Abstract:
A telecommunications system for providing mobile communications to mobile user equipment is provided which includes an internet protocol access network and a cellular mobile radio network. The internet protocol access network provides a facility for mobile communications to mobile user equipment within a coverage area for wireless communication provided by the internet protocol access network, the mobile communications being provided using an internet protocol. The cellular mobile radio network provides a facility for mobile communications to mobile user equipment within a coverage area for wireless communication provided by the cellular mobile radio network. The mobile communications are provided using at least one of a packet switched communications channel in which data is divided into packets and communicated on a communications bearer shared with other data packets, and a circuit switched communications channel in which data is communicated on a communications channel having a dedicated bandwidth. Mobile user equipment is provided and may activate a connection on the cellular mobile radio network using the packet switched communications channel whilst receiving a communications service via the internet protocol access network. The mobile user equipment communicates via the connection on the packet-switched channel of the cellular mobile radio network in response to a handover event for changing affiliation of the mobile user equipment fro m the internet protocol access network to the cellular mobile radio network. The mobile user equipment also establishes a connection on the cellular mobile radio network using the circuit switched channel in response to the handover event, continues to communicate via the connection on the packet-switched channel while the connection on the circuit-switched channel has been established, and then communicates via the connection on the circuit-switched channel once it has been established. In this way, improved continuity of service can be provided and the incidence of dropped calls can be reduced.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to telecommunications apparatus and methods operable to facilitate the hand-over of a call from an internet protocol access network to a cellular mobile radio network. The present invention also relates to a mobile user equipment. 
       BACKGROUND OF THE INVENTION 
       [0002]    Mobile telecommunications networks are arranged to provide a facility for communicating to and from a user equipment (UE) within a coverage area of the network for wireless communications. A user equipment can be within the coverage area of more than one network at the same time, and a communications session provided to a user equipment over a first network may need to be re-routed, or handed over, to the user equipment via a second network if the user equipment leaves the coverage area of the first network. For instance, a short-range wireless network may have a coverage area which is a subset of a coverage area of a cellular radio network and may provide greater bandwidth communications than the cellular radio network. In this case, a user equipment present in the short-range wireless network may preferentially use the short-range wireless network to benefit from the greater bandwidth provided. However, if the user equipment were to exit the coverage area of the short-range wireless network, a communications session commenced in the short-range wireless network can be continued in the cellular radio network by providing a hand-over of the communications session. In some circumstances such a hand-over can be performed without any serious interruption to the communications session. However, in other circumstances an interruption in service may result or the communications session may even be dropped. 
       SUMMARY OF INVENTION 
       [0003]    According to an aspect of the present invention there is provided a telecommunications system for providing mobile communications to mobile user equipment. The system comprises an internet protocol access network and a cellular mobile radio network. The internet protocol access network provides a facility for mobile communications to mobile user equipment within a coverage area for wireless communication provided by the internet protocol access network, and these mobile communications are provided using an internet protocol. The cellular mobile radio network provides a facility for mobile communications to mobile user equipment within a coverage area for wireless communication provided by the cellular mobile radio network. The mobile communications are provided using at least one of a packet switched communications channel and a circuit switched communications channel. In the packet switched communications channel data is divided into packets and communicated on a communications bearer shared with other data packets. In the circuit switched communications channel data is communicated on a communications channel having a dedicated bandwidth. The system also comprises a mobile user equipment which is operable to activate a connection on the cellular mobile radio network using the packet switched communications channel whilst receiving a communications service via the internet protocol access network. The mobile user equipment communicates via the connection on the packet-switched channel of the cellular mobile radio network in response to a handover event for changing affiliation of the mobile user equipment from the internet protocol access network to the cellular mobile radio network. The mobile user equipment also establishes a connection on the cellular mobile radio network using the circuit switched channel in response to the handover event, and continues to communicate via the connection on the packet-switched channel while the connection on the circuit-switched channel is being established. Once the circuit-switched channel has been established, the mobile user equipment communicates via the connection on the circuit-switched channel. 
