PATENT DOCUMENT

Publication Number: US-8717876-B2
Application Number: US-201213487881-A
Country: US
Kind Code: B2

Title: Providing packet-based multimedia services via a circuit bearer

Abstract:
A packet-based multimedia service is provided to a terminal in a network. A packet signaling connection is established between the terminal and the network. Signaling information for the multimedia service is transferred via the packet signaling connection using Session Initiation Protocol (SIP) or a similar protocol. A circuit bearer connection is also established with the terminal. Data for the multimedia service is transferred via the circuit bearer connection. This allows the data to be carried across networks which do not support the required QoS functionality for the packet-based service, or which cannot efficiently carry packet-based data. The circuit bearer connection can be established by a network entity or by the terminal. The circuit bearer can be interworked to a packet-switched bearer at some point in the network, such as at a gateway, so as to provide a remote party with the appearance that a fully packet-switched connection is being used.

Claims:
The invention claimed is: 
     
       1. A method for using a packet-based multimedia service defined by a telecommunications standard between a terminal and a network, the method comprising:
 the terminal establishing (i) a packet signaling connection between the terminal and the network and (ii) a circuit bearer connection between the terminal and a gateway of the network, wherein the circuit bearer connection is established while maintaining the packet signaling connection; 
 the terminal transferring signaling information for the multimedia service via the packet signaling connection; and 
 the terminal transferring data for the multimedia service via the circuit bearer connection, wherein the network does not support packet quality of service (QoS) functionality required by the telecommunications standard. 
 
     
     
       2. The method of  claim 1 , further comprising:
 receiving a signaling message via the packet signaling connection which includes the identity of the network entity towards which the circuit bearer connection should be established; and, 
 initiating the circuit bearer connection towards that network entity. 
 
     
     
       3. The method of  claim 1 , further comprising:
 sending a notification that the terminal supports a circuit bearer connection; 
 waiting for a response from the network that a circuit bearer connection should be used; and 
 if no response is received, establishing a circuit bearer connection for at least part of the path between the terminal and a called party. 
 
     
     
       4. The method of  claim 1 , further comprising:
 if a response is received from a called party indicating that the called party supports a circuit bearer connection, establishing a circuit bearer connection between the terminal and the called party. 
 
     
     
       5. The method of  claim 1 , wherein the network further comprises a gateway which performs conversion between the circuit bearer connection and a packet-switched bearer, and wherein:
 a circuit bearer connection is established by instructing the gateway to establish a circuit bearer call with the terminal. 
 
     
     
       6. The method of  claim 1 , wherein the network further comprises a gateway which performs conversion between the circuit bearer connection and a packet-switched bearer, and wherein:
 a circuit bearer connection is established between the terminal and the gateway; and 
 the gateway is instructed to establish a packet-switched bearer for transferring the data between the gateway and a remote party. 
 
     
     
       7. A non-transitory, computer accessible memory medium storing program instructions for using a packet-based multimedia service defined by a telecommunications standard between a terminal and a network, wherein the program instructions are executable by a processor of the terminal to:
 establish (i) a packet signaling connection between the terminal and the network and (ii) a circuit bearer connection between the terminal and a gateway of the network, wherein the circuit bearer connection is established while maintaining the packet signaling connection; 
 transfer signaling information for the multimedia service via the packet signaling connection; and 
 transfer data for the multimedia service via the circuit bearer connection, wherein the network does not support packet quality of service (QoS) functionality required by the telecommunications standard. 
 
     
     
       8. The non-transitory, computer accessible memory medium of  claim 7 , wherein the program instructions are further executable to:
 receive a signaling message via the packet signaling connection which includes the identity of the network entity towards which the circuit bearer connection should be established; and, 
 initiate the circuit bearer connection towards that network entity. 
 
     
     
       9. The non-transitory, computer accessible memory medium of  claim 7 , wherein the program instructions are further executable to:
 send a notification that the terminal supports a circuit bearer connection; 
 wait for a response from the network that a circuit bearer connection should be used; and 
 if no response is received, establish a circuit bearer connection for at least part of the path between the terminal and a called party. 
 
     
     
       10. The non-transitory, computer accessible memory medium of  claim 7 , wherein the program instructions are further executable to:
 if a response is received from a called party indicating that the called party supports a circuit bearer connection, establish a circuit bearer connection between the terminal and the called party. 
 
     
     
       11. The non-transitory, computer accessible memory medium of  claim 7 , wherein the network further comprises a gateway which performs conversion between the circuit bearer connection and a packet-switched bearer, and wherein
 a circuit bearer connection is established by instructing the gateway to establish a circuit bearer call with the terminal. 
 
     
     
       12. The non-transitory, computer accessible memory medium of  claim 7 , wherein the network further comprises a gateway which performs conversion between the circuit bearer connection and a packet-switched bearer, and wherein:
 a circuit bearer connection is established between the terminal and the gateway; and 
 the gateway is instructed to establish a packet-switched bearer for transferring the data between the gateway and a remote party. 
 
     
     
       13. An apparatus for providing a packet-based multimedia service defined by a telecommunications standard between a terminal and a network, wherein the apparatus comprises:
 communication circuitry for performing communication with the terminal and the network; and 
 processing hardware coupled to the communication circuitry, wherein the processing hardware is configured to:
 establish (i) a packet signaling connection between the terminal and the network and (ii) a circuit bearer connection between the terminal and a gateway of the network, wherein the circuit bearer connection is established while maintaining the packet signaling connection; 
 transfer signaling information for the multimedia service via the packet signaling connection; and 
 transfer data for the multimedia service via the circuit bearer connection, wherein the network does not support packet quality of service (QoS) functionality required by the telecommunications standard. 
 
 
     
     
       14. The apparatus of  claim 13 , wherein the processing hardware is further configured to:
 receive a signaling message via the packet signaling connection which includes the identity of the network entity towards which the circuit bearer connection should be established; and, 
 initiate the circuit bearer connection towards that network entity. 
 
     
     
       15. The apparatus of  claim 13 , wherein the processing hardware is further configured to:
 send a notification that the terminal supports a circuit bearer connection; 
 wait for a response from the network that a circuit bearer connection should be used; and 
 if no response is received, establish a circuit bearer connection for at least part of the path between the terminal and a called party. 
 
     
     
       16. The apparatus of  claim 13 , wherein the processing hardware is further configured to:
 if a response is received from a called party indicating that the called party supports a circuit bearer connection, establish a circuit bearer connection between the terminal and the called party. 
 
     
     
       17. The apparatus of  claim 13 , wherein the network further comprises a gateway which performs conversion between the circuit bearer connection and a packet-switched bearer, and wherein
 a circuit bearer connection is established by instructing the gateway to establish a circuit bearer call with the terminal. 
 
     
     
       18. The apparatus of  claim 13 , wherein the network further comprises a gateway which performs conversion between the circuit bearer connection and a packet-switched bearer, and wherein:
 a circuit bearer connection is established between the terminal and the gateway; and 
 the gateway is instructed to establish a packet-switched bearer for transferring the data between the gateway and a remote party.

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/108,062, filed May 16, 2011 which is a continuation of U.S. patent application Ser. No. 10/843,402, filed May 11, 2004, now U.S. Pat. No. 7,961,714, which is a continuation-in-part of U.S. patent application Ser. No. 10/630,999, filed Jul. 30, 2003, now U.S. Pat. No. 7,746,849. 
    
