WO2006045326A1

WO2006045326A1 - Method and devices for actice call inter-system handover

Info

Publication number: WO2006045326A1
Application number: PCT/EP2004/011967
Authority: WO
Inventors: Joerg Christian Ewert; Andreas Witzel; Martin Stümpert; Ralf Keller
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2004-10-22
Filing date: 2004-10-22
Publication date: 2006-05-04

Abstract

The invention relates to methods and devices for the handling of calls to a dual mode mobile station when a connection via a first of two access networks is lost.

Description

METHOD AND DEVICES FOR ACTIVE CALL INTER-SYSTEM HANDOVER

[technical field]

The present invention relates to methods and devices for handling a call involving a 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 was increasing. Today 11% of the households in Germany are equipped with a broadband connection (mainly ADSL), hi 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 Computers), PDA (Personal Digital Assistants), mobile phones etc. to their fixed broadband connection.

Furthermore, it is expected that the number of public WLAJN-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 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- 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.

CONFIRMATION CQPΥ 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 claims 1, 5 and 10, the control node of claim 14, the device of claim 20 and the mobile station of claim 23. Further advantageous embodiments can be derived from the dependent claims.

One advantage is that possibility to keep a call active while the access network is changed. It is further an advantage that this is implemented in a very simple way.

The invention introduces a method for handling a call nrvolving a 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. The method comprises the step of determining a request for changing the access network;, for example because the quality of the connection via the first access network is below a predefined threshold value, a user request or a request from a network, and the step of initiating the sending of a connection request towards a device of the cellular access network.

The connection request can comprise an indication that this is a connection request for re-establishing the call.

The connection request can also comprise an identification of the call. In an embodiment of the invented method, a call leg towards at least one other party involved in the call is kept.

The invention further introduces a method for handling a call involving a 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. The method comprises the steps of receiving a request for establishing a connection via a cellular telecommunications access network to the mobile station, detecting that the connection is requested for re-establishing the call, and initiating the set-up of a connection via the cellular telecommunications access network to the mobile station.

The method can comprise the additional step of suppressing call set-up signalling towards at least one other party involved in the call.

At the initiation of the set-up of a connection an indication can be sent, indicating that the connection is requested for re-establishing the call.

In an embodiment of the invention the method comprises the additional step of sending an announcement to at least one further party involved in the call.

The mobile station can comprise 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 invention further introduces a method for handling a call in a 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. The method comprises the steps of losing connection via the first access network, receiving a connection request or an offering of a connection via the second access network, and accepting the offered connection. The method can comprise the additional step of determining that the connection is requested or offered to re-establish the call, and the connection is accepted automatically after said determination.

The method can also comprise the steps of receiving a connection request or offering of a connection via the first access network, accepting the offered connection, storing an identification of the call, detecting that the connection via the first access network has been lost, and sending an initiation message via the second access network comprising the stored identification of the call.

In an embodiment of the invention, the mobile station comprises 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 and the method comprises the additional steps of receiving data via. the second access network in the second part of the mobile station, unpacking the received data, and forwarding them to the first part.

The invention further introduces a control node for performing call control for a telecommunications network. The telecommunications network comprises or is connected to a first access network reachable by the control node via a device operating according to a session initiation protocol and a second access netrwork wherein the second access network is a cellular access network. The contro 1 node comprises a detection unit for detecting that a connection is requested for ttie re- establishment of a call, and an insertion unit for inserting an indication that the connection is requested for the re-establishment of a call in at least one message, sent from the control node to the second access network for setting up the connection towards a mobile station.

The control node can comprise a suppression unit for suppressing connection set-up signalling messages towards further parties involved in the call.

The control node can also comprise a freezing unit for keeping a call leg to -wards at least one further party involved in the call. In an embodiment of the invention, the control node comprises an initiation, unit for initiating an announcement towards at least one further party involved in the call.

