GB2450322A

GB2450322A - Inter-working between packet switched and circuit switched radio systems or sub-systems

Info

Publication number: GB2450322A
Application number: GB0711751A
Authority: GB
Inventors: Chen-Ho Chin; Craig Kelvin Bishop; Taesun Yeoum
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2007-06-18
Filing date: 2007-06-18
Publication date: 2008-12-24
Anticipated expiration: 2027-06-18
Also published as: GB2450322B; GB0711751D0; WO2008156258A1

Abstract

In a packet switched wireless communications system for providing voice calls to user equipment, said user equipment also being operable with a circuit switched wireless network, said packet switched network being arranged to hand over a voice call with said user equipment to said circuit switched network during said call, a signalling method comprising the step of transmitting from the packet switched network to the user equipment a signal containing circuit switched call data for use in the circuit switched network.

Description

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MOBILE COMMUNICATIONS

This invention relates to mobile communications. More specifically, this invention relates to mobile communications with both packet switched and circuit switched radio systems or sub-systems, and to inter-working between such sub-systems.

Many mobile communication systems (public land mobile networks or PLMNs) at present provide voice communications via a circuit-switched arrangement, in which a circuit connection to mobile user equipment is established, and maintained as the user moves. GSM networks, for example, provide circuit switched voice communications. Such networks may also provide data communications which operate on a packet switched basis; for example, GPRS is an example of a packet switched data network. Packet switched voice communications are known in non-mobile communication systems. For example, IP phones such as SKYPETM, operating over connected lIP networks are becoming well known. However, packet switched data communications have not until recently been considered for mobile voice communications.

Currently under development is an all IP, packet switched, PLMN system known as SAE (system architecture evolution), described on the website of the third generation partnership project (3GPP), intended to work over a new air interface referred to as 3GPP Long Term Evolution (LTE), see

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http://www.3gpp.org/highlights/lte/lte.htm. Another such packet switched system is 3GPP2 (for which see www.3gpp2.org) It is expected that, in the longer term, voice services will be carried over packet switched radio networks, and that the need for circuit switched voice communications will be reduced. However, in the meantime, whilst packet switched voice communications are rolled out, packet switched voice communications will co-exist with circuit switched networks, and user equipment will need to be able to communicate using both. Additionally, mobile user equipment conducting a voice call will need to be capable of handovers between a packet switched domain (provided by an SAE network) and a circuit switched domain (provided by, for example, a GSM network).

As the SAE system is deployed, there will be "islands" of SAE packet switched voice coverage surrounded by "oceans" of GSM circuit switched voice coverage, so that there will need to be handovers from the packet switched domain to the circuit switched domain.

Handovers in GSM (GERAN-based) and UMTS (UTRAN-based) systems are defined in 3GPP document http://www.3gpp.orglftp/Specs/html-info/23009.htrn.

The problem of handing over a voice call from SAE/LTE networks to GSM or IJMTS networks has not yet been fully solved. Some options concerning signalling to initiate the handover are discussed in 3GPP TR 23.882 and some of these are more fully explored in 3GPP document S2- 072102 rev2 available at: http://www.3gpp.org/flp/tsg_sa/WG2_ArchITSGS2_57_Beijing/docs/ S2-072 I 02.zip The present invention is concerned with improvements in such handover. A handover between two circuit switched networks maintains continuity of the circuit switched call -the handover only affects the radio access stratum handled by the radio access components of the network (i.e. the base stations) and not the call control which is handled in the non-access stratum, so that from the point of view of the call control protocols operating in the mobile terminal, it is as if no handover had occurred; likewise, at the network level, the originating network continues to "anchor" the call, and passes on such call control data as is necessary for the switching centre of the target network to relay the call to the relevant base station during the handover preparation signalling, so again there is continuity at the call control level. However, in handover from a packet switched call, there is no circuit switched call in existence that can be handed over, so the assumption of that handover is merely a matter for the radio access stratum, with continuity at higher levels, breaks down.

