Abstract:
Techniques and apparatus are described for controlling set up of a terminating call from an IMS to a UE, in a Circuit Switched Fallback (CSFB) situation. A first MSC receives a terminating call from IMS and tries to page the UE. The UE has moved to the second MSC and performs a Location Update to the second MSC. The second MSC informs the first MSC with a Send Identification message comprising an address of the second MSC. The first MSC maintains the IMS subscription of the UE at Cancel Location and forwards the terminating call to the second MSC. The first MSC acts as relay for subsequent call control messages for calls of the UE. The first and second MSCs are supervising the ongoing calls and as soon as all calls are completed the second MSC registers the UE into IMS and the first MSC de-registers the UE from IMS.

Description:
RELATED APPLICATION 
     Pursuant to 35 U.S.C. §120, this application claims priority to and the benefit of International Patent Application PCT/EP2014/059482, filed 8 May 2014, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure generally relates to telecommunications and in particular to Circuit Switched Fallback (CSFB) in an IMS Centralized Services (ICS) scenario. A technique for controlling set up of a terminating call to a User Equipment (UE) in a CSFB situation is described. 
     BACKGROUND 
     CSFB is specified today in the 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 23.272 (see, e.g., V12.2.0 of March 2014). In brief, CSFB permits in an Evolved Packet System (EPS) the provisioning of voice and other CS domain services (such as services pertaining to Unstructured Supplementary Service Data, or USSD) by re-use of CS infrastructure when the UE is served by the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). A CSFB-enabled UE, connected to E-UTRAN, may thus use 2nd or 3rd Generation (2G or 3G) technologies such as GSM Edge RAN (GERAN) or UTRAN to connect to the CS domain. 
     The mechanism for handling a terminating call needed to be modified in certain CSFB scenarios. Specifically, CSFB is only available in case E-UTRAN coverage (as defined by Tracking Areas, or TAs) is overlapped by either GERAN or UTRAN coverage (as defined by, for example, Location Areas, or LAs). A general problem results from the fact that there exists no 1:1 mapping between TAs and LAs. 
     For a terminating call the lacking congruency between TAs and LAs may have the consequence that the UE, when falling back from E-UTRAN to GERAN or UTRAN, may land in a LA that is not controlled by the “old” MSC-S towards which the SGs interface association for the UE has been established by the MME. In such a case the “old” MSC-S will not be able to terminate the call. 
     Mobile Terminating Roaming Forwarding (MTRF) is a procedure that has been specified in 3GPP TS 23.018 (V12.2.0 September 2013), in order to deliver mobile terminating calls to subscribers that change MSC area during the setup of a terminating call. The MTRF procedure may be applied for CS calls towards mobile subscribers roaming in GERAN or UTRAN access, but also towards mobile subscribers performing CS fallback from E-UTRAN to GERAN or UTRAN, as specified in 3GPP TS 23.272. 
     In addition, 3GPP specifies the concept of IMS centralized services (ICS) in TS 23.292, TS 24.292 and TS 29.292. ICS allows IMS subscribers to get services executed in IMS when they are connected via CS radio access. One way to deploy ICS is to enhance the MSC with ICS support, and such an MSC is called ‘MSC server enhanced for ICS’ in the 3GPP standards. The MSC Server enhanced for ICS is connected with the IMS domain via a so called I2 interface. 
     When the UE registers in the MSC Server enhanced for ICS via a CS Location Update procedure and the subscriber is an ICS subscriber (for example determined by an indication received from the HLR as part of the subscriber data), the MSC Server enhanced for ICS shall register the subscriber in IMS. 
     When the MSC has registered the ICS subscriber in IMS, then IMS may route terminating calls (voice or video call) for that subscriber directly to that MSC based on the information received at the IMS Registration procedure, as defined in 3GPP TS 23.292 for IMS calls. 