In an embodiment of the invention, the control node can be implemented hy means of a mobile services switching centre or a gateway mobile services switching centre server.

The control node can comprise a media gateway control function.

The invention also introduces a device operating according to a session initiation protocol for controlling connections via a first access network. The device i s connectable to a second access network that is a cellular access network, and wherein the device is adapted to send a connection request towards the second access network. The device comprises an insertion unit for inserting an indication that the connection is requested for the re-establishment of a call into a message sent for requesting the connection.

The device can comprise a freezing unit for keeping a call leg towards at least one further party involved in the call.

In an embodiment of the invention, the device comprises a determining unit for determining that the quality of the connection via the first access network is below a predefined threshold value.

The invention further introduces a mobile station comprising a first transmission unit for a connection via a first access network and a second transmission uαiit for a connection via a second access network, wherein the second access network is a cellular access network. The mobile station comprises a detection unit for detecting that a connection request or offer is sent for re-establishing a call and an accept unit for automatically accepting an offered connection.

The mobile station can comprise means for determining that the quality of a connection via the first access network is below a predefined threshold value, and a requesting unit for requesting a connection via the second access network. The mobile station can also comprise 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.

In an embodiment of the invention the mobile station comprises a second transmission unit that is adapted to operate according to at least one of a Global System for Mobile Telecommunication, Code Division Multiple Access, Wideband Code Division Multiple Access, American Mobile Phone System, Digital- American Mobile Phone System, or Pacific Digital Cellular standards.

The mobile station can further comprise a first transmission unit that is adapted to operate according to at least one of Bluetooth, ISDN, 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 WLAN coverage failure detection,

Fig. 3 depicts an implementation of a call handling according to the invention, Fig. 4 depicts an alternative implementation of a call handling according to the invention, Fig. 5 depicts a further alternative implementation of a call handling according to the invention,

Fig. 6 depicts a control node according to the invention, Fig. 7 depicts a device according to the invention, and Fig. 8 depicts a mobile station 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, 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 further access.

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.

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 switching centre, a combined mobile services switching centre/media gateway control function 103 which are connected to each other. In another embodiment of the invention the mobile services switching centre and the media gateway control function are not combined but just interconnected. The combined mobile services switching centre/media gateway control function 103 is further connected to an internet domain 104 comprising a first SIP (Session Initiation Protocol) proxy (proximity server) L 05 and a second SEP proxy 106 that is connected to telephone 107 adapted to send signals according to SIP. 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 106 is connected via a WLAN radio connection to the WLAN Access Point and via a GSM or WCDMA radio connection to the mobile services switching centre/media gateway control function.

According to the invention, the mobile station 116 can use the WLAN radio connection to register in the first SIP proxy 105. The details of the registration will be explained in figure Ia. Figure Ia depicts a registration of the mode mobile station 116 with the first SIP proxy 105. This is state of the art. To that end the mobile station 106 sends a SIP register message towards the SIP proxy 105 via. the WLAN radio connection. After successful registration the SEP proxy returns a 200 OK message to the mobile station. After receiving this confirmation that the registration has been successful the mobile station sends an indication that it can be reached via the SD? proxy to the mobile services switching centre 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 an embodiment of the invention the indication is stored in a home location register or a home subscriber server.

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 a preferred embodiment of the invention the SIP proxy is identified by an identification of trie mobile station. To that end, an identification of trie 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 IDentifϊcation), a domain name or a WLAN location area identification.