In one embodiment, the present invention provides signalling from the packet switched network to the user equipment to allow the user equipment to set up its end of a circuit switched voice call. Preferably, a circuit switched transaction identifier for the call is created at the packet switched network and transmitted. Preferably, other non-access stratum data such as called and/or calling party numbers and/or bearer capability data and the current "call state"

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is also transmitted. Thus, the user equipment can set itself up to the notional state in which a circuit switched call would be. Of course, it would be possible to provide many other arrangements providing voice call continuity on handover from a packet switched domain to a circuit switched domain: for example, the user equipment could be permitted to calculate the relevant parameters itself and notify them to the target circuit switched network after handover, or the packet switched network could communicate them to the circuit switched network which could then instruct the user equipment to set up a new call using a conventional call setup command. However, it is convenient to allow the packet switched network to calculate the parameters of the forthcoming circuit switched call and transmit them, as they can then be made available to both the user equipment and the target circuit switched network in advance of handover (the timing of which is initiated by the packet switched network) to minimise the risk of a delay or glitch on handover.

In one embodiment the formulation of the information is at the time the packet switched network initiates the handover request. In another embodiment, the formulation of the information is at the time the packet switched network has received an acknowledgment that the circuit switched network accepts the handover request. By providing in each case that the information is formulated close to the handover (rather than, say, at the beginning of the voice session in the packet network) the likelihood that the circuit switched network call will closely match the packet switched call is increased. Further, as many calls will not be handed over out of the packet

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switched network, unneccessary operations are avoided. In an embodiment, the transmission is contained within a handover command message sent to the user equipment. That reduces unneccessary transmissions, and increases the chance that the call state set up in the user equipment for the circuit switched call will match that in the packet switched network at the time of handover.

In an embodiment, the transmission contains a call setup command defined within the circuit switched network.

The invention is conveniently used with the embodiments described in our co-pending application GB, filed on the same day and having the same assignee as the present invention, agents' reference J000050097GB, the contents of which are incorporated herein by reference in their entirety.

Other aspects, embodiments and preferred features of the invention, and their associated advantages, will become apparent from the following

description, claims and drawings.

Embodiments of the invention will now be disclosed, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows schematically the interconnection of a packet switched network according to the present invention with other networks and with terminals; Figure 2 shows schematically the elements of a packet switched network according to the present invention (known per se); Figure 3 shows schematically the elements of circuit switched networks of Figure 1 (known per se);

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Figure 4 shows schematically the layers into which communications protocols are divided in networks according to the present invention (known per se); Figure 5 is a signal flow diagram showing schematically the handover process performed between two circuit switched networks (known per se); Figure 6 is a message flow diagram showing schematically the process performed by the packet switched network according to a first embodiment of the invention and a circuit switched network; Figure 7 is a corresponding flow diagram showing the operation of the core of the packet switched network in the first embodiment, Figure 8 is a corresponding flow diagram showing the operation of the user equipment in the first embodiment; Figure 9 is a diagram showing the contents of an information element transmitted during the process of the first embodiment; Figure 10 is a message flow diagram showing schematically the process performed by the packet switched network according to a second embodiment of the invention and a circuit switched network; Figure 11 is a corresponding flow diagram showing the operation of the core of the packet switched network in the second embodiment; and Figure 12 shows the structure of a handover command message transmitted according to a third embodiment of the invention.

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FIRST EMBODIMENT

Figure 1 depicts schematically a number of inter-operating communications systems. A fixed line telephone 2 is connected to a plain old telephony system (POTS) or Integrated Services Digital Network (ISDN) public switched telephone network (PSTN) 4. A computer 6 is connected to the Internet 8, as is an Internet phone 10. Mobile telephony user equipment 12 is in radio communication with a circuit switched (CS) public land mobile network (PLMN) 14 such as a GSM network, and with a packet switched (PS) PLMN 16 such as an SAIE/LTR 3G network. Each of the networks are connected together via gateways. Thus, the user of the user equipment 12 can conduct a voice call with the user of telephone 2 or the Internet phone 10 or another mobile user equipment (not shown).

