Abstract:
A video telecommunication connection is established via a landline telecommunications network and an IP multimedia subsystem telecommunications network using a media gateway control function of at least one Internet multimedia-media gateway device in the IP multimedia subsystem telecommunications network. An H.223 multiplex level usable for a telecommunication connection is exposed to an H.223 protocol negotiation, in carrying out an H.245 protocol negotiations for setting up the H.223 protocol negotiations connection for a telecommunication connection. A media gateway control function device is informed when the H.223 logic channels are open by H.245 signaling, thereby enabling the Internet multimedia-media gateway device of the IP multimedia subsystem telecommunications network to carry out at least part of the setup.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and hereby claims priority to German Application No. 10 2005 050 588.0 filed on 21 Oct. 2005, the contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Described below are methods and devices for setting up a telecommunications connection. 
     Alongside the so-called ‘Circuit-Switched (CS) domain’ of a mobile communications network based on the 3rd Generation Partnership Project (3GPP), the so-called ‘IP Multimedia Subsystem’ (IMS) can also be utilized for voice and video telephony, and a so-called ‘Interworking’ of the relevant services, i.e. connecting of the services by a suitable conversion of the signaling used and the transport format used for the data, between the IMS and the CS domain, is required. The IMS is also used, alongside the 3GPP Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) access networks, for other access networks, for example the Wireless Local Area Network (WLAN) and Digital Subscriber Line (DSL). Precisely in these scenarios, it can initially be anticipated that voice and video telephony are effected by way of the IMS. Video telephony can also be utilized in a public telephone network, i.e. a so-called ‘Public Switched Telephone Network (PSTN)’, with the same inband video telephony specific protocols as in the 3GPP CS domain being utilized for transport and signaling in this instance as a rule. An interworking with respect to the IMS is also required from the PSTN. 
     Up to now, just an interworking between the IMS and the CS domain or the PSTN, and only for voice telephony, has been described in the standard. The method described below relates to the corresponding interworking of video telephony. A demand for this can be foreseen since video telephony is gaining in importance both in the 3GPP CS domain and also in the IMS, in this instance particularly for access networks such as WLAN or DSL, or newly arising network access options (e.g. Worldwide Interoperability for Microwave Access (WiMAX)). 
     The interworking between the IMS and a CS network, that is to say a PSTN or a 3GPP CS domain, is specified, as from 3GPP Release 6, only for pure voice telephony in 3GPP TS 29.163. 
     As defined by TS 29.163, the interworking of the so-called ‘call control’ signaling is effected in the so-called ‘Media Gateway Control Function’ (MGCF). The interworking of the payload connection, that is to say the passing on and repacking and also where necessary the transcoding of the payload data, is effected in the so-called ‘Internet Multimedia-Media Gateway’ (IM-MGW). The MGCF controls the IM-MGW by using the H.248 protocol standardized by the ITU-T, by way of the so-called ‘Mn’ interface, as is described further in 3GPP TS 29.332. 
     In the CS network, Bearer Independent Call Control (BICC) or ISDN User Part (ISUP) is used for call control signaling. In this case, if the call control signaling is routed separately from the transport connections, this method is also designated as ‘out-of-band’ signaling. As a further consequence, the possibility also exists of exchanging signaling messages within the transport connection, which messages are designated as ‘in-band’ signaling operations. In the case of ISUP, Time Division Multiplex (TDM) is utilized as the transport in the CS network and in the case of BICC, packet transport by using Internet Protocol (IP) or Asynchronous Transfer Mode (ATM). The negotiation as to whether pure voice telephony or video telephony is used can be effected for ISUP during the call control signaling for setting up the call, by using the so-called ISUP ‘UDI Fallback’ procedure. For BICC, this negotiation can be effected by using the ‘Service Change and UDI Fallback’ (SCUDIF) standardized in 3GPP TS 23.172, which enables a switch between voice telephony and video telephony even during the call. Both ‘UDI Fallback’ and SCUDIF utilize out-of-band signaling. Alongside this, it is possible for ISUP and also BICC not to use the aforesaid procedures, and to attempt a call setup only for video telephony, and in the case that video telephony is not supported, to terminate the call setup. In contrast to the optional negotiation between voice and video, the negotiation of the voice and video codecs used for video telephony is effected in-band, after video telephony has already been selected previously and a corresponding transport connection (‘bearer’) has been set up. For video telephony, a so-called ‘BS30’ data connection with 64 kbyte/s bandwidth is used in the CS network. Within this data connection, the H.324 protocol suite standardized by the ITU-T is used, the variant H.324M adapted for mobile communication being selected in the 3GPP CS domain. In this respect, following the setup of the data connection, the configuration of the multimedia connection is negotiated inband by way of the H.245 protocol standardized by the ITU-T, particularly the video codec and voice codec used and the details of the respective codec configuration. Voice and video are multiplexed within the same transport connection by using the H.223 protocol. For the 3GPP CS domain, TS 26.110 describes the use of the H.245 protocol suite in further detail, the so-called ‘H.324M’ configuration in particular being selected. 
     The main sequences during the setting up of a 3G-324M connection (‘session’) are as follows:
     1. Following the beginning of the ISUP or BICC call setup signaling, the reservation of the necessary resources that are needed for the desired bearer is effected, and as a further consequence, the setup of the transport connection.   2. Beginning of the in-band negotiation. First, negotiation as to which H.223 multiplexer level is to be used for this transport connection.   3. Detection of the leading terminal which opens the multistream connection by using H.245 negotiation, if necessary. This function is only necessary if a conflict arises in the context of the opening of a bidirectional logical channel. This function is designated as ‘Master or Slave Determination’ (MSD).   4. By using so-called ‘Terminal Capability Set’ H.245 messages, the capabilities of the terminal sending the message are transmitted. Such messages are sent independently by both terminals. These described capabilities encompass the following information: audio and video codecs and their specific properties or their forms respectively, functional scope of the multiplexer, which adaptation layer is supported in detail (e.g. ‘simple’ or ‘nested’ multiplexing), and its mobile specific enhancements.   5. Setup of ‘logical’ channels for each media stream by using H.245 signaling. As from this point in time, either with or without MSD, the terminal or the IM-MGW is ready to open logical channels in order to enable the exchange of voice and/or video payload data. During the creation of a bidirectional logical channel, the channel number and the definitive media capabilities to be used are defined.   6. Definition of the multiplexing properties by using H.245.   7. Beginning of the transfer of video, audio/voice or data.   