Figure 2 depicts trie architecture of figure 1 from the WLAN access point 113 to the SIP proxy 105 as well as the further SIP proxy 106 and the SIP telephone 107. Furthermore the cellular telecommunications network is depicted by means of a media gateway control function 202 and a mobile services switching centre 201. The mobile station 116 is currently involved in a call with a SIP -telephone 107, wherein the call is routed via the first access network, the WLAN access network. However, as shown by means of an X, the WLAN connection is disturbed, this means that the quality of the connection is below a predefined threshold value or it is foreseen that the quality will decline below such value. This may be to an extent that the connection is lost. Such disturbance can result for example from a malfunction of the WLAN access point 113, a too large distance ^"between the WLAN access point and the mobile station 116, interference, reflections and further reason known to the person skilled in the art. The occurrence of such disturbance, can be detected in the SIP proxy or the mobile station for example by means of sending and receiving heart beat messages, a monitoring of the payload stream, measurements of a field strength or monitoring of a bandwidth of the connection. The mobile station can react to the detection of the disturbance of the connection by sending an indication that the connection is disturbed, for example in a location update message via the second access network, the cellular access network. If the disturbance is foreseen, for example because the user enters a request for a handover or because of location information, the indication can also be serxt via the first access network, in the depicted example via the WLAN access network.

Figure 3 depicts an implementation of a call handling according "to the invention. The figure shows depicts the architecture of figure 1 from the WXAN access point 113 to the SIP proxy 105 as well as the further SIP proxy 106 and the SIP telephone 107. It furthermore depicts some selected nodes of the cellular telecommunications network, which are a media gateway control function 303, a gateway mobile services switching centre 302 and a mobile services switching centre 301. The media gateway control function could be collocated with both the mobile services switching centre or the gateway mobile services switching centre. The media gateway control function 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). Therefore it could in all embodiments of the figures 2, 3, 4, and 5 be replaced by another signalling gateway, performing the said functions.

In the depicted example, the SIP proxy 105 detected that the connection via the first access network is or will be lost. It initiates a re-establishment of the call between the S EP -telephone 107 and the mobile station 116 by requesting a coπunection to the mobile station via the second access network.

The SEP 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. Alternatively, there can be a single address stored in the SD? proxy to which all initiation messages are sent. 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.

In a preferred embodiment of the invention, the SIP proxy keeps its call leg towards the SEP telephone 105. In the preferred embodiment, the SEP proxy suppresses any connection set up signalling towards the SIP phone, however, it can link in an announcement machine for sending an announcement towards a user of the SEP phone to keep the connection. This linking in can also be performed by a mobile services switching centre involved in the set up of the connection. Trie initiation message sent by the SIP proxy is converted by the media gateway control function 303 and forwarded to the gateway mobile services switching centre 302. The gateway mobile services switching centre requests routing information from a home location register, not depicted, and receives an identification of the mobile services switching centre that can be used for reaching the mobile station.

In an embodiment of the invention, the SIP proxy sends an indication that the mobile station cannot be reached via the first access network to the cellular telecommunications network. Said indication is used to toggle any indicator stored, for example in a visitor location register or an home location register., that indicates that the mobile station can be used via the first access network.

The gateway mobile services switching centre contacts the mobile services switching centre and requests the set up of a connection to the mobile station. The mobile services switching centre requests the set up of the connection and returns an message indicating the successful set up of the connection. In a preferred embodiment of the invention, the connection request messages sent by the SIP proxy, the media gateway control function, the gateway mobile services switching centre, and the mobile services switching centre comprise an indication that the connection is requested to re-establishing a call. In the preferred embodiment the mobile station determines the indication and accepts the connection automatically.

In an embodiment of the invention the mobile station sent an identification of the call to the SIP proxy, for example during call set up.

In a preferred embodiment of the invention, the mobile station comprises a first transmission unit operating according to standards used for communication with the first access network, and a second transmission unit operating according protocols used for communication with the second access network. The mobile station further comprises a first part for communicating via the first access network, for example a SIP client and a second part for communication via the cellular telecommunications network, for example a client as specified by 3GPP standards.

In a preferred embodiment the first part presents received data of the call to the user. After the call has been re-established via the second access network, the second transmission unit inter acts with the second part for handling the call and the second part forwards received data to the first part that presents the data to a user or an application.

