WO2016075510A1

WO2016075510A1 - Terminating a mobile call

Info

Publication number: WO2016075510A1
Application number: PCT/IB2014/065936
Authority: WO
Inventors: Jari Mutikainen; Juha Matias Kallio
Original assignee: Nokia Technologies Oy; Nokia Usa Inc.
Priority date: 2014-11-10
Filing date: 2014-11-10
Publication date: 2016-05-19

Abstract

A terminating access domain selection function in an internet protocol multimedia subsystem (IMS) is transmitting a first session setup request associated with a user according to session initiation protocol (SIP), and is receiving a session initiation protocol (SIP) response to the first session setup request, the response comprising an indication indicating that the user is not reachable via a session control entity that received the session setup request. During awaiting a period of time it is determined if the user performs a registration during the period of time, if it is determined that the user has performed the registration during the period of time, a second session setup request associated with the user according to session initiation protocol (SIP) is transmitted.

Description

TERMINATING A MOBILE CALL

Technical Field of the Invention

The invention relates to a method, an apparatus and a computer program product for terminating a mobile call in a communication system.

Background of the Invention

Within the IP (Internet Protocol) Multimedia Subsystem (IMS) as defined by 3rd Generation Partnership Project (3GPP) the Session Initiation Protocol (SIP) as defined by Internet Engineering Task Force (IETF) is used for controlling communication. SIP is an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. These sessions may include Internet multimedia conferences, Internet telephone calls, and multimedia distribution. Members in a session can communicate via multicast or via a mesh of unicast relations, or a combination of these.

The 3GPP specifies IMS centralised services (ICS), Session Continuity (SC) and

Single radio Voice Call Continuity (SRVCC). The basic idea of these activities is to have the whole service control for a user purely in the 3GPP IMS independent from the access type being used, i.e. even if the serving access network for a speech bearer is in the 3GPP circuit switched (CS) domain the IMS is controlling the session and provides associated services like supplementary services. One of the specified features is to provide session delivery via 3GPP and non-3GPP accesses wherever user equipment (UE) is reachable and select the access network such that it is able to carry the session. UE may have active sessions via more one than one access network in parallel or can be reachable by more than one access network in parallel. The functional element within the IMS, which is responsible to select the most suitable access network, is called terminated access domain selection function (T-ADS). In order to determine the best suited access network, a T-ADS may use several input parameters, e.g. attach/registration status of a user or user/operator preferences

Mobile services switching centre (MSC) Server architecture splits traditional monolithic MSC into control and user plane parts. Control plane is handled by MSC

Server and user plane switching is handled by media gateways (MGW). Also in-band announcements, tones, dual-tone multifrequency (DTMF) generation and detection as well as multiparty conference bridge functionalities are located at MGW. This layered architecture enables operators to locate MGW network elements within own network topology into locations where actual user plane switching can be done most optimally. Control plane related tasks can be centralized into large core network sites by using IP based core network infrastructure.

MSC server based mobile network architecture can enable operators to have more efficiency from core network (CN) resource usage, for example, use of IP transmission within core network instead of using expensive time-division multiplexing (TDM) as well as additional cost savings by re-using same equipment in the future for IMS integration as well for example by providing plain IMS-CS interworking as well as voice services when acting as service gateway.

When deploying circuit switched 2G and 3G radio access networks in such manner that single MSCs or MSC servers control only either one of the radio access technology and not both, problems can be faced with mobile terminating calls which result in paging failures due to unreachable user equipment (UE). The reason why UE may be unreachable is due to the fact that the change from one radio access technology to another may take relatively long time (seconds) and if such procedure is executed many times per hour then it may happen that terminal is unreachable when call arrives from the network and paging is executed. Summary of the Invention

The invention is trying to overcome above drawback by providing a method, a computer program product and a terminating access domain selection function in an internet protocol multimedia subsystem (IMS), comprising transmitting a first session setup request associated with a user according to session initiation protocol (SIP), receiving a session initiation protocol (SIP) response to the first session setup request, the response comprising an indication indicating that the user is not reachable via a session control entity that received the session setup request, awaiting a period of time if the indication indicating that the user is not reachable via the session control entity is received, determining if the user becomes reachable during the period of time, and transmitting a second session setup request associated with the user according to session initiation protocol (SIP), if it is determined that the user has become reachable during the period of time.