         [0004]    Accordingly, by temporarily using the packet-switched channel of the cellular mobile radio network while the circuit-switched channel is being established, improved continuity of service can be provided and the incidence of dropped calls can be reduced. 
         [0005]    A hand-over event may be for example one of the mobile user equipment or the access network to which it is currently affiliated detecting that communications may be more efficiently conducted via another access network, for instance because the mobile user equipment is losing the radio coverage of the access network to which it is currently affiliated. 
         [0006]    Embodiments of the present invention can provide a facility for improving service continuity when a hand-over is performed from an internet protocol access network to a cellular mobile radio network. The cellular mobile radio network may be for example a Universal Mobile Telecommunications System (UMTS) network providing longer range and larger coverage than a shorter range internet protocol access network which may be for example a WiMAX or WiFi wireless local area network (WLAN). The cellular mobile radio network is therefore arranged to provide mobility to the mobile user equipment in which the mobile user equipment can move throughout a coverage area provided by base station transceivers (known as Node Bs in UMTS), handing over from one base station to another in order to maintain communications. The internet protocol wireless access network may provide a short range communications facility within so called wire-less “hot spots”, with a communications bandwidth which is typically higher than that of the cellular mobile radio network. 
         [0007]    As those familiar with mobile communications technology will appreciate, some cellular mobile radio networks such as GSM (referred to as 2G networks) are able to provide a circuit switched communications channel to mobile user equipment, providing a dedicated bandwidth to the mobile user equipment for communicating data to provide a communications service. Other cellular networks such as those operating in accordance with GPRS or UMTS are able to support packet data communications in which data is communicated in packets, which are multiplexed with data packets from other mobile user equipment and sources via a common communications bearer. A packet data connection established via the cellular network can therefore support internet protocol communications. Furthermore, bandwidth is not consumed by a mobile user equipment unless data packets are communicated. It is desirable to use a circuit switched communications channel rather than a packet switched communications channel because the circuit switched communications channel provides better quality for voice communications, is cheaper for the operator, and may be the only choice in terms of coverage. 
         [0008]    Embodiments of the present invention are arranged to provide a facility for a mobile user equipment to hand-over between an internet protocol access network and a cellular mobile radio network, in which continuity of a communications service is improved by reducing a likelihood of losing data packets. For time sensitive services such as voice or video communications, then improvement in continuity is particularly useful. To provide the improvement in the continuity of communications, a connection is provided in a packet switched communications channel of a cellular mobile radio network which is established concurrently with or before a communications service is established via an internet protocol access network. Since the connection established with the cellular mobile radio network is for a packet switched channel, a bandwidth available for communications on the cellular mobile radio network is not consumed until a data packet is communicated. As such no bandwidth is consumed whilst the mobile user equipment is communicating via the internet protocol access network. However, in the event of a loss of connection with the internet protocol access network, data packets may be communicated via the packet switched connection provided by the cellular mobile radio network. The loss of connection may be as a result of the mobile user equipment moving out of a coverage area or losing sharply the radio signal provided by the internet protocol access network. For some applications such as a 3G video call or voice communications, the packet switched connection may provide sufficient continuity until a circuit switched communications channel can be established in order to maintain communications. 
         [0009]    In one example the mobile user equipment controls the communication via the packet data communications channel and the establishment of the circuit switched communications channel in response to a hand-over event. To this end, the mobile user equipment may be provided with a protocol stack which can be used to tear-down an internet protocol communications channel via which the mobile user equipment receives a communications session from the internet protocol access network. The mobile user equipment can thereby tear-down the internet protocol communications session which was established via the internet protocol communications channel, and communicate signalling data with a media gateway controller to tear-down the communications channel established via the packet data gateway of the WLAN to the UMTS network. 