    
     BACKGROUND OF THE INVENTION 
     The present disclosure relates generally to supporting packet-based multimedia services in a communications system. 
     Telecommunications systems, such as Universal Mobile Telecommunications System (UMTS) wireless networks, are evolving into systems that may carry both voice and data traffic via fixed, wireless, and satellite networks. Part of this evolution includes developing and providing packet frameworks for the delivery of IP based, real-time, conversational, multimedia services. For example, an IP multimedia subsystem (IMS) standard has been defined as part of a third generation partnership project (3GPP) to provide such services. 
     Standards (such as IMS) that address the delivery of multimedia services via a packet based network generally require quality of service (QoS) mechanisms that are intended to ensure a certain level of quality. However, most wireless packet networks require relatively substantial enhancements before such QoS mechanisms can be provided, which slows down the implementation of the associated standards. For example, while IMS provides a framework to support the delivery of multimedia services in a wireless network, most wireless networks need upgrades to their access/radio layers, as well as to their packet core/general packet radio service (GPRS) subsystems before IMS can be properly supported. Implementing these upgrades may involve a considerable amount of time and expense, as the upgrades will need to be developed, deployed and tested. 
     Accordingly, what is needed is an improved system and method to provide for the delivery of IP based, real-time, conversational, multimedia services. It is desirable to deliver these services to mobile devices via networks that may not support QoS mechanisms specified for the delivery of such services, or networks which are not capable of efficiently carrying IP based traffic. 
     SUMMARY OF THE INVENTION 
     Accordingly, an aspect of the present invention provides a method for providing a packet-based multimedia service to a terminal in a network. A packet signaling connection is established between the terminal and the network. A circuit bearer connection is established between the terminal and the network. Signaling information for the multimedia service is transferred via the packet signaling connection and data for the multimedia service is transferred via the circuit bearer connection. 
     Using a circuit bearer connection to carry data for the packet-based multimedia service has several advantages. Where the packet-switched capabilities of a network do not support the quality of service (QoS) demanded by the multimedia service, the circuit bearer can be used to ensure that data is carried in a manner that meets the required QoS. Alternatively, where data for the multimedia service can be carried more efficiently by the circuit bearer, it is advantageous to use a circuit bearer to carry the data, even if the packet-switched capabilities of the network would support the required QoS. An example of this is where the multimedia service is a voice-based service. Many networks are optimised to efficiently carry voice data, and it is more efficient to carry voice data over a circuit bearer connection rather than packing the voice data into IP packets where voice-specific optimizations may not be applied. 
     The circuit bearer can be used on just one leg of the total path between the terminal and a called party. The circuit bearer can be local to an end-user terminal and is treated simply as a bearer for the first ‘hop’ of a Media Component in a Session Initiation Protocol (SIP) session. The circuit bearer connection can be established by a network entity or by the terminal. Preferably, the circuit bearer is interworked to a packet-switched bearer at some point in the network, such as at a gateway, so as to provide a remote party with the appearance that a fully packet-switched connection is being used. Thus, the remote party can continue to use the normal packet-switched protocols for signalling and bearer traffic. 
     The method can be used at the start of a call. The method can also be used part-way through an existing call to transfer an ongoing packet-based call (e.g. a SIP VoIP call) to the circuit-switched domain over at least one leg of the call. This situation can arise if a mobile user moves away from an area where coverage is provided by a wireless local area network (WLAN) or 3G network, which supports a packet-switched bearer connection, to an area where coverage is provided by an alternative network which is not optimised to support a packet-switched bearer connection. 
     In one embodiment, a method is provided for providing a packet-based multimedia service to a mobile device in a network. The service is defined by a telecommunications standard, and the network does not support efficient packet quality of service (QoS) functionality as required by the standard. The method comprises establishing a packet signaling connection and a circuit bearer connection between the mobile device and network. Signaling information for the multimedia service is transferred via the packet signaling connection in alignment with the standard. Data for the multimedia service is transferred via the circuit bearer connection in alignment with the standard. This provides the multimedia service to the mobile device via the network as specified by the standard, even though the network does not support the required QoS functionality. The circuit bearer is used in a manner which complies with normal circuit bearer standards and so there is no requirement to change the way in the circuit network operates. 
     In addition to the data which is carried over the circuit bearer connection, other data for the multimedia service can be carried via a packet-switched connection. Typically, this is data which does not have strict QoS requirements, or data which is best carried in packet form rather than via a circuit bearer, such as Instant Messaging. 
     The invention is particularly useful with wireless communications systems which lack the packet QoS functionality required by a service or which cannot efficiently carry the data in packet form, although it can equally be applied to any situation in which a terminal has access to both a packet-based connection (such as a low-bandwidth IP connection) and also a circuit bearer (such as a telephony) connection. 
     Further aspects of the invention relate to methods of operating a control entity at a terminal and to a method of operating a control entity in the network. Still further aspects of the invention relate to a control entity which implements these methods. 
     The functionality described here can be implemented in software, hardware or a combination of these. Accordingly, further aspects of the invention provide a computer program product for implementing any combination of the steps of the methods according to the invention. It will be appreciated that the software can be installed on the host apparatus (e.g. a network entity such as an application server or the terminal) at any point during the life of the equipment. The software may be stored on an electronic memory device, hard disk, optical disk or other machine-readable storage medium. The software may be delivered as a computer program product on a machine-readable carrier or it may be downloaded directly to the host via a network connection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a flowchart of an exemplary method for providing multimedia services to a mobile device using a circuit bearer; 
         FIG. 2  shows an exemplary UMTS wireless network in which the method of  FIG. 1  may be implemented; 
         FIGS. 3 and 4  show embodiments of an architecture that may be used to implement the method of  FIG. 1  within the system of  FIG. 2 ; 
         FIG. 5  shows a terminal for implementing the method according to the invention; 
         FIGS. 6 and 7  shows call flows of a call set-up in which a circuit bearer is requested by a network via a media gateway within the architecture of  FIG. 3 ; 
         FIG. 8  shows a call flow of the terminating leg of a call where a circuit bearer is initiated by the network; 
         FIG. 9  shows a call flow of a call set-up in which a circuit bearer is requested by a mobile device within the architecture of  FIG. 3 ; 
         FIG. 10  shows a call flow in which a mobile device establishes an outgoing circuit bearer; 
         FIG. 11  shows a call flow for a terminating leg of a call in which a terminal establishes an outgoing circuit bearer; 
         FIG. 12  shows a call flow for a call set-up in which call control is performed by the mobile device; 
         FIG. 13  shows a call flow for a call set-up in which call control is performed by the mobile device, and devices at both ends of the connection can support a circuit bearer; 
         FIGS. 14-17  show call flows where a circuit bearer is established during an existing call session; 
         FIG. 18  shows another embodiment of an architecture that may be used to implement the method of  FIG. 1  within the system of  FIG. 2 ; 
         FIG. 19  shows a call flow of a call set-up in which a circuit bearer is requested by a network via an intelligent gateway within the architecture of  FIG. 18 ; 
         FIG. 20  shows yet another embodiment of an architecture that may be used to implement the method of  FIG. 1  within the system of  FIG. 2 ; 
         FIG. 21  shows a call flow of a call set-up in which a mobile device initiates a call to a network within the architecture of  FIG. 20 ; and 
         FIG. 22  shows a call flow of a call set-up in which a network initiates a call to a mobile device within the architecture of  FIG. 20 . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates generally to supporting Internet protocol (IP) based multimedia services using a combination of an IP connection and a circuit bearer. While the main embodiments describe the application to wireless communications systems it could equally apply to any situation in which there exists a packet-based connection (such as a low-bandwidth IP connection) and also a circuit bearer (such as a telephony) connection. It is understood that the following disclosure provides many different embodiments or examples. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Referring to  FIG. 1 , in one embodiment, a method  100  may be used to provide a packet-based multimedia service to a mobile device in a network. As will be described later in greater detail, the service is defined by a telecommunications standard that specifies quality of service (QoS) functionality for packet-based data transfers. However, the network does not efficiently support such QoS functionality. Accordingly, the method  100  may be used for providing the multimedia service in accordance with the standard on the non-compliant network. 
     In step  102 , a packet signaling connection may be established between the mobile device and network. This signaling connection may use, for example, a signaling protocol that provides call setup, routing, authentication, and other messages to endpoints within an IP network. In step  104 , a circuit bearer connection is established between the mobile device and network. Because the circuit bearer and packet signaling connections exist simultaneously, the mobile device should have functionality that supports this dual connection operation. 
     In step  106 , signaling information and data associated with the multimedia service may be transferred between the network and the mobile device. For example, in step  108 , signaling information for the multimedia service may be transferred via the packet signaling connection in alignment with the standard. In step  110 , data for the multimedia service may be transferred via the circuit bearer connection in alignment with the standard. It is understood that steps  108  and  110  may occur simultaneously, as signaling and data transfer may occur throughout a communication session. Accordingly, the method  100  enables the multimedia service to be provided to the mobile device via the network as specified by the standard, even though the network does not support the specified QoS functionality. 
     Referring now to  FIG. 2 , a telecommunications network  200  illustrates a system in which the method  100  described in reference to  FIG. 1  may be practised. In the present example, the network  200  is a wireless network that supports both voice and data packet communications using General Packet Service Radio (GPRS) and/or Universal Mobile Telecommunications System (UMTS) technologies. 
     The network  200  comprises a Radio Access Network (RAN)  202  and a core network  204 . The core network  204  further comprises a circuit domain  206  and a packet domain  208 . Other networks may be accessible to the network  200 , such as a Public Switch Telephone Network (PSTN)  210  (connected to the circuit domain  206 ), Internet  212 , and an X.25 network  214  (both connected to the packet domain  208 ). 
     The RAN  202  includes a plurality of cells (not shown) serviced by base transceiver stations (BTS)  216 ,  218 , and  220 . The BTS  216  is connected to a base station controller (BSC)  222  to provide a second-generation wireless network. The BTSs  218 ,  220  are accessible to radio network controllers (RNCs)  224 ,  226 , respectively, to provide a third-generation wireless network. A mobile switching center/visitor location register (MSC/VLR)  228  may be used to connect the core network  204  with other networks, such as the PSTN  210 . A home location register (HLR)  230  may be accessible to the MSC/VLR  228  and also to a serving GPRS support node (SGSN)  232  and a gateway GPRS support node (GGSN)  234  in the packet domain  208 . 
     The network  200  enables at least one mobile device  236  to establish a communication session with another device via the BTS  216 . For example, a request to establish a communication session with the mobile device  236  may be directed by the MSC/VLR  228  to (1) a second mobile device  238 , (2) a voice terminal (not shown) coupled to the PSTN  210 , or (3) a data terminal (not shown) coupled elsewhere to the telecommunications network  200 . For example, if the communication session is a circuit data transfer session, the request may be to connect the mobile device  236  to a computer or other data device via the network  200 . If the communication is a packet data transfer session, the request may be routed through the SGSN  232 , the GGSN  234 , and to the Internet  212 . It is noted that the mobile devices  236  and  238 , while illustrated as mobile telephones, may be any mobile device capable of communicating via the network  200 . Furthermore, the mobile devices  236 ,  238  may be capable of simultaneous circuit/data (e.g., packet) connections. It is understood that the network  200  is for purposes of illustration and the present disclosure may be equally applicable to other networks. 