Further a method, a computer program product and a terminating access domain selection function in an internet protocol multimedia subsystem (IMS) is provided, comprising updating an address of the terminating access domain selection function as an address of a mobile switching centre (MSC) in a subscriber database (HLR) according to mobile application part (MAP), transmitting a first session setup request associated with a user according to session initiation protocol (SIP), receiving a resume call handling (RCH) mobile application part (MAP) response, the response comprising an indication indicating that the user is not reachable via a session control entity that received the first session setup request, awaiting a period of time if the indication indicating that the user is not reachable via the session control entity is received, determining if the user becomes reachable during the period of time, and transmitting a second session setup request associated with the user according to session initiation protocol (SIP), if it is determined that the user has become reachable during the period of time.

Determining if the user becomes reachable during the period of time can comprise determining

- if the user becomes reachable in a domain.

- detecting an internet protocol multimedia subsystem (IMS) registration of the user.

- receiving a send routing information (SRI) message of mobile application part (MAP) associated with the user.

The send routing information (SRI) message can comprise a mobile station roaming number (MSRN) or a circuit switched roaming number (CSRN) of the user.

Transmitting the second session setup request can comprise addressing the user with the mobile station roaming number (MSRN) or the circuit switched roaming number (CSRN) of the user.

The terminating access domain selection functions and the methods can comprise selecting a domain for transmitting the first session setup request.

The updating can comprise requesting mobile terminating roaming retry procedure.

The terminating access domain selection functions and the methods can comprise checking, before the awaiting the period of time, if the user has an internet protocol multimedia subsystem (IMS) registration, and wherein if the user has the internet protocol multimedia subsystem (IMS) registration, the second session setup request associated with the user according to session initiation protocol (SIP) is transmitted without waiting the period of time.

Further a method and an a session control entity in a communication network are provided, comprising receiving a session setup request associated with a user according to session initiation protocol (SIP), communicating with a subscriber database (HLR) according to mobile application part (MAP), comprising configuring to receive a cancel location signal, transmitting a session initiation protocol (SIP) response to the session setup request responsive to receiving the cancel location signal, and including in the response an indication indicating that the user is not reachable via the session control entity. The indication can comprise at least one of a session initiation protocol header (SIP), an extensible markup language (XML) body or a session initiation protocol (SIP) response code.

The method and the session control entity can comprise attempting to reach the user by performing paging. The session control entity comprises a mobile switching centre (MSC).

Embodiments of the invention can have one or more of following advantages • Improving end user service experience because mobile terminating call attempt failures due to UE transitioning are reduced.

· Terminating a mobile is not visible to the applications executed prior to the

terminating.

Description of Drawings

Figures 1 - 4 illustrate example signaling flows according to aspects of the invention.

Figure 5 illustrates an example structure and functions of devices implementing aspects of the invention.

Detailed Description of the Invention

A mobile services switching centre (MSC) is a mobile network element which performs switching functions in its area of operation and controls interworking with other networks. MSC server (MSS) is providing the call control and the control of multimedia gateways (MGW), and is allowing the same circuit-switched services as the mobile services switching centre (MSC). The MSC server (MSS) can integrate the functions of a mobile services switching centre (MSC), visitor location register (VLR), service switching point (SSP), and MGW control into a single entity.

Mobile application part (MAP) is a part of the signalling system of a mobile network used for signalling between mobile network centres and registers. The MAP can be used for location updates, call control of incoming calls to a mobile station, and transmission of short messages. MAP is not a protocol by itself, but a set of non-call- related signalling protocols. Resume Call Handling (RCH) is a MAP operation which the terminating visited MSC can use to request the gateway MSC to resume call handling and to forward the call to the specified destination. Send routing information (SRI) is a MAP message which the gateway MSC can use to obtain the routing information when terminating the call set-up. A cancel location message is a MAP message that the home location register (HLR) can send to a serving network entity, such as MSC, when a subscriber is deactivated relating to the network entity, after which the mobility management (MM) context and packet data protocol (PDP) context can be removed in the network entity. A mobile station roaming number (MSRN) is a number allocated to a mobile station, when registered with a visitor location register (VLR), for the purpose of re-routing calls to that station when it has roamed out of the area covered by the public land mobile network (PLMN) in which the station is permanently registered. Provide roaming number (PRN) is a procedure which a home location register (HLR) can use to request the visitor location register (VLR) for a roaming number.