Figure 4 depicts an alternative implementation of a call handling according to the invention. The figure depicts the architecture of figure 1 from trxe WLAN access point 113 to the SIP proxy 105 as well as the further SP proxy 106 and the SIP telephone 107. The cellular telecommunications network is depicted by means of a gateway mobile services switching centre 401 and a combined mobile services switching centre/media gateway control function 402. The combination of mobile services switching centre and media gateway control function is just one possible embodiment, the nodes could certainly be implemented as separate nodes. The mobile station 116 has an active call through the cellular telecommunications network which had been transmitted via the first access network through WLAN radio 114. The connection had been disturbed and a request for a connection had been sent to the mobile services switching centre/media gateway control function. The mobile services switching centre detects for example by means of a call identification received in the connection request, that it already handles the call that is to be re-established and therefore drops the call back. That is the SIP proxy is removed from the call and the connection is set up directly from the mobile services switching centre through the second access network GSM or WCDMA radio 115 to the mobile station 116.

In a preferred embodiment of the invention, the connection request messages sent by the mobile services switching centre comprises an indication that the connection is requested to re-establishing a call. In the preferred embodiment the mobile station determines the indication and accepts the connection automatically.

hi a preferred embodiment of the invention, the mobile station comprises a first transmission unit operating according to standards used for communication with the first access network, and a second transmission unit operating according protocols used for communication with the second access network. The mobile station further comprises a first part for communicatήig via the first access network, for example a SIP client and a second part for communication via the cellular telecommunications network, for example a client as specified by 3GPP standards.

Figure 5 depicts an implementation of a call handling according to the invention. The figure shows depicts the architecture of figure 1 from the WLAN access point 113 to the SIP proxy 105 as well as the further SIP proxy 106 and the SIP telephone 107. The cellular telecommunications network is depicted by means of a gateway mobile services switching centre 501 and a combined mobile services switching centre/media gateway control function 502. The combination of mobile services switching centre and media gateway control function is just one possible embodiment, the nodes could certainly be implemented as separate nodes. The mobile station 116 has an active call through the cellular telecommunications network which had been transmitted via the first access network through WLAN radio 114.

In the depicted example the mobile station had an ongoing call with the SEP telephone 107 via the first access network, the WLAN access. The connection through the first access network is disturbed, illustrated by an "X". Both, the mobile station 116 and the SEP proxy 105 detect that the connection is interrupted. The mobile station sends a request for a connection via the second access network, the GSM or WCDMA radio access 115, to the combine mobile services switching centre/media gateway control function 502 for being connected to the SEP proxy again. As described above, this request can include an indication that the connection is requested for the re-establishment of a call. In a preferred embodiment, the request comprises an identification of the call. The combined mobile services switching centre/media gateway control function 502 forwards the connection request after a protocol conversion to the SEP proxy 105. The SIP proxy receives the request and re-establishes the call. In a preferred embodiment of the invention, the received connection request comprises the identification of the call. The SIP proxy 105 determined that the connection to the mobile station via the first access network was disturbed and linked in an announcement machine for indicating to the user of the SEP telephone 107 that he or she shall keep the call, for example after the expiration of a timer.

In an alternative embodiment of the invention, the mobile station was involved in a call from the cellular telecommunications network. Ln that example the connection to the SEP proxy is terminated while the combined mobile services switching centre / media gateway control function 502 keeps the call leg towards the other party involved in the call. The re-establishment to the mobile station is performed by the mobile services switching centre via the second access network, the GSM or WCDMA radio access network 115.

In a preferred embodiment of the invention, the mobile station comprises a first transmission unit operating according to standards used for communication with the first access network, and a second transmission unit operating according protocols used for communication with the second access network. The mobile station further comprises a first part for communicating via the first access network, for example a SIP client and a second part for commixnication via the cellular telecommunications network, for example a client as specified by 3GPP standards.