     Referring now to  FIG. 3 , an architecture  300  may be used to implement a call session representing the method  100  of  FIG. 1 . In the present example, the call session may be requested by the network via a media gateway or by an end user, such as a mobile station. The session is to provide IP based, real-time, conversational, multimedia services. For example, the services may be provided using an IP multimedia subsystem (IMS), which is defined as part of a third generation partnership project (3GPP). Providing these services in compliance with 3GPP may require certain QoS mechanisms that may not exist in some networks, such as QoS for packet and access layers associated with the telecommunications network  200  of  FIG. 2 . Accordingly, the architecture  300  enables the use of 3GPP IMS services prior to the introduction of the IP QoS mechanisms as follows, although it is understood that the present disclosure may also be implemented in a network in which such QoS mechanisms do exist. 
     The Session Initiation Protocol (SIP) has been adopted by the 3GPP IMS for the transport of multimedia services. SIP messaging is based on a request-response paradigm and may be divided into SIP request messages and SIP response messages. SIP request messages include INVITE (which initiates a call or changes call parameters), ACK (which confirms a final response for INVITE), BYE (which terminates the call), CANCEL (which cancels an ongoing INVITE), OPTIONS (which queries a server about its capabilities), REGISTER (which registers with the location service), and INFO (which sends in-progress information). The SIP response messages may contain response codes such as  100  (continue),  180  (ringing),  200  (OK),  302  (moved temporarily),  401  (unauthorized), and  600  (busy). The use of SIP enables flexibility in the call session, and may also serve to align the call session with known standards, such as 3GPP IMS. A part of SIP is the Session Description Protocol (SDP) which is used, inter alia, to declare capabilities of network entities. 
     The architecture  300  comprises a signaling path  302  and a bearer path  304  between a mobile station (MS)  306  (which will also be called a User Equipment UE-A) and another party  308  (which will also be called a User Equipment UE-B). Mobile station  306  can be a dual mode mobile phone which is capable of simultaneous circuit and data connections. The MS  306  is connected to a GGSN  310  via a packet domain connection  312  (e.g., using dynamic host configuration protocol (DHCP), domain name service (DNS), etc.) The GGSN  310  is connected to a Proxy Call Session Control Function (P-CSCF)  314  which, in turn, may communicate with a Serving Call Session Control Function (S-CSCF)  316 . It is understood that other network entities may be used, such as an Interrogating Call Session Control Function (I-CSCF) (shown with the S-CSCF  316 ). The P-CSCF  314  may provide a point of contact in a visited network after the MS  306  is registered in the network. The S-CSCF  316  may be used to identify privileges associated with the MS  306 , as well as for selecting and providing access to a home network application server ( 315 ). The I-CSCF (also  316 ) may be used to locate the S-CSCF and hide the S-CSCF&#39;s network architecture. The P-CSCF  314  and I/S-CSCF  316  may be viewed as functional blocks that may be located on any of a plurality of network nodes, including within the GGSN  310 . The I/S-CSCF  316  communicates with the called party  308  via SIP messaging. A SIP Application Server (AS)  315  supports various SIP services. The SIP AS is connected to the I/S-CSCF via an ISC interface which carries SIP messaging. A Home Subscriber Server (HSS) acts as a repository for data relating to subscribers, including authentication information and information on the services that each subscriber is authorized to access. The I-CSCF and S-CSCF communicate with the HSS using the Cx interface, in order to obtain this information so that subscribers can be authenticated and authorized for access to the network and services. Similarly, the AS  315  communicates with the HSS using the Sh interface for the same purposes. A Media Gateway Control Function (MGCF) provides for routing of calls outside the IP Multimedia Subsystem. The MGCF communicates with SIP entities in the IMS (CSCFs) and also with call handling entities within the circuit switched network. It also communicates with a Media Gateway to control the mapping of user data (media) between IMS and Circuit Switched domains. 
     The MS  306  is also connected to a media gateway (MGW)  318  via a circuit domain connection  320 . The MGW  318  can communicate with the called party (UE-B)  308  via an IP bearer path  322 . In the present example, the MGW  318  converts the circuit-switched bearer traffic received from the MS  306  via the circuit domain connection  320  into IP packet based bearer traffic. The circuit-switched bearer  320  may be initiated by the MS  306  or by an intelligent node in the network, such as the MGW  318 . As will be shown later in greater detail, the messaging used to establish the call session within the architecture  300  enables the session to accommodate later network changes, such as the implementation of QoS mechanisms. It is noted that the circuit domain connection  320  is used solely for bearer traffic to and from the MS  306 , while signaling information is routed via the P-CSCF  314 . Called party  308  (and any other entities to the right hand side of  FIG. 3 ) see a conventional VoIP call  330 . In the following description the provision of a Circuit Bearer connection for at least part of the bearer path between a calling party ( 306 ) and called party ( 308 ), as part of a SIP session, will be referred to as a SIP Circuit Bearer (SCB). 
     A fully packet-switched SIP service would firstly establish a connection between the MS  306  and called party  308  using SIP signalling, identifying both parties, such that data packets which carry data for the service can be routed across the network via an IP bearer. Where a SIP Circuit Bearer is used a part of the bearer path—in this case between the mobile device MS  306  and gateway MGW  318 —is provided by a circuit-switched connection. As part of the SIP signalling, the called party UE-B  308  is provided with the address of the gateway MGW  318  so that data packets are correctly routed to the gateway  318  rather than the mobile device  306 . 
     Three functional elements are added to a conventional network to support a SIP circuit bearer (SCB). Firstly, a Circuit Bearer Control Function (CBCF) performs third party call control to mediate between the SIP Circuit Bearer  320  and IP bearer  322 . The CBCF sits in the signalling path  302  and presents the appearance of a standard SIP call using IP bearers  322  to the terminating SIP user agent (called party UE-B  308 ). It also instructs a Circuit Bearer Originating Function (CBOF), which may reside in the network or the terminal, to initiate a circuit bearer call  320 . 
     Secondly, a Circuit Bearer Originating Function (CBOF) originates the Circuit Bearer portion  320  of the call towards a telephone number (an E. 164  number) provided by the CBCF. The CBCF may already be configured with the E. 164  number (in the case that it is fixed as part of the network coordination) or the CBCF will have to obtain the number from the CBTF. If the CBOF is within the network, as shown in  FIG. 3 , (rather than at the client as shown in  FIG. 4 ), then it will need to provide the IP address/port of the gateway MGW to the CBCF in order for the CBCF to provide them to the called party. 
     Finally, a Circuit Bearer Termination Function (CBTF) terminates the Circuit Bearer portion of the call. It can allocate a circuit number that can be used to route a call from the CBOF, receives the incoming circuit call and notifies the CBCF (if required). If the CBTF is in the network, then the “call” is routed through the MGW/MGCF (which turns it from a circuit call to a SIP call) and onwards to the CBTF. For compatibility reasons, it is desirable that the MGW/MGCF is not modified to provide any additional functionality specific to SIP Circuit Bearer, and just converts the call as it would any other call. The CBTF in this case will act as a terminating SIP User Agent. 
     There are various ways in which these functional elements can be added to the network.  FIG. 3  shows a solution where most of the functionality is provided in the network. The call control (CBCF) and circuit bearer originating functions (CBOF) are provided in the network, and a convenient place for these is a SIP Application Server (AS)  315 . The mobile device MS is modified to include a circuit bearer terminating function (CBTF) so that it can respond to a circuit bearer initiated by the network.  FIG. 4  shows a solution where most (or all) of the functionality is provided at the terminal. The call control (CBCF) and circuit bearer originating and terminating functions (CBOF/CBTF) are part of the terminal. As a further alternative, both the network and terminal can have one or more of the CBCF, CBOF and CBTF functional elements. This allows either the terminal or network to control the SIP circuit bearer call, and either the terminal or network to establish the circuit bearer leg  320  of the call. Signalling messages (SDP) can indicate whether the terminal wishes to make or receive the Circuit Bearer call (or whether it doesn&#39;t care) and this can be negotiated between the network and terminal. If the terminal supports a CBCF, then it can indicate support for a standard VoIP media component within the Session Description (SDP). If this media component is accepted by the called party, the terminal will invoke the CBCF to establish the circuit bearer and obtain the VoIP parameters (address/port of the MGW) that need to be passed to the called party. In the case that the terminal supports CBTF or CBOF, then it can indicate that it supports SIP Circuit Bearer within the Session by including a separate media component with SCB parameters. The network may then invoke its own CBCF. If the network supports a network-based Circuit Bearer Control Function, then it is preferred that this is used to control the SCB. If the network does not have it&#39;s own Circuit Bearer Control Function (CBCF) then the terminal can detect this and perform the CBCF function itself 
     It is preferable that a terminal supports the SIP Circuit Bearer mechanism in a manner which is transparent to the user such that a user is not made aware of the difference between a SIP Circuit Bearer call and a normal SIP call in any way. To achieve this, it is preferred that the terminal includes the following: standard 3GPP IMS call control capabilities; the ability to request SIP Circuit Bearer in its SDP, the ability to recognise a SIP Circuit Bearer request in incoming SDP; the ability to recognise and silently accept an incoming SIP Circuit Bearer call or to automatically make an outgoing SIP Circuit Bearer call; to associate a SIP Circuit Bearer Call with the corresponding SIP session in a transparent manner; and the ability to support simultaneous CS domain and PS domain connections. 
       FIG. 5  shows the main parts of a terminal which is adapted to support SIP Circuit Bearer. An IMS Client  250  supports SIP signalling and real-time packet-switched multimedia services  255 . The IMS Client  250  is connected to a packet-switched (PS) domain interface  280  which formats signals for transmission and reception along a PS connection. The IMS client  250  includes one or more of the CBCF, CBOF and CBTF functions described above to support the SIP Circuit Bearer. IMS Client  250  is connected to a circuit-switched telephony function and a circuit-switched domain interface  290 , which allows the terminal to transmit and receive CS data over a CS connection when a SIP Circuit Bearer is required. Both the PS domain interface  280  and CS domain interface  290  connect to a radio interface  295  which includes conventional coding and modulation functions for transporting the PS and CS signals over a radio interface. A user interface  260  includes conventional features such as a microphone, loudspeaker, display and keypad. The functions of the IMS Client  250  and blocks  270 ,  280 ,  290 ,  295  are typically realised as software executed by a microprocessor, by a combination of hardware and software, or by dedicated processing equipment. 
     A number of different call flows will now be described to illustrate the various ways in which embodiments of the invention can operate. In each of these call flows it is assumed that the terminal is aware that a SIP Circuit Bearer (SCB) is required, i.e. the terminal knows that the current access system is not optimised for supporting native VoIP. All call flows begin with the terminal sending an INVITE message with the SCB SDP, requesting a SCB. If the terminal supports it&#39;s own CBCF then it will indicate in the SDP that it would like either a SCB connection or a normal VoIP connection—the ‘normal VoIP’ SDP is included because the terminal can convert this into a SIP Circuit Bearer by using it&#39;s own CBCF. There are four main possibilities:
         1. The terminal has indicated that it accepts incoming SCB calls. The network detects the SCB SDP and invokes the Circuit Bearer Control Function. The network makes the CS call to the terminal and links it to the SIP session. The network provides information in the form of SDP of the Media Gateway to the far endpoint. This includes the IP address of the Media Gateway.   2. The terminal has indicated that it wishes to make an outgoing SCB call. The network detects the SCB SDP and invokes the Circuit Bearer Control Function. The network CBCF responds to the terminal with the E. 164  number that should be used for the circuit bearer connection. The terminal makes the CB call and the CBTF in the network receives the call and forwards to the CBCF. The network CBCF then provides the SDP of the Media Gateway MGW to the far endpoint.   3. The network does not recognise the SCB SDP, i.e. the network does not have a CBCF, and the SDP is forwarded to the far endpoint, which also does not recognise it but accepts the VoIP part of the call. The calling terminal invokes it&#39;s own CBCF and the call flows continue as in (1) or (2) with the terminal performing the CBCF function and either CBOF or CBTF.   4. The network does not recognise the SCB SDP and it is forwarded to the far endpoint, which does recognise it. According to the preference indicated by the calling terminal, the called or calling terminal invokes a CBOF to establish a Circuit Bearer directly between the two endpoints. As the Circuit Bearer is established directly between CBOF and CBTF at the two endpoints there is no need for mediating between a Circuit Bearer and a VoIP connection and hence no need for a CBCF. A further possibility in this case is that the SCB parameters negotiated in the session description identify an already established CS domain call between the two endpoints. In this way, an IMS session can be established making use of a pre-existing CS domain call between the two users.       