An MSC supporting a mobile terminating roaming retry feature can include the Call Reference Number, the MSC address and the MT Roaming Retry Supported Information Element (IE) in the first SRI sent to the HLR. A HLR supporting the mobile terminating roaming retry feature can include the Call Reference Number, the MSC address and the MT Roaming Retry Supported IE in the PRN sent to the MSC/VLR if received in the SRI.

Receipt of the MT Roaming Retry Supported IE in the PRN indicates that the MSC can support the Resume Call Handling (RCH) procedure and the mobile terminating roaming retry feature. When UE has moved, an old MSC/VLR can, if it supports the mobile terminating roaming retry feature and did receive the MT Roaming Retry

Supported IE in the PRN, send an RCH message to the MSC with the MT Roaming Retry IE. Upon receipt of the RCH message with the MT roaming retry IE, the MSC

acknowledges the RCH message, releases the call towards the old MSC/VLR, and can re-send a new SRI to the HLR using a new call reference number.

Customised applications for mobile network enhanced logic (CAMEL) is a network architecture that provides the mechanisms to support intelligent network (IN) supported services to a roaming subscriber.

An MSC Server (MSC-S) is based on 3GPP defined architecture for Bearer

Independent Circuit Switched Core Networks (BICSCN). This layered/split architecture was introduced in 3GPP release 4 and continues to improve in later 3GPP releases with new functionalities related to the voice call continuity, Service Change UDI Fallback feature; use of IP based transmission including SIP for call control as well as use of Circuit Switched bearers with IMS to name a few enhancements.

Nature of these enhancements is related to optimization of cost efficiency of CS network solution without introducing any new end user services which is in line with the idea related to IMS Centralized Service model that was introduced into 3GPP Release 8 content.

3GPP has defined procedure named as Mobile Terminated Roaming Retry Call according to which, if a terminal is moved into coverage of another MSC/MSC Server while paging is still ongoing in another MSC/MSC Server, the terminating call can still be re-routed by gateway MSC/MSC Server to the correct visited MSC/MSC Server in order to complete the call. Without such procedure paging would fail and call can either be released or forwarded to another destination (such as voice mail) resulting poor end user experience.

IMS Centralized Services (ICS) architecture enables use of IMS architecture to provide e.g. Multimedia Telephony services to end users that use any access technology (xDSL, cable, fixed CS, mobile broadband and mobile CS). These services can be both consumer and enterprise services. Architecture to support UEs, which are not enhanced with ICS capabilities can also be handled with two different solutions. The first solution is to enhance an MSC Server network element with ICS capability (i.e. ICS enhanced MSC Server approach) and the second approach is to use Intelligent Network (IN) based routing mechanisms (e.g. CAMEL homing), which is also the only approach available today to provide e.g. Voice Call Continuity solution. It is assumed that in the future ICS enhanced MSC server approach will be gradually taken into use by operators that want to deploy ICS for non-ICS UEs. Use of ICS enhanced MSC Servers can also provide benefits when used with ICS enabled UEs but that has not been the main reason for such solution in the network side.

Different types network entities and functions exist in the IMS network. Call Session Control Functions (CSCF) implement a session control function in SIP layer. The CSCF can act as Proxy CSCF (P-CSCF), Serving CSCF (S-CSCF) or Interrogating

CSCF (l-CSCF). The P-CSCF is the first contact point for the User Equipment (UE) within the IMS; the S-CSCF actually handles the session states in the network; the l-CSCF is mainly the contact point within an operator's network for all IMS connections destined to a subscriber of that network operator, or a roaming subscriber currently located within that network operator's service area.

