Abstract:
The invention relates to an arrangement and a methods for controlling communication connections in a packet-oriented communication system (CSY), comprising at least one communication master computer (SRV 1 ,SRV 2 ) for controlling communication connections based on exchanged signaling messages of a respectively preset communication protocol and at least one service provider unit (PRX 1 , PRX 2 ) for supplying communication services for establishing and controlling connections based on bi-directional communication service calls with at least one service user unit (SC 1 ,SC 2 ). Interfaces are provided in the at least one service provider unit (PRX 1 , PRX 2 ) with the at least one service user unit (SC 1 , SC 2 ) and the at least one communication master computer (SRV 1 , SRV 2 ). Means are also provided for characterizing a received signaling message and for extracting parameters contained in the signaling message and for assignment to a communication service as are means for generating at least one signaling message further to a communication service call of the at least one service user unit (SC 1 ,SC 2 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to the German application No. 10345548.5 DE, filed Sep. 30, 2003 and which is incorporated by reference herein in its entirety.  
       FIELD OF INVENTION  
       [0002]     The invention relates to an arrangement and a method for controlling communication connections for packet-oriented real-time communication between two communication users.  
       BACKGROUND OF INVENTION  
       [0003]     As well as transmitting data, current packet-oriented networks are used increasingly for transmitting communication data or real-time communication data. The transmission of communication data via a packet-oriented network is often also referred to as Voice over IP or VoIP. The term VoIP thereby refers to a technology for transmitting useful data, such as voice, and an exchange of signaling data for controlling connections via packet-oriented networks.  
         [0004]     Two standard protocols for the implementation of VoIP, specifically H.323 and SIP, are known for the packet-oriented transmission of communication data between two users and for switching users and providing performance features.  
         [0005]     SIP (Session Initiation Protocol) is a standard proposed by the Internet Engineering Task Force (IETF) for signaling connections via packet-oriented networks and is currently predominantly deployed in the field of internet telephony and for further services such as conference interactions, event notification, message transmission, etc.  
         [0006]     The standard H.323 is an international ITU-T standard (International Telecommunication Union—Telecommunication Standardization Sector) for voice, data and video communication via packet-oriented networks, ensuring mutual interoperability of proprietary products.  
         [0007]     For the implementation of packet-oriented communication technologies in a communication system, the problem frequently occurs that the above-mentioned protocols H.323 und SIP have to coexist, in order to be able to link communication terminals operating according to the respective communication protocol. It is not however possible for a SIP communication terminal to communicate with an H.323-compatible communication terminal without the provision of further technical devices.  
         [0008]     Previous VoIP solutions have been based on so-called standalone software. Standalone software refers to software installed in the communication terminal or—in more general form—in a client to process both signaling data and payload data. One disadvantage of such a solution is that any change to the communication protocol and any introduction of new additional performance features can only be implemented by updating said software at every terminal. For a company this measure requires a great deal of configuration work and therefore a significant cost expenditure.  
         [0009]     A further problem is the complexity of said software. As new performance and service features are introduced, said software becomes more and more complex and extensive. The software implemented in the communication terminal—client software—not only has to identify newly added performance features, it must also be able to implement methods to execute said performance features.  
         [0010]     With extensive standalone software, integration into applications not provided for communication purposes is problematic. To develop said applications, already existing VoIP functions generally have to be adapted or developed from new. Furthermore the integration of additional performance features from third party manufacturers is almost excluded.  
         [0011]     Some VoIP applications provide programming interfaces for integration into other applications and these are known in specialist circles as Application Program Interfaces, API. However a further compilation process is generally required to integrate said programming interfaces into other applications.  
         [0012]     The use of so-called gateways is known as a means to ensure the interoperability of the VoIP communication protocols H.323 and SIP. A gateway thereby converts signaling data based on the SIP protocol to a format suitable for H.323 and vice-versa. It is often the case here however that protocol elements of a respective communication protocol are incompatible with protocol elements of the other communication protocol. It is also often the case that one of the two communication protocols supports services, applications, processing methods, etc. which are not supported in the other communication protocol.  
         [0013]     To avoid the problems associated with updating stand-alone software as discussed above, some service providers offer VoIP solutions as so-called web applications. Obtaining a so-called applet via a packet-oriented data network and integrating it in a browser environment in the form of a so-called plug-in allows the execution of a VoIP functionality via a web browser. Such an above-mentioned execution improves the above-mentioned update problem but offers no opportunity for seamless integration into any applications.  