     This means that for terminating calls from IMS no GMSC and no related roaming number retrieval procedure is applied. But the current solutions specified for MTRF do not cope for the case that the MSC is enhanced for ICS and that the user is an ICS user. 
     SUMMARY 
     There is a clear need for an improved MTRF procedure, taking into account the situation that the MSCs are enhanced for ICS and that the user is an ICS user. 
     It is an object of the present invention to improve the control of the set-up of a terminating call from an IMS to a UE in a CSFB situation for ICS users. This object is achieved by the independent claims. Advantageous embodiments are described in the dependent claims. 
     According to an exemplary aspect of the invention, a method in a Mobile Switching Center (MSC) for controlling set up of a terminating call from an IP Multimedia Subsystem (IMS) to a User Equipment (UE) in a Circuit Switched Fallback (CSFB) situation, is provided. The MSC supports IMS Centralized Services, ICS. The method comprises paging of the UE and receiving a Send Identification message from a further MSC, wherein the further MSC also supports ICS. The method also comprises receiving a Cancel Location message from a Home Location Register, HLR and forwarding the terminating call set up request to the further MSC. The method further comprises relaying further messages related to ongoing calls of the UE between the further MSC and the IMS and determining that all calls of the UE have ended. The method also comprises triggering de-registration of the UE from the IMS, when all calls of the UE have ended. 
     According to another exemplary aspect of the invention, a method in a Mobile Switching Center (MSC) for controlling set up of a terminating call from an IP Multimedia Subsystem (IMS) to a User Equipment (UE) in a Circuit Switched Fallback (CSFB) situation, is provided. The MSC supports IMS Centralized Services, ICS. The method comprises receiving a location update request from the UE and sending a Send Identification message to a further MSC, wherein the further MSC also supports ICS. The method comprises receiving a terminating call set up request from the further MSC. The method also comprises receiving further messages related to ongoing calls of the UE from the IMS via the further MSC and sending further messages related to ongoing calls of the UE to the IMS via the further MSC. The method further comprises determining that all calls of the UE have ended and triggering registration of the UE into the IMS, when all calls of the UE have ended. 
     According to another exemplary aspect of the invention, a Mobile Switching Center (MSC) for controlling set up of a terminating call from an IP Multimedia Subsystem (IMS) to a User Equipment (UE) in a Circuit Switched Fallback (CSFB) situation is provided. The MSC supports IMS Centralized Services, ICS. The MSC is capable to page the UE and to receive a Send Identification message from a further MSC, wherein the further MSC also supports ICS. The MSC is also capable to receive a Cancel Location message from a Home Location Register, HLR, and to forward the terminating call set up request to the further MSC. The MSC is further capable to relay further messages related to ongoing calls of the UE between the further MSC and the IMS and to determine that all calls of the UE have ended. The MSC is also capable to trigger de-registration of the UE from the IMS, when all calls of the UE have ended. 
     According to another exemplary aspect of the invention, a Mobile Switching Center (MSC) for controlling set up of a terminating call from an IP Multimedia Subsystem (IMS) to a User Equipment (UE) in a Circuit Switched Fallback (CSFB) situation, is provided. The MSC supports IMS Centralized Services, ICS. The MSC is capable to receive a location update request from the UE and to send a Send Identification message to a further MSC, wherein the further MSC is also supporting ICS. The MSC is further capable to receive a terminating call set up request from the further MSC. The MSC is also capable to receive further messages related to ongoing calls of the UE from the IMS via the further MSC and to send further messages related to ongoing calls of the UE to the IMS via the further MSC. The MSC is further capable to determine that all calls of the UE have ended, and to trigger registration of the UE into the IMS, when all calls of the UE have ended. 