The functions performed by the l-CSCF are, for example, assigning an S-CSCF to a user performing SIP registration and routing SIP requests received from another network towards the S-CSCF. The S-CSCF performs the session control services for the UE. It maintains a session state as needed by the network operator for support of the services and may be acting as Registrar, i.e. it accepts registration requests and makes its information available through the location server (e.g. HSS). The S-CSCF is the central point to users that are hosted by this S-CSCF. The S-CSCF provides services to registered and unregistered users when it is assigned to these users. This assignment is stored in the Home Subscriber Server (HSS). Application Servers (AS) offer services to an user, e.g. presence or supplementary telephony services like call forwarding.

The HSS and a Home location Register (HLR) is a master database for a given user. It is the entity containing the subscription-related information to support the network entities actually handling calls/sessions. As an example, the HLR/HSS provides support to the call control servers (MSC, CSCF) in order to complete the routing/roaming procedures by solving authentication, authorization, naming/addressing resolution, location dependencies, etc.

The HLR/HSS can be responsible for holding the following user related

information:

User Identification, Numbering and addressing information

User Security information: Network access control information for authentication and authorization, such as password information

- User Location information at inter-system level: the HLR/HSS supports the user registration, and stores inter-system location information, etc.

User profile information.

A media gateway control function (MGCF) acts as an interworking point between a circuit switched (CS) network and an IP based network in the control plane of the network. The MGCF controls the parts of the call state related to connection control for media channels in a media gateway (MGW), communicates with call state control, and performs protocol conversion between the call control protocols, such as SIP and integrated services digital network (ISDN) user part (ISUP).

A Terminating Access Domain Selection (T-ADS) function can direct an incoming session to an ICS User, or for more UEs of the ICS User. The T-ADS can influence the selection of one or more contacts amongst the registered contacts and can influence the selection of an access network for delivery of the incoming session to the selected contact. T-ADS can be implemented as part of an AS called Session Continuity and Centralisation AS (SCC AS).

Global System for Mobile Communications (GSM) / enhanced data rates for global evolution (EDGE) radio access network (GERAN) is a radio access network that includes general packet radio service (GPRS) and EDGE technologies and connects the A, Gb, and lu interfaces to a core network, thus enabling real-time IP-based services. UMTS terrestrial radio access network (UTRAN) is a radio access network (RAN) which consists of radio network controllers (RNC) and base transceiver stations (BTS) and which is located between the lu interface and the wideband code division multiple access

(WCDMA) radio interface. Paging is a procedure by which a mobile network attempts to reach the mobile station (MS) or a mobile subscriber within its location area, before any other network-initiated procedure can take place. Radio access technology (RAT) is an access technology that is used to connect different terminals and applications to telecommunication networks by using radio frequency signals. In case Circuit Switched 2G/GERAN and 3G/UTRAN radio access technologies are controlled by different ICS enhanced MSC Servers part of the same ICS architecture it may occur that (non-)ICS UE may move from one radio access technology to another while Mobile Terminated call is being offered from IMS towards UE. It has been witnessed today that transition between 2G/UTRAN and 3G/GERAN may take several seconds from UE before it is again reachable. Some tuning can be done in the radio access network (e.g. in triggering conditions for such transition) but it does not solve the problem totally. Therefore in the current CS core network the 3GPP has defined specific procedures for Mobile Terminated call retry. However these procedures are only relevant for non-ICS architecture in which gateway MSC based mobile terminated call routing is executed based on CS network principles.

Four different scenarios can be defined in which mobile terminated call need to be re-routed by T-ADS:

1 . Source MSC and target MSC supports IMS Centralized Services (ICS)

2. Source MSC supports ICS but target MSC does not support ICS

3. Source MSC does not support ICS but target MSC supports ICS

4. Source MSC and target MSC does not support ICS

According to aspects of the invention, a SIP response can be provided by an MSC that is ICS enhanced to a T-ADS in order to indicate that during ongoing paging a CANCEL has been received from an HLR which indicates that UE has moved into area of another MSC. Further, a new call attempt by T-ADS towards new target MSC can be triggered.

The SIP response can be introduced, for example, as a new SIP header, as a new body (e.g. extensible markup language (XML) body) into existing SIP response or as new SIP response code.