         [0010]    In one example, and in particular if a correspondent node to which a communications session has been established is within the circuit switched network, data communicated using internet packets is routed through a media gateway, so that a correct internet protocol session can be identified for routing internet protocol packets (IP packets) to the correspondent node. The media gateway is controlled by a media gateway controller provided in an IP core network. Signalling data such as SIP messages are communicated to the media gateway controller which responds to the messages which are communicated for controlling the communications session to configure the media gateway in accordance with the messages. 
         [0011]    In one example the correspondent node is attached to an IP network, and therefore before the handover the call is voice-over IP, and after the handover the call is routed through the media gateway. In another example the correspondent node is attached to the cellular or public switched telephone network (PSTN), in which case before the handover, the call is routed through the media gateway, because the user equipment is initially present in an IP network, and after the handover, the call is a circuit switched call and is not routed through the media gateway. 
         [0012]    Whilst the packet switched communications channel and the circuit switched communications channel could be established using the radio network infrastructure and common core network infrastructure, they could be provided using separate radio network and/or core network infrastructure. 
         [0013]    Various further aspects and features of the present invention are defined in the appended claims and include a mobile user equipment and a telecommunications method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0014]    Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, where like parts are provided with corresponding reference numerals, and in which: 
           [0015]      FIG. 1A  schematically illustrates a telecommunications system according to an embodiment of the present invention prior to a hand-over from one access network to another access network; 
           [0016]      FIG. 1B  schematically illustrates a telecommunications system according to another embodiment of the present invention prior to a hand-over from one access network to another access network; 
           [0017]      FIG. 2A  schematically illustrates the telecommunications system of  FIG. 1A  during the hand-over procedure; 
           [0018]      FIG. 2B  schematically illustrates the telecommunications system of  FIG. 1B  during the hand-over procedure 
           [0019]      FIG. 3A  schematically illustrates the telecommunications system of  FIGS. 1A and 2A  after the hand-over procedure has been completed; 
           [0020]      FIG. 3A  schematically illustrates the telecommunications system of  FIGS. 1B and 2B  after the hand-over procedure has been completed; 
           [0021]      FIG. 4  is a schematic flow diagram illustrating a hand-over procedure according to an embodiment of the invention; 
           [0022]      FIG. 5  schematically illustrates the signalling data flow involved in a network controlled hand-over procedure in accordance with an embodiment of the invention; 
           [0023]      FIG. 6  schematically illustrates the signalling data flow for a hand-over procedure which is controlled by a mobile user equipment in accordance with an embodiment of the invention; and 
           [0024]      FIG. 7  schematically illustrates the functional architecture of a mobile user equipment which controls the hand-over procedure of  FIG. 6 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]    Referring to  FIG. 1A , a telecommunications system is shown to comprise an internet protocol (IP) core network  10 , a wireless local area network (WLAN)  20 , and a UMTS network  30 . The wireless local area network provides local area networking via, for example, a radio link. The UMTS network provides wide area communications via both packet-switched and circuit-switched channels. The IP core network  10  controls internet protocol communications between mobile user equipment affiliated with the attached access networks. In particular, in the present example the IP core network  10  controls internet protocol communications between a User Equipment (UE)  22  which is within the coverage areas of both the WLAN  20  and the UMTS network  30  and a correspondent node (CN)  42  which is affiliated with an IP communications network (not shown). In  FIG. 1A , the communications links between the various network elements which communicate media data between the user equipment  22  and the correspondent node  42  are illustrated by solid lines. In contrast, communications links not used to communicate media data between the user equipment  22  and the correspondent node  42 , which includes communication links which communicate signalling data rather than media data, are illustrated by dashed lines. It can therefore be seen that the user equipment  22  is able to communicate with the correspondent node  42  via the WLAN  20 . Specifically, the user equipment  22  can transmit data to and receive data from an access point (AP)  24  over a radio link. The access point  24  is in communication with a packet data gateway  26  which can communicate signalling messages to the IP core network  10 , in particular to an internet protocol mobility manager  12  in the IP core network  10 , and can communicate media data between the user equipment  22  and external networks, and in particular to the correspondent node  42 . 