     According to another exemplary aspect of the invention, a system for controlling set up of a terminating call from an IP Multimedia Subsystem (IMS) to a User Equipment (UE) in a Circuit Switched Fallback (CSFB) situation, is provided. The system comprises a first Mobile Switching Center (MSC) a second (MSC) an IMS, a UE, and a Home Location Register, HLR. The foregoing and other objects, features and advantages of the present invention will become more apparent in the following detailed description of embodiments of the invention illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages of the invention will become better apparent from the detailed description of particular but not exclusive embodiments, illustrated by way of non-limiting examples in the accompanying drawings, wherein: 
         FIG. 1  shows a diagram illustrating a system for control of the set-up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to the invention; 
         FIG. 2  shows a sequence diagram for control of the set-up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to prior art; 
         FIG. 3  shows a flow diagram in a first MSC for control of the set-up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to the invention; 
         FIG. 4  shows a flow diagram in a second MSC for control of the set-up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to the invention; 
         FIG. 5  shows a block diagram illustrating a first MSC for control of the set-up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to the invention; 
         FIG. 6  shows a block diagram illustrating a second MSC for control of the set-up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, a system, methods, nodes, and computer programs for controlling set up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to the invention are described in more detail. 
     Within the context of the present application, the term “communication network” or short “network” may particularly denote a collection of nodes or entities, related transport links, and associated management needed for running a service, for example a telephony service or a packet transport service. Depending on the service, different node types or entities may be utilized to realize the service. A network operator owns the communication network and offers the implemented services to its subscribers. Typical examples of a communication network are radio access network (such as 2G/GSM, 3G/WCDMA, CDMA, LTE, WLAN, Wi-Fi), mobile backhaul network, or core network such as IMS, CS Core, PS Core. 
     Within the context of the present application, the term “user equipment” (UE) refers to a device for instance used by a person for his or her personal communication. It can be a telephone type of device, for example a telephone or a SIP phone, cellular telephone, a mobile station, cordless phone, or a personal digital assistant type of device like laptop, notebook, notepad equipped with a wireless data connection. The UE may also be associated with non-humans like animals, plants, or even machines. A UE may be equipped with a SIM (Subscriber Identity Module) comprising unique identities such as IMSI (International Mobile Subscriber Identity) and/or TMSI (Temporary Mobile Subscriber Identity) associated with a subscriber using the UE. The presence of a SIM within a UE customizes the UE uniquely with a subscription of the subscriber. 
     Within the context of the present application, the term “MSC” refers to a control node of the circuit switched communication network, for example a MSC. The MSC may be combined with a Visited Location Register (VLR) and be therefore also called MSC/VLR. The MSC node may also be enabled to control a remote Media Gateway (MGW) and therefore act as a server and may therefore be also called MSC Server or short MSC-S. Within the context of the present application MSC, MSC/VLR and MSC-S are functional equivalent. The term MSC also corresponds to the ‘MSC server enhanced for ICS’ in the 3GPP standards. 
     Within the context of the present application, the term “HLR” refers a subscriber database of the circuit switched communication network, for example a HLR. The HLR may be combined with or may be integrated part of a Home Subscriber Server (HSS). 
     Referring to  FIG. 1 , this figure shows a system for controlling set up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to the invention. 
     Here it is assumed that both MSC, MSC  1   120  and MSC  2   140 , support ICS functionality. The figure shows a delivery of a terminating call from IMS  100  to a UE  160  in a communication network  10 . The UE  160  is registered in IMS  100 , and as part of a previous registration procedure the IMS  100  learned the address of MSC  1   120  as the as the address to be contacted for terminating calls. 
     When now a terminating call is to be delivered, the IMS  100  forwards the terminating call set up request to the MSC  1   120 . The MSC  1   120  then pages the UE  160 . In a CSFB situation, the lacking congruency between TAs and LAs have the effect that the UE  160 , when falling back from E-UTRAN to GERAN or UTRAN, lands in a LA that is not controlled by the MSC  1   120 . In such a case the MSC  1   120  will not be able to terminate the call, so the paging request by MSC  1   120  towards the UE  160  is not responded to. 