     For simplicity, some network entities have been omitted from the call flows, such as the various CSCFs that signaling messages would be routed through. Where there is a network based CBCF, it is shown as an individual entity although it will, in reality, be provided at the P-CSCF, S-CSCF or SIP AS. The call flows are the same, although the discovery of information such as E. 164  numbers may differ. The terminal UE-A has a CS domain subscription with a Mobile Station ISDN number (MSISDN) +447710875525. When the CBCF is network-based, it operates a full back-to-back User Agent, meaning that it acts as a SIP User Agent Server towards the originating SIP User Agent and as a SIP User Agent Client towards the terminating SIP User Agent, mediating between the two. 
     We consider the case that UE-A wants to use a SIP Circuit Bearer. This is transparent to UE-B in Cases  1 - 3 . UE-B may also use SCB, in which case (for cases  1 - 3 ) the call flow for UE-B is essentially the mirror of that for UE-A. Messages that are tagged with an asterisk symbol ‘*’ indicate that pre-conditions are not yet met. SDP extensions are required for describing a Circuit Bearer. These are outlined in the Appendix. 
       FIG. 6  generally shows a first call flow in which the network manages the call flow, and the network initiates the SCB. Data is carried over a first leg of the session by circuit domain connection  320 . This may be accomplished by establishing a signaling PDP context between the MS  306  and the P-CSCF  314  (via the GGSN  310 ). SIP signaling then occurs between the MS  306  and the P-CSCF  314  to establish a call session. Network services may be executed using the S-CSCF  316 , and a circuit bearer is requested to establish the circuit domain connection  320 . A second leg of the connection is established to the other party  308  via the MGW  318  using either a packet or circuit connection. The MGW  318  then bridges both the first and second legs to connect the MS  306  and other party  308 . In the call flow  400 , the MOW  318  is used to establish the circuit domain connection  320 . As shown in  FIG. 6 , the call flow  400  includes the MS  306 , the P-CSCF  314 , the I/S-CSCF  316 , and the MGW  318 . The call flow  400  relies heavily upon SIP messaging. 
     The call flow  400  begins in step  402  when the MS  306  sends a SIP INVITE message to the P-CSCF  314 . The INVITE message includes an initial session description protocol (SDP) packet in the SIP INVITE message body. SDP is a protocol that may be used to indicate a multimedia session, and may include such information as a session name and purpose. The SIP INVITE message is forwarded from the P-CSCF  314  to the MGW  318  via the I/S-CSCF  316 . 
     The MS  306 , P-CSCF  314 , I/S-CSCF  316 , and MGW  318  conduct SDP negotiations via SIP messages in step  404 . These negotiations may include SDP answer, SDP offer, SDP success, and SDP answer exchanges. The SDP negotiations include a reservation of circuit resources by the MGW  318 , as indicated by step  406 . 
     In step  408 , the P-CSCF  314  utilizes a Policy Control Function (PCF) mechanism to authorize QoS resources requested during the SDP negotiations in step  404 , which may occur multiple times during the SDP negotiations. In the present example, this a NULL operation because no QoS is being requested (i.e., conversational grade QoS is inherent in the circuit domain connection  320  and need not be requested). In step  410 , the MGW  318  sets up first and second circuit legs to the MS  306  and the other party  308 , respectively. 
     The MGW  318  receives a ringing indication in step  412  and maps the ringing indication to a SIP ringing response message, which is then sent to the MS  306  via the I/S-CSCF  316  and P-CSCF  314 . When the MGW  318  receives an answer indication in step  414 , it relays this information as a SIP OK message to the P-CSCF  314  via the I/S-CSCF  316  in step  416 . The P-CSCF  314  utilizes the PCF to commit the requested QoS in step  418 , which is a NULL operation because no QoS was requested. The P-CSCF  314  then forwards the SIP OK message to the MS  306  in step  420 . The MS  306  may then begin using the media resources authorized and committed in the call set-up in step  422 . In step  424 , the MS  306  sends a SIP ACK message to the I/S-CSCF  316  via the P-CSCF  314 . 
       FIG. 7  shows a similar call flow in greater detail. The Circuit Bearer Control Function (CBCF) is network-based and the Circuit Bearer is initiated by a CBOF in the network. For clarity the CBOF and CBTF are not explicitly shown but the CBOF is collocated with the CBCF and the CBTF is at UE-A. The call flow begins at step  1  when UE-A sends a SIP INVITE message which is addressed to UE-B (the called party). UE-A knows that the access network is not optimised to support VoIP and so it includes SDP declaring that UE-A supports SIP Circuit Bearer. This SDP also indicates that UE-A supports the Circuit Bearer Terminating Function (i.e. that the UE is capable of receiving an incoming Circuit-switched connection for SCB) and the address to which the SCB call should routed (which will be a MSISDN allocated to UE-A). This number may be a special number allocated to the UE for SIP Circuit Bearer purposes or it may be the normal MSISDN of the terminal. This information can be indicated using suitable extensions to the Session Description Protocol, as described in the Appendix. Alternatively, the capabilities of UE-A may already be known to the network, based for example on the user identity. In this case, the network may automatically retrieve the relevant parameters from a database of subscription information, such as the HSS ( FIG. 3 ). 
     The INVITE message is received by the CBCF, which recognises that UE-A supports SCB. The CBCF removes the SCB field from the SDP, adds the standard VoIP field, and then forwards the modified INVITE message to UE-B at step  2 . UE-B replies, at step  3 , with a message indicating that it supports VoIP. The CBCF then forwards a modified message to UE-A, at step  4 , indicating that the network CBCF supports SCB. This message also includes the CLI of the CBOF that will be making the circuit bearer call. 
     The CBCF then instructs the CBOF to initiate the circuit bearer leg of the call. A SIP INVITE message is sent to the Gateway, at step  5 , which includes the circuit bearer number of UE-A. The Gateway initiates, at step  6 , a circuit call to UE-A. UE-A recognises the circuit bearer call and responds, preferably without ringing to alert the user. The terminal knows to associate this call with the existing SIP call as it has already been told the CLI of the CBOF that will be making an incoming call and is expecting the incoming call. 
     Having established a circuit bearer call between UE-A and the Gateway, the CBCF informs UE-B of the IP address of the Gateway in a SIP UPDATE message. A VoIP connection is established between the Gateway and UE-B. In parallel with the above steps, on receiving and answering the incoming Circuit call, UE-A sends an UPDATE message (Step  14 ) towards the CBCF. This message updates session description information indicating that the pre-conditions for the session have now been met. This is according to standard procedures in which sessions are established with “pre-conditions” which are then cleared when the resources necessary for the session have been established. The CBCF forwards the UPDATE towards UE-B, making appropriate modifications to the session description to replace the references to SCB with standard VoIP parameters. It should be noted that steps  14 - 17  could occur either before or after steps  12 - 13 . In messages to UE-B then pre-conditions will only be marked as ‘met’ when both messages  10  and  14  have been received by the CBCF. 
     According to standard procedures, on receipt of an indication that pre-conditions have been met, UE-B can begin alerting the terminating user UE-B. UE-B returns a message (at step  18 ) to indicate that called party alerting has begun. This is passed to UE-A by the CBCF. When the called user answers the call, this is indicated by messages at steps  20  and  22 . At step  23  the connection is cut-through in both directions, to establish an end-to-end connection which comprises the circuit domain connection between UE-A and the Gateway and the packet-domain connection between the Gateway and UE-B. It should be noted that UE-B is not aware that a SIP Circuit Bearer is in use between UE-A and the network. 
       FIG. 8  shows a call flow for the situation where a SIP circuit bearer is established at the terminating leg of a call to terminal UE-B. Terminal UE-A initiates a call by sending a SIP INVITE message towards UE-B, indicating that a VoIP connection is required. The CBCF intercepts the message. Knowing that the network which will be used for the terminating leg of the call is not optimised for VoIP, the CBCF forwards the message at step  2 , substituting an indication that SCB should be used. Note that, alternatively, the CBCF may add an indication that SCB is available, leaving it to UE-B to determine whether SCB or standard VoIP should be used. UE-B responds at step  3 , indicating that it supports SCB. The CBCF instructs a Gateway to initiate a circuit call to UE-B at step  5 . The circuit call is set up between the Gateway and UE-B at steps  6 - 8  in a similar manner to the previously described call flow. 
     At step  10 , UE-A is made aware of the session description information from the Gateway. UE-A responds with its own session description according to standard procedures. Again, according to standard procedures, when pre-conditions at UE-A are met, UE-A will send an UPDATE to update the session description (Step  12 ). This is passed on to UE-B (Step  13 ), with appropriate modifications to replace the VoIP parameters with SCB parameters. UE-B responds (Step  14 ) and the response is forwarded to UE-A (Step  15 ), again with appropriate modifications to replace the SCB parameters with the VoIP parameters of the gateway. 
     Referring to  FIG. 9 , in another embodiment, the architecture  300  may be requested by the mobile device  306  (rather than by the network via the MGW  318 ), as described above. As stated previously, the architecture  300  may be used for a call session that provides IP based, real-time, conversational, multimedia services using 3GPP IMS prior to the introduction of IP QoS mechanisms in the network. 
     In operation, a signaling PDP context may be established between the MS  306  and the GGSN  310 . SIP signaling then occurs between the MS  306  and the P-CSCF  314  to establish a call session. Network services may be executed using the S-CSCF  316 . A first leg of the session may be established over either a packet or circuit connection, and may use a SIP VoIP or SIP circuit bearer call setup. The second leg of the connection (the circuit domain connection  320 ) is set up by the MS  306 . During SIP/SDP signaling that occurs between the MS  306  and P-CSCF  314 , which indicates that a circuit connection is to be established. The MS  306  recognizes this signalling and requests a circuit connection via the MGW  318 . The MGW  318  then bridges both the first and second legs to connect the MS  306  and other party  308 , and may remain in the session for mid-call service control. 
     Call flow  500  illustrates a sequence of messages that may be used within the architecture  300  of  FIG. 3 . In the call flow  500 , the MS  306  is used to establish the circuit domain connection  320 . The call flow  500  includes the MS  306 , the P-CSCF  314 , the I/S-CSCF  316 , and the MGW  318 . The call flow  500  relies heavily upon SIP messaging. The call flow  500  begins in step  502  when the MS  306  sends a SIP INVITE message to the P-CSCF  314 . As described previously, the INVITE message includes an initial SDP packet in the SIP INVITE message body. The SIP INVITE message  302  is forwarded from the P-CSCF  314  to the MGW  318  via the I/S-CSCF  316 , and may also be forwarded to another network entity, such as a terminator CSCF. 
