511  is sent to the MSC  502 , which upon reception of the attach message requests from the HLR further subscriber subscription data. Said data is provided with the message  512  to the MSC. In a preferred example the MAP protocol might be used to insert the required data to the MSC, which might be for example information that the CAMEL mobility triggers are set as de-activated per default. Further the basic CS speech service subscription (TS11/TS12) and the CAMEL call routing triggers might also be provided. The subscriber data is needed inter alia to perform the tracking procedure in particular in order to receive mobility event notifications related to the circuit-switched capable terminal. Independent of the provision of the different subscriber data, the result of the attach procedure is that the user is attached to the circuit-switched domain. Subsequently, since the terminal has the packet-switched functionality, the ICS client, said client starts to trigger registration of the user in the packet-switched domain over the established circuit-switched bearer. 
     The registration procedure is described in the following in relation to  FIG. 6 , wherein an embodiment is presented with an ICS client performing an IMS registration in the IMS over USSD. Thus according to  FIG. 6 , there is a first user UE  601   a  being located in a home domain and communicating with a first MSC,  605   a  and a second user UE  601   b  being located in a visited network and communicating with a second MSC,  605   b.  Both MSC are connected to an HLR  604  which provides on the other side a connection to a packet-switched adapter IA,  602 . The packet-switched adapter is adapted to provide an access to the IMS network  603 . Further the packet-switched adapter has a connection,  614 , to an HSR database,  606 . 
     After attaching to a CS network, the user&#39;s UE, independent whether it is located in a home or in a visited network, sends a registration message,  611   a  and  611   b.  In a preferred embodiment it is an ICS registration message defined in the ICS protocol, wherein said message is packed in a USSD message. The transport protocol used on the link between a terminal and a MSC is the DTAP protocol. Consequently the IMS registration packed according to the available protocols is sent to the corresponding MSC,  605   a  and  605   b.  The MSCs have a USSD Relay logic which is adapted to interpret the received USSD message and to process them. According to  FIG. 6  the USSD messages are packed into a MAP protocol being available on the interface between the MSC and the HLR and are forwarded to the HLR,  612   a,    612   b.  Upon reception of the messages the HLR forwards them,  613  to the packet-switched adapter IA,  602 . The forwarding might be performed in any suitable way. It is proposed to use the ICS protocol for the forwarding purpose, since the interface between the HLR and the IA is not defined yet. Preferably the IA verifies the ICS subscription in the HSR,  614  and if the verification yields in a positive result, the registration is forwarded to the IMS,  615 . After successful IMS registration, the IA starts a timer,  616 , to supervise when a re-registration is needed in order to keep the IMS registration alive. The result of the above described procedure is that the user is registered in IMS for receiving the IMS services, wherein the registration is performed in the circuit-switched domain through the ICS channel over USSD bearer. General speaking the user is registered in a packet-switched domain through a packet-switched control channel over a circuit-switched transport bearer. 
     In the following the embodiment is described when a user loses the connectivity through the packet-switched control channel. This might occur for example when a roaming operator decides not to support the packet-switched control channel over USSD or after a SIM swap. 
     In case a SIM-swap takes place, the new terminal, without the ICS client, performs a standard IMSI attach at power-up procedure and as an effect thereof the MSC is notified, however the information is not spread out to other nodes within the system. However, although the packet-switched control channel does not exist, the IMS registration may still be alive and the re-registration timer in the IA may still be running. Therefore, since there is no ICS client on the terminal anymore and therefore no packet-switched control channel available anymore, the present invention proposes that the packet-switched adapter handles on behalf of the user, in particular that the terminal performs a registration procedure in the packet-switched domain. 
     In respect to  FIG. 7  the initiation of the registration procedure is described in more details. According to  FIG. 7 , there is a terminal UE  701  communicating via a MSC,  705  with the HLR  704 . The HLR  704  is connected on the other side with the packet-switched adapter LA,  702 . The packet-switched adapter LA is adapted to provide an access to the IMS network  703 . 
     In a first step,  711 , the IA detects that the re-registration timer expires since no re-registration message has been received from the user. In the next phase it is to be determined whether the user is still reachable. The information about user&#39;s reachability might be available already, for example when the CAMEL mobility triggers are activated per default at provisioning of subscriber data in HLR. In this case no further actions are needed in the IA to check user&#39;s availability. Otherwise it is proposed that the IA activates the provisioning of the mobility event notifications but at the first stage the IA should determine the user&#39;s state. This might be performed in any suitable way. In one embodiment it is proposed to that the packet-switched adapter IA uses the standardized procedures for this. For example, MAP Any Time Interrogation message is sent towards the HLR,  712 . The HLR requests the state from the serving VLR using standard MAP Provide Subscriber Information procedure,  713 . The VLR checks the subscriber state and returns either reachable or not-reachable as a result,  714 , which is then returned to the requesting IA,  715 . 
     In another embodiment, not shown in  FIG. 7 , it is proposed that the IA uses an OSA/Parlay/Parlay-X User Status service interface provided by the CS domain to determine the state of the subscriber. 