     In the IMS, the negotiation for video telephony is effected out-of-band with the aid of the so-called ‘Session Description Protocol’ (SDP), IETF RFC 2327, which is transported by using the so-called ‘Session Initiation Protocol’ (SIP), IETF RFC 3261. In this respect, the negotiation as to whether voice telephony or video telephony is used is connected with the negotiation of the codecs used, and is effected before or during the setup of the bearers. The so-called SDP ‘offer-answer’ mechanism as defined by RFC 3264 is used. In this respect, the offering party sends a list of supported codecs in the SDP ‘offer’ message. Following receipt of this message, the answering party sends an SDP ‘answer’ message that contains those codecs from the list that it also supports and wishes to use. The answering party must not specify any codecs that were not contained in the list in the SDP ‘offer’. In contrast to the CS domain, two separate transport connections (bearers) are utilized for voice and video, which use the so-called ‘Real Time Transport Protocol’ (RTP), IETF RFC 3550 in each case. For the 3GPP IMS by way of the General Packet Radio Service (GPRS) access network, 3GPP TS 26.235 describes the codecs to be used for video telephony. 
     The protocols and codecs used for video telephony on the CS domain side and on the IMS side are summarized once again in the following: 
     CS Network (in Particular 3GPP CS Domain): 
                                     Call control:   BICC or ISUP           Negotiation between pure voice telephony and           video telephony can be effected by using           ‘UDI Fallback’ for ISUP and by using           ‘SCUDIF’ for BICC.       Multimedia   H.324M (H.324 Annex C):       Protocol suite:       Codec negotiation:   H.245 inband negotiation by way of the set-up           CS bearer at 64 kbit/s       Video codec:   Support for H.263 prescribed           H.261 optional           MP4V-ES (simple video profile level 0) optional       Voice codec:   Support for NB-AMR prescribed           WB-AMR optional           G.723.1 recommended       Transport:   Multiplexing of voice and video on a bearer as           defined by H.223           Annex A-B                    
IMS (Codecs for GPRS Access Network):
 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Call control: 
                 SIP 
               
               
                   
                 Contains both negotiation between pure voice 
               
               
                   
                 telephony and video telephony, and also codec 
               
               
                   
                 negotiation. 
               
