WO2006058553A1

WO2006058553A1 - Handing off a mobile station between a first access network supporting sip (session initiation protocol) and a second cellular access network

Info

Publication number: WO2006058553A1
Application number: PCT/EP2004/013768
Authority: WO
Inventors: Ralf Keller; Joerg Christian Ewert; Andreas Witzel; Martin Stümpert
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2004-12-03
Filing date: 2004-12-03
Publication date: 2006-06-08

Abstract

The invention relates to a method, and device, a mobile station, a database and a mobile services switching center for the handover of calls in a telecommunications system comprising two access networks, a first access network supporting SIP (session initiation protocol), like a WLAN and a second, cellular access network. A demand to change from the first (WLAN) to the second (cellular) network is determined, a connection is set-up to the second network, and the connection to the first access network is released. A device supporting SIP and call control functionality for circuit switch is disclosed. A Data base is disclosed wherein two location identities associated to a mobile station are stored together with an indication on which is the preferred access network via which the mobile station should be reached. In addition, when routing a connection to a mobile station, information on a target mobile services switching centre will be extracted from a message received at an MSC and a paging will be suppressed.

Description

HANDING OFF A MOBILE STATION BETWEEN A FIRST ACCESS NETWORK SUPPORTING SIP (SESSION INITIATION PROTOCOL) AND A SECOND CELLULAR ACCESS NETWORK

[technical field]

The present invention relates to methods and devices for handling a call involving a 5 mobile station adapted to co-operate with a first access network comprising a device operating according to a session initiation protocol and a second access network wherein the second access network is a cellular access network.

[related art]

In recent years the percentage of household having a broadband connection at home 10 was increasing. Today 11% of the households in Germany are equipped with a broadband connection (mainly ADSL). In Sweden the percentage is already 22% (50% ADSL, 50 % other technologies). It is expected that these percentages will increase significantly over the next years. Many of these users will install private WLAN(Wireless Local Area Network)-access points to connect their PCs (Personal 15 Computers), PDA (Personal Digital Assistants), mobile phones etc. to their fixed broadband connection.

Furthermore, it is expected that the number of public WLAN-hot spots in Europe will increase from approx. 18.000 (2003) to 36.000 (2009).

Parallel, to the increased number of broadband connections the users are starting to 20 use IP (Internet Protocol) telephony over their WLAN-broadband connection via PCs and PDAs. In the next years mobile phone vendors will launch so-called GSM/WLAN dual mode phones (e.g. Nokia 2005).

For these users of the GSM/WLAN dual mode phones it will be quite easy to originate calls via the SIP-client over the WLAN-radio interface towards other SIP- 25 clients connected to the same network or even to the internet.

Today several architectures are proposed how to integrate cellular networks with WLAN-access points. The main focus is the integration of the WLAN-access point towards the packet switching network. For example Mobile @home proposes an architecture where the WLAN-access point is connected via a broadband connection to the BSC. The disadvantage of this solution is that the same operator has to supply the cellular mobile network and the broadband connection. Furthermore, Mobile @home proposes a solution based on the ADSL (Asymmetric Digital Subscriber Line) technology.

However none of the current solutions provides mechanisms and devices for handling a call that comprises a connection via a WLAN access network to a mobile station in the case that the mobile station leaves a coverage area of the WLAN.

Therefore it is object of the invention to provide methods and devices to overcome the shortcomings of the prior art.

[summary]

This will be solved advantageously by the methods of claim 1, the device of claim 7 the mobile station of claim 8, the database of claim 10 and the mobile services switching centre of claim 12. Further advantageous embodiments can be derived from the dependent claims.

The invention introduces a method for handling a call involving a mobile station and a telecommunications system, wherein the system comprises a core network, a first access network, a second access network that is a cellular radio access network, and a device operating according to a session initiation protocol towards said first access network. The method comprises the following steps determining a demand for changing the access network used for the call from the first to the second access network, initiating the set-up of a connection to the mobile station via the second access network, and initiating the release of the connection through the first access network. The method can comprise the additional steps of contacting a call control node of the core network that serves a service area of the second access network wherein the mobile station is located, and linking the call to said call control node. In an embodiment of the invention the method comprises the additional step of modifying an indication that the first network is preferred for reaching the mobile station.