According to an aspect of the invention, a T-ADS can support MAP Resume Call Handling (RCH) procedure defined by the 3GPP, for example, similarly as a gateway MSC function. The T-ADS can request so-called Mobile Terminating Roaming retry procedure (as defined in 3GPP specification 23.018 ) by adding an address of the T-ADS and an indication into MAP Send Routing Information procedure when executing an HLR enquiry in order to obtain a MS/CS Roaming Number (MSRN/CSRN) of a user for terminating a call via an MSC that does not support ICS. The indication can be MT Roaming Retry Supported information element (IE). The HLR can transmit the indication and the address of the T-ADS to the MSC, for example, as part of a PRN procedure. If the MSC that does not support the ICS receives a MAP Cancel signal from the HLR, the MSC can determine based on the received indication about the Mobile Terminating Roaming retry that this procedure is applicable, and the MSC can invoke a Resume Call Handling (RCH) procedure towards the T-ADS.

The T-ADS can, after receiving the Resume Call Handling (RCH), check whether there exists an IMS registration for the user from another (target) MSC that is ICS capable. If an IMS registration exists, the T-ADS can re-execute a call termination towards the MSC supporting the ICS and from which the registration has been received. If there is no registration, the T-ADS can start a timer in order to wait a possible registration from another MSC. If no registration of the user is performed during the timer, the T-ADS can assume that the target MSC is not ICS capable and can route the call terminating by re-executing an HLR enquiry. If a registration of the user is received before the timer expires, the T-ADS can re-execute the call termination towards the MSC supporting the ICS and from which the registration has been received.

According to aspects of the invention, a T-ADS which has been involved in the mobile terminating call towards a "source" MSC is able to complete the call to the called subscriber in previously mentioned different alternative scenarios.

Figure 1 illustrates an example message flow according to aspects of the invention. In this example, both a first contacted MSC 4 (source MSC) and next contacted MSC 5 support IMS Centralized Service (ICS). An S-CSCF 2 receives an INVITE 1 01 request originating from a calling user 7. The S-CSCF process 102 the INVITE 101 and transmits the INVITE 103 to a T-ADS 1 . The T-ADS 1 can perform a domain selection 104 for selecting a domain (e.g. IMS or CS) for terminating the call. After the domain selection, the T-ADS 1 can transmit the INVITE 105 to the S-CSCF 2 which in turn transmits the INVITE 106 to a source MSC Server 4. The source MSC 4 can start paging 107 to locate UE 6, which however may be in the middle of the process of moving 108 from the control of the (source) MSC 4 to control of a target MSC 5. The UE 6 can perform a location update 109 to the MSC 5 which updates 1 10 the location of the UE 6 in a HLR 3. At this phase the UE 6 can become reachable via the MSC 5. The HLR 3 can send a cancel location command 1 1 1 to the MSC 4 which then can stop the paging 1 12. The MSC 4 can send a SIP response 1 1 3 to the INVITE 106 via the S-CSCF 2 to the T- ADS 1 , including an indication indicating that the user is not reachable. After receiving the

SIP response 1 14 and detecting the indication, the T-ADS can start a timer 1 15 to wait a period of time to check if an IMS registration is received for the UE 6 within this time. If the UE 6 performs an IMS registration with the MSC 5, the IMS registration 1 1 6 is transmitted via the S-CSCF 2 to the T-ADS 1 . When receiving the IMS registration 1 1 7 of the UE 6, the T-ADS 1 can decide to re-try to terminate the call 1 18 by transmitting a new INVITE 1 1 9 request to the S-CSCF 2 which transmits the INVITE 120 to the MSC 5 which can then start paging and terminate the call towards the UE 6. Figure 2 illustrates another example signalling flow according to aspects of the invention in which the source MSC 4 support the ICS, however, the target MSC 5 does not support the ICS. Steps 201 - 21 5 corresponds steps 1 01 - 1 1 5 of Figure 1 . Since the MSC 5 does not support ICS, it cannot perform IMS registration as in the example of Fig 1 . Instead, the MSC 5 can initiate provide roaming number (PRN) 21 6 procedure with the HLR 4, which in turn can perform send routing information (SRI) 217 procedure towards the T-ADS 1 . The T-ADS can now determine 21 8 that the target MSC 5 does not support ICS and can decide 218 to terminate the call using an MSRN/CSRN of the UE 6 which the T-ADS 1 can have received from the HLR 4 in the SRI 21 7. The T-ADS 1 can transmit a new INVITE 21 9 with the MSRN/CSRN to the CSCF 2 which transmits the INVITE 220 to an MGCF 8. The MGCF 8 can convert SIP INVITE 220 to an ISUP Initial Address Message (IAM) 221 and transmit it 221 to the MSC 5 which can start paging the UE 6 for terminating the call.