               
                 Codec negotiation: 
                 Before setup of the bearer out-of-band by 
               
               
                   
                 using SDP, which is transported in SIP 
               
               
                 Video codec: 
                 Support for H.263 prescribed 
               
               
                   
                 H.264 optional 
               
               
                   
                 MP4V-ES (simple video profile level 0) optional 
               
               
                 Voice codec: 
                 Support for NB-AMR and WB-AMR prescribed 
               
               
                 Transport: 
                 Two separate RTP bearers for voice and video with 
               
               
                   
                 the use of different so-called RTP ‘payload’ 
               
               
                   
                 formats - 
               
               
                   
                 Voice: Nb-AMR + WB-AMR: IETF RFC 3267 
               
               
                   
                 Video: H.263: IETF RFC 2429 
               
               
                   
                 H.264 (AVC): IETF RFC 3984 
               
               
                   
                 MPEG-4: IETF RFC 3016 
               
               
                   
                 Synchronization of parallel RTP media streams is 
               
               
                   
                 effected by using so-called RTP ‘timestamps’, 
               
               
                   
                 which are negotiated by the Real Time Control 
               
               
                   
                 Protocol (RTCP, see IETF RFC 3550). 
               
               
                   
               
             
          
         
       
     
     Alongside or in place of the codecs specified in this instance, however, other codecs can also be supported by the terminals, in particular if the CS terminals are located in the PSTN or if the IMS terminals are not using GPRS as the access network. 
     It is desirable to use the same video codec and if possible also the same voice codec on the CS side and in the IMS, in order to avoid a transcoding operation. A transcoding of the video codec in particular, but to a lesser extent also of the voice codec, would require considerable computing power and resources in the IM-MGW. Additionally, the transmission would become delayed and the quality of the image or the voice respectively would become worse. If the required bandwidth for the codecs is different on the CS domain side and the IMS side, additional bandwidth would be used on one side without the image or voice quality improving as a result. 
     For this purpose, it is a requirement that the MGCF and the IM-MGW exchange suitable information referring to the negotiation of the voice and video codecs by using H.245 and SIP/SDP, and referring to the setup of the transport connection by using H.223. 
     A method for exchanging suitable information referring to the negotiation of the voice and video codecs by using H.245 and the setup of the transport connection by using H.223 between the MGCF and the IM-MGW forms the subject matter of the method. As a result, a transcoding for video telephony is largely avoided. The MGCF and the IM-MGW connect a CS network, that is to say a PSTN or a 3GPP CS domain, and also an IP network, which uses SIP and SDP for negotiating the codecs, that is to say for example the IMS. 
     SUMMARY 
     An aspect is to support a setup of a suitable telecommunications connection by way of a circuit-switched network and an IMS network as efficiently as possible and to avoid a video telephony transcoding operation as far as possible. 
     Accordingly, the so-called ‘H.245 client’, that is to say the functional unit that terminates the H.245 protocol, is located in the IM-MGW. The H.245 client exchanges information referring to the selection of the codecs and the sequence of the call setup with the functional components in the MGCF in charge of the so-called ‘call control’, preferably with the functional component(s) that are in charge of the handling of the SIP and the SDP on the IMS side. For this purpose, the protocol of the ‘Mn’ interface is suitably enhanced. 
     For example, if the MGCF detects or presumes from the call control signaling during the call setup that video telephony as defined by the H.324 is required on the CS side, the MGCF instructs the IM-MGW:
         to implement the H.223 negotiation of the so-called ‘multiplex level’ autonomously, and   to implement the H.245 negotiation necessary for the setup of H.324 video telephony, and   to inform the MGCF as soon as logical H.223 channels are opened by using H.245, and the IM-MGW carries out the corresponding instructions.       