         [0026]    The IP core network  10  is able to control a media gateway  50  using a media gateway controller (MGC)  14 . In  FIG. 1A , the user equipment  22  is not currently communicating using the UMTS network  30 , and therefore the network elements and their interconnections are shown using dashed lines. A Node B  31 , a Node B  32  and a Node B  33  are shown and are operable to communicate with a mobile user equipment within their coverage area via a radio link. A radio network controller (RNC)  34  is arranged to control communications using the Node B  31  and a radio network controller  35  is arranged to control communications using the Node B  32  and the Node B  33 . 
         [0027]    Data being communicated to or from a mobile user equipment in radio contact with one of the Node Bs will be communicated via a respective RNC  34 ,  35  to either a Mobile Switching Centre (MSC)  36  or a Serving GPRS Support Node (SGSN)  37  depending on whether a circuit switched communications channel or a packet switched communications channel is being used to support a communications session. In particular, when a packet switched communications channel is used, the data being communicated to and from a mobile user equipment is routed via the SGSN  37  to a Gateway GPRS Support Node (GGSN)  38  and then on to the media gateway  50 . Alternatively, when a circuit switched communications channel is used, the data being communicated to and from a mobile user equipment is routed via the MSC  36  to the media gateway  50 . 
         [0028]    Referring to  FIG. 1B , the telecommunications system of  FIG. 1A  is shown, but with the correspondent node  42  of  FIG. 1A  being replaced with a correspondent node  42 ′ which is present in, and affiliated to, the UMTS network  30  as shown in the diagram. Alternatively, the correspondent node could be part of a public switched telephone network (PSTN). Accordingly, media data is communicated from the packet data gateway  26  to the media gateway  50  via the IP core network  10 . The media data is then communicated from the media gateway  50  to the correspondent node  42 ′ via the MSC  36 , the RNC  34  and the Node B  31 . 
         [0029]    Referring to  FIG. 2A , the telecommunications system of  FIG. 1A  is illustrated for a state in which a hand-over of the communications session between the user equipment  22  and the correspondent node  42  from the WLAN  20  to the UMTS network  30  is occurring, with corresponding features of  FIG. 1A  and  FIG. 2A  being provided with the same reference numerals. It can be seen from  FIG. 2A  that the user equipment  22  is no longer within the coverage area of the WLAN  20 . The user equipment  22  is now served only by the UMTS network  30 . As illustrated by a solid line in  FIG. 2 , media data is communicated between the various network elements between the user equipment  22  and the correspondent node  42 , and therefore follows a different route from that illustrated in  FIG. 1A . In particular, while in  FIG. 1A  media data was communicated to the media gateway and onwards to the correspondent node  42  via an access point  24  and a packet data gateway  26 , this signal path is no longer used by the mobile user equipment  22 , and so the media data is communicated instead over the UMTS network  30 . The UE  22  is in radio contact with the Node B  32  which operates under the control of the RNC  35 . During the hand-over procedure, media data is communicated between the user equipment  22  and the media gateway  50  via the SGSN  37  and the GGSN  38 . 
         [0030]    Control of the hand-over procedure will now be described. When the user equipment  22  moves out of the coverage area of the WLAN  20 , the UE  22  will communicate a message indicating the loss to the IPMM  12  of the IP core  10 . The UE  22  or the IPMM  12 , by issuing control signals to the UE  22 , will then initiate the hand-over procedure from the WLAN to the UMTS network so that the MGC routes the communication session to the media gateway  50  via the UMTS network. In  FIG. 2A , the communications session is provided by a packet-switched communications channel of the UMTS network  30 , and in particular is routed to the media gateway (if the correspondent node is in the internet protocol access network) via the SGSN and GGSN, which are packet switched elements of the UMTS network  30 . A time required to transfer data on an already PDP context-activated UMTS packet switched session is very small, and so comparatively uninterrupted service can be provided. This is because the packet switched communications channel is always “on” but not used unless data is actually being sent. 