Figure 3 illustrates a further example message flow according to aspects of the invention in which the source MSC 4 does not support ICS, but the target MSC 5 support ICS. Steps 301 - 304 corresponds steps 1 01 - 104 of Figure 1 . After determining 304 that the call must be terminated in the CS domain (since the MSC 4 does not support ICS), the T-ADS 1 can transmit a SRI 305 to the HLR 3. The SRI 305 can include an indication requesting MT Roaming Retry procedure, for example, an MT Roaming Supported Information Element (IE). The SRI 305 can further include and identify an address of the T-ADS 1 as an MSC address. The HLR 3 can transmit above information to the MSC 4 as part of provide roaming number (PRN) 306 procedure. The T-ADS 1 can transmit an INVITE 307 in which UE 6 has been identified with a MSRN/CSRN which the T-ADS 1 can have received from the HLR 3 as part of the SRI 305 message exchange. The S-CSCF 2 can transmit the INVITE 308 to an MGCF 8 which can convert the SIP

INVITE 308 into an ISUP Initial Address Message (IAM) 309 and transmit it 309 to the MSC 4 which can start paging 310 the UE 6 for terminating the call. Steps 31 1 - 315 corresponds steps 108 - 1 1 2 of Figure 1 . The MSC 4 can initiate resume call handling procedure 316 towards the T-ADS 1 based on the information the MSC 4 has received from the HLR 3 in PRN 306. After receiving the RCH 316, the T-ADS 1 can start 31 7 a timer to wait a period of time to check if an IMS registration is received for the UE 6 within this time. Steps 317 - 323 corresponds steps 1 15 - 121 of Figure 1 .

Figure 4 illustrates a further example message flow according to aspects of the invention in which neither of the source MSC 4 or the target MSC 5 support ICS. Steps 401 - 41 7 corresponds steps 301 - 31 7 of Figure 3. Since the MSC 5 does not support ICS, no IMS registration for UE 6 is received in the T-ADS 1 , instead the MSC 5 performs provide roaming number 418 procedure and the HLR 3 initiates a SRI 41 9 procedure towards the T-ADS 1 . Steps 41 8 - 424 corresponds steps 21 6 - 222 of Figure 2.

Figure 5 illustrates an example internal structure and function of network elements implementing aspects of the invention. A session control entity, such as MSC 4, can comprise a receiving unit 51 configured to receive a session setup request, associated with a user, such as SIP INVITE or ISUP IAM. The session control entity can comprise a location unit 52 configured to communicate with a subscriber database, such as HLR, according to mobile application part (MAP). The location unit 52 can be configured to receive a cancel location message according to MAP. The session control entity can comprise a transmitting unit 53 configured to transmit a session initiation protocol (SIP) response to the session setup request. The response can be transmitted responsive to receiving the cancel location signal from the by the location unit 52. The session control entity can comprise an including unit 54 configured to include in the response an indication indicating that the user is not reachable via the session control entity. The indication can be for example a specific SIP header, new body (e.g. XML) in an existing SIP response or a specific SIP response code. The session control entity can comprise a paging unit 55 configured to perform paging to reach the mobile station (UE) within its location area. The paging unit 55 can be configured to start paging after the receiving unit 51 has received a session setup request, and can be configured to stop paging if the location unit 52 receives a cancel location message.