     Preferably, the MGCF configures the IM-MGW correspondingly by using so-called H.248 ‘Add’ or ‘Modify’ messages, for example if it is establishing the so-called ‘termination’ in charge of the handling of the multiplexed H.223 protocol. Preferably, the MGCF inserts one or more so-called H.248 ‘signals’, which are to be newly standardized, in these messages, which state that the H.223 and/or H.245 negotiation should be implemented. 
     In order to demand a notification about the setup of logical H.223 channels, the MGCF preferably utilizes a so-called H.248 ‘event’, which is to be newly standardized for the purpose, which the MGCF preferably inserts in the same H.248 message. If in the following a logical H.223 channel is opened by using H.245 signaling, the IM-MGW utilizes a so-called H.248 ‘Notify’ message according to the method, in which it specifies the newly defined ‘event’. 
     It is advantageous if the IM-MGW specifies the selected codec and also the so-called ‘Logical Channel Number’ in the notification about the opening of a logical H.223 channel, preferably as parameters of the event used in the notification. The MGCF utilizes this information according to the method to configure the IM MGW in such a way that it passes the media stream through between the CS network side and the IMS side. The MGCF specifies for both sides which codecs have been chosen. On the CS network side, the specification ‘Logical Channel Number’ is required as defined by the existing H.248 standard. If the same codec has been chosen in the same configuration on both sides, the IM-MGW does not need to use a transcoder. 
     It is advantageous if the IM-MGW also notifies the MGCF if the H.223 negotiation of the multiplex level has been terminated or if this negotiation has failed respectively. The MGCF can establish by the absence of the notification or with the aid of an error message that the CS-side transport connection is not or not yet being used for video telephony, and respond to this in the call control signaling, for example by reconfiguring the call to another service such as voice telephony or terminating the connection. 
     In order to demand a notification about the result of the H.223 negotiation of the multiplex level, the MGCF preferably utilizes a so-called H.248 ‘event’, which is to be newly standardized for the purpose, which the MGCF preferably likewise inserts in the H.248 message for beginning the H.223 negotiation. If in the following the negotiation has been terminated, the IM-MGW utilizes a so-called H.248 ‘Notify’ message according to the method, in which it specifies the newly defined ‘event’. 
     In the case of a call setup from the CS network side in the direction of the IMS, it can happen that the connection setup is forwarded by the IMS to another MGCF. In this case, it is advantageous if the MGCF configures the IM-MGW in such a way that it passes on the BS30 data service transparently, for example by making use of the so-called ‘Clearmode’ codec, IETF RFC 4040. The MGCF negotiates the transparent transport of the data service by using the SIP/SDP signaling exchanged with the other MGCF. In one embodiment, the MGCF configures the IM-MGW only for the BS30 service initially, and does not yet establish the data connection. As soon as the MGCF receives signaling referring to the selected codec from the IMS side, the MGCF can detect whether video telephony is involved and in this case configures the IM-MGW in such a way that it begins the inband H.223 negotiation. If, on the other hand, a transparent transport is selected, no reconfiguration of the IM-MGW is required. 
     The IM-MGW preferably implements the H.245 procedures for the so-called ‘Master Slave Determination’ autonomously. 
     In a simple embodiment, the IM-MGW also sends and receives the so-called H.245 ‘Terminal Capability Set’ messages autonomously. These contain information about, among other things, audio and video codecs supported and their specific properties and also about the functional scope of the multiplexer (e.g. which adaptation layer is supported, the multiplexing nesting depth supported, that is to say so-called ‘simple’ or ‘nested’ multiplexing, and information about mobile specific enhancements). The IM-MGW specifies preconfigured values in the Terminal Capability Set message sent, which values reflect its own capabilities. 
     In a simpler embodiment, the transfer of codecs between the MGCF and the IM-MGW is omitted, and the MGCF and the IM-MGW are configured in such a way that they select the same voice and video codecs in the SIP/SDP out-of-band negotiation on the IMS side or in the H.245 inband negotiation on the CS network side respectively. This embodiment can be sufficient since, as defined by 3GPP TS 26.235 and TS 26.110, the same voice and video codecs have to be supported in the CS domain and the IMS. 
     In an alternative embodiment, however, the IM-MGW exchanges information referring to the codecs supported with the MGCF. 
     In an advantageous embodiment, the MGCF advises the IM-MGW as to which codecs the IM-MGW should specify in the Terminal Capability Set message sent, and in each case also advises the details of the codec configuration. Preferably, the MGCF selects codecs for this which it has received from the SIP/SDP signaling on the IMS side. In one embodiment, the MGCF also takes account, during the selection, of which codecs are supported at the IM-MGW. The MGCF either possesses configured knowledge about these capabilities or it retrieves these capabilities from the IM-MGW by using a so-called H.248 ‘AuditCapabilities’ message. In an alternative embodiment, the IM-MGW removes those among the codecs received from the MGCF that it does not itself support before it sends the codecs in a Terminal Capability Set message. 