The user equipment 12 is mobile, and may therefore provide voice communications via the packet switched PLMN where coverage is available, or the circuit switched PLMN 14 (for which coverage is almost universal) where there is no packet switched coverage. The user equipment 12 is therefore capable of registering on either the CS-PLMN 14 or the PSPLIVIN 16, and those networks are arranged to be capable of handing over a voice call between the networks whilst it is in progress. It will be understood that the user equipment 12 may also be capable of non-voice data communications in various modes, not here discussed.

Referring to Figure 2, the components of an SAE network 16 are illustrated. The user equipment 12 communicates with an evolved node B

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(eNB) which provides a radio relay or base station defining one or more cells.

A plurality of eNBs 162 are provided. The eNB communicates with a serving gateway ( S-GW) 164 and with a mobility management entity (MME) 166.

The serving gateway is the point through which packets to and from the user equipment 12 are routed, via the eNB 162. The serving gateway 164 is connected to a packet data network (PDN) gateway 168 via which it communicates with IP services provided by the network operator (e.g. IP multimedia services (IMS), packet switched services and others, indicated genencally as 170). The mobility management entity 166 tracks the location of user equipment 12 and distributes paging messages to the relevant eNBs.

The MME also manages and stores the UE control plane context, handles UE authentication, and various other functions.

A policy and charging rules function (PCRF) node 172 stores rules indicating which services can be used by different users (not relevant to the present invention). A home subscnber service (HSS) node 174 provides the main data storage for subscriber and service related data including user identities, registration information, access parameters and service-tnggering information.

The serving gateway 164, mobility management entity 166, packet data network gateway 168 and home subscriber service database 174 comprise the elements of the evolved packet core (EPC) of the SAE network.

The EPC together with the eNB 162 makes up the enhanced packet system (EPS) of the SAE communications network.

A serving GPRS support node (SGSN) 176 interconnects the packet switched network with the packet switched domains of a GPRS/EDGE radio access network (GERAN) 14a and a UMTS radio access network (UTRAN) 1 4b.

As shown in Figure 3, the GERAN 14a compnses a plurality of base transceiver stations (BTS) 142a, 142b each generating one or more respective radio cells, coupled to a base station controller (BSC) 144. Similarly, in the UTRAN 14b, there are provided a plurality of node Bs 143a, 143b coupled to a radio network controller (RNC). The base station controller 144 and RNC 145 are connected via circuit switched links to a mobile switch centre (MSC) 146 and circuit switched media gateway (CS-MGW) 148. The MSC server 146 comprises the call control and mobility parts of a mobile switching centre and the CS-MGW 148 interfaces the data transport from the BSC 144 or RNC with the core network. The CS-MGW therefore contains the codecs, echo cancellers, and other format conversion components (not shown) necessary for the network to ineroperate with others.

The various nodes of the networks shown comprise switching and control logic, in practice provided by one or more computers performing each function, together with network interface chips for data reception and transmission. Each of the identified functions may be provided by a physically separate computer, a single function may be distributed between several computers, or several computers collectively may provide one or more of the functions, as is generally known in the art.

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The user equipment 12 may take a variety of forms, but is typically a handheld mobile communications terminal comprising conventional components (omitted for clarity) including an aerial, radio transmission and reception circuits, a codec, a modem, a keypad for dialling or data input, a screen for control and data display, a microphone and loudspeaker or earpiece for telephonic voice communications, a battery, a computer interface, a subscriber identity module (SliM), and so on. Controlling these are one or more suitably programmed control devices such as microcomputer or DSP chips, executing a control program that implements network signalling and data communications protocols.

Referring to Figure 4, it is well known that communications in communications systems such as the mobile systems with which the present invention is concerned, are handled by computer programs which execute protocols arranged in various layers. Thus, at the physical layer 102 is the air interface. Also within the "access stratum" (handled by the radio access network components) are the media and radio link control layers (grouped together here as 104). Above these is the radio resource control layer 106, and above that (in the "non access stratum") are the mobility management layer and the call or session control layers 108, 110. Above the non-access stratum is the application layer 112, with which applications such as email 114, chat 118 and other applications 120 communicate via application programming interfaces (APi) 122.

