WO2007144028A1

WO2007144028A1 - Method and apparatus for controlling intersystem handover in a mobile communication system

Info

Publication number: WO2007144028A1
Application number: PCT/EP2006/063300
Authority: WO
Inventors: Magnus HALLENSTÅL; Jari Tapio Vikberg; Tomas Nylander
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2006-06-16
Filing date: 2006-06-16
Publication date: 2007-12-21

Abstract

A method of handing over an ongoing real-time call from a current cell to a target cell belonging to one or more radio access networks, where the ongoing call was established using the services of an IP Multimedia Subsystem and is transported in the current cell over a packet switched bearer. The method comprises, at the radio access network responsible for said current cell, identifying one or more target cells for the handover, and, if a packet switched handover of the ongoing call cannot be performed to the target cell or a preferred one of the target cells, signalling to the IP Multimedia Subsystem the identity of the target cell or the preferred target cell. A handover instruction is then sent to the radio access network responsible for the target cell or the preferred target cell and a circuit switched bearer to which the ongoing call can be transferred is established within the target cell or preferred target cell.

Description

METHOD AND APPARATUS FOR CONTROLLING INTERSYSTEM HANDOVER IN A MOBILE

COMMUNICATION SYSTEM

Field of the Invention This invention relates to a handover control mechanism for a mobile communication system. In particular, the invention relates to a method and apparatus for triggering the seamless handover of a real-time session, for example a Voice-over-IP call.

Background to the Invention IP Multimedia (IPMM) services provide a dynamic combination of voice, video, messaging, data, etc, within the same session. By growing the numbers of basic applications and the media which it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, including so-called "combinational IP Multimedia" services.

IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks (3GPP TS 22.228, TS 23.228, TS 24.229, TS 29.228, TS 29.229, TS 29.328 and TS 29.329 Releases 5 to 7). IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly. Other protocols are used for media transmission and control, such as Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP), Message Session Relay Protocol (MSRP), and Hyper Text Transfer Protocol (HTTP). IMS requires an access network which would typically be a 3GPP Packet Switched (PS) network, but which might be some other access network such a fixed broadband or WiFi network.

Figure 1 illustrates schematically how the IMS fits into the mobile network architecture in the case of a 3GPP PS access network. Call/Session Control Functions (CSCFs) operate as SIP proxies with the IMS. The 3GPP architecture defines three types of

CSCFs: the Proxy CSCF (P-CSCF) which is the first point of contact within the IMS for a SIP terminal; the Serving CSCF (S-CSCF) which provides services to the user that the user is subscribed to; and the Interrogating CSCF (I-CSCF) whose role is to identify the correct S-CSCF and to forward to that S-CSCF a request received from a SIP terminal via a P-CSCF.

A fundamental requirement for real-time service provision is the seamless handover of services for subscribers roaming across cell boundaries of the radio access network (RAN). Traditional circuit switched (CS) based call services have been designed to meet this requirement. In the case of 2G and currently implemented 3G networks, PS real time handover with low latency is not provided for although service continuity is achieved at the terminal side by ordering a session to be moved from one cell to another, i.e. there is no prepare phase to shorten latency when moving cell.

Real time PS handover is standardized in 3GPP for 3G networks, but the feature has not yet been deployed. It is expected that when High-Speed Downlink Packet Access (HSDPA) is deployed, or shortly thereafter, the mechanisms needed for fast PS handover will be also be deployed. In the initial implementation stage, roll-out of this feature across 3G networks will inevitably be patchy. For 2G networks, fast and efficient PS handover procedures in the packet switched (PS) domain within the 2G network (and between 2G and 3 G networks) have only recently been standardized in 3GPP TS 43.129 for GSM/EDGE networks but are not yet deployed. Support for PS handover in 2G networks is never likely to be comprehensive (if implemented at all), yet handover of PS calls would be desirable as 2G networks will continue to provide a fallback network for 3G subscribers in the case of limited 3G network coverage. It can also be expected that the next generation radio and core network which are currently being specified under the name LTE (Long Term Evolution) and SAE (System Architecture Evolution) in 3GPP will also have limited coverage, and that these networks will also require fallback to 3G and 2G networks.