     The MGCF preferably specifies the codecs in an ‘Add’ or ‘Modify’ message as parameters of the signal that instructs the IM-MGW to implement the H.245 negotiation. 
     The MGCF can already specify the codecs if it instructs the IM-MGW to start the H.245 negotiation, or in a separate H.248 message at a later point in time, for example if it receives corresponding information in an SIP message. If the MGCF wishes to only specify the codecs at a later point in time, it preferably instructs the MGCF to wait for this message. For this purpose, the MGCF preferably utilizes a special parameter of the signal that instructs the IM-MGW to implement the H.245 negotiation. If the MGCF has not sent any codecs and the MGCF has not made a request to wait, the IM-MGW sends a Terminal Capability Set message containing preconfigured values. 
     In an advantageous embodiment, the IM-MGW also advises the MGCF as to which codecs it has received in a Terminal Capability Set message, and in each case also specifies the detailed configuration of the codec in this respect. Preferably, however, the IM-MGW does not advise the MGCF of received codecs that it does not itself support. The MGCF preferably passes this information on to the terminal on the IMS side by using SIP/SDP. 
     In order to demand a notification about the codecs received, the MGCF preferably utilizes a so-called H.248 ‘event’, which is to be newly standardized for the purpose, which the MGCF preferably inserts in the same H.248 message in which it instructs the IM-MGW to start the H.245 negotiation. If in the following the IM-MGW receives a Terminal Capability Set message, the IM-MGW utilizes a so-called H.248 ‘Notify’ message according to the method, in which it specifies the newly defined ‘event’, and specifies the codecs as parameters of this event. 
     In one embodiment, the IM-MGW, after the exchange of the H.245 Terminal Capability Set messages and the completion of the H.245 Master Slave Determination, opens logical H.223 channels autonomously assuming that it is in charge of such according to the outcome of the Master Slave Determination. In this respect, the IM-MGW chooses suitable codecs from the set of the codecs transferred in the Terminal Capability Set messages, preferably one voice codec and one video codec in each case, and then opens the channels by using so-called H.245 ‘Open Logical Channel’ messages. As described above, the IM-MGW informs the MGCF about the opening of the channels. 
     In an alternative embodiment, the MGCF instructs the IM-MGW to set up the logical channels for specific codecs, preferably by specifying the codecs as special parameters of a suitable signal, which is to be newly standardized, within an H.248 ‘Modify’ message. If the MGCF wishes to use such a signal, it preferably instructs the MGCF to wait for this message. For this purpose, the MGCF preferably utilizes a special parameter of the signal that instructs the IM-MGW to implement the H.245 negotiation. 
     In an embodiment, the IM-MGW also informs the MGCF if the inband negotiation using H.245 has failed, for example because no matching capabilities were found that would have permitted a transmission of video telephony. If the inband negotiation was unsuccessful, the MGCF can either clear down the call or initiate a fallback to voice, it being possible for the MGCF to use the so-called ‘Service Change and UDI Fallback’ (SCUDIF), 3GPP TS 23.172, on the CS side, and also a SIP/SDP codec renegotiation by using a SIP re-INVITe message on the IMS side. 
     The MGCF can instruct the IM-MGW, either as early as during the first configuring of the so-called ‘terminations’ or after notification about a successful inband negotiation, to establish the connection. As soon as the MGCF instructs the IM-MGW to connect the video call through, the IM-MGW compares the settings for the video codec and the voice codec in order to decide whether transcoding is necessary for the video signal and/or the voice signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages These and other aspects and advantages will become more apparent and more readily appreciated from the following description of an exemplary embodiment, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a block diagram of a network configuration, 
         FIG. 2  is a block diagram of the key components, 
         FIG. 3  is a signal flow diagram illustrating the principle of the interworking with respect to an embodiment, and 
         FIG. 4  is a block diagram illustrating the context for a video call. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  shows a typical network configuration. The network configuration represented is the one that is necessary for a mobile terminal MS 1  connected to the 3GPP CS domain to be able to communicate with a mobile terminal MS 2  connected to the IMS. The CS domain is connected to the IMS with the aid of a Media Gateway Control Function (MGCF) and an IMS Media Gateway (IM-MGW). The MGCF controls the IM-MGW by using the H.248 protocol standardized by the ITU-T, by way of the so-called ‘Mn’ interface. On the CS domain side, so-called ‘Mobile Switching Center’ (MSC) servers, which communicate with each other and with the MGCF by way of BICC signaling, are located in the core network. They control CS MGWs in each case. The CS MGWs are connected among themselves and to the IMS MGW by way of the so-called ‘Nb’ interface. For video telephony, the so-called ‘BS30’ data transport service (‘bearer service’) is used. MS 1  is connected to an MSC server of a CS MGW by using a so-called ‘radio access network’, for example a UTRAN. From the IMS side, the MGCF communicates with the aid of the SIP call control protocol with so-called ‘Call Session Control Functions’ (CSCF), which pass on the signaling by way of the Gateway GPRS Support Node (GGSN) and a radio access network, for example a UTRAN, to the mobile terminal MS 2 . Data is transported from the IMS Media Gateway by way of the ‘Mb’ interface to the GGSN, which likewise passes it on by way of the radio access network to the MS. 