Grouping protocols into layers, separate of each other, allows ready inter-connection of different systems. The protocols in the stack within the user equipment 12 therefore communicate with equivalent peer protocols within the network, each working transparently through those at lower levels.

Different functional entities within the network execute the different protocols. The physical layer protocols are handled within the base stations (i.e. the BTS of a GSM system, or the node B or eNB of a UMTS system or LTE system respectively). Mobility management is handled by the mobility management entity (MME). Applications themselves are provided at service providers. The application layer 112 is handled at gateways such as the serving gateway 164, the PDN gateway 168 and the SGSN 176. Call control is handled at the Mobile Switching Centre 146 of the circuit switched network, and the equivalent session control within a packet switched network is handled within the EPC thereof Handover procedures are discussed in 3GPP Technical Standard TS 23.009 (available at www.3gpp.org/flp/specslhtml-info/23009.htm). When handing over user equipment 12 from a packet switched network 16 such as an SAE network to the circuit switched domain of another network 14 such as a GERAN or UTRAN network, the EPC part of the SAE network 16 must be able to replicate the traditional handover functionality described in 3GPP Technical Standard IS 23.009; there has to be an entity within the EPC of the packet switched network 16 which can emulate the mobile switching centre (MSC) of a GERAN within the EPC, and typically as a function within the

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MME 166, to emulate an anchoring MSC. As part of the handover requirements, this entity must therefore perform the specified handover preparation procedures of TS 23.009, to prepare the target circuit switched system to take over the voice call. It will therefore be connected to the MSCs 146 of the circuit switched system 14a, 14b as if it were another MSC.

In handovers within, or between, circuit switched systems, the call remains, to the user equipment, the same call before and after the handover.

In other words, the state machine which controls the call (i.e. controls the set up, progress and clear down of the call and all other aspects of the call according to telephone call protocols) remains in the same state, as the handover is handled at lower protocol levels.

Within the circuit switched network, call sessions are set up and handled (mostly by the MSC) using call control protocols which are adaptations of the ISDN signalling system 7 (SS7) call control protocols. The user equipment 12 is capable of initiating or terminating a call, in response to a user request or a network message, and therefore contains call control protocols. The call control protocols move from one defined call state to another in response to defined external stimuli, and therefore constitute a "state machine", identical or similar to the ISDN Q93 I call control state machine. Within the network, a peer protocol handles the other end of the call.

On the other hand, in the packet switched domain, the voice call is handled by voice-over-Internet-protocols (VoW) and in particular Session

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Initiation Protocol (SIP). No "call" in the conventional sense exists; voice communications are camed as packets in an Internet session.

Figure 5 is an extract from 3GPP technical standard TS 23.009, showing the message sequence for an inter-MSC handover within a circuit switched call. The source or origin is MSC-A and the destination or target is MSC-B. Messages with prefix "A" are over the A interface and those with prefix "MAP" are through the MAP protocol. Those in dotted lines are ISIJP messages. These messages are based on their SS7 equivalents.

In this embodiment, the EPC of the packet switched network emulates the performance of MSC-A in Figure 5, and responds to the messages expected by MSC-B which is located in the circuit switched network 14. The exact details of the operation are not relevant to the present invention but will be apparent to the skilled person familiar with circuit switched PLMN systems.

Figure 6 illustrates the process performed according to this embodiment. Figure 7 shows the corresponding processes performed at the network side and Figure 8 the corresponding processes performed at the user equipment side. A packet switched voice call is in progress (step 1002) and as indicated generally at step 1003, the packet switched session protocol at the user equipment is handling the call.

hi the meantime the user equipment is making periodic radio strength measurements for both the packet switched network 16 and surrounding circuit switched networks 14. The measurements are signalled to the packet

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switched network (step 1006) and received in step 1007. In step 1008, a decision is taken to hand over the user equipment 12 to a circuit switched network 14. In step 1010, according to this embodiment, the EPC of the packet network 16 generates call control information which will be used for the set up of a circuit switched call.