     The MS  306 , P-CSCF  314 , I/S-CSCF  316 , and MGW  318  conduct SDP negotiations via SIP messages in step  504 . These negotiations may include SDP answer, SDP offer, SDP success, and SDP answer exchanges. In the present example, one of the SDP packets may include a codec value to indicate that a circuit bearer is being used. In step  506 , the P-CSCF  314  utilizes a PCF mechanism to authorize QoS resources requested during the SDP negotiations  304 , which may occur multiple times during the SDP negotiations. In the present example, this a NULL operation because no QoS is being requested (i.e., conversational grade QoS is inherent in the circuit domain connection  320  and need not be requested). In step  508 , the MGW  318  sets up a first call leg with the other party  308 , and the MS  306  sets up a circuit call (a second call leg) to the MGW  318  via the circuit domain connection  320 . The MGW  318  then bridges the first and second call legs. 
     In step  510 , the MGW  318  sends a ringing indication to the MS  306  via the I/S-CSCF  316  and P-CSCF  314 . When the MGW  318  receives an answer indication in step  512 , it relays this information as a SIP OK message to the P-CSCF  314  via the I/S-CSCF  316 . The P-CSCF  314  utilizes the PCF to commit the requested QoS in step  514 , which is a NULL operation because no QoS was requested. The P-CSCF  314  then forwards the SIP OK message to the MS  306  in step  516 . The MS  306  may then use the media resources authorized and committed in the call set-up in step  518 . In step  520 , the MS  306  sends a SIP ACK message to the I/S-CSCF  316  via the P-CSCF  314 . 
       FIG. 10  shows a further embodiment in which the CBCF function is network-based and the Circuit Bearer is initiated by a CBOF in the terminal (UE-A). For clarity the CBOF and CBTF are not explicitly shown but the CBOF is collocated with UE-A and the CBTF is at the CBCF. Steps  1 - 4  are the same as those in  FIGS. 7 and 8 . At step  5  UE-A sets up a circuit call by contacting the Gateway, using the E. 164  number it has been provided with in the session description at Step  4 . Alternatively, if UE-A knows the E. 164  number needed for setting up the circuit bearer connection, then it can begin step  5  earlier than shown. At step  13  the CBCF informs UE-B of the IP address of the Gateway in a SIP UPDATE message. A VoIP connection is established between the Gateway and UE-B. The remaining steps are similar to those already described at  FIG. 7 . 
       FIG. 11  shows a call flow for the situation where a SIP circuit bearer is established at the terminating leg of a call to terminal UE-B. The CBCF knows that the access network which will be used to support the terminating leg of the call is not optimised for VoIP. The CBCF sends an initial SIP INVITE message (at step  2 ) specifying that SCB is to be used. Alternatively, the CBCF may indicate that either SCB or VoIP may be used, leaving the decision to UE-B. Terminal UE-B responds, indicating that it supports SCB, and then initiates a CB call at step  4  to the Gateway. The remaining steps are similar to those described previously with reference to  FIG. 8 . 
     In the above call flows the Circuit Bearer Control Function (CBCF) is network-based. In contrast, in the two following examples the CBCF forms part of the terminal. For clarity the CBCF is not shown as a separate entity, but is collocated with the terminal UE-A. An entirely terminal-based solution has the advantage that a network operator does not need to upgrade their network equipment to support the SCB service. In  FIG. 12  the call flow begins at step  1  by terminal UE-A sending a SIP INVITE message to terminal UE-B. SDP in the message indicates that UE-A can support SIP Circuit Bearer or VoIP. UE-B responds at step  2  indicating that it supports a VoIP connection. 
     UE-A now begins a sequence of steps to establish a circuit call with a Gateway and to inform UE-B of the Gateway&#39;s identity. Two alternative ways of setting up a circuit call are shown as steps  3   a - 6   a  and steps  3   b - 6   b . Taking the first method, UE-A sends, at step  3   a , a SIP INVITE message to the Gateway. This message includes the UE-A&#39;s own circuit number and effectively requests a circuit call with itself. The ‘SDP (UE-B VoIP)’ are the VoIP parameters of UE-B that were provided in step  2 —i.e. the IP address and port to which the gateway should send VoIP packets towards UE-B. The Gateway initiates a circuit call with UE-A at step  4   a.  UE-A recognises the CLI and answers the call. The routing of the INVITE message is automatic, i.e. terminal UE-A does not need to know the IP address of the gateway. At step  6   a  the terminal receives the SDP of the gateway. 
     The alternative method of establishing a circuit call with the Gateway is shown at steps  3   b - 6   b . Firstly, at step  3   b , UE-A initiates a CB call with itself via the Gateway. The UE initiates this by sending a CS domain SETUP message addressed to an MSISDN which has been allocated to the UE for incoming IMS calls. According to standard call routing and procedures, the gateway interworks this to a SIP INVITE addressed to the UE (Step  4   b ). This message includes the VoIP parameters of the gateway within the session description. At Step  5   b , the UE responds to this INVITE with a SIP  180  Ringing indication into which it places the VoIP parameters it has received from UE-B. At Step  6   b , the gateway interworks this to a CS domain alerting indication, whilst also noting the VoIP parameters it has been provided with. 
     Having established a CB call between UE-A and the Gateway, the CBCF of UE-A, at step  7 , informs UE-B of the VoIP parameters of the Gateway using a SIP UPDATE message. This ensures that UE-B establishes a VoIP connection with the Gateway rather than UE-A. The CBCF of UE-A has now established the two legs of the path between itself and UE-B: a first (circuit bearer) leg between UE-A and the Gateway and a second (IP bearer) leg between the Gateway and UE-B. SIP messaging continues to occur directly between UE-A and UE-B. 
     In the call flow of  FIG. 13  UE-A sends a SIP INVITE message to UE-B, including SDP which indicates that it is capable of supporting SCB or VoIP. UE-B receives the message, indicates that it too supports SCB and initiates a circuit switched call at steps  3   a - 6   a.  Alternatively, UE-A can initiate the circuit-switched call at steps  3   b - 6   b . In this case there is no IP bearer or Gateway involved in the call. 
     In a final alternative, UE-A and UE-B have a pre-existing CS domain connection between them (i.e. they are engaging in a normal CS domain voice call). UE-A wishes to convert this call into an IMS session with SIP Circuit Bearer. This may be achieved by following the call flow of  FIG. 13 . UE-A indicates within the session description in Step  1  that it is prepared to initiate a SIP Circuit Bearer Call, and the CLI that it will use. On receiving this message, UE-B detects that it already has a call in progress from a matching CLI, which must be UE-A. UE-B can respond in Step  2 , indicating that it accepts incoming SIP Circuit Bearer calls and providing it&#39;s destination number. UE-A can similarly detect that a call is already ongoing to that number, and proceeds to associate this call with the IMS Session, in preference to establishing a new circuit call. 
     The above scenarios have assumed that the terminal is aware that a SIP Circuit Bearer is required. If the terminal is unable to determine this by itself, then the terminal will offer both SCB and VoIP SDP as described above. If the network is aware that SCB is required (cases 1 and 2 above), then this will be provided (and the terminal will receive an SDP answer indicating parameters of the SCB). If the network is aware that SCB is not required (i.e. native VoIP is supported by the network), then the network may reject the SCB indication in the SDP using the mechanisms for applying policy to SDP requests. The terminal will retry with a normal VoIP session. 
     In order to deploy terminals which do not automatically detect the access type, the network operator must deploy network equipment which can handle the SCB decision. The inability of the local network to support VoIP can be indicated to the terminal simply by refusing a VoIP resource reservation. A final possibility is the case in which the network recognises that SCB is required, but does not itself provide the CBCF. Further SDP extensions may be required to allow the network to indicate this case to the UE, or the UE could automatically detect this case when it&#39;s VoIP resource reservation fails. 
     In each of the above call flows the SIP Circuit Bearer (SCB) is established at the start of a call. However, the method is not limited to use at the beginning of a call but can be applied to an ongoing VoIP session to transfer part of the IP bearer path to a SIP Circuit Bearer.  FIG. 14  shows a call flow which begins with an existing VoIP session between terminals UE-A and UE-B. The network includes a CBCF and UE-A supports incoming CB calls. At step  1  UE-A sends a SIP INVITE message to UE-B, indicating that it supports SCB. Step  1  can be triggered by a need for UE-A to transfer to an access network which is not optimised for carrying VoIP calls. At steps  3 - 6  the CBCF initiates a CB call between the Gateway and UE-A. At step  8  the CBCF updates UE-B with the IP address and capabilities of the Gateway. This will establish the necessary parameters at UE-B to allow it to subsequently send IP packets to the Gateway rather than UE-A. At Step  7 , UE-B responds to the re-invite request, indicating its own VoIP parameters (and possibly responses to other elements in the SDP). The VoIP parameters are provided to the gateway in Step  10  and other parameters to UE-A in Step  9 . 
     At Step  12 , UE-B responds to the gateway&#39;s VoIP parameters that were provided in Step  8 . This message may include changes to UE-B&#39;s VoIP parameters, which then need to be communicated to the gateway in Step  13 . It should be noted that, up to this point, the session descriptions exchanges between UE-A, the CBCF and UE-B have indicated outstanding pre-conditions. Therefore, according to standard SIP procedures, these exchanges will not yet cause any change in the media flows for the session. However, they will establish all the necessary parameters at the correct nodes to enable a rapid switch from the previous media connection to the new one. 
     At Step  15 , UE-A determines that it needs to switch to the SIP Circuit Bearer connection. This could occur, for example, because UE-A&#39;s previous VoIP-optimised IP connection is disconnected. UE-A sends an UPDATE containing its session description, modified to indicate that pre-conditions are now met. At Step  16 , UE-B responds, indicating similarly that the preconditions are now met. Since this is an automatic update of a session in progress, there is no need to wait for the user to answer the session at UE-B and the 200 OK to the original Re-INVITE can be sent immediately (Step  19 ). 
       FIG. 15  shows another call flow in which part of an existing VoIP path is transferred to a SCB. In this call flow the CBCF is network-based and the SCB is established by the terminal UE-A.  FIG. 16  shows another call flow in which part of an existing VoIP path is transferred to a SCB. In this call flow the call control functions (CBCF) are resident in terminal UE-A and operation is similar to that previously described in  FIG. 12 . In this call flow, steps  19 - 22  could be completed as early as step  7 . Note that in this case, no further SIP signalling is needed to complete the switch to SCB. This means the switch could be completed even if IP connectivity is lost after step  1  (so long as the CBCF is tolerant to the loss of messages  4  and  19 .) However there is no possibility to establish the CB before it is needed.  FIG. 17  shows a further call flow in which part of an existing VoIP path is transferred to a SCB. This call flow is similar to  FIG. 12  in that the VoIP connection is replaced entirely by a CS connection. The call flows for VoIP to SCB switching work best if there is IP connectivity at UE-A throughout the process. In general, the following cases support loss of the IP connectivity after the first SIP message initiating the switch is sent:
         Case  1 : Network-based CBCF with network-initiated CS calls   Case  2 : Network-based CBCF with user-initiated CS calls
 