     Instead the UE  160 , after having received the page request via E-UTRAN, falls back to GERAN or UTRAN, and performs a Location Update procedure towards MSC  2   140  (as it is new in this LA and in that MSC  2   140 . The HLR  180  is then informed and subscriber data are downloaded from HLR  180  to the MSC  2   140 . The HLR  180  then cancels the CS registration in MSC  1   120 . 
     However, MSC  1   120  keeps the IMS  100  registration of the UE  160  active and MSC  2   140  does not yet register the UE  160  into IMS  100 . The MSC  1   120  then forwards the terminating call set up request to MSC  2   140  and MSC  2   140  delivers the terminating call set up request to the UE  160 . 
     While the terminating call is ongoing, the UE  160  may initiate new calls, which are then relayed by MSC  1   120  from MSC  2   140  to IMS  100 . As long as there is at least one call of the UE  160  still ongoing, MSC  1   120  relays any related call control signaling between the IMS  100  and the MSC  2   140 . 
     When all calls of the UE  160  have ended, MSC  2   140  triggers registration of the UE  160  into IMS  100 , and MSC  1   120  triggers de-registration of the UE  160  from IMS  100 . From then onwards any terminating call set up requests will straight go from IMS  100  to MSC  2   140 . 
     This procedure allows to successfully deliver terminating calls in a CSFB situation, if both MSC, MSC  1   120  and MSC  2   140  support ICS functionality and the user of the UE  160  is a subscriber with ICS subscription. 
     Further details of the signaling sequence are shown in  FIG. 2 . 
     Referring to  FIG. 2 , this figure shows a sequence diagram for controlling set up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to the invention. 
     The sequence diagram is simplified and focuses on a high-level flow of events and related information. Some real messages are omitted in order to improve the clarity. The shown messages are named according to their function and do not mean real protocol messages. 
     The IMS  100  has to deliver a terminating call to the UE  160 . From recent IMS registration procedures the IMS  100  knows the contact address of the MSC  1   120 . So the IMS  100  sends the terminating call set up by sending an Invite message  200  to the MSC  1   120 . This Invite message may correspond to a SIP Invite message. 
     The MSC  1   120  receives the Invite message  200  from the IMS  100 . In order to deliver the terminating call request to the UE  160  the MSC  1   120  pages  205  the UE  160 . This may be done via corresponding E-UTRAN paging mechanisms. 
     The UE  160  receives the paging indication via E-UTRAN. In order to accept the terminating call, the UE  160  falls back to UTRAN or GERAN radio. In such a CSFB scenario, the UTRAN or GERAN radio cell and location area used by the UE  160  belongs to MSC  2   140 , therefore the UE  160  performs first a location update procedure by sending Location Update Request  210  message to the MSC  2   140 . This Location Update Request message may be a corresponding DTAP (Direct Transport Application Part) or RANAP (Radio Access Network Application Part) message. 
     The Location Update message may comprise a Circuit Switched Mobile Terminating (CSMT) flag, indicating to the MSC  2   140  that the Location Update is due to CSFB. 
     The MSC  2   140  receives the Location Update Request  210  message. From the information in that message the MSC  2   140  derives the previous MSC, MSC  1   120 . MSC  2   140  then sends a Send Identification  215  message to the MSC  1   120 . This Send Identification  215  message may be a corresponding MAP (Mobile Application Part) message or also a SIP message or any other message capable of informing MSC  1   120  that MSC  2   140  supports ICS and to deliver a contact address of MSC  2   140  to MSC  1   120 . 
     By alternative the MSC  1   120  may know the MSC  2   140  address from configuration within the node, inserted by the network operator. In this case the information that MSC  2   140  supports ICS is enough for MSC  1   120  to trigger the forwarding of the terminating call request. The MSC  1   120  may also know that the MSC  2   140  supports ICS from configuration within the node, inserted by the network operator. In this case no further information is needed in the Send Identification  215  message to the MSC  1   120 . The support of ICS in MSC  2   140  may also be used as an implicit indication that the MSC  2   140  supports a SIP interface for forwarding of the terminating call request. 