In particular, a circuit switched call reference identity (in other words the Transaction Identifier or TI defined in 3GPP TS 24.007, at paragraph 11.2.3.1.3) is generated. That Transaction Identifier identifies and relates a given call on both the user equipment side and the network side, regardless of which side originated the call, and is needed to synchronise the call control state machines in the user equipment and the network.

Also generated at this time are other call control non-access stratum data (i.e. data to be passed through the radio access components and on to the core of the network). Specifically, the called and calling party numbers, coded in binary coded decimal (BCD), in accordance with 3GPP TS 24.008 and the bearer capability (likewise defined) for the forthcoming call are generated from information held within the packet switched network relating to the user equipment, and to the current SIP session carrying the voice call.

Figure 9 shows the contents of the new CS CALL NAS information element which includes Transaction Identifier, Called and Calling party BCD number, Bearer Capabilities, and CS Call State. The CS call state is derived from the state of the IP session.

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Further access stratum radio capability data of the user equipment to be used for the circuit switched call are preferably also generated. Suitable access and non-access data may be based on the IP parameters of the call in the IP network 16, so that the quality of service experienced by the user does not undergo a sudden change.

Next, the packet switched network 16 initiates handover preparation signalling to the target network (step 1012), in the same manner as between two circuit switched networks, and, where the target switched network accepts the handover, it signals acceptance back to the packet switched network 16 (step 1014). The handover preparation signalling includes the Transaction Identifier and some of the other access and non-access stratum data sent to the user equipment 12, to the same extent as would be received in a handover from another circuit switched network.

When the handover preparations between the networks are complete, then in step 1016, the network generates a radio interface HANDOVER COMMAND message. The handover command message in this embodiment is a modification of that presently described in TS 44.018, table 9.1.15. 1 as shown in Table I below, to include an extra information element (JE) shown here as "circuit switched call NAS information" which includes the information referred to above generated in step 1010.

IE Information element Type I Reference Presence Format length RR management Protocol Protocol Discriminator M V 1/2 -Discriminator 10.2 _________ ________ Skip Indicator Skip Indicator M V 1/2 ______________________ 10.3.1 _________ ________ ______ Handover Command Message Type M V 1 -Message Type 10.4 ________ _______

Cell Description Cefi description M V 2

_____________________ 10 5.2 2 _________ _______ _____ Description of the first Channel Description 2 M V 3 channel, after time 10 5.2 5a _________ _______ _____ Haridover Reference Handover Reference M V 1 _____________________ 10.5.2.15 ________ _______ _____ Power Command and Power Command and M V 1 Access type Access type _____________________ 1 0.5.2.28a ________ _______ _____ D-Synchronization Indication Synchronization Indication 0 TV 1 ______________________ 10.5.2 39 _________ _______ ______ 02 Frequency Short List, after Frequency Short List C TV 10 time 10.5.2.14 ________ _______ ______ Frequency List, after time Frequency List C TLV 4-131 ____________________ 105.2.13 ________ _______ _____ 62 Cell Channel Description Cell Channel Description C TV 17 ____________________ 10.5.2 lb ________ _______ _____ Description of the multislot Multislot Allocation C TLV 3-12 -configuration 10.5.2.21b ________ _______ ______ 63 Mode of the First Channel Mode 0 TV 2 -Channel(Channel Set 1)) 10.5.2.6 _________ _______ ______ 11 Mode of Channel Set 2 Channel Mode 0 TV 2 ______________________ 10.5.2.6 _________ _______ ______ 13 Mode of Channel Set 3 Channel Mode 0 TV 2 ____________________ 10.5.26 ________ _______ _____ 14 Mode of Channel Set 4 Channel Mode 0 TV 2 ____________________ 105.2.6 ________ _______ _____ Mode of Channel Set 5 Channel Mode 0 TV 2 ______________________ 10.5.2.6 _________ _______ ______ 16 Mode of Channel Set 6 Channel Mode 0 TV 2 ______________________ 10.5.2.6 _________ ________ ______ 17 Mode of Channel Set 7 Channel Mode 0 TV 2 _____________________ 10.5.2.6 ________ _______ ______ 18 Mode of Channel Set 8 Channel Mode 0 TV 2 ______________________ 10.5.2.6 _________ ________ ______ 64 Description of the Second Channel Description 0 TV 4 Channel, after time 10.5.2.5 ________ _______ ______ 66 Mode of the Second Channel Mode 2 0 TV 2 -Channel 10.5.2.7 ________ _______ ______ 69 Frequency Channel Frequency Channel C TV 10 Sequence, after time Sequence _____________________ 10.5.2.12 ________ _______ ______ 72 Mobile Allocation, after Mobile Allocation C TLV 3-10 -time 10 5.2.21 _________ ________ ______ 7C Starting Time Starting Time 0 TV 3 _____________________ 10.5.2.38 ________ _______ ______ 7B Real Time Difference Time Difference C TLV 3 _____________________ 10.5.2.4 1 ________ _______ ______ 7D Timing Advance Timing Advance C TV 2 ____________________ 10.5.240 ________ _______ _____ 12 Frequency Short List, Frequency Short List C TV 10 -before time 10.5.2.14 ________ _______ _____ 19 Frequency List, before Frequency List C TLV 4-131 time 10.5.2.13 _________ ________ ______ 1 C Description of the First Channel Description 2 0 TV 4 -Channel, before time 10.5.2.5a _________ ________ ______ Description of the Second Channel Description 0 TV 4 Channel, before time 10.5.2.5 ________ _______ ______