       FIG. 2  shows a block diagram of the key components. Functional key components in the MGCF and the IM-MGW are represented. Accordingly, the so-called ‘H.245 client’, that is to say the functional unit that terminates the H.245 protocol, is located in the IM-MGW and exchanges, by using the H.248 protocol by way of the ‘Mn’ interfaces, information referring to the selection of the codecs and the sequence of the call setup with the functional components in the MGCF in charge of the so-called ‘call control’, preferably with the functional component(s) that are in charge of the handling of the SIP and the SDP on the IMS side. Accordingly, H.245 messages received within the H.223 protocol from the CS side are passed on to the H.245 client by the H.223 multiplexer. Accordingly, the H.245 client also exchanges information referring to the H.223 protocol with the H.223 multiplexer/demultiplexer. In the IM-MGW, so-called ‘media streams’ for audio and video are handled separately. Depending on the audio and video codecs selected on the IMS side and the CS side and the details of their transport formats in these networks, a transparent passing-on of the data, a so-called ‘Re-framing’, i.e. a simple change of the transport format, or a complete conversion of the data between different codecs by using a so-called ‘transcoder’ can optionally be required. The method is aimed at largely avoiding transcoding, particularly for video codecs. 
       FIG. 3  (Interworking Principle) represents, with the aid of the course of the signaling, the principle of the interworking between the H.245 signaling on the CS side and the H.248 signaling between the MGCF and the IM-MGW. 
     In detail, the signaling steps are as follows:
     1. The MGCF decides to establish an H.324 connection for video telephony on the CS side. The MGCF initially configures the physical so-called ‘termination’ on the CS network side. For packet transport, the MGCF generates for this purpose a new termination in a new so-called H.248 ‘context’ with the aid of an H.248 ‘Add’ command. For TDM transport, the MGCF can, in place of this, shift an existing termination, which represents a fixed timeslot on a physical link, into a new context. The termination is allocated a so-called H.248 ‘stream’.   2. The IM-MGW establishes the termination correspondingly and returns the identifiers T 1  for the termination and C 1  for the context.   3. The CS-side transport connection is set up.   4. The MGCF establishes, as defined by existing H.248.1 and H.248.20 standard, a special logical H.248 termination for describing the multiplexing in the same context C 1  and states by the so-called ‘Mux’ parameter that the multiplexing is described in termination T 1  and takes place as defined by the H.223 standard. It describes the logical channel of the H.223 protocol that should be used for the H.245 signaling by using a dedicated stream, which is allocated the so-called ‘Logical Channel Number’ (LCN) with value 0. Accordingly, the MGCF instructs the IM-MGW to start the H.223 negotiation of the so-called ‘multiplexing level’, preferably by using a new so-called H.248 ‘signal’, which is referred to as ‘H223Negotiation’ in this instance.
       Accordingly, the MGCF instructs the IM-MGW to subsequently start the H.245 negotiation, preferably by using a new so-called H.248 ‘signal’, which is referred to as ‘H245Negotiation’ in this instance. In a variant, ‘H223Negotiation’ and ‘H245Negotiation’ are combined into one signal. Accordingly, the MGCF preferably also instructs the IM-MGW to send the MGCF a message as soon as the H.223 negotiation of the so-called ‘multiplexing level’ has been completed, preferably by using a new so-called H.248 ‘event’, which is referred to as ‘H223Establishment’ in this instance. Accordingly, the MGCF also instructs the IM-MGW to send the MGCF a message containing codecs supported on the CS side as soon as the IM-MGW receives an H.245 Terminal Capability Set message. Preferably, the MGCF utilizes for this purpose a new so-called H.248 ‘event’, which is referred to as ‘H245Capabilities’ in this instance. Accordingly, the MGCF also instructs the IM-MGW to send the MGCF a message containing the selected codec and the associated H.245 Logical Channel Number as soon as a logical H.223 channel is opened by using H.245. Preferably, the MGCF utilizes for this purpose a new so-called H.248 ‘event’, which is referred to as ‘H245Channel’ in this instance.   
       5. The IM-MGW establishes the new termination correspondingly and returns the identifier T 2 .   6. The IM-MGW establishes the H.223 connection and in so doing negotiates the multiplexing level, in Example 2.   7. The IM-MGW advises the MGCF that the negotiation of the H.223 multiplexing level has been completed. Preferably, the IM-MGW utilizes for notifying the MGCF a so-called H.248 ‘Notify’ message with the new event ‘H223Establishment’.