Note: in case  1  is it possible for the UE to establish the CS bearer first and then confirm the switch with a CS ANSWER message. This is not possible in case  2 —the switch is triggered by the initial CS SETUP.
   Case  4 : Both UE&#39;s support SCB, no CBCF at all
 
Again, if the far end (UE-B) initiates the CS call, then UE-A can confirm the switch with a CS ANSWER message. If UE-A initiates the CS call, then the switch is triggered by just the SETUP.
       

     To support this mode with Case  3  (Client based CBCF), a new network service is be required. This new service automatically generates the UPDATE to UE-B on UE-A&#39;s behalf as a result of the ANSWER indication received at the gateway. This new network service is based on a SIP proxy in the network being made aware of the linkage between UE-A&#39;s session with UE-B and UE-A&#39;s session with the gateway. The proxy can then determine that the answer of the session with the gateway is the pre-condition that the session with UE-B is waiting for. This may be achieved by UE-A sending the UPDATE ‘early’ with an instruction to the proxy to store it until some other condition is met, before processing. 
     The invention can be applied to other network layouts.  FIG. 18  shows an architecture  600  which illustrates another possible implementation of a call session. A communication session within the architecture  600  may be requested by the network via an intelligent gateway (rather than by the MS  306  or by the network via the MGW  318 , as described above). The architecture  600  may be used to provide IP based, real-time, conversational, multimedia services using 3GPP IMS prior to the introduction of IP QoS mechanisms in the network. 
     The architecture  600  is similar to the architecture  300  of  FIG. 3 , but includes an intelligent network gateway (IN gateway)  602  and a MSC  604 . The IN gateway  602  is positioned between the P-CSCF  314  and the MSC  604 . The MSC  604  is positioned between the circuit domain connection  320  and the MGW  318 . It is understood that the positions of the IN gateway  602  and the MSC  604  are for purposes of illustration, and that they may be positioned elsewhere within the architecture  600 . Furthermore, the IN gateway  602  may be represented by a gateway function located on another network entity, such as the MSC  604 , and so the IN gateway  602  may not be an independent physical network entity. The IN gateway  602  provides functionality for mapping between IP/SIP messages and SS 7 /IN messages. 
     In operation, as will described in greater detail in  FIG. 19 , a signaling PDP context may be established between the MS  306  and the P-CSCF  314  (via the GGSN  310 ). SIP signaling then occurs between the MS  306  and the P-CSCF  314  to establish a call session. Network services may be executed using the S-CSCF  316 . A first leg of the session may be established using either a packet or circuit connection via an IN protocol message to the MSC  604 . In the present example, the first leg is established using a standard SIP VoIP or SW circuit bearer call setup. 
     The second leg of the connection is requested by the MS  306  via an IN message to the MSC  604 . The first and second legs are then bridged to connect the MS  306  and other party  308 . If both legs are circuit based, the bridging may be done by the MSC  604  without the need for the MGW  318 . However, if the first leg is packet based, then the MGW  318  may be needed to complete the bridging in conjunction with the MSC  604 . 
     With additional reference to  FIG. 19 , a call flow  700  illustrates a sequence of messages that may be used to establish the communication session described previously, in which the network requests the circuit domain connection  320  via the IN gateway  602 . The call flow  700  includes the MS  306 , the P-CSCF  314 , the I/S-CSCF  316 , and the IN gateway  602  and MSC  604  (which are combined in this illustration and denoted by the reference number  604 ). 
     The call flow  700  begins in step  702  when the MS  306  sends a SIP INVITE message to the P-CSCF  314 . As described previously, the INVITE message includes an initial SDP packet in the SIP INVITE message body. The SIP INVITE message is forwarded to the I/S-CSCF  316 , which sends a corresponding IN message to the MSC  604  (via the IN gateway  602 ) to request a circuit connection in step  704 . 
     In step  706 , the MS  306 , P-CSCF  314 , and I/S-CSCF  316  conduct SDP negotiations via SIP messages. These negotiations may include SDP answer, SDP offer, SDP success, and SDP answer exchanges. In the present example, one of the SDP packets may include a codec value to indicate that a circuit bearer is being used. In step  708 , which may occur simultaneously with step  706 , interaction occurs with the IN gateway/MSC  604  to request answer notifications. In step  710 , the P-CSCF  314  utilizes a PCF mechanism to authorize QoS resources requested during the SDP negotiations, which may occur multiple times during the SDP negotiations. In the present example, this is a NULL operation because no QoS is being requested (i.e., conversational grade QoS is inherent in the circuit domain connection  320  and need not be requested). In step  712 , the MSC  604  sets up first call leg (which is circuit based in the present example) with the other party  308 , and sets up a second call leg with the MS  306  via the circuit domain connection  320 . 
     In step  714 , the I/S-CSCF  316  sends a ringing indication to the MS  306  via the P-CSCF  314 . When the MSC  604  receives an answer, it reports this to the I/S-CSCF  316  in step  716 . In step  718 , the I/S-CSCF  316  relays this information as a SIP OK message to the P-CSCF  314 . The P-CSCF  314  utilizes the PCF to commit the requested QoS in step  720 , which is a NULL operation because no QoS was requested. The P-CSCF  314  then forwards the SIP OK message to the MS  306  in step  722 . The MS  306  may then use the media resources authorized and committed in the call set-up in step  724 . In step  726 , the MS  306  sends a SIP ACK message to the I/S-CSCF  316  via the P-CSCF  314 . 
     Referring now to  FIG. 20 , in another embodiment, an architecture  800  illustrates yet another possible architecture within which a call session representing the method  100  of  FIG. 1  may be executed. As with previous examples, the architecture  800  may be used to provide IP based, real-time, conversational, multimedia services. For example, the services may be provided using IMS prior to the introduction of IP QoS mechanisms generally needed for the provision of such services. Connections within the architecture  800  may be circuit switched (CS) or packet switched (PS). 
     The architecture  800  includes an MS  802  and an MS  804 . Disposed between the two mobile stations is a hybrid service gateway (HSG)  806 . A network  808  (which may include network entities such as an SGSN, a GGSN, and/or other entities as described in  FIG. 2 ) and an MSC  810  are positioned between the MS  802  and the HSG  806 . An S-CSCF or SIP proxy/AS  812  is positioned between the MS  804  and the HSG  806 , although not all connections between the MS  804  and the HSG  806  may go through the SIP AS  812 . 
     The HSG  806  includes a plurality of different functions, which may be represented as actual independent physical components or may be represented merely as a functional module of the HSG  806 . For purposes of clarity, these functions will be referred to as independent components that are combined within the HSG  806 . In the present example, the HSG  806  acts as a P-CSCF  816  (e.g., it provides no services and controls the media in the local network). The HSG  806  also encompasses a SIP Primary Rate Interface (PRI) gateway  818 , a Real-Time Protocol (RTP) portal  820 , an IMS media server  822 , and various other media servers  824 . 
     The PRI gateway  818  may provide access to and from a network (such as an IP network) by acting as a signaling and media gateway between a VoIP network and a circuit based network, using the ISDN Primary Rate Interface. To provide access, the PRI gateway  818  generally converts packet-based voice streams to circuit-based voice streams and vice versa. The RTP portal  820  may enable elements in a private SIP network to securely communicate with elements in a public network in both directions. The RTP portal  820  may also serve as an anchor point for the RTP media stream. This provides additional flexibility by, for example, enabling the architecture  800  to work with voice broadcast types of services. 
     In operation, as will be described in greater detail with reference to  FIGS. 21 and 22 , services may be provided at the S-CSCF (or the SIP proxy/AS)  812 . Because of this, the MS  804  may be unaware that a circuit switched leg is being used (e.g., SIP services, such as call forwarding and web based provisioning, are entirely re-used from IMS). Furthermore, the PRI gateway  818  may not “call” the MS  804 . Instead, SIP signaling may be used to provide the MS  804  with a port number at the RTP portal  820 , which provides a “true” VoIP SIP session set up. In addition, while the architecture  800  may enable both legs to be circuit switched, it may be desirable to add an additional PRI gateway. Access methods may be mixed by registering with a different P-CSCF (e.g., using R6 VoIP method, WLAN, LAN, etc.). 
     With additional reference to  FIG. 21 , a call flow  900  illustrates a sequence of messages that may be used to provide a communication session initiated by the MS  802  within the architecture  800  of  FIG. 20 , where one leg is circuit based and one leg is packet based (e.g., VoIP). As shown in  FIG. 21 , the call flow  900  includes the MS  802 , the SIP AS  812 , the HSG PRI Gateway  818 , the HSG RTP Portal  820 , and the MS  804 . 