         [0031]    Referring to  FIG. 2B , the telecommunications system of  FIG. 2A  is shown, but with the correspondent node  42  of  FIG. 2A  being replaced with a correspondent node  42 ′ which is present in, and affiliated to, the UMTS network  30 . That is,  FIG. 2B  illustrates the telecommunications system of  FIG. 1B  for a state in which a hand-over of the communications session between the user equipment  22  and the correspondent node  42  from the WLAN  20  to the UMTS network  30  is occurring. Media data in this case is communicated between the UE  22  and the correspondent node  42 ′ via the node B  32 , the RNC  35 , the SGSN  37 , the GGSN  38 , the media gateway  50 , the MSC  36 , the RNC  34  and the Node B  31 . 
         [0032]    Referring to  FIG. 3A , the telecommunications system illustrated in  FIGS. 1A and 2A  is illustrated for a state in which a hand-over of the communications session between the user equipment  22  and the correspondent node  42  from the WLAN  20  to the packet-switched interface of the UMTS network  30  has been completed, with corresponding features being provided with the same reference numerals. As with  FIG. 2A , the communications session is provided using the UMTS network  30 , but is now provided using a circuit switched channel rather than a packet switched channel. Accordingly, signalling data and media data continue to be communicated over the radio link between the user equipment  22  and the Node B  32  under the control of the RNC  35 , but in  FIG. 3A , rather than communicating via the SGSN and GGSN, the MSC  36  is used instead to provide a circuit switched communications channel to the media gateway  50 . Typically, the time required to set up this circuit switched communication channel will be approximately 4 to 6 seconds. The set up procedure is commenced along with the initial hand-over from the WLAN  20  to the packet-switched interface of the UMTS network  30 , and until the set up procedure is completed and the circuit switched channel is established, communication between the user equipment  22  and the correspondent node  42  will be provided by the packet switched communication channel as illustrated in  FIG. 2A . As with  FIG. 2A , control of the media gateway to route media data between the UMTS network  30  and the correspondent node  42 , if it is in the internet protocol access network, is provided the MGC  14 . Comparison of  FIGS. 2A and 3A  reveals that much of the network infrastructure for the packet switched and circuit switched channels of the UMTS network are the same. In the example of  FIGS. 2A and 3A  only the use of the SGSN  37  and the GGSN  38  is dependent on whether a packet switched or circuit switched communications channel is being used. Other elements are common to communications on either the packet switched or circuit switched communications channel. 
         [0033]    It will be appreciated that while the correspondent node  42  in  FIGS. 1A to 3A  is not affiliated with either of the WLAN  20  or the UMTS network  30 , it could equally be provided in either of these networks. If the correspondent node were to be present in the same or another internet protocol access network then calls would be managed inside the internet protocol access network without a need to communicate via the media gateway  50  when the user equipment  22  is present in the WLAN coverage area. If the correspondent node were to be present in the UMTS network  30  then calls would be managed within the UMTS network  30  when the user equipment has moved out of the WLAN coverage area. This latter scenario is that which is illustrated by  FIGS. 1B ,  2 B and  3 B. 
         [0034]    Referring to  FIG. 3B , the telecommunications system of  FIG. 3A  is shown, but with the correspondent node  42  of  FIG. 3A  being replaced with a correspondent node  42 ′ which is present in, and affiliated to, the UMTS network  30 . That is,  FIG. 3B  illustrates the telecommunications system of  FIGS. 1B and 2B  for a state in which a hand-over of the communications session between the user equipment  22  and the correspondent node  42  from the WLAN  20  to the UMTS network  30  has been completed. Media data in this case is communicated between the UE  22  and the correspondent node  42 ′ via the node B  32 , the RNC  35 , the MSC  36 , the RNC  34  and the Node B  31 . 