       SUMMARY OF INVENTION  
       [0014]     The object of the invention is to provide a communication environment, the deployment of which avoids the problems of the prior art.  
         [0015]     The object is achieved by the claims.  
         [0016]     According to the invention at least one communication master computer is provided in a packet-oriented communication system to control communication connections based on exchanged signaling messages—for example SIP or H.323—of a respectively preset communication protocol. At least one service provider unit is also provided to supply communication services for establishing and controlling connections based on bi-directional communication service calls with at least one service user unit. These communication services are provided particularly as web services. The service provider unit is configured with interfaces with the at least one service user unit and with the at least one communication master computer and includes means for characterizing a received signaling message and for extracting parameters contained in the signaling message and for assigning to a communication service. The service provider unit also includes means for generating at least one signaling message further to a communication service call of the at least one service user unit.  
         [0017]     One significant advantage of the inventive method is that the communication services for establishing and controlling connections are moved from a communication terminal—such as a so-called SIP phone or a communication terminal operating according to the standard H.323.—to a central service provider unit.  
         [0018]     The complex and time-consuming update measures that previously had to be taken in the service user units or clients are therefore not necessary with the inventive means.  
         [0019]     The inventive communication services advantageously take over all the signaling tasks of the client, from the point of view of which the inventive service provider unit acts as proxy. The role of proxy means that all communication with the client takes place in both directions between said client and the proxy.  
         [0020]     The inventive arrangement proves to be particularly advantageous even with the provision of additional performance features that have to be supported in the communication system. This frequently occurring update measure also only affects the service provider units, while only new or modified abstract communication service calls occur for the service user units.  
         [0021]     One particular advantage of the preferably 3-tier architecture with a level arranged between the communication master computers and the service user units acting as communication terminals—implemented in the service provider unit—is the loose coupling and simple integration of communication services—in some instances in the form of so-called web services—into other applications, whereby said integration is advantageously both independent of the programming language used for the application and also independent of the operating system provided for execution of the service user units or the application.  
         [0022]     The communication services are advantageously independent of the communication protocols existing or co-existing in the communication system and are therefore also suitable for deployment in communication systems with heterogeneous communication protocols.  
         [0023]     Advantageous developments of the invention are specified in the dependent claims.  
         [0024]     An exemplary embodiment with further advantages and configurations of the invention is described in more detail below with reference to the drawing, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1 : shows a block diagram to illustrate schematically the logical levels within an inventive communication system;  
         [0026]      FIG. 2 : shows a block diagram to illustrate schematically the signaling data and useful data exchanged within the communication system and also communication service calls;  
         [0027]      FIG. 3 : shows a chronological flow diagram to illustrate schematically an exchange of signaling messages and control messages to establish a connection between a service user unit and a communication unit;  
         [0028]      FIG. 4 : shows a chronological flow diagram to illustrate schematically an exchange of signaling messages and control messages to establish a connection between two service user units;  
         [0029]      FIG. 5 : shows a chronological flow diagram to illustrate schematically an exchange of signaling messages and control messages to establish a connection between a communication unit and a service user unit; 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0030]      FIG. 1  shows a communication system CSY divided into three logical levels LYR 1 , LYR 2 , LYR 3 . The communication system CSY is part of a packet-oriented network (not shown). This means that each of the functional components described in more detail below has the capability of a packet-oriented data exchange with the packet-oriented network.  
         [0031]     A first master computer SRV 1  assigned to a first logical level LYR 1  has—in addition to further functional units (not shown)—a first processing unit BL 1 . 1  and a first interface IF 1 . 1 . A second master computer SRV 2  also assigned to the first logical level LYR 1  correspondingly has a second processing unit BL 1 . 2  and a second interface IF 1 . 2 .  
         [0032]     A proxy master computer unit assigned to a second logical level LYR 2  and hereafter also referred to as the proxy server PRX, is connected to the first interface IF 1 . 1  of the first master computer SRV 1  and the second interface IF 1 . 2  of the second master computer SRV 2  via the packet-oriented network (not shown). In the drawing this “connection” via a packet-oriented network is shown with a solid line. Such a “connection” thereby serves only to illustrate a communication relation via a—by its nature connectionless per se—packet-oriented network.  