     The call flow  900  begins in step  902  when the MS  802  sends a SIP INVITE message to the P-CSCF  816 . It is understood that SIP messaging may be transferred via the SIP AS  812 . The SIP INVITE message contains a SDP packet stipulating a null codec, so that no voice packets are sent over a packet switched network. In step  904 , the P-CSCF  816  and RTP Portal  820  perform a handshake to reserve call resources. The P-CSCF  816  then sends a SIP INVITE message containing a SDP packet identifying an RTP portal port number to the MS  804  in step  906 . In step  908 , the MS  804  responds by sending a SIP OK message containing a SDP packet identifying the MS  804  to the P-CSCF  816 . The P-CSCF  816  forwards the SIP OK message containing a SDP packet with a dummy domain name to the MS  802  in step  910 . A VoIP bearer leg  912  is established between the MS  804  and the RTP portal  820 . In the present example, the VoIP bearer leg  912  is established using a standard VoIP call set-up. 
     The MS  802  may use native device call setup procedures to initiate a circuit switched call in step  914  through the PRI gateway  818 . The PRI gateway  818  sends a SIP INVITE message containing a SDP packet identifying the PRI gateway  818  to the P-CSCF  816  in step  916 . In step  918 , the P-CSCF  816  sends a SIP OK message identifying the RTP portal  820  to the PRI gateway  818 . A VoIP bearer leg  920  is then established between the RTP portal  820  and the PRI gateway  818 . As before, the VoIP bearer leg  920  may be established using a standard VoIP call set-up. The PRI gateway  818  sends a call set-up ACK message to the MS  802  in step  922 . A circuit switched bearer leg  922  may then be established between the MS  802  and the PRI gateway  818 . In step  924 , the MS  802  sends a SIP ACK message to the P-CSCF  816 , which forwards the message to the MS  804 . 
     Referring now to  FIG. 22  and with continued reference to  FIG. 20 , a call flow  1000  illustrates a sequence of messages that may be used to provide a communication session initiated by the network to the MS  802  within the architecture  800  of  FIG. 20 , where one leg is circuit based and one leg is packet based (e.g., VoIP). As shown in  FIG. 22 , the call flow  1000  includes the MS  802 , the SIP AS  812 , the HSG PRI Gateway  818 , the HSG RTP Portal  820 , and the MS  804 . 
     The call flow  1000  begins in step  1002  when the MS  802  sends a SIP INVITE message to the P-CSCF  816 . It is understood that the SIP messaging may be transferred via the SIP AS  812 . The SIP INVITE message contains a SDP packet stipulating a null codec, so that no voice packets are sent over a packet switched network. In step  1004 , the P-CSCF  816  and RTP Portal  820  perform a handshake to reserve call resources. The P-CSCF  816  sends a SIP INVITE message containing a SDP packet identifying the MS  802 &#39;s domain name and the RTP Portal to the PRI gateway  818  in step  1006 . In step  1008 , the PRI gateway  818  sends a circuit switched call set-up message to the MS  802 , which then returns a call set-up ACK message to the PRI gateway  818  in step  1010 . The PRI gateway  818  sends a SIP OK message containing a SDP packet identifying the PRI gateway  818  to the P-CSCF  816  in step  1012 . A VoIP circuit bearer  1014  is then established between the RTP portal  820  and the PRI gateway  818 . In the present example, the VoIP bearer leg may be established using a standard VoIP call set-up. A circuit switched bearer leg  1016  is established between the PRI gateway  818  and the MS  802 . In the present example, it may be desirable for the MS  802  to suppress incoming ringing. 
     In step  1018 , the P-CSCF  816  sends a SIP INVITE message containing a SDP packet identifying the RTP portal  820  to the MS  804 . In response, the MS  804  sends a SIP OK message containing a SDP packet identifying the MS  804  to the P-CSCF  816 . The P-CSCF  816  inserts a dummy domain name into the packet and forwards the SIP OK message to the MS  802  in step  1022 . A VoIP bearer path  1024  is established between the MS  804  and the RTP portal  820 . The MS  802  sends a SIP ACK message to the P-CSCF  816  in step  1024 , which forwards the SIP ACK message to the MS  804 . 
     The above embodiments and the Appendix describe the use of code, within SIP messaging flows, which explicitly signals SIP Circuit Bearer. In an alternative technique, one or more of the SDP packets may include a special codec value to indicate that a circuit bearer is being used, or is required. 
     In a further development of the invention, there can be some intelligence in choosing the location of the gateway (e.g. MGW  318  in  FIG. 3 ) which bridges the Circuit Bearer and IP bearer legs of the call. Where the circuit and packet switched networks are owned by the same operator the location of the gateway is not so important. However, they can be independently owned as, for example, where the packet switched network is a private network belonging to an enterprise. Users can still access such networks from wireless local or wide area networks using a Virtual Private Network (VPN). In this case, the owner of the private network would wish the gateway to be located as close as possible to the physical location of the user, so that the Circuit Switched leg of the session (which they have to pay another operator for) is as short as possible. This can be achieved as follows. Firstly, the CBCF obtains information from the terminal about it&#39;s physical location (using the SIP signalling, if the CBCF is in the network). The CBCF uses a database to look up the PSTN number of the gateway that is nearest to the client. Finally, the CBCF instructs the CBOF to originate a call to a CBTF E. 164  number which will route through the chosen gateway to the CBTF. The above works wherever the CBCF, CBOF and CBTF are located. If the CBCF is at the terminal then the CBCF requires a protocol to perform the database lookup. If the CBCF and CBOF are not co-located (e.g. CBCF in network, CBOF at terminal) then the CBTF number is communicated between them, such as by using the modified SDP messages that have been described. In the case of a multi-national enterprise network, with gateways in multiple countries, this allows the enterprise&#39;s employees to bypass mobile roaming charges, which is a significant advantage. 
     It is understood that the present disclosure provides for setting up a call using SIP or a similar protocol, and then using circuit switched network elements to provide part of the bearer path. This enables multimedia services, such as those defined by IMS, to be delivered in such a way that a client looking into the network has no knowledge that a circuit switched bearer forms part of the bearer path. This enables such services to be delivered via a network that has not implemented all the specified QoS mechanisms, and aligns the provision of the services with defined standards, such as the 3GPP IMS architecture. Furthermore, this approach permits a simple migration to a full VoIP bearer path as QoS mechanisms are introduced into a network. In addition, this enables the provision of a service control, billing, and authentication architecture that is identical to that proposed in 3GPP IMS or other standards. In the present disclosure, the bearer may be any type of real time, conversational service (e.g. voice, video, gaming sessions, application sharing, etc.). 
     It is preferred that a terminal is fully adapted to support the SIP Circuit Bearer method just described as this allows the service to be provided in a transparent manner to the network and a user of the terminal. However, it is possible to provide an intermediate level of support for SIP Circuit Bearer by a less capable terminal. If the terminal cannot recognise and silently accept an incoming SIP Circuit Bearer call and transparently associate it with the SIP session, the consequences are as follows. For outgoing calls, after invoking the SIP call, the user will receive an incoming call which they must then answer manually. Locally generated ring-tone (generated by the SIP client) may be overridden by the incoming CS domain call, depending on terminal implementation. As a result, for IMS to IMS calls, in-band ring tone must be provided on the CS call. For IMS to PSTN calls, the PSTN should provide the in-band ring tone. For incoming calls, the user will see an incoming SIP call and an incoming CS domain call. Neither call will be presented to the user until pre-conditions at the originating party are met. Both calls must be manually answered. 
     While the present disclosure uses 3GPP GPRS and UMTS technologies for purposes of illustration, it is understood that it may be equally applicable to other technologies, including fixed wireline access, wireless local area network (LAN) access such as 802.11, CDMA (Code Division Multiple Access)/1×RTT (Radio Transmission Technology)/1×EV-DO (Evolution Data Only) access, and/or other technologies. 
     Accordingly, while the preceding description shows and describes one or more embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present disclosure. For example, the type of protocols used in the preceding description may vary, and it is understood that substitutions may be made. Similarly, different network configurations may be used for different types of digital devices. Furthermore, terms such as “first leg” and “second leg” are used for purposes of example, and do not necessarily denote a particular sequential or chronological order. In addition, it is understood that messages may be sent to or from different network entities than those shown. For example, although SIP INVITE messages are illustrated as being sent from the MS, a SIP INVITE may be sent to the MS in some embodiments. Therefore, the claims should be interpreted in a broad manner, consistent with the present disclosure. 
     Appendix 
     Circuit Bearer Control Function (CBCF) Logic 
     Whilst the call flows above appear rather complex, the Circuit Bearer Control Function logic can be summarised quite simply. The CBCF operates at the level of SDP offers and answers. The SIP messages to use at any given time are determined by the SIP Call State Machine. It is the CBCF logic which defines the CBCF operation, not a particular call flow. The call flows above, therefore, are just examples. In the following overview of the CBCF logic the following terms are used:
         Local user: the local user to whom Sip Circuit Bearer Control functions are provided.   Remote user: the remove VoIP user with whom the session is taking place   Gateway: the gateway providing the VoIP &lt;&gt; Circuit interworking   Local SDP of X: the SDP that the CBCF sends to X   Remote SDP of X: the SDP that X sends to the CBCF
 