It is expected that in the future a major user of PS services will be Voice-over-IP (VoIP) applications. VoIP calls will be particularly sensitive to even relatively minor service interruptions caused by inter-cell handovers. As long as a terminal engaged in a VoIP call can perform PS handover to another cell (the "target cell"), the interruption can be kept short enough to avoid any noticeable drop in perceived quality. However, if either the current cell or the target cell do not support PS handover, a noticeable interruption is likely to occur as packets will be lost during the transition period. Consequently, until all RAN cells support PS handover, the provision of IMS services such as voice and video calls utilising the PS domain is likely to result in users receiving a reduced quality of service when crossing cell boundaries.

International patent application number PCT/EP04/05333 describes a process for allowing the IMS to automatically establish a VoIP call over a CS network when a user requests the VoIP call using signalling sent over a PS network. In this way, the IMS ensures that the appropriate Quality of Service (QoS) is applied to the call. In addition, the VoIP CS call will benefit from the seamless handovers afforded by the CS domain, if and when the user moves into a neighbouring cell. The procedure involves the establishment of a CS leg between the IMS and the MSC in order to link the CS access network leg to the IMS. This is achieved using an inter MSC handover procedure.

The disadvantage associated with the above proposal is that a VoIP call is automatically carried over the CS network even if the PS network is able to provide a satisfactory QoS for the VoIP call. In addition, the process of transferring the call to the CS network will consume significant resources within the network and, where the PS network can support the call, this will be wasted effort.

Summary of the Invention

According to a first aspect of the present invention there is provided a method of handing over an ongoing real-time call from a current cell to a target cell belonging to one or more radio access networks, where the ongoing call was established using the services of an IP Multimedia Subsystem and is transported in the current cell over a packet switched bearer, the method comprising: at the radio access network responsible for said current cell, identifying one or more target cells for the handover; if a packet switched handover of the ongoing call cannot be performed to the target cell or a preferred one of the target cells, signalling to the IP Multimedia Subsystem the identity of the target cell or the preferred target cell; and sending a handover instruction to the radio access network responsible for the target cell or the preferred target cell and establishing within the target cell or preferred target cell a circuit switched bearer to which the ongoing call can be transferred.

Embodiments of the invention enable the seamless handover of an ongoing real-time call between cells, regardless of whether or not both the current and the target cell support packet switched handover. In the case where one of the cells does not support such handover, handover involves handing over the call from the packet switched bearer in the current cell to a circuit switched bearer in the target cell.

In an embodiment of the invention, the radio access network responsible for said current cell identifies a target cell, and signals the identity directly to the IP Multimedia Subsystem together with a handover instruction. This may be sent via a GPRS Gateway Support Node (GGSN) of a General Packet Radio Service (GPRS) core network.

In an alternative embodiment of the invention, the radio access network signals the identity of the target cell, or a list of target cells, to the mobile terminal at which the ongoing call terminates. The mobile terminal selects from the list if necessary, and sends the target cell identity to the IP Multimedia Subsystem over the established packet switched bearer, e.g. as a SIP information message.

In the case where the target cell is a cell of a GSM radio access network, the handover instruction is sent by the IP Multimedia Subsystem to the appropriate Base Station Controller via a Mobile Switching Centre connected to the Base Station Controller. The IP Multimedia Subsystem subsequently establishes a circuit switched leg to the Mobile Switching Centre which is linked to the circuit switched bearer reserved in the target cell.

Where the current cell is a cell of a 3G radio access network, e.g. a UTRAN, a Radio Network Controller is responsible for the current cell. Where the current cell is a cell of a 2G radio access network, e.g. a GPRS RAN, then it is a Base Station Controller that is responsible for the current cell. A typical use case of the present invention involves performing a handover of a real-time call from a cell of a 3 G radio access network or a 3GPP LTE radio access network to a cell of a 2G radio access network, where the former supports packet switched handover but the latter does not.

The invention is applicable in particular, though not exclusively, to Voice-over-IP real time calls.

Preferably, the radio access network responsible for said current cell, identifies one or more target cells for the handover based upon a knowledge of the packet switched handover capabilities of the current cell and of neighbouring cells, and, if two or more candidate target cells are available, the radio access network selects the cell that supports packet switched handover. This may be done regardless of whether or not the current cell supports packet switched handover to facilitate future packet switched handovers.

Preferably, the target cell identity or set of target cell identities is received within the IP Multimedia Subsystem by a functional entity which may be a standalone entity or an entity integrated into an existing node, for example a Proxy Call/Session Control Function.