A call control node that serves a service area of the second access network wherein the mobile station is located may perform the steps of the method.

The method may comprise the additional step of performing functions of an anchor call control node.

The call control node may be a mobile services switching centre or a mobile services switching centre server.

The invention further provides a device for a telecommunications network comprising a first unit operating according to a session initiation protocol and a second unit for providing call control functionality for a circuit switched call.

A further aspect of the invention is the provision of a mobile station comprising a first transmission unit for a connection via a first access network, a second transmission unit for a connection via a second access network that is a cellular access network, a first part operating according to at least one protocol used via the first access network and a second part operating according at least one protocol used via the cellular access network. The mobile station is adapted to perform the steps of setting up or receiving a call via the first transmission unit, operating the call in the first part, receiving the call via the second transmission unit and linking the call from the second transmission unit to the first unit.

The mobile station may further comprise means for determining that the quality of a connection via the first access network is below a predefined threshold value, and means for signalling a demand for changing the access network.

It is also object of the invention to provide a database for storing subscriber data comprising an input output unit, a processing unit and a storage unit, wherein the storage unit is adapted to store at least two location identities for a mobile station and an indication which location identity should be used preferably to reach the mobile station, and that the processing unit is adapted to receive a message indicating that a mobile station shall be reached preferably by one of the two location identities and to initiate the storing of such indication.

In an embodiment of the database the storing of the indication is implemented by using a particular way or location of storing the preferred location identity.

Such database for storing subscriber data can be for example a home location register, a home subscriber server, or any other AAA (Authentication, Authorization and Accounting) server.

A further aspect of the invention is the provision of a mobile services switching centre with an extraction unit for extracting an indication from a message that a connection to the mobile station shall be routed via a target mobile services switching centre, and a suppression unit for suppressing a paging after receiving a request from a database storing subscriber data.

In an embodiment of the invention, the storing of the indication is implemented by using a particular way or location of storing the preferred location identity.

The first access network may be adapted to operate according to at least one of Bluetooth, ISDN, optical, infrared, fixed network or Wireless Local Area Network recommendations.

[brief description of the drawings]

The following figures show:

Fig. 1 depicts an architecture for enabling a mobile station to access a telecommunications network via a further access network,

Fig. Ia depicts a registration of a dual mode mobile station with a SIP proxy, (state of the art) and the indicating of this to the telecommunications network,

Fig. 2 depicts a flow of a method according to the invention, Fig. 2a depicts a flow of a further method according to the invention, Fig. 3 depicts a flow of a first alternative for a call set-up,

Fig. 4 depicts a flow of a second alternative for a call set-up,

Fig. 5 depicts a call set-up,

Fig. 6 depicts a hand over according to the invention,

Fig. 7 depicts a further hand over according to the invention

Fig. 8 depicts a device according to the invention,

Fig. 9 depicts a mobile station according to the invention,

Fig. 10 depicts a database for storing subscriber data according to the invention, and

Fig. 11 depicts a mobile services switching centre according to the invention.

[detailed description]

In the following the invention will be further described by means of examples and by means of figures.

It should be noted that the WLAN access is just an exemplary embodiment and could be replaced by any other access technology as for example Bluetooth, optical connection as e.g. infrared, short range radio or fixed line connection. In particular access technology with lower costs on transmission resources like bandwidth than the GSM or WCDMA technology can be used advantageously for the first access network.

It should be further noted that for an implementation of the invented methods, the access point adapted to communicate over a radio, fixed or optical connection to mobile station, may be connected by any means to a domain operating according to a session initiation protocol and transmitting data according to an internet protocol.