     By alternative, also other call/session control protocols may be used between MSC  1   120  and MSC  2   140  such as a BICC (Bearer Independent Call Control) protocol. 
     In yet another alternative, the MSC  2   140  may not support ICS functionality, but still support a SIP interface and MTRF functionality. In this case the MSC  2   140  may indicate a contact address but no ICS support indicator. 
     MSC  1   120  receives the Send Identification  215  message and extracts the contained information, such as ICS support of MSC  2   140  and/or a contact address of MSC  2   140 . 
     Triggered by the reception of Location Update request  210  message (and the fact that UE  160  is new in this MSC  2   140 ), the MSC  2   140  sends an Update Location  220  message to the HLR  180 . The Update Location  220  message may be a corresponding MAP message. 
     The HLR  180  receives the Update Location  220  message and triggers canceling of the old CS registration in MSC  1   120 , by sending Cancel Location  225  message to the MSC  1   120 . The Cancel Location  225  message may be a corresponding MAP message. 
     MSC  1   120  receives the Cancel Location  225  message, but MSC  1   120  maintains  230  the IMS  100  registration of the UE  160 , and responds with Cancel Location Acknowledgement  235  message to the HLR  180 . In the prior art the MSC  1   120  must de-register the UE  160  from IMS  100 . The Cancel Location Acknowledgement  235  message may be a corresponding MAP message. 
     After that the downloading of subscriber data is performed by one or several Insert Subscriber Data/Insert Subscriber Data Acknowledgement  240  message exchanged between HLR  180  and MSC  2   140 . The Insert Subscriber Data/Insert Subscriber Data Acknowledgement  240  messages may be corresponding MAP messages. The received subscriber data may also comprise an indication that the subscriber using the UE  160  is an ICS subscriber having a subscription to ICS. 
     In a next step the MSC  1   120  forwards the terminating call set up request by sending a SIP Invite  245  message to the MSC  2   140 . MSC  1   120  uses the contact address of the MSC  2   140  as received in the Send Identification  215  message to address the MSC  2   140 . Also other protocols may be used such as BICC, in this case the message would be an IAM (Initial Address Message). 
     In the meantime MSC  2   140  receives the SIP Invite  245  message. After successful downloading of subscriber data to MSC  2   140 , MSC  2   140  accepts the UE&#39;s  160  location update request by sending Location Update Accept  250  message to the UE  160 . The Location Update Accept  250  message may be a corresponding DTAP or RANAP message. 
     The UE  160  receives the Location Update Accept  250  message. 
     The MSC  2   140  at this point suppresses  255  the IMS  100  registration of the UE  160 . So the UE  160  remains to be registered in IMS  100  with MSC  1   120  as the contact address. In the prior art MSC  1   120  must register the UE  160  into IMS  100 , if the subscriber has a subscription to ICS. 
     In the meantime MSC  1   120  has started to act as relay  260  for any subsequent call control messages, between the IMS  100  and the MSC  2   140  and vice versa. 
     The MSC  2   140  then delivers the terminating call setup request to the UE  160  by sending Setup  265  message to the UE  160 . The UE  160  receiving the Setup  265  message responds with Call Confirmed  270  message to the MSC  2   140 . Both messages may be a corresponding DTAP or RANAP message. The MSC  2   140  receives the Call Confirmed  270  message. 
     After that the normal mobile terminating call procedures  275  takes place. These details are omitted for simplification reasons. 