Continuing with Fig 5, a terminating access domain selection function can comprise a session unit 61 configured to receive a session setup request associated with a user (e.g. UE 6), for example, a SIP INVITE request. The terminating access domain selection function can have a domain selection unit 62 configured to determine the preferred access network, for example, based on various input parameters, such as. attach/registration status of a user (UE 6), type of service, or user/operator preferences. A transmitting unit 63 can be configured to transmit a first session setup request associated with the user, such as SIP request. A receiving unit 64 can be configured to receive a session initiation protocol (SIP) response to the first session setup request. The response can comprise an indication indicating that the user is not reachable via a session control entity that received the session setup request. The indication can be for example a specific SIP header, new body (e.g. XML) in an existing SIP response or a specific SIP response code. Alternatively or in addition the receiving unit 64 can be configured to receive MAP messages relating to, for example, resume call handling (RCH) operation. The terminating access domain selection function can comprise an awaiting unit 65 configured to wait a period of time if the indication indicating that the user is not reachable via the first session control entity, or the resume call handling (RCH) operation has been received by the receiving unit 64. The awaiting unit 65 can comprise a timer. A determining unit 66 can be configured to determine if the user performs a registration during the period of time. A registration unit 67 can be configured to receive registration related information, such as IMS registration of the user or SRI MAP message associated with the user, and transmit the information relating to the received registration related information to the determining unit 66. A re-transmitting unit 68 can be configured to transmit a second session setup request associated with the user, such as SIP INVITE, if it is determined by the determining unit 66 that the user has performed the registration during the period of time defined by the awaiting unit 65. In the second session setup request the user can be addressed with a MSRN/CSRN. The terminating access domain selection function can comprise an updating unit 69 configured to update an address of the terminating access domain selection function as an address of a mobile switching centre to a subscriber database, such as HLR, according to mobile application part (MAP). The updating unit 69 can include an indication requesting mobile terminating roaming retry procedure, for example, an MT Roaming Supported Information Element (IE), in MAP signalling to the subscriber database.

All units described above may be implemented for example using

microprocessors, chips and/or other electrical components and/or by software.

The network elements of a communication network may be physically

implemented in a switch, router, server or other hardware platform or electronic equipment which can support data transmission and processing tasks, or can be implemented as a component of other existing device.

Functions of a terminating access domain selection function can be implemented as part of a service centralization and continuation application server (SCC AS).

Functions of the terminating access domain selection function and the session control entity described above may be implemented by code means, as software, and loaded into memory of a computer.

Claims

Claims:

1 . A terminating access domain selection function in an internet protocol multimedia subsystem (IMS), comprising

transmitting means for transmitting a first session setup request associated with a user according to session initiation protocol (SIP),

receiving means for receiving a session initiation protocol (SIP) response to the first session setup request, the response comprising an indication indicating that the user is not reachable via a session control entity that received the session setup request, awaiting means for awaiting a period of time if the indication indicating that the user is not reachable via the session control entity is received,

determining means for determining if the user becomes reachable during the period of time, and

re-transmitting means for transmitting a second session setup request associated with the user according to session initiation protocol (SIP), if it is determined that the user has become reachable during the period of time.

2. A terminating access domain selection function of claim 1 , further comprising a domain selection means for selecting a domain for transmitting the first session setup request.

3. A terminating access domain selection function of claim 1 or 2, wherein determining if the user becomes reachable during the period of time comprises determining if the user becomes reachable in a domain.

4. A terminating access domain selection function of any of preceding claims, wherein the determination if the user becomes reachable comprises detecting an internet protocol multimedia subsystem (IMS) registration of the user.

5. A terminating access domain selection function of any of preceding claims, wherein the determination if the user becomes reachable comprises receiving a send routing information (SRI) message of mobile application part (MAP) associated with the user.

6. A terminating access domain selection function of any of preceding claims, wherein the send routing information (SRI) message comprises a mobile station roaming number (MSRN) or a circuit switched roaming number (CSRN) of the user, and wherein the transmitting the second session setup request comprises addressing the user with the mobile station roaming number (MSRN) or the circuit switched roaming number (CSRN) of the user.

7. A terminating access domain selection function in an internet protocol multimedia subsystem (IMS), comprising

updating means for updating an address of the terminating access domain selection function as an address of a mobile switching centre (MSC) in a subscriber database (HLR) according to mobile application part (MAP),

receiving means for receiving a resume call handling (RCH) mobile application part (MAP) response, the response comprising an indication indicating that the user is not reachable via a session control entity that received the first session setup request,

awaiting means for awaiting a period of time if the indication indicating that the user is not reachable via the session control entity is received,

8. A terminating access domain selection function of any of claim 7, wherein the updating comprises requesting mobile terminating roaming retry procedure.