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IE Frequency channel Frequency channel C TV 10 sequence before time sequence _____________________ 10.5.212 _________ _______ ______ 21 Mobile Allocation, before Mobile Allocation C TLV 3-10 -time 10.5.2.21 _________ _______ ______ 9-Cipher Mode Setting Cipher Mode Setting 0 TV 1 ___________________ 105.2.9 ________ _______ _____ 01 VGCS target mode VGCS target mode 0 TLV 3 Indication Indication _____________________ 1 0.5.2.42a _________ _______ ______ 03 Multi-Rate configuration MultiRate configuration 0 TLV 4-8 _____________________ 10.5.2.2laa _________ _______ ______ 76 Dynamic ARFCN Mapping Dynamic ARFCN Mapping 0 TLV 6-34 ___________________ 10.5.2.llb ________ _______ ______ 04 VGCS Ciphering VGCS Ciphering 0 TLV 3-15 Parameters Parameters ____________________ 10.5 2.42b ________ _______ ______ 51 Dedicated Service Dedicated Service 0 TV 2 Information Information ____________________ 10 5.2.59 ________ _______ ______ xy ICircuit Switched Call CS Call NAS Information -NAS Information _______________________ _________ _______ ______ Table 1: Modified CS HANDOVER message sent by PS network The handover command message is then transmitted to the user equipment in step 1018. On receipt (step 1019), the user equipment 12 starts the handover at radio and radio level and radio resource control level to move on to the circuit switched system 14, initiating communications with the BTS thereof. In parallel, in step 1024, the user equipment creates a state machine data structure for a circuit switched call, using the transaction identifier, called and calling party numbers and call state. Thus, the user equipment 12 is put into a state to communicate in a circuit switched call, and the call is set up in the state in which it was within the packet switched system (typically the call will be in progress on handover, but it might be handed over during set up or tear down).

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Once the radio level handover preparations are complete (step 1022) and the call control process to set up the state machine on the user equipment 12 is complete (step 1024), handover is complete (step 1026) and the userequipment 12 then passes control of the call to the circuit switched call control state machine (step 1028) and voice communications with the circuit switched network control (step 1029).

Although not shown here, the remainder of the handover at the circuit level proceeds as in a conventional circuit switched handover, with the packet switched network continuing to anchor the call, conversion to a circuit switched call taking place at the gateway to the GERAN or UTRAN, and the MSC of the circuit switched network routing the call on to the user equipment.

In this embodiment, the call control information to be used in the circuit switched call is gathered by the packet switched network 16 as part of the handover preparation process. This is convenient in that, at this stage, the packet switched network will also be collecting the information to be passed to the circuit switched network, and minimises the risk of any additional delay during the handover as information is collected at the earliest possible moment.