     Both, MSC  1   120  and MSC  2   140  then start to supervise the ongoing call. Since the IMS  100  registration has not been changed yet, further terminating call requests may be received from IMS  100  in MSC  1   120 . MSC  1   120  may relay also these new terminating call set up requests to MSC  2   140 , and also relay their related call control signaling. In order to limit this signaling relay functionality, MSC  1   120  may reject any further terminating call set up request by responding with an “user busy” indication back to IMS  100 . In that case only one call needs to be relayed. In the same way the MSC  2   140  may not allow establishment of further calls via the MSC  1   120 . This rejection may be based on an operator preference, the current load situation in the MSC node, or on the type of subscriber, so whether the subscriber is an own subscriber or a visited subscriber. 
     While the terminating call is ongoing, the UE  160  may initiate a further call in parallel to the ongoing terminating call (also known as enquiry call). Since the UE  160  is still registered in IMS  100  via the contact in MSC  2   140 , such parallel calls and their related call control signaling are to be relayed via MSC  1   120  as well. So MSC  2   140  forwards such parallel calls to MSC  1   120  which then relays them to IMS  100 . 
     While the originating call is ongoing, the original terminating call may be released and the originating call may remain as the only ongoing call. In the same manner new originating or terminating call requests may come in, resulting in any combination of originating and terminating calls as ongoing calls of the UE  160 . 
     By alternative, the MSC  2   140  may reject any parallel originating call requests from the UE  160  in order to limit this signaling relay functionality of MSC  1   120 . 
     Then in  280  any ongoing call of the UE  160  is completed, i.e. any call has ended. A check  285  is performed in MSC  1   120  and MSC  2   140  whether there is no ongoing call of the UE  160  anymore. 
     If there is no ongoing call of the UE  160  anymore in MSC  2   140 , MSC  2   140  triggers registration of the UE  160  into IMS  100  by sending a Register  290  message (which may correspond to SIP Register message). 
     If there is no ongoing call of the UE  160  anymore in MSC  1   120 , the MSC  1   120  triggers de-registration  295  of the UE  160  from IMS  100 , after applying a delay period  293 . MSC  1   120  does the de-registration  295  by sending a SIP Register message to IMS  100  with a zero expiration value, so immediate expiry. The delay period applied by MSC  1   120  may be adjusted to be long enough to ensure that the re-registration from the MSC  1   120  arrives at the IMS  100  after the registration request from MSC  2   140  has been handled. This may optimize the registration handling in IMS  100  as IMS  100  may keep up the UE  160  registration and just replace the contact address of MSC  1   120  with contact address of MSC  2   140 . The delay period value may be configured by the network operator or may depend on whether the subscriber is a visited or a home subscriber. 
     Referring to  FIG. 3 , this figure shows a flow diagram in a first MSC  120  for controlling set up of a terminating call from an IMS  100  to a UE  160  in a CSFB situation for ICS users according to the invention. 
     The flow starts with the reception of a terminating call request from IMS  300 . This may be a SIP Invite message. The first MSC  120  then pages  310  the UE  160 . 
     In the next step  320  the first MSC  120  receives a Send Identification message. This Send Identification message may be a corresponding MAP message or a SIP message. The message comprises an indication that the second MSC  140  supports ICS functionality and a contact address of the second MSC  140 . This information may also be omitted if the corresponding information is already available in the first MSC  120  by configuration of the node by the network operator. 
     In the next step  330  the first MSC  120  receives a Cancel Location message from HLR  180 . In this case the first MSC  120  maintains  340  the IMS  100  registration of the UE  160  in IMS  100 . 
     Then the first MSC  120  forwards the terminating call set up request to the second MSC  140  by sending forward  350  the SIP Invite to the second MSC  140 . 
     The flow then enters a loop. The first MSC  120  relays  360  any subsequent call control messages received from the second MSC  140  to the IMS  100  and vice versa. Then the first MSC  120  checks whether the call has ended  370 . Note that in this flow only a single terminating call at a time is considered. In an alternative solution also further parallel calls of the UE  160  may be handled. So if the terminating call is still active, the flow loops back to step  360  and keeps on relaying  360  subsequent call control messages. 