       The MGCF can use the information received to establish whether an inband H.245 negotiation is possible at the given point in time, which is advantageous because, for example, so-called ‘early media’ can be passed through or blocked by the call control signaling on the CS side, depending on the networks involved, before the completion of the call setup. If no H.223 signaling is received over a lengthy period of time, the MGCF can also establish the error situation that it has waited for video telephony incorrectly.   
       8. The MGCF confirms the receipt of the ‘Notify’ message.   9. The IM-MGW receives a so-called ‘Terminal Capability Set’ H.245 message, which can be combined with a so-called ‘Master-Slave Determination’ H.245 message. The Terminal Capability Set H.245 message contains, among other things, information with respect to the voice and video codecs supported by the terminal on the CS network side and the details of their configuration.   10. Accordingly, the IM-MGW passes the received information on by way of the codecs. Preferably, the IM-MGW utilizes for this purpose a so-called H.248 ‘Notify’ message containing the new event ‘H245Capabilities’, which contains suitable parameters for specifying the codecs, for example as SDP or encoded in the form of the so-called H.248 ‘SDP equivalents’.
       In an advantageous variant, the IM-MGW deletes codecs that it does not support before it passes the information on to the MGCF.   
       11. The MGCF confirms the receipt of the ‘Notify’ message.   12. The IM-MGW confirms the Terminal Capability Set H.245 message and the Master-Slave Determination H.245 message.   13. The MGCF instructs the IM-MGW to offer specific codecs in the H.245 negotiation, for example because the MGCF has received corresponding information within SIP/SDP messages on the IMS side. It is advantageous if the MGCF also takes account of the codecs supported by the IM-MGW. The MGCF either possesses configured knowledge about these capabilities or it retrieves these capabilities from the IM-MGW by using a so-called H.248 ‘AuditCapabilities’ message. In an advantageous variant, the MGCF does not itself take account of the codecs supported by the IM-MGW, but instead passes all eligible codecs on to the IM-MGW. The IM-MGW then itself deletes codecs that it does not support before it passes the information on in H.245 signaling.
       Preferably, the MGCF utilizes the new so-called H.248 ‘signal’, ‘H245Negotiation’, within an H.248 ‘Modify’ message, and specifies the codecs as suitable parameters of the signal, for example as SDP or encoded in the form of the so-called H.248 ‘SDP equivalents’.   
       14. The IM-MGW sends a Terminal Capability Set H.245 message in which it provides information with respect to capabilities supported on the IM-MGW side, for example referring to H.223 protocol options, which message passes on the information referring to the codecs that was received in message  13  from the MGCF and also takes account of the H.223 multiplexing level defined in Step 6. In the example, the IM-MGW combines this message with a Master-Slave Determination H.245 message.   15. The IM-MGW sends a confirmation of the H.248 ‘Modify’ message.   16. The IM-MGW receives a confirmation of the Terminal Capability Set H.245 message and the Master-Slave Determination H.245 message.   17. The MGCF selects codecs for the video telephony, taking account of information from the SIP/SDP signaling on the IMS side and also the information contained in message  10  referring to the CS-side terminal. Preferably, the MGCF selects codecs which are supported both on the IMS side and the CS network side in order to avoid a transcoding operation.
       Accordingly, the MGCF instructs the IM-MGW to configure logical channels for these codec(s). Preferably, the MGCF utilizes the new so-called H.248 ‘signal’, ‘H245Selection’, within an H.248 ‘Modify’ message, and specifies the codecs as suitable parameters of the signal, for example as SDP or encoded in the form of the so-called H.248 ‘SDP equivalents’.   In a variant, not represented in this instance, the MGCF omits the definitive selection of the codecs and leaves this decision to the IM-MGW. The IM-MGW would then select from the set of the codecs contained in messages  9  and  13  and also the codecs that it supports itself.   
       18. The MGCF sends a so-called ‘Open Logical Channel’ H.245 message for the codecs received in message  17 , in which message it uses the H.223 Logical Channel Number (LCN) corresponding to the selected codec, which was defined previously by using the Terminal Capability Set H.245 messages  9  or  14 .
       In a case not represented in this instance, it can also happen that the IM-MGW receives an ‘Open Logical Channel’ H.245 message from the CS network. In this case, a selection has been made from the capabilities offered in the Terminal Capability Set message  14  in the CS network.   
       19. The IM-MGW confirms the ‘Modify’ message. Should the IM-MGW not be in a position to establish the requested logical channels, for example because it is the H.245 ‘Slave’, it preferably advises this fact in this message.   20. The IM-MGW receives a so-called ‘Open Logical Channel Ack’ H.245 message.   21. Accordingly, the IM-MGW informs the MGCF as soon as a logical channel has been opened by using H.245 and in so doing forwards the H.223 Logical Channel Number used and also the corresponding codec. The IM-MGW ascertains the codec with the aid of the Logical Channel Number signaled in the corresponding ‘Open Logical Channel’ message and the information assigned to it in the Terminal Capability Set H.245 messages  9  or  14 .   