Figure 6 depicts a control node CN6 according to the invention. Such control node can be implemented for example by means of a serving GPRS support node, a combined GPRS support node, a mobile services switching centre, a gateway mobile services switching centre, a SIP Proxy or any other node performing call control functions. The depicted node comprises a classical unit CU6 that represents the usual functionality expected in a call control node. It further comprises a detection unit DU6 or detecting that a connection is requested for a re-establishment of a call. The detection can be performed for example by extracting an indicator from a message sent for requesting a connection. It could also be performed by extracting an identification of a call and. comparing it with identifications for calls that are already handled by the control node. The node further comprises an insertion unit IU6 for inserting an indication that the connection is requested for the re-establishment of a call in at least one message, sent by the control node.

Further depicted is the optional suppression unit SU6 for suppressing connection set-up signalling messages towards a party involved in a call. This signalling is suppressed for example if a connection on one call leg is disturbed and the call is to be re-established. In that case only for the call leg on whicti a connection has been lost will receive the call set-up signalling whereas it is suppressed towards the other at least one call leg.

Further depicted is a freezing unit FU6 that can be included in the control node. In the described case that a connection is lost, the freezing unit keeps a call leg towards at least one party that has an active connection to the control node and is involved in a call. The freezing unit for example avoids the release of transmission resources of the at least one call legs that did not loose a connection.

The control can also comprise a initiation unit INI6 for initiating an announcement towards at least one party involved in a call. This initiation can be performed for example by requesting an announcement machine that shoxild be linked in.

The control node can further comprise a media gateway control function MGCF6 for performing signal conversion from a standard used in a. cellular telecommunications network to a SIP.

Figure 7 depicts a device D7 adapted to operate according to a session initiation protocol for controlling connections via a first access network. The device is connectable to a second access network that is a cellular access network, and the device is adapted to send a connection request towards the second access network, for example via the input output and control unit IOCU7. Xhe device comprises an insertion unit IU7 for inserting an indication that the connection is requested for the re-establishment of a call, into a message sent for requesting the connection.

The device can comprise a freezing unit FU7 for keeping a call leg towards at least one further party involved in a call. The freezing unit for example avoids the release of transmission resources of the at least one call legs that did not loose a connection.

The device can also comprise a determining unit DU7 for determining that the quality of the connection via the first access network is below a predefined threshold value. Figure 8 depicts a mobile station MS 8 comprising a first transmission unit 1TU8 for a connection via a first access network and a second transmission unit 2TU8 for a connection via a second access network, that is a cellular access network. The mobile station comprises a detection unit DU8 for detecting that a connection request or offer is sent for re-establishing a call and an accept unit AU8 for automatically accepting an offered connection. Tie detection can be performed for example by determining an indicator in a message sent for requesting or offering a connection or by comparing a call identification from a message and comparing it with a call identification of a call that is already handled in the mobile station.

The mobile station can comprise means for determining that the quality of a connection via the first access network is below a predefined threshold value DR8, and a requesting unit for requesting a connection via the second access network DR8. in the depicted embodiment, both means are implemented in the same device, however, they can be implemented independent of each other. The detection can be performed by means of heartbeat messages or by monitoring the payload stream. It could also be implemented by measurement of a field strength, of transmission delays or of a bandwidth of a connection. The requesting unit can perform the request by sending a connection request message or a location update message.

In a preferred embodiment of the invention, the mobile station comprises a first part operating 1P8 according to at least one protocol used via the first access network and a second part 2P8 operating according at least one protocol used via the cellular access network.

The units and means of the control node, the device and the mobile station can be implemented by means of software, e.g. a software module, hardware, or a combination of both.

Claims

1. Method for handling a call involving a 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, said method comprising the following steps: determining a request for changing the access network, initiating the sending of a connection request towards a device of the cellular access network.

2. Method according to claim 1, wherein the connection request comprises an indication that this is a connection request for re-establishing the call.