In an embodiment of the invention, the identity of the target cell or preferred target cell is provided to a Voice Call Continuity entity within the IP Multimedia Subsystem. It is this entity that will then be responsible for sending a handover instruction to the radio access network responsible for the target cell or the preferred target cell. The identity may be sent to the Voice Call Continuity entity either directly by the mobile terminal or the radio access network responsible for said current cell, or via an intermediate node, e.g. an extended Proxy Call/Session Control Function.

Embodiments of the present invention allow for the mobile terminal and or the IP Multimedia Subsystem to reject a request for a handover based upon local policy.

It is envisaged that the radio access network responsible for the current cell may be the same as the radio access network that is responsible for the target cell, or may be a different.

According to a second aspect of the present invention there is provided an IP Multimedia Subsystem based apparatus comprising: means for receiving the identity of a target cell associated with a handover of a real-time call from a packet switched in a current cell to a circuit switched bearer in the target cell; and means for sending a handover instruction to the radio access network responsible for the target cell.

The apparatus of the second aspect of the invention may be an IP Multimedia Subsystem based server. For example, the apparatus may be integrated into a Proxy Call/Session Control Function. Alternatively, the apparatus may comprise a Call Control Continuity Functional application server.

According to a third aspect of the present invention there is provided apparatus for use in a radio access network of a mobile communications system, the network controller comprising: means for storing the packet switched handover capabilities of cells under the control of the network controller and of other neighbouring cells; means for identifying one or more target cells for the handover of an ongoing real-time call by a mobile terminal, based upon the stored handover properties of cells, the call being carried over a packet switched bearer; and means for signalling the identified target cell(s) to an IP Multimedia Subsystem responsible for establishing the ongoing call or to said mobile terminal.

The apparatus of the third aspect of the present invention may be a Radio Network Controller (3G), Base Station Controller (BSC), or merely a Base Station (LTE/SAE).

According to a fourth aspect of the present invention there is provided a mobile terminal arranged to communicate via a radio access network of a mobile communications system, the terminal comprising: means for communicating with an IP Multimedia Subsystem network to set-up a real-time call over a packet switched bearer provided within a current cell of the radio access network; means for receiving from the access network the identity of one or more target cells for a handover of the call to a circuit switched bearer; and means for signalling to the IP Multimedia Subsystem the identity of the target cell or a selected one of the target cells.

Brief Description of the Drawings

Figure 1 illustrates schematically the integration of an IP Multimedia Subsystem into a

3 G mobile communications system;

Figure 2 illustrates the relationship and interfaces between an IP Multimedia Subsystem, radio access network, and circuit switched and packet switched core networks;

Figure 3 is a signalling diagram illustrating signalling associated with a packet switched to circuit switched handover;

Figure 4 illustrates schematically a first implementation of a Packet Circuit Anchor Point functional entity with an IP Multimedia Subsystem;

Figure 5 illustrates schematically a second implementation of a Packet Circuit Anchor

Point functional entity with an IP Multimedia Subsystem; Figure 6 illustrates a third implementation of a Packet Circuit Anchor Point functional entity with an IP Multimedia Subsystem;

Figure 7 illustrates an implementation using a VCC architecture; and Figure 8 illustrates the current system architecture in 3GPP for VCC.

Detailed Description of Certain Embodiments

With reference to Figure 2, there is illustrated a mobile station (MS) which is assumed to be attached to a cell of a radio access network (RAN) that supports packet switched (PS) handover (this is the upper cell shown in the Figure). Typically, the RAN is a UMTS Terrestrial RAN (UTRAN) comprising a Radio Network Controller (RNC) which will allocate transmission bandwidth to subscribers.

Within the 3GPP 2G and 3G PS core network, PS services are facilitated by a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support Node (GGSN). [In 3GPP SAE/LTE the current standard's view is that the PS service will be facilitated by a so- called a-GW (access GateWay).] For the 2G and 3G case, the GGSN is coupled to the IMS core network, and in particular to a Proxy Call/Session Control Function (P-CSCF) node. In order to access the IMS, the MS must first register with the IMS using the Session Initiation Protocol (SIP) REGISTER message. To establish and control an IMS service, the MS exchanges further SIP messages with the S-CSCF. SIP messages are transported via a physical GPRS transport channel. Within the IMS, the P-CSCF is responsible for requesting and releasing PS network resources at the GGSN. It does this via a Policy & Charging Resource Function (PCRF).