     Preferably, the IM-MGW utilizes for this purpose a so-called H.248 ‘Notify’ message containing the new event ‘H245Channel’, which contains suitable parameters for specifying the codecs and also the Logical Channel Number, for example as SDP or encoded in the form of the so-called H.248 ‘SDP equivalents’.
     22. The MGCF confirms the receipt of the ‘Notify’ message.   23. The MGCF instructs the IM-MGW to establish the logical H.223 channel, which has already been agreed by way of H.245 signaling with the aid of the messages  21  to  26 . For this purpose, the IM-MGW sends an H.248 ‘Modify’ message referring to the multiplexing termination T 2  in which it describes a new ‘stream  3 ’, specifying the LCN and the codec as in message  21 .   24. The IM-MGW sends a confirmation of the H.248 ‘Modify’ message.   25. The MGCF instructs the IM-MGW to establish a termination on the IMS side to which stream  3  should be connected, so that the IM-MGW passes on the data assigned to stream  3  that is received on the IMS side or the CS network side to the other side in each case. For this purpose, the IM-MGW sends an H.248 ‘Add’ message referring to context C 1  in which it specifies that stream  3  is to be transported and which codec is to be used for this. If the same codec is specified in messages  23  and  25 , the IM-MGW detects the fact that no transcoding is required.   26. The IM-MGW confirms the ‘Modify’ message.   27. The steps  17  to  26  are also implemented in order to configure the stream  4  for the bearer for the transport of the voice, and the corresponding voice codec for a termination T 4 .   

     The H.248 context for a video call therefore appears as shown in  FIG. 4 . 
     The system also includes permanent or removable storage, such as magnetic and optical discs, RAM, ROM, etc. on which the process and data structures of the present invention can be stored and distributed. The processes can also be distributed via, for example, downloading over a network such as the Internet. The system can output the results to a display device, printer, readily accessible memory or another computer on a network. 
     A description has been provided with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in  Superguide v. DIRECTV,  358 F3d 870, 69 USPQ2d 1865 (Fed. Cir. 2004).