It should also be noted that the terms subscriber and mobile station are used as synonyms as for storing data related to them or location information as well as target point for a communication there is no need to differ between them for understanding the invention. According to the invention the call control is kept at the control node setting up the connection, even after a handover. For example for a call received via the circuit switched network or the core network the call control node performing call control, e.g. an mobile services switching centre, operates as an anchor call control node. ■ Accordingly keeps the SIP proxy the call control received via a SIP domain.

Usually the handover is triggered for the signalling before the user plane is set up.

Figure 1 depicts an architecture for enabling a mobile station to access a telecommunications network via a further access network, comprising an home location register 101, a gateway mobile services service centre 102, a mobile services switching centre 103 which are connected to each other. The mobile services switching centre 103 is further connected to an internet domain 104 comprising a combined SIP (Session Initiation Protocol) proxy (proximity server) MSC 105 and a SIP proxy 106 that is connected to telephone 107 adapted to send signals according to SIP. In another embodiment of the invention a media gateway control function is linked in somewhere in the path between the gateway mobile services switching centre 102 and the MSC/SIP proxy for protocol conversions. A media gateway control function as used for the invention performs a conversion of signalling from signalling used among nodes of the first access network towards signalling used in the cellular telecommunications network, for example from SIP to ISUP (ISDN User Part) and or BICC (Bearer Independent Call Control) and vice versa. The combined SIP proxy/MSC provides MSC functionality at least to the extent that it is capable of communicating with further control nodes for call control purposes, and that it can provide call control functions for controlling circuit switched calls. The internet domain is connected via an ISP (Internet Service Provider) 108 and a DSLAM (Digital Subscriber Line Access Multiplexer) 109 to a local exchange 110 which is connected to a public switched telephony network 111. The local exchange is further connected via a DSL (Digital Subscriber Line) modem 112 to a WLAN-access Point 113. A mobile station 116 can be connected via a WLAN radio connection 114 to the WLAN Access Point and via a GSM or WCDMA radio connection 115 to the mobile services switching centre 103.

According to the invention, the mobile station 116 can use the WLAN radio connection to register in the combined SIP proxy/MSC105. The details of the registration will be explained in figure Ia. Figure Ia depicts a registration of the mode mobile station 116 with the combined SIP proxy/MSC 105. To that end the mobile station 116 sends a SIP register message towards the combined SIP proxy/MSC 105 via the WLAN radio connection 114. After successful registration the combined SIP proxy/MSC returns a 200 OK message to the mobile station. The mobile station sends an indication that it can be reached via the combined SIP proxy/MSC to the mobile services switching centre 103 via the GSM or WCDMA connection by sending a location update message. The mobile services switching centre stores the indication in a visitor location register. It returns an accepted message to indicate the reception of the location update. In a preferred embodiment of the invention, the core network treats the first access network as a location area or a service area of the cellular telecommunications network. The indication is in that case an entry as described for example for GSM and UMTS networks, e.g. an identity of the combined MSC/SIP proxy or the service area served by it, and is stored in a home location register or a home subscriber server. To improve the readability the identity of an MSC as well as of a service area or another identity of a location are summarised under the term location identity. The handling of the indication are described in more detail by means of figures 3 and 4.

The HLR can be adapted to store two location identities for a mobile station, for example a first location identity for the first access network and a second identity for the second access network. It can be further adapted to store an indication which location identity, and by that which access network shall be used preferably to reach the mobile station. Alternatively an indication is stored which access network is to be used for reaching the mobile station. The HLR uses then the appropriate location information.

The sending of the indication can be implemented in several ways. It can be sent for example in a USSD (Unstructured Supplementary Service Data) message or any other signalling means between mobile station and mobile services switching centre. In the preferred embodiment it is included in a location update message. Independent from the means used for transporting the information from mobile station to mobile services switching centre, the information can be sent as a simple indication, for example a flag, indicating that an alternative access technology is available. In that case one can always use the same predefined device for routing calls from the mobile services switching centre/media gateway control function to the mobile station. Such device can be for example an interrogating call session control function as defined in the 3GPP recommendation 29.163 version 6.2.0 of March 2004. If not always the same device is used a device must be identified.