9. A terminating access domain selection function of any of claim 7 or 8, further comprising checking means for checking, before the awaiting the period of time, if the user has an internet protocol multimedia subsystem (IMS) registration, and wherein if the user has the internet protocol multimedia subsystem (IMS) registration, the second session setup request associated with the user according to session initiation protocol

(SIP) is transmitted without waiting the period of time.

10. A terminating access domain selection function of any of claims 7-9, further comprising a domain selection means for selecting a domain for transmitting the first session setup request.

1 1 . A terminating access domain selection function of any of claims 7-10, wherein determining if the user becomes reachable during the period of time comprises determining if the user becomes reachable in a domain.

12. A terminating access domain selection function of any of claims 7-1 1 , wherein the determination if the user becomes reachable comprises detecting an internet protocol multimedia subsystem (IMS) registration of the user.

13. A terminating access domain selection function of any of claims 7-12, wherein the determination if the user becomes reachable comprises receiving a send routing information (SRI) message of mobile application part (MAP) associated with the user.

14. A terminating access domain selection function of any of claims 7-13, wherein the send routing information (SRI) message comprises a mobile station roaming number (MSRN) or a circuit switched roaming number (CSRN) of the user, and wherein the transmitting the second session setup request comprises addressing the user with the mobile station roaming number (MSRN) or the circuit switched roaming number (CSRN) of the user.

15. A session control entity in a communication network, comprising

receiving means for receiving a session setup request associated with a user according to session initiation protocol (SIP),

location means for communicating with a subscriber database (HLR) according to mobile application part (MAP), wherein the location means in configured to receive a cancel location signal,

transmitting means for transmitting a session initiation protocol (SIP) response to the session setup request responsive to receiving the cancel location signal, and including means for including in the response an indication indicating that the user is not reachable via the session control entity.

16. A session control entity of claim 1 5, further comprising paging means for attempting to reach the user by performing paging.

17. A session control entity of claim 1 5 or 16, wherein the indication comprises at least one of a session initiation protocol header (SIP), an extensible markup language

(XML) body or a session initiation protocol (SIP) response code.

18. A session control entity of any of claims 15 - 17, wherein the session control entity comprises a mobile switching centre (MSC).

19. A method for terminating a mobile call, comprising

transmitting a first session setup request associated with a user according to session initiation protocol (SIP),

receiving a session initiation protocol (SIP) response to the first session setup request, the response comprising an indication indicating that the user is not reachable via a session control entity that received the session setup request,

awaiting a period of time if the indication indicating that the user is not reachable via the session control entity is received,

determining if the user becomes reachable during the period of time, and

transmitting a second session setup request associated with the user according to session initiation protocol (SIP), if it is determined that the user has become reachable during the period of time.

20. A method for terminating a mobile call, comprising

updating, by terminating access domain selection function, an address of the terminating access domain selection function as an address of a mobile switching centre (MSC) in a subscriber database (HLR) according to mobile application part (MAP),

receiving a resume call handling (RCH) mobile application part (MAP) response, the response comprising an indication indicating that the user is not reachable via a session control entity that received the first session setup request,

determining if the user becomes reachable during the period of time, and

21 . A method for controlling a mobile call, comprising

receiving a session setup request associated with a user according to session initiation protocol (SIP),

receiving, from a subscriber database (HLR), a cancel location signal according to mobile application part (MAP), transmitting a session initiation protocol (SIP) response to the session setup request responsive to receiving the cancel location signal, and

including in the response an indication indicating that the user is not reachable via a session control entity sending the response.

22. A method for terminating a mobile call, comprising

receiving, from a subscriber database (HLR), a cancel location signal for the user according to mobile application part (MAP),

transmitting a session initiation protocol (SIP) response to the session setup request responsive to receiving the cancel location signal, and

determining if the user becomes reachable during the period of time, and

23. A computer program product comprising code means adapted to produce steps of any of claims 20 - 22 when loaded into the memory of a computer.