The present proposal aims to ensure seamless service provision for a user by transferring data calls such as VoIP calls from the PS domain to the CS domain only when PS handover is not possible within the current location. The RAN is able to make an informed cell selection in the cases where a handover is required, based upon a knowledge of the PS handover capabilities of the current and neighbouring cells.

Assuming that a VoIP call is in progress via the PS handover enabled UTRAN cell, the call having been established using the services of the IMS network, the MS will be continuously measuring the quality of the in-use radio link. The MS will be told on which neighbouring cell(s) it should measure signal strength. The MS will periodically report the results of the measurements to the RNC within the UTRAN cell. Based upon the received reports, the RNC may decide to instigate a handover of the VoIP call to a neighbouring cell in which case it will notify the MS of the identity of the new, target cell. This is normal handover behaviour. However, according to this proposal, in addition to the conventional capability data, the RNC is provided with information regarding the PS handover capabilities of all cells for which it is responsible as well as for other neighbouring cells handled by other RANs. The RNC uses the cell PS handover capability knowledge when making a handover decision, in addition to signal strength observed by the MS, the type of session (i.e. voice/text) and the location and movement of the MS, coverage and capabilities of the neighbouring cells, etc. The same type of information is stored in all other PS capable RANs that support handover to the CS domain, e.g. BSS and LTE.

In the case of the ongoing VoIP call in the PS domain, it is preferable that the session continues in the PS domain as long as possible. Thus, if an appropriate neighbouring cell supports PS handover, this cell would be preferred. However, if it is necessary to change from a cell which supports PS handover to one which does not as a result of the mobility of the MS, then the network controller initiates a handover of the VoIP call from the PS domain in the current cell to the CS domain in the target cell. Referring again to Figure 2, this illustrates a target cell (the lower cell in the Figure) with CS handover support, but without PS handover support. Typically, this cell belongs to a 2G RAN, e.g. a GSM BSS.

In order to implement PS to CS handover of the ongoing VoIP call, a new element is configured within the IP Multimedia Subsystem (IMS), referred to as a Packet Circuit Anchor Point (PCAP). This is illustrated in Figure 2. The PCAP could be an enhanced P-CSCF with user plane functionality, or a P-CSCF that controls a User plan Anchor Point (UAP). The UAP entity within the PCAP shown in the Figure is responsible for switching over from a PS leg (the connection to the MS over the PS domain) to a CS leg (a CS connection to the MS). The PCAP is notified of the target cell identity and the need for a PS to CS handover, and initiates the appropriate actions. The PCAP may receive this notification either from the MS or directly from the RAN. Thus, the PCAP node acts essentially as an anchor MSC requesting resources in a target MSC.

Considering firstly the case where it is the MS that notifies the PCAP of the handover requirement, the RAN must first inform the MS of the decision. This can be done, for example, using a new message ('HOtoCS') in Radio Resource Control (RRC) protocol. Also included in the message sent to the MS is the identity of the "best" target cell or a list of possible target cells. A cell may be identified by, for example, a Cell Global Identity (CGI) in the case of a GSM network (or a Location area and a 3G cell identity in the case of a 3 G network).

Assuming that SIP is used for signalling/call control, the MS then forwards the above message to the PCAP in a SIP Information Message in the established (GPRS) SIP session used for the PS (voice call) session. The MS or the PCAP node may be capable of terminating the handover to CS procedure if it is decided that handover is not needed, e.g. if the session is not a voice session.

Once the PCAP has been notified of the target cell location (CGI) by the MS, the process of controlling the allocation of time slots utilises a handover procedure to establish a CS call for the MS, and to handover the VoIP call from the PS domain to the CS domain. This approach requires that the PCAP implement standard handover functionality for a GSM handover. The PCAP acts effectively as an anchor MSC and generates information which would usually be initiated by a BSC, i.e. the information which a BSC would include in a handover-required message sent to an MSC in order to trigger a handover at the MSC. This approach causes the target BSC (in the GSM BSS) to allocate CS time slots to the MS on the basis of the information contained in the handover-required message.