In an embodiment of the invention the SIP proxy is identified by an identification of the mobile station. To that end, an identification of the mobile station, e.g. the MSISDN or the IMSI, is stored in a table or a database in association with an identification of the SIP proxy, conveniently this can be the address of the SIP proxy. In a further embodiment of the invention, the mobile station identification can be related to an operator and the operator identification can be stored in relation to the SIP proxy identification.

Alternatively, a device can be identified for example by determining location information of the mobile station like a location area or geographical coordinates and using a mapping table that maps said location information to an address. A further way to address the mobile station is the use of an ENUM (tElephone NUmber Mapping) database as defined in IETF (Internet Engineering Task Force) RFC 3761. The ENUM database maps the telephone number, for example the IMSI (International Mobile Subscriber Identity) of the mobile station, to an IP address. A further way is the transmission of a location area identifier in the location update message. The location area identifier carries an identification of the SIP proxy or of an access point, for example an ESSID (Extended Services Set IDentification), a domain name or a WLAN location area identification.

In a preferred embodiment of the invention the combined MSC/SIP-proxy is identified in the same way as any other MSC of the system.

Figure 2 depicts a flow according to the invention. In a first step SOOl the method starts. In a succeeding step S002 a demand for changing the access network is determined. Said determination can be implemented by means of measurements either in a radio access network or a mobile station and/or by sending a message to the determining entity. The measured matter can be for example a relative signals strength or a bandwidth of a connection as well as the required transmission power. In a succeeding step S003 a set-up of a connection via the second access network is initiated. The set-up of a connection for hand-over purposes is well known for cellular radio networks operating according to UMTS, CDMA, WCDMA or GSM standards. In a further step S004, the release of the connection via the first access network is initiated. In a further step S005 an indicator that the first access network is preferred for reaching the mobile station is toggled. This indicator may be stored in a home location register or a mobile services switching centre. The method ends in a step S006. The method can be executed for example by a mobile services switching centre of the core network of the telecommunications system. In a preferred embodiment, it is executed by the mobile services switching centre serving the service area of the second access network, wherein the mobile station is located.

Figure 2a depicts a flow for a further method according to the invention. In a first step SlOl the method starts. In a succeeding step S 102 a demand for changing the access network is determined. Said determination can be implemented by means of measurements either in a radio access network or a mobile station and/or by sending a message to the determining entity. The measured matter can be for example a relative signals strength or a bandwidth of a connection as well as the required transmission power. In a succeeding step S 103 a set-up of a connection via the second access network is initiated. To that end in a step S 104 a control node is contacted, serving the location area or service area of the second access network, wherein the mobile station is located. The control node is linked into the connection in a step S 105. The set-up of a connection for hand-over purposes is well known for cellular radio networks operating according to UMTS, CDMA, WCDMA or GSM standards. In a further step S 106, the release of the connection via the first access network is initiated. In a further step S 106 an indicator that the first access network is preferred for reaching the mobile station is toggled. This indicator may be stored in a home location register or a mobile services switching centre. The method ends in a step S006. The method as depicted in figure 2a can be executed for example by a device operating according to a SIP protocol towards the first access network. The device can be adapted to operate like a mobile services switching centre towards nodes of the core network of the telecommunications system, or a media gateway control function can be linked in on a path from the device to a call control node of the core network. In a preferred embodiment the device is adapted to operate like a mobile services switching centre or a mobile services switching centre server at least to the extent, that it can communicate with further control nodes of the core network.

Figure 3 depicts in more detail the steps performed by a call control node, e.g. a mobile services switching centre and a database for storing subscriber data, e.g. a home location register at call set-up. In the depicted method, the database is adapted to store two location identities for a single mobile station or a MSISDN associated to the mobile station at the time of call set-up. This solution has the advantage that the principle to store subscriber related information shall be stored at home location register is supported, and the disadvantage that the home location register has to be modified for implementing the invention. The steps S201 to S210 are performed by the call control node, the steps S301 to S313 are performed by the database.