SECOND EMBODIMENT

In this embodiment, the features of the first embodiment also apply, except that instead of gathering and formulating the information for setting up

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the user equipment for the circuit switched call in step 1010 during handover preparation, the information is gathered after the target circuit switched network 14 signals acceptance of the handover, and handover formalities are complete (step 1030, referring to Figures 10 and 11). This has the advantages that it is unnecessary to collect the information if the handover does not proceed, and also that the information is collected closer to the actual moment of handover so that there is less chance of a change of call state between collection and handover. On the other hand, there is a slightly higher chance of additional delay during the handover whilst the information is collected.

THIRD EMBODIMENT

In the preceding embodiments, the HANDOVER command is expanded to include an additional information element relating to the circuit switched call parameters to be used by the mobile user equipment 12. In this embodiment, use is made of the SETUP COMMAND which is defined within the circuit switched network 14, see 3GPP TS 24.008, 9.3.23.1. The set up message includes the transaction identifier, called and calling party numbers of bearer capability information, amongst other things, for the user equipment 12 to set up a call. Accordingly, the packet switched network 16 is arranged, rather than generating a new information element, as descnbed above, to populate the fields of a circuit switched SETUP command, and to include that message within the handover command message it transmits. Figure 12 shows the resulting handover command schematically. At the user equipment end, when the HANDOVER command is received containing a circuit switched SETUP command, the data is extracted from the SETUP command to establish the state machine for a circuit switched call corresponding to the ongoing packet switched call, in the same manner as in the first and second embodiments.

OTHER EMBODIMENTS AND VARIATIONS

It will be understood that the foregoing embodiments are merely examples, and that the invention extends to any and all modifications, variations and substitutions which would be obvious to the skilled person in the light thereof. In particular, names of messages, data and systems are purely exemplary. Although use of a message corresponding to a modified form of the SAE HANDOVER message as described above, it will be apparent that a separate message could be sent to inform the user equipment to set up a circuit switched call state machine, including either new information element as described above in the first and second embodiments, or a circuit switched set up command as described in the third embodiment above. Whilst preparing the information at two separate points in time have been described above as alternatives, it will be apparent that some call information could be formulated at one time and some at another: for example, called and calling party numbers could be formulated in BCD at the time of call setup arid stored for subsequent use in any potential handover to a

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CS network, and call state could be formulated at the time of handover acknowledgment.

Claims

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1. In a packet switched wireless communications system for providing voice calls to user equipment, said user equipment also being operable with a circuit switched wireless network, said packet switched network being arranged to hand over a voice call with said user equipment to said circuit switched network during said call, a signalling method comprising the step of transmitting from the packet switched network to the user equipment a signal containing circuit switched call data for use in the circuit switched network.

2. The method of claim 1, wherein said circuit switched call data includes further non access stratum data.

3. The method of claim 1, wherein said circuit switched call data includes access stratum data.

4. The method of claim 1, wherein said circuit switched call data includes an indication of the current call state and the user equipment is arranged to establish a state machine using said current call state.

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5. The method of claim 1, wherein said circuit switched call data includes a call identifier which will identify the call to both the circuit switched network and the user equipment.

6. The method of claim 1, wherein said circuit switched call data includes the called and/or calling party numbers.

7. The method of claim 1, wherein said circuit switched call data includes bearer capability information.

8. The method of any preceding claim, wherein said signal comprises a signal instructing the user equipment to perform a handover.

9. The method of any preceding claim, wherein said circuit switched call data is in the form of a call setup command defined within the circuit switched network.

10. The method of any preceding claim, further comprising the step, performed by the packet switched network, of formulating the circuit switched call data.

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11. The method of claim 10, wherein the formulating step is performed as part of the handover preparations performed by said packet switched network.

12. The method of claim 10, wherein the formulating step is performed after acceptance by said circuit switched network of said handover.

13. User equipment for performmg all or part of the method of any preceding claim.

14. A network core component of a packet switched network for performing all or part of the method of any of claims 1 to 12.