Thus, in the case of a VoIP call, upon receipt of the SIP message from the UE to the PCAP containing the CGI, the PCAP looks up a mapping table which maps CGIs to MSCs in order to identify the MSC which is responsible for BSC owning the target cell. The PCAP then sends a "MAP-PREPARE HANDOVER-REQUEST" message (including the CGI) to this MSC, which reacts by sending a HANDOVER REQUEST message to the target BSC. The BSC then allocates a CS timeslot within the cell to the MS, and a handover command information including a handover reference is returned to the MSC in a HANDOVER REQUEST-ACK message. The handover command information is then sent back to the PCAP in a MAP-PREP ARE-HANDOVER- RESPONSE message. The PCAP initiates a CS connection to the MSC (illustrated by the line connecting the PCAP to the MSC in Figure 2) by exchanging ISUP IAM and ACM messages with the MSC. At the same time, the PCAP notifies the MS of the handover command information in a SIP 200OK Message. When the MS receives this information in the SIP-application layer, the RR layer in the MS is requested to activate and perform the standard procedure to tune to the new timeslot. Once the MS is in contact with the base station of the transmission network, a voice channel is activated from the BSC via the MSC to the PCAP. Figure 3 illustrates the signalling associated with this procedure.

The GPRS PDP used to carry the VoIP call in the PS domain may be retained in a modified or unmodified state, or may be torn down depending upon the requirements of the MS. However, normal terminal and/or network procedures will be able to deal with this.

Considering secondly the case where the network notifies the PCAP of the handover requirement directly, the RNC will send a message to the GGSN to request the initiation of a CS handover. This message may be a new message sent in GPRS Tunnelling Protocol-c (GTP-c), e.g. 'HOtoCS'. Again, the message to the GGSN includes the identity of the best target cell or a list of possible target cells, e.g. identified by way of a CGI. The GGSN will then forward this message/event in a DIAMETER message to the Policy & Charging Resource Function (PCRF) which will subsequently forward it using DIAMETER to the PCAP. As described above, the PCAP node may be capable of terminating the handover to CS procedure if it is decided that handover is not needed, e.g. if the session is not a voice session. The handover procedure then continues as described above with reference to Figure 3 (from the MAP-PREPARE HANDOVER- REQUEST ONWARDS). The PCAP may specifically ask for notifications from the PS domain regarding the need to handover to CS. When the PCAP started the session it would have requested a specific IP bearer that would support VoIP, from the PCRF. It would also have asked for notifications regarding HO to CS. The PCRF would then in turn have requested that a PDP context should be set-up, and also asked for HO to CS events for that PDP context.

Figure 4 illustrates schematically a first implementation of the PCAP entity within the IMS. According to this implementation, the PCAP is enabled with CS call control signalling and MAP signalling functionality and it anchors the user plane and converts PS to CS when necessary in the user plane. The functions can be bundled according to the rectangles shown.

Figure 5 illustrates an alternative implementation of the PCAP entity which makes use of existing IMS components such as the Media Gateway Control Function (MGCF),

Media Gateway Function (MGF), and the Media Resource Function Processor (MRFP).

Here the PCAP uses normal SIP signalling to set-up a call leg towards the CS network.

The MGCF translates this SIP signalling to ISUP or other CS call control signalling protocol. The Signalling Gateway Function (SGWF) translates IP based signalling bearers to CS signalling bearers. The PCAP entity uses the MRFP to switch between different PS user plane paths. For a PS path to MGF, the MGF translates it to CS user plane. PCAP will also signal MAP, but still use IP as bearer, the SGWF will translate this to CS signalling bearer.

Figure 6 illustrates yet another alternative approach to implementing the present invention which makes use of a Voice Call Continuity (VCC) application within the IMS. [VCC is specified in 3GPP TR 23.806, 23.206, and 24.206.] Figure 8 shows the current version of the VCC architecture from 3GPP 23.206. The VCC application introduces a CCCF (Call Control Continuity Function) server, which is enhanced with the required (PCAP) functionality. Here, the VCC server will act in a similar way to the PCAP in Figure 5, i.e. it will act as an anchor MSC towards the CS domain. Figure 7 illustrates an alternative approach using the VCC to establish the CS leg. When the radio access network indicates to the terminal that a HO to CS is required, then the MS will release the voice path in the PS domain and make a CS call. The CCCF application server will then co-ordinate activities so that this call is linked in instead of the now released PS voice leg. In this embodiment there is no target cell list sent, only an indication that the voice part of the call shall be transferred. Thus, there is no real time handover at the PS to CS transfer, which means that there will be a short period of silence during the transfer.