         [0035]    In  FIG. 4 , a flow diagram is provided which schematically illustrates an example hand-over procedure that can be used for the communications system of  FIGS. 1A ,  2 A and  3 A. At a step S 1 , the mobile user equipment  22  is switched on when in both a WLAN  20  and UMTS  30  coverage areas. At a step S 2  a communications session is established either from the user equipment  22  or to the user equipment  22  to or from the correspondent node  42 . As a result, at a step S 3 , the communications channel is established which causes the WLAN  20  to communicate media data. Meanwhile, at a step S 4 , a packet switched communications channel is set up on the UMTS network as a back up for the WLAN  20  in the event that a hand-over event should occur such as the mobile user equipment moving out of a coverage area of the WLAN. Subsequently, at a step S 5 , the user equipment  22  leaves the WLAN coverage area while the communications session is still in progress. To provide continuity of the call, the UMTS packet switched communication channel set up in step S 4  is activated at a step S 6 . This activation step happens very quickly, so a dropped call should not result, nor should there be a severe discontinuity in the call. Meanwhile, at a step S 7 , a UMTS circuit switched communications channel is set up. This procedure is slower, typically requiring 4 to 6 seconds to complete. Once the set up of the circuit switched channel is completed at a step S 8 , the call is transferred from the packet switched channel to the circuit switched channel at a step S 9 . As described above, to provide low delay communication of media data, a UMTS network communications channel should ultimately be provided using a circuit switched channel because the packet switched channel is limited in bandwidth and hence is not resource effective for the operator. However, until the circuit switched channel has been set up the packet switched channel can be used to provide continuity of service, albeit possibly at a lower quality or with some service limitation. 
         [0036]    In  FIG. 5  the signalling data flow involved in a network controlled communications session hand-over procedure is schematically illustrated. The functional network entities present in  FIG. 5  are the User Equipment (UE), a packet data gateway (PDG), a Gateway GPRS Support Node (GGSN), a circuit switched network element (CS) and an internet protocol mobility manager (IPMM). The network controlled hand-over procedure uses an internet protocol mobility manager (IPMM) in the core network to control the hand-over from a WLAN to a circuit switched cellular network. The UE  22  and, if used, the IPMM requires a Packet Data Protocol (PDP) context to be activated at all times (wherever the user equipment is located, be it the WLAN IP domain or the CS domain) in order to exchange signalling messages which allow the decision and control of the hand-over. Accordingly, when a voice-over-IP (VoIP) session is running on an IP network (under WLAN coverage), a PDP context is activated on the packet switched UMTS interface of the UMTS network. Specifically, referring to  FIG. 5 , at a step S 101  the user equipment communicates PDP context activation message to the GGSN in the UMTS network to activate the PDP context and establish a communications session using the packet switched channel of the UMTS network. As described above, no bandwidth will be used by this channel unless it is actually used to communicate media data. At a step S 102 , the user equipment has an active call session on the WLAN, however, at a step S 103  the user equipment moves away from the WLAN and thereby loses coverage. After coverage has been lost, the user equipment communicates a monitoring message to the IPMM at a step S 104 . As a result of this message, the IPMM decides to hand-over the communications session to the circuit switched domain of the UMTS network at a step S 105 . A hand-over instruction is then issued back from the IPMM to the user equipment at a step S 106 . The hand-over procedure involves two main stages. The first stage, carried out at a step S 107 , is the transfer of the VoIP session from the WLAN to the packet switched domain of the UMTS network which has already been established in step S 101  by the PDP context activation message. A hand-over instruction acknowledgement message is communicated from the user equipment to the IPMM at a step S 108  to indicate that the user equipment has received the hand-over instruction from the IPMM and has performed the first stage of the transfer by switching to the packet switched domain. Concurrently with the transfer of the VoIP session to the packet switched domain, a call setup request is communicated from the user equipment to the circuit switched interface at a step S 109  to establish a communications channel in the circuit switched domain. At a step S 110  the user equipment establishes the circuit switched call, following which, at a step S 111  the circuit switched interface communicates a setup complete message back to the user equipment. The second stage of the hand-over procedure occurs at a step S 112  where the VoIP session is transferred from the packet switched domain of the UMTS network to the circuit switched domain of the UMTS network. Finally, at a step S 113 , the user equipment communicates a hand-over instruction acknowledgement message to indicate that the user equipment has performed the second stage of the transfer by switching to the circuit switched domain. 