         [0033]     The proxy server PRX has—in addition to further functional components (not shown)—an integration unit IL, a processing unit BL 2  and an interface IF 2 .  
         [0034]     The proxy server PRX is connected via its interface IF 2  to a first and a second communication terminal SC 1 , SC 2 . The first communication terminal SC 1  and the second communication terminal SC 2  are assigned to a third logical level LYR 3 .  
         [0035]     To illustrate interoperability with different communication protocols, it is assumed below that the first master computer SRV 1  is a central administrative entity operating according to the H.323 communication protocol, also known in specialist terminology as a gatekeeper.  
         [0036]     The second master computer SRV 2  corresponds to an administrative entity operating according to the SIP communication protocol. In the case of the communication protocol SIP this administrative entity is generally subdivided into a plurality of units (not shown) such as a SIP proxy server, a redirect server, a registrar and a location server, while one unit in the form of the second master computer SRV 2  is used as the basis for the drawing for reasons of clarity.  
         [0037]     The first master computer SRV 1  exchanges messages configured according to the H.323 communication standard with H.323-compatible terminals (not shown) via an interface (not shown). In a similar manner the second master computer SRV 2  exchanges messages configured according to the SIP communication standard with SIP-compatible terminals (not shown) via an interface (not shown).  
         [0038]     At the first interface IF 1 . 1  of the first master computer SRV 1  messages exchanged in the form of data packets are however not configured according to the H.323 communication standard. In a similar manner the messages exchanged at the second interface IF 1 . 2  of the second master computer SRV 2  are not configured according to the SIP communication standard. The messages exchanged at the first or second interface IF 1 . 1 , IF 1 . 2  are specifically exchanged in a different format that can be processed for the integration unit IL of the proxy server PRX, as will be examined in more detail below.  
         [0039]     After this illustration of hardware components to describe the physical structure of the communication system CSY, the functional units shown in  FIG. 1  are considered below with regard to their function and communication relations in respect of each other. The three logical levels LYR 1 , LYR 2 , LYR 3  define an organization form according to a so-called 3-tier model, in which units in the form of hardware and/or software exchange data with each other.  
         [0040]     For the purposes of structural consideration, the units referred to above as communication terminals within the third logical level LYR are introduced as first and second service users SC 1 ,SC 2 . A service user SC 1 ,SC 2  or a service user unit SC 1 ,SC 2  is often also referred to in specialist circles as a client and it implements calls for functions, which are provided for example by a so-called service provider or service provider unit—in specialist circles often also referred to as a server.  
         [0041]     The functions called by the service user units SC 1 ,SC 2  are provided by the service provider unit PRX previously referred to as the proxy server. For the service user units SC 1 ,SC 2 , 
        this service provider unit PRX therefore acts as a so-called service provider.        
 
         [0043]     The mode of operation of the logical levels LYR 1 ,LYR 2 ,LYR 3  can now be summarized as follows based on this architecture of the communication system CSY: 
    The first logical level LYR 1  provides native signaling functions of communication protocols such as H.323 and SIP.     The second logical level LYR 2  abstracts these native signaling functions and supplies these to the third logical level LYR 3  in the form of (VoIP) communication services as so-called web services.     The third logical level LYR 3  corresponds to client software. Based on the supplied implementation of said client software a designer is able to implement a VoIP solution using the communication services provided by the second logical level LYR 2 . The client software thereby includes only the functions which are essential for the clients, i.e. the service users SC 1 ,SC 2 , such as the management of a useful data exchange and the codec functions used for this. Service users SC 1 ,SC 2  use the communication services provided by the second logical level LYR 2  for execution in their own applications. These applications are implemented for example in a communication terminal or on a computer system.    
 
         [0047]     The respective service user SC 1 ,SC 2  sends requests for the establishment of a connection to the service provider PRX and awaits confirmation or rejection of said request. This request is independent of the communication protocol—SIP or H.323—used in the first logical level LYR 1  for the actual establishment of the connection but is configured as a call for communication services. This call for communication services is protocol-neutral and is effected for example based on the XML standard (Extended Markup Language). If the request is confirmed, the service user SC 1 ,SC 2  receives parameters with the confirmation such as