In a first step S201 the method is started. In a succeeding step S202, an indication is received that the mobile station can be reached via the second access network. This is implemented for example by the registration procedure as described in figure Ia. The indication is forwarded to the database in a step S203, which receives it in a step S303. A location identity is stored in a step S304, for the location of the mobile station in the second access network, for example a location identity associated to the second access network like a location area identifier or a service area identifier or an identity of a mobile services switching centre or another node serving the location.

In a step S204 an indication is received that the mobile station can be reached via the first access network. This indication can be sent for example by a device as described in figure 1. The indication is forwarded to the database in a step S205 and received in a step S305. The database stores a location information of the first access network for the mobile station in a step S306. In a preferred embodiment of the invention, there is at least one location area identity associated to the first network in a format as used for the second network. In a step S307 the database determines a preferred access network for reaching the mobile station. Such determination can be implemented e.g. by reading predefined settings of a user or an operator, by checking a load situation for the access network or by random.

In a step S308 the database receives a request for routing information. It analyses the stored data for the mobile station the call shall be routed to in a step S309, and sends a request for a MSRN (Mobile Station Roaming Number) to a call control node determined by the analysis, in a step S310. The request for a MSRN includes an indicator, that the contacted mobile services switching centre shall contact a further mobile services switching centre that is used as a target mobile services switching centre for reaching the mobile station. The contacted mobile services switching centre shall operate as an anchor mobile services switching centre for the call. The mobile services switching centre receives the request in a step S206. It interprets the indicator in a step S207 and as a result suppresses a paging for the mobile station in a step S208. It provides a MSRN to the database in a step S209. The database receives the MSRN in a step S311 and provides routing information in a step S312. The methods end in steps S209 and S313. Figure 4 depicts in more detail the steps performed by a call control node, e.g. a mobile services switching centre and a database for storing subscriber data, e.g. a home location register at call set-up. It differs from the method as depicted in figure 3 in that the database does not store two location identities for a single mobile station or a MSISDN associated to the mobile station at the time of call set-up. This solution has the disadvantage that the principle to store subscriber related information shall be stored at home location register is violated, and the advantage that the home location register has not to be modified for implementing the invention. The steps S401 to S412 are performed by the call control node, the steps S501 to S509 are performed by the database.

In a first step S401 and S501 and the methods are started. In a succeeding step S402, an indication is received that the mobile station can be reached via the second access network. This is implemented for example by the registration procedure as described in figure Ia. The indication is forwarded to the database in a step S403, which receives it in a step S502. A location identity is stored for the location of the mobile station in the second access network, for example a location identity associated to the second access network like a location area identifier or a service area identifier or an identity of a mobile services switching centre or another node serving the location in a step S503.

In a step S304 an indication is received that the mobile station can be reached via the first access network. This indication can be sent for example by a device as described in figure 1. The indication is stored in the control node by means of an indicator in a step S305.

In a preferred embodiment of the invention it is determined by the mobile services switching centre which access network is preferred for reaching the mobile station e.g. by reading predefined settings of a user or an operator, by checking a load situation for the access network or by random.

In a step S504 the database receives a request for routing information. It analyses the stored data for the mobile station the call shall be routed to in a step S505, and sends a request for a MSRN (Mobile Station Roaming Number) to a call control node deteπnined by the analysis, in a step S506.

The mobile services switching centre receives the request in a step S306. It interprets the indicator it stored for the mobile station or an identification associated to the mobile station like the MSISDN or an IMSI (International Mobile Subscriber Identity) in a step S307 and as a result suppresses a paging for the mobile station in a step S308. It provides a MSRN to the database in a step S309. The database receives the MSRN in a step S311 and provides routing information in a step S31_.2. The methods end in steps S209 and S313. Also in this embodiment the mobile services switching centre shall operate as an anchor mobile services switching centre and contact a target mobile services switching centre that serves the mobile station.