High Level CBCF Operation:
       

     The CBCF maintains a copy of the local and remote SDP of each of the Local user, Remote user and the Gateway. Whenever a new copy of the Remote SDP of one of these three arrives some CBCF logic is applied. This may result in changes in the Local SDP of one or more users. Any such changes trigger the sending of this new Local SDP to that user. 
     Intialisation: 
     The Remote SDP of the Gateway is initialised to indicate a VoIP session with a dummy IP address. 
     The Remote SDP of the Remote User is initialised to indicate a VoIP session with a dummy IP address. 
     New call from Local User: 
     
         
         
           
             1) Set Local SDP of Gateway to VoIP portion of Remote SDP of Remote User
 
New Remote SDP from Local User:
 
             1) Remove SIP Circuit Bearer indications from received SDP 
             2) Add Remote SDP of Gateway 
             3) Set Local SDP of Remote User to the result
 
New Remote SDP from Remote User:
 
             1) Remove VoIP portion of received SDP. 
             2) Set Local SDP of Gateway to this VoIP portion 
             3) Add SIP Circuit Bearer response (based on gateway status) and set Local SDP of Local User to the result
 
New Remote SDP from Gateway:
 
             1) Use received SDP to update VoIP portion of Local SDP of Remote User 
           
         
       
    
     Note: the above is just outline logic, additional procedures for cut-through and for handling provision of in-band ringtone to the originating party over the CS connection (required for non-SCB UEs) may be required. 
     SDP Elements 
     New SDP elements are defined to indicate that the connection is not a VoIP one, but a CS domain connection. It is also necessary to indicate:
         The CS domain establishment direction—we use the a=direction attribute from the comedia draft (http://www.ietf.org/internet-drafts/draft-ietf-mmusic-sdp-comedia-05.txt)   The called and calling party numbers—we define a new network type ‘GSTN’ and address type ‘e164’ for use in the c=line of the SDP   An indication that two media lines (the SCB one and a normal VoIP one) are offered as alternatives.       

     EXAMPLE 1 
     This example represents an endpoint which is prepared to receive a CS domain call for the audio part of a SIP session.
     v=0   o=   s=   c=GSTN e164 447710875525   t=   m=audio 9999 gstn   a=direction: passive   

     EXAMPLE 2 
     This example represents an endpoint which is prepared to establish a CS domain call for the audio part of a SIP session and will use IP as normal for an Instant Messaging session according to http://www.ietf.org/internet-drafts/draft-ietf-simple-message-sessions-01.txt.
     v=0   o=   s=   c=IP IP6 aaaa:bbbb:cccc:dddd   t=   m=audio 9999 gstn   c=GSTN e 164 447710875525   a=direction: active   m=message 9999 msrp/tcp message/cpim text/plain text/html

Metadata:
Filing Date: 20120604
Publication Date: 20140506
Grant Date: 20140506
Priority Date: 20030730
Inventors: WATSON MARK
BODIN RICHARD
LEEDER MICHAEL
Assignee: APPLE INC
CPC Classifications: [{"code": "H04L12/66", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L12/66", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 44121937