     If the terminating call has ended, the first MSC  120  applies a delay. The value of delay may be configured by the network operator in the first MSC  120 . When the delay has expired, the first MSC  120  triggers  390  the de-registration of the UE  160  from IMS  100 . This may be done by sending a SIP Registration with a zero expiration value, so immediate expiry, to IMS  100 . This optimized the registration handling in IMS  100  as IMS  100  may keep up the UE  160  registration and just replace the contact address of MSC  1   120  with contact address of MSC  2   140 . Then the flow ends. 
     Referring to  FIG. 4 , this figure shows a flow diagram in a second MSC  140  for controlling set up of a terminating call from an IMS  100  to a UE  160  in a CSFB situation for ICS users according to the invention. 
     The flow starts with the reception  400  of a Location Update Request message from the UE  160 . This message may comprise a CSMT flag indicating that the Location Update is related to a CSFB scenario. 
     The second MSC  140  then sends  410  a Send Identification message to the first MSC  120 . This may be a corresponding MAP message or also a SIP message. The message may comprise information that the second MSC  140  supports ICS and the contact address of the second MSC  140 . 
     Then the second MSC  140  requests  420  Update Location towards HLR  180  and subscriber data is received from HLR  180  in step  430 . This subscriber data may also comprise information that the subscriber using the UE  160  has a subscription to ICS. 
     Then a terminating call set up request is received in step  440 . This then triggers the second MSC  140  to suppress  450  the registration of the UE  160  in IMS  100 . 
     Then the second MSC  140  sends  460  the Location Update Accept to the UE  160  and continues with the set-up of the terminating call. 
     The flow then enters a loop. The second MSC  140  sends and receives further messages related to the terminating call. 
     Note that in this flow only a single terminating call at a time is considered. In an alternative solution also further parallel calls of the UE  160  may be handled. Then the second MSC  140  checks whether the call has ended  480 . So if the terminating call is still active, the flow loops back to step  470  and keeps on sending/receiving receives further messages related to the terminating call. 
     If the terminating call has ended, the second MSC  140  triggers  490  the registration of the UE  160  into IMS  100 . This is done by sending a SIP Registration message to IMS  100 . Then the flow ends. 
     Referring to  FIG. 5 , this figure shows a block diagram illustrating a MSC  1  for controlling set up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to an embodiment. The illustrated entity may correspond to the MSC  1   120 . The MSC  1   120  may be adapted to perform one or more steps of the above described method shown in  FIG. 3 . 
     The MSC  120  may comprise a number of functional units, which are described in further detail below and which are adapted to perform respective method steps. 
     A processing unit  500  of the MSC  120  may be adapted to page a UE  160 , receive a Send Identification message from a second MSC  140 , receive a Cancel Location message from a HLR  180 , forward the terminating call set up request to the MSC  2   140 , relay further messages related to terminating calls between the MSC  2   140  and the IMS  100 , and trigger de-registration of the UE  160  from the IMS  100 . In a practical implementation the processing unit  500  may be one processor taking care of all the above functions, or may also be distributed over more than one processor, wherein the functions are distributed over the available processors. 
     The MSC  120  may further comprise a sending unit  502  and a receiving unit  504  via which the MSC  120  can communicate with other entities of the communication network  10  such as the MSC  2   140 , the HLR  180 , or the IMS  100 . The sending unit  502  may send out signaling messages composed by the processing unit  500 . The receiving unit  504  may receive signaling messages from the MSC  2   140 , the HLR  180 , or the IMS  100  and forward the received signaling messages to the processing unit  500  for handling. The MSC  120  may comprise more than one sending unit and receiving unit for signaling capacity and redundancy reasons. 