         [0037]    It will therefore be understood that the IPMM temporarily switches the communications session of the user equipment from the WLAN IP network to the packet switched UMTS interface of the cellular network using standard IPMM mechanisms. In other words, the VoIP/video session is switched from the WLAN VoIP/video channel to a packet switched UMTS VoIP/video channel. In parallel, the establishment of a call in the circuit switched UMTS domain is commenced. The packet switched UMTS VoIP/video channel lasts until the circuit switched session is established, then the call is switched from the packet switched UMTS VoIP/video session interface to the circuit switched cellular interface using standard 3GPP defined mechanisms. The packet switched VoIP/video channel can then be stopped. 
         [0038]    In  FIG. 6  the signalling data flow involved in a mobile user equipment controlled communications session hand-over procedure is schematically illustrated. The functional network entities present in  FIG. 6  are the User Equipment (UE), a packet data gateway (PDG), a Gateway GPRS Support Node (GGSN) and a circuit switched network element (CS). While  FIG. 5  related to a network-controlled hand-over procedure,  FIG. 6  relates to a mobile user equipment controlled hand-over procedure. Several of the initial steps of the two procedures are identical, in particular steps S 201 , S 202  and S 203  are identical with steps S 101 , S 102  and S 103  of  FIG. 5 , and shall not be repeated here. However, at a step S 204  the network and mobile user equipment controlled procedures diverge, and in the mobile user equipment controlled procedure of  FIG. 6  the user equipment decides for itself to hand over to the circuit switched domain of the UMTS network. As a result of this decision, a call setup request message is communicated from the user equipment to the circuit switched interface at a step S 205  to initiate the set up process for the circuit switched domain. As with the network-controlled hand-over procedure, two stages are present. The first stage, at a step S 206 , is to transfer the VoIP session from the WLAN to the packet switched domain of the UMTS network which has already been established in the step S 201  by the PDP context activation message. Following this, the user equipment establishes a circuit switched call at a step S 207  and the circuit switched interface communicates a setup complete message to the user equipment at a step S 208 . At the second stage of the hand-over procedure, the VoIP session is transferred from the packet switched domain to the circuit switched domain. 
         [0039]    In  FIG. 7  the functional architecture of a mobile user equipment which controls the communications session hand-over procedure of  FIG. 6  is schematically illustrated. Hand-over decision handling logic  710  is provided which performs the decision processing required to action the step S 204  of  FIG. 6 . The hand-over decision handling logic  710  also selects an appropriate one of a VoIP application  720  and a circuit switched voice application  730  depending on the type of communications channel provided, and in particular in dependence on whether the communications channel is an IP-based channel or a circuit switched channel. When the VoIP application is selected, it will provide communications functionality with whichever IP-based communications channel is provided. This could be either an IP-based WLAN channel  740  or a packet switched UMTS channel  750 . When the circuit switched voice application is selected, it will provide communications functionality with a circuit switched communications session  760 . 
         [0040]    Various modifications may be made to the embodiments described above without departing from the scope of the present invention. Various further aspects and features of the present invention are defined in the appended claims.