It will be appreciated by persons skilled in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention. For example, whilst the above discussion has been concerned with a 2G access network, the invention is equally applicable to a 3 G access network and to overlapping 2G and 3G networks. In the case of a 3G network, the UTRAN is responsible for notifying the MS (or UE in 3G parlance) or the PCAP of a handover decision and the identity of the target cell. The invention is also applicable to future generation networks such as 3GPP LTE/SAE.

Claims

CLAIMS:

1. A method of handing over an ongoing real-time call from a current cell to a target cell belonging to one or more radio access networks, where the ongoing call was established using the services of an IP Multimedia Subsystem and is transported in the current cell over a packet switched bearer, the method comprising: at the radio access network responsible for said current cell, identifying one or more target cells for the handover; if a packet switched handover of the ongoing call cannot be performed to the target cell or a preferred one of the target cells, signalling to the IP Multimedia

Subsystem the identity of the target cell or the preferred target cell; and sending a handover instruction to the radio access network responsible for the target cell or the preferred target cell and establishing within the target cell or preferred target cell a circuit switched bearer to which the ongoing call can be transferred.

2. A method according to claim 1, wherein the radio access network responsible for said current cell identifies a target cell, and signals the identity directly to the IP Multimedia Subsystem together with a handover instruction.

3. A method according to claim 2, wherein said identity is signalled via a GPRS Gateway Support Node of a General Packet Radio Service core network.

4. A method according to claim 1, wherein the radio access network signals the identity of the target cell, or a list of target cells, to the mobile terminal at which the ongoing call terminates.

5. A method according to claim 4, wherein the mobile terminal selects from the list if necessary, and sends the target cell identity to the IP Multimedia Subsystem over the established packet switched bearer.

6. A method according to claim 5, wherein said identity is conveyed in a SIP information message.

7. A method according to any one of the preceding claims, wherein the target cell is a cell of a GSM radio access network, and the handover instruction is sent by the IP Multimedia Subsystem to the appropriate Base Station Controller via a Mobile Switching Centre connected to the Base Station Controller, the IP Multimedia Subsystem subsequently establishing a circuit switched leg to the Mobile Switching Centre which is linked to the circuit switched bearer reserved in the target cell.

8. A method according to any one of claims 1 to 7, wherein the current cell is a cell of a 3 G radio access network.

9. A method according to any one of the preceding claims, wherein said real-time call is a Voice-over-IP call.

10. A method according to any one of the preceding claims, wherein the radio access network responsible for said current cell, identifies one or more target cells for the handover based upon a knowledge of the packet switched handover capabilities of the current cell and of neighbouring cells, and, if two or more candidate target cells are available, the radio access network selects the cell that supports packet switched handover.

11. A method according to any one of the preceding claims, wherein the entity within the IP Multimedia Subsystem responsible for handling the handover request is integrated into a Proxy Call/Session Control Function.

12. A method according to any one of claims 1 to 10, wherein the entity within the IP Multimedia Subsystem responsible for handling the handover request comprises a Voice Call Continuity entity.

13. An IP Multimedia Subsystem based apparatus comprising: means for receiving the identity of a target cell associated with a handover of a real-time call from a packet switched in a current cell to a circuit switched bearer in the target cell; and means for sending a handover instruction to the radio access network responsible for the target cell.

14. Apparatus for use in a radio access network of a mobile communications system, the network controller comprising: means for storing the packet switched handover capabilities of cells under the control of the network controller and of other neighbouring cells; means for identifying one or more target cells for the handover of an ongoing real-time call by a mobile terminal, based upon the stored handover properties of cells, the call being carried over a packet switched bearer; and means for signalling the identified target cell(s) to an IP Multimedia Subsystem responsible for establishing the ongoing call or to said mobile terminal.

15. Apparatus according to claim 14, wherein said apparatus is one of a Radio Network Controller, Base Station Controller, or Base Station.

16. A mobile terminal arranged to communicate via a radio access network of a mobile communications system, the terminal comprising: means for communicating with an IP Multimedia Subsystem network to set-up a real-time call over a packet switched bearer provided within a current cell of the radio access network; means for receiving from the access network the identity of one or more target cells for a handover of the call to a circuit switched bearer; and means for signalling to the IP Multimedia Subsystem the identity of the target cell or a selected one of the target cells.