Fig. 5 depicts a call set-up, in the architecture of figure 1. In a first step 501 a call request message, for example an initial address message is received in the gateway mobile services switching centre 102. The gateway mobile services switching centre requests routing information in a step 502 from the database storing subscriber information, in the depicted embodiment the home location register 101. The home location register checks the stored subscriber data and requests a MSRN (Mobile Station Roaming Number) from the mobile services switching centre associated to the stored data. The home location register requests said MSRN from the mobile services switching centre 103, which returns the MSRN in a step 504. The home location register 101 sends the requested routing information in a step 505 to the gateway mobile services switching centre 102. The alternative embodiments of the steps 503 and 504, depending on whether the home location register is adapted to store two location identities for a mobile station or not have been described by means of figure 3 and 4. In a next step 506 the gateway mobile services switching centre 102 forwards the call request message, for example an initial address message according to ISUP (ISDN User Part), to the mobile services switching centre 103. In both of the alternatives as described by means of figure 3 and 4, the mobile services switching centre stores an indicator that it shall act as an anchor mobile services switching centre and forward the call to a target mobile services switching centre, which is done in a step 507. In the depicted embodiment the target mobile services switching centre is a combined SIP proxy/mobile services switching centre 105. In an other embodiment of the invention, the target mobile services switching centre is implemented by means of a SIP proxy and a media gateway control function converting messages sent by the anchor mobile services switching centre into messages that can be operated by a SIP proxy and vice versa.

The call is then set up via the first access network in a step 508.

Figure 6 depicts a hand over according to the invention in the architecture of figure 1. The set-up of the call is symbolised by the steps 601 and 602. The anchor mobile services switching centre 103 determines a demand that the access network for reaching the mobile station or used for the call should be changed from the first to the second access network. The mobile services switching centre 103 initiates the release of the connection via the first access network in a step 604, and it initiates the set-up of a connection to the mobile station via the second network in a step 605, for example by sending a connection request message to a node, e.g. a base station controller or a radio network controller, of the second access network.

Fig. 7 depicts a further hand over according to the invention. For the given situation the mobile station registered itself as described by means of figure Ia and a call has been received from the SIP telephone 107 in the SIP proxy 701. The SIP 701 proxy sets up a connection to the mobile station via the first access network. The SIP proxy determines a demand for changing the access network used for the call from the first to the second access network. It initiates the set-up of a connection to the mobile station via the second access network. To that end it contacts the transit mobile services switching centre 103 that serves the service area of the second access network the mobile station is located in, via the media gateway control function 702. In another embodiment of the invention the SIP proxy is a combined mobile services switching centre/SIP proxy that can communicate without using a media gateway control function. The SIP proxy stored the address of the mobile services switching centre in advance. Alternatively it could be derived by using a location information of the mobile station in combination with a mapping table for mapping location identities to mobile services switching centre addresses. The SIP proxy can comprise a database that maps an identification of the mobile station, e.g. an IMSI or a SIP identification, to an address to send an initiation message to. A further solution is the use of location information like geographical co-ordinates or the identity of the WLAN access node that are mapped to an address.

The transit mobile services switching centre sets up a connection via the second access network to the mobile station. The SIP proxy links the call to the transit mobile services switching centre. How the mobile station handles the incoming and handed over call is described by means of figure 9.

Figure 8 depicts a device DEV8 according to the invention, comprising an input output unit IOU8 for sending and receiving messages and data, a processing unit for co-ordinating the other units and for executing a stored program, a first unit 1U8 operating according to a session initiation protocol and a second unit 2U8 for providing call control functionality for a circuit switched call.