     The MSC  120  may also comprise a storing unit  506  for storing information related to controlling set up of a terminating call from an IMS to a UE in a CSFB situation for ICS users. The storing unit  506  may comprise various types of memory such as volatile memory, non-volatile memory, hard disk drives, solid state drives, a network interface to a database or a data center, secure digital cards, or hardware such as smart cards, non-reversible chips, security chips, security modules, or trusted platform module devices. The storing unit  506  may be used by the processing unit  500  to store information, for example program code or a MSC  2   140  contact address. 
     The MSC  120  may also comprise a call supervision unit  508  for determining whether the terminating call relayed to the MSC  2   140  has ended. This supervision unit  508  may also implement the delay timer to delay the de-registration trigger of the UE  160  from IMS  100 . A timer may for example be implemented by a periodic interrupt signal from the processing unit  500  and a counter or a shift register. 
     Referring to  FIG. 6 , this figure shows a block diagram illustrating a MSC  2  for controlling set up of a terminating call from an IMS to a UE in a CSFB situation for ICS users according to an embodiment. The illustrated entity may correspond to the MSC  2   140 . The MSC  2   140  may be adapted to perform one or more steps of the above described method shown in  FIG. 4 . 
     The MSC  140  may comprise a number of functional units, which are described in further detail below and which are adapted to perform respective method steps. 
     A processing unit  600  of the MSC  140  may be adapted to receive a location update request from a UE  160 , receive a terminating call set up request from a MSC  1   120 , receive further messages related to terminating calls from the IMS  100  via the MSC  1   120 , send further messages related to terminating calls to the IMS  100  via the MSC  1   120 , and trigger registration of the UE  160  into the IMS  100 ). In a practical implementation the processing unit  600  may be one processor taking care of all the above functions, or may also be distributed over more than one processor, wherein the functions are distributed over the available processors. 
     The MSC  140  may further comprise a sending unit  602  and a receiving unit  604  via which the MSC  140  can communicate with other entities of the communication network  10  such as the MSC  1   120  or the HLR  180 . The sending unit  602  may send out signaling messages composed by the processing unit  600 . The receiving unit  604  may receive signaling messages from the MSC  1   120 , or the HLR  180  and forward the received signaling messages to the processing unit  600  for handling. The MSC  140  may comprise more than one sending unit and receiving unit for signaling capacity and redundancy reasons. 
     The MSC  140  may also comprise a storing unit  606  for storing information related to controlling set up of a terminating call from an IMS to a UE in a CSFB situation for ICS users. The storing unit  606  may comprise various types of memory such as volatile memory, non-volatile memory, hard disk drives, solid state drives, a network interface to a database or a data center, secure digital cards, or hardware such as smart cards, non-reversible chips, security chips, security modules, or trusted platform module devices. The storing unit  606  may be used by the processing unit  600  to store information, for example program code or subscriber data. 
     The MSC  140  may also comprise a call supervision unit  608  for determining whether the terminating call from the MSC  1   120  has ended. When the call has ended, this may trigger the registration of the UE  160  into the IMS  100 . 
     According to another embodiment, a computer program is provided. The computer program may be executed by the processing units  500  and/or  600  of the above mentioned entities  120  and/or  140  respectively such that a method for controlling set up of a terminating call from an IMS to a UE in a CSFB situation for ICS users as described above with reference to  FIG. 3 or 4  may be carried out or be controlled. In particular, the entities  120  and/or  140  may be caused to operate in accordance with the above described method by executing the computer program. 
     The computer program may be embodied as computer code, for example of a computer program product. The computer program product may be stored on a computer readable medium, for example a disk or the storing unit  506  and/or  606  of the entities  120  and/or  140 , or may be configured as downloadable information. 
     One or more embodiments as described above may enable at least one of the following technical effects:
         Allows to use MTRF for ICS subscribers that are registered in an MSC enhanced for ICS   The solution does not impact the HPLMN network, changes are needed only in MSCs of the VPLMN   Minimal additional call setup delay   Relaying resources in the first MSC are used only for the duration of the call       

     Modifications and other embodiments of the disclosed invention will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the embodiments are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.