3. Method according to any of the preceding claims, wherein the connection request comprises an identification of the call.

4. Method according to any of the preceding claims, with the additional step of keeping a call leg towards at least one other party involved in the call.

5. Method for handling a call involving a 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, said method comprising the following steps: receiving a request for establishing a connection via a cellular telecommunications access network to the mobile station, detecting that the connection is requested for re-establishing the call, initiating the set-up of a connection via the cellular telecommunications access network to the mobile station.

6. Method according to claim 5, with the additional step of suppressing call set¬ up signalling towards at least one other party involved in the call.

7. Method according to claim 5 or 6, wherein at the initiation of the set-up of a connection an indication is sent, that the connection is requested for re¬ establishing the call.

8. Method according to claim 5, 6, or 7, with the additional step of sending an announcement to at least one further party involved in the call.

9. Method according to any of the preceding claims, wherein the mobile station comprises 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.

10. Method for handling a call in a 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, said method comprising the following steps: losing connection via the first access network, receiving a connection request or offering of a connection via the second access network, and

- accepting the offered connection.

11. Method according to claim 10, with the additional step of deteπnining that the connection is requested or offered to re-establish the call, and wherein the connection is accepted automatically after said determination.

12. Method according to any of claims 10 or 11, with the steps of

- receiving a connection request or offering of a connection via the first access network, accepting the offered connection, and storing an identification of the call, detecting that the connection via the first access network has been lost, and sending an initiation message via the second access network comprising the stored identification of the call.

13. Method according to any of claims 10, 11 or 12, wherein the mobile station comprises 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 with the additional steps of

- receiving data via the second access network in the second part of the mobile station,

- unpacking the received data, and

- forwarding them to the first part.

14. Control node for performing call control for a telecomirxunications network comprising a first access network reachable by the control node via a device operating according to a session initiation protocol and a second access network wherein the second access network is a cellular" access network characterised by that the control node comprises a detection unit for detecting that a connection is requested for the re-estabLishment of a call, and an insertion unit for inserting an indication that the connection is requested for the re-establishment of a call in at least one message, sent from the control node to the second access network for setting up the connection towards a mobile station.

15. Control node according to claim 14, wherein the control node comprises a suppression unit for suppressing connection set-up signalling messages towards further parties involved in the call.

16. Control node according to claim 14 or 15, comprising freezing unit for keeping a call leg towards at least one further party involved in the call.

17. Control node according to claim 14, 15 or 16, comprising an initiation unit for initiating an announcement towards at least one further party involved in the call.

18. Control node according to any of claims 14 to 17, that is implemented b;y means of a mobile services switching centre or a gateway mobile services switching centre server.

19. Control node according to any of claims 14 to 18, wherein the control node comprises a media gateway control function.

20. Device adapted to operate according to a session initiation protocol for controlling connections via a first access network, wherein the device is connectable to a second access network that is a cellular access network., and wherein the device is adapted to send a connection request towards the second access network, characterised by that the device comprises an insertion unit for inserting an indication that the connection is requested for the re-establishment of a call, into a message sent for requesting the connection.

21. Device according to claim 20, comprising a freezing unit for keeping a call leg towards at least one further party involved in the call.

22. Device according to claim 20 or 21 comprising a determining unit for determining that the quality of the connection via the first access network is below a predefined threshold value.

23. Mobile station comprising a first transmission unit for a connection via a first access network and a second transmission unit for a connection via a second access network, wherein the second access network is a cellular access network:, characterised by that the mobile station comprises a detection unit for detecting that a connection request or offer is sent for ore- establishing a call and an accept unit for automatically accepting an offered connection.

24. Mobile station according to claim 23 with means for determining that th.e quality of a connection via the first access network is below a predefined 99

threshold value, and a requesting unit for requesting a connection via the second access network.

25. Mobile station according to claim 23 or 24, comprising 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.