Figure 9 depicts a mobile station MS9 according to the invention, comprising a first transmission unit 1TU9 for connecting via a first access network, a second transmission unit 2TU9 for connecting via a second access network that is a cellular access network, a first part 1P9 operating according to at least one protocol used via the first access network and a second part 2P9 operating according at least one protocol used via the cellular access network. The mobile station is adapted to perform the steps of setting up or receiving a call via the first transmission unit, operating the call in the first part, receiving the call via the second transmission unit and linking the call from the second transmission unit to the first unit. In the scenario as described by means of figure 6 or 7, the mobile station has an active connection from the first part 1P9 via the first transmission unit 1TU9 and via the first access network. Later the connection via the first access network is released and a connection via the second access network is offered. After the connection via the second access network has been set up successfully, the call is linked from the second transmission unit 2TU9 that receives and sends messages and data for the connection via the second access network, to the first unit 1TU9 as depicted by the ^• curved arrow.

Figure 10 depicts a database DBlO for storing subscriber data according to the invention, comprising an input output unit IOU10 for sending and receiving messages and data, a processing unit for controlling and co-ordinating the units and for receiving and processing a message indicating which access network shall be preferred for reaching a mobile station. The database further comprises a storage SUlO for storing subscriber data. The storage SUlO is adapted to store two location identities for a subscriber and an indication which access network is preferred for reaching the mobile station or, as explained above the subscriber.

Fig. 11 depicts a mobile services switching centre MSCl 1 according to the invention comprising an input output unit IOU11 for sending and receiving messages and data, a processing unit PUl 1 for controlling the mobile services switching centre and co-ordinating the other units, an extraction unit EUl 1 for extracting an indication from a message that a connection to the mobile station shall be routed via a target mobile services switching centre, and a suppression unit SUl 1 for suppressing a paging after receiving a request from a database storing subscriber data.

The units of the figures can be implemented by means of software, hardware and/or a combination of both.

Claims

1. Method for handling a call involving a mobile station and a telecommunications system, wherein the system comprises a core network, a first access network, a second access network that is a cellular radio access network, and a device operating according to a session initiation protocol towards said first access network, wherein the method comprises the following steps: - determining a demand for changing the access network used for the call from the first to the second access network, initiating the set-up of a connection to the mobile station via the second access network, and initiating the release of the connection through the first access network.

2. Method according to claim 1, with the additional steps of contacting a call control node of the core network that serves a service area of the second access network wherein the mobile station is located, and linking the call to said call control node.

3. Method according to claim 1, with the additional step of modifying an indication that the first network is preferred for reaching the mobile station.

4. Method according to any of the preceding claims, wherein a call control node that serves a service area of the second access network wherein the mobile station is located performs the steps of the method.

5. Method according to any of the preceding claims, wherein the method comprises the additional step of performing functions of an anchor call control node.

6. Method according to any of the preceding claims, wherein the call control node is a mobile services switching centre or a mobile services switching centre server.

7. Device for a telecommunications network comprising a first unit operating according to a session initiation protocol characterised by a second unit for providing call control functionality for a circuit switched call.

8. Mobile station comprising a first transmission unit for a connection via a first access network, a second transmission unit for a connection via a second access network that is a cellular access network, a first part operating according to at least one protocol used via the first access network and a second part operating according at least one protocol used via the cellular access network characterised by that the mobile station is adapted to perform the steps of setting up or receiving a call via the first transmission unit, operating the call in the first part, receiving the call via the second transmission unit and linking the call from the second transmission unit to the first unit.

9. Mobile station according to claim 8, comprising means for determining that the quality of a connection via the first access network is below a predefined threshold value, and means for signalling a demand for changing the access network.

10. Database for storing subscriber data comprising an input output unit, a processing unit and a storage unit, characterised by that the storage unit is adapted to store at least two location identities for a mobile station and an indication which location identity should be used preferably to reach the mobile station, and that the processing unit is adapted to receive a message indicating that a mobile station shall be reached preferably by one of the two location identities and to initiate the storing of such indication.

11. Database according to claim 10 wherein the storing of the indication is implemented by using a particular way or location of storing the preferred location identity.

12. Mobile services switching centre characterised by an extraction unit for extracting an indication from a message that a connection to the mobile station shall be routed via a target mobile services switching centre, and a suppression unit for suppressing a paging after receiving a request from a database storing subscriber data.