     Consequently the result of the subscriber state query is received in the IA. Depending on the state of the subscriber the IA can take different actions. If the subscriber is not-reachable, the IA either does not take any action and the IMS registration timer expires, or the IA can actively de-register the subscriber from IMS. In any case the IA keeps track on the state of the subscriber in order to take appropriate action by using appropriate registration procedures as soon as the user becomes reachable again. The checking of the user&#39;s availability might be performed in any suitable way. It is proposed that the IA permanently polls the subscriber state, or the IA activates the Mobility Triggers in order to be informed of an attach event of the subscriber&#39;s terminal. 
     In case the result of the user&#39;s state is that the user is available, and due to the fact that no re-registration via USSD took place, the IA starts the registration procedure on behalf of the user and restarts the re-registration timer. The effect of the registration procedure is that the packet-switched registration is bound to the circuit-switched attach status. 
     Additionally, since the user is reachable, the IA activates also the provisioning of CAMEL mobility triggers, as an example of the mobility event notifications. The activation procedure is presented in the following in respect to  FIG. 8 . According to  FIG. 8  the same nodes as in the aforementioned figures are depicted, a MSC  805 , HLR  804 , IA  802  and IMS core  803  and a user&#39;s terminal without the ICS client,  801 . 
     In step  811  the IA initiates the standardized MAP Any Time Modification procedure towards HLR to activate the CAMEL Mobility Triggers. Since on the interface between HLR and IA the MAP protocol is used, it is preferable to use this protocol to send the MAP Any Time Modification message. The HLR translates the received message into the standard MAP Insert Subscriber Data to change the VLR subscriber data accordingly. This message is sent to MSC in step  812 , and the result whether it has been activated or not, is forwarded to the HLR in message  813 . In case the Mobility Triggers have been successfully activated, the HLR forwards the result to IA, step  814 . 
     The outcome of the activation procedure is that attach, detach and/or location update (including periodic location update) events are reported from the serving MSC to the IA. For example in case of changing the location area, the user&#39;s terminal  801 , sends in step  815  a location update message to MSC. The location update message might be sent by means of the DTAP protocol being available on the UE-MSC interface, DTAP: Location Update. This triggers in the MSC the related mobility detection point and the MSC sends a corresponding mobility event notification, which is directed to the IA. In the present embodiment the MSC might send a MAP: Note-MM-Event message,  816  as a carrier of a CAMEL Mobility Trigger Notification. The IA maps the received mobility event notification to the corresponding registration status. In this embodiment, apparently the user is still available, and therefore the IA aligns the subscription status in the IMS by performing a re-registration procedure towards the IMS,  803  at the next expiry of the re-registration timer. 
     In other embodiment the user&#39;s terminal might send an attach message to the MSC,  815 , which initiates a CAMEL Attach Notification as a Trigger towards the IA,  816 . It is proposed that the IA upon reception of this message recognizes that the status of the user is reachable and therefore a registration of the user in the IMS domain is performed. Additionally a re-registration timer is started. 
     In a further embodiment the user&#39;s terminal might send a detach message,  815 , being translated in the MSC into CAMEL detach notification,  816 , which is mapped in the IA into a status that the user is not reachable anymore and consequently a de-registration procedure of the user in the IMS is performed and the re-registration timer is stopped. 
     In another embodiment it is proposed to provide a procedure when the user swaps back the SIM card to a packet-switched capable terminal. In the following an embodiment is presented with reference to  FIG. 9 . According to  FIG. 9  the following nodes are depicted: a user&#39;s terminal with the ICS client,  901 , an MSC  905 , HLR  904 , IA  902  and IMS core  903 . 
     After swapping back the SIM card into a ICS capable terminal, a DTAP: IMSI attach message is send from the user&#39;s terminal to the MSC,  911 , whereupon the MSC sends a CAMEL Mobility notification, MAP: Note-MM-Event,  912  to the IA. After the attachment to the network is completed, the ICS client on the terminal starts a IMS Registration over USSD, thus a DTAP (USSD) “ICS Registration” message is sent,  913  to the MSC, which forwards said message as MAP (USSD) message in step  914  to the HLR and using the ICS protocol in step  915  to the IA. Due to the fact that an IMS Registration request is received from the terminal via USSD, this indicates to the IA that the user is accessible in the circuit-switched domain via a packet-switched connection over a circuit-switched bearer, ICS over USSD. Consequently, the IA performs a re-registration of the user in the IMS,  916  and starts the re-registration timer,  917 , upon reception of the re-registration message from the user. Further the fact that the user is connected via ICS over USSD leads to the conclusion that the CAMEL Mobility triggers, if activated are not longer needed and therefore it is proposed that the IA deactivates said triggers. 
     The deactivation procedure of the mobility event notifications is not shown in any figure. It is proposed that the IA sends a MAP: Any Time Modification with CAMEL Mobility Trigger deactivate as a parameter to the HLR. The HLR translates the received message into a MAP: Insert Subscriber Data and sends it to the VLR by means of the CAMEL Mobility Trigger deactivate message. The result of the deactivation is sent back to the IA via HLR. 
     Although embodiments of the present invention have been illustrated in the accompanying drawings and described in the description, it will be understood that the invention is not limited to the embodiment disclosed herein. In particular it is to be emphasized that the ICS over USSD is one example of implementing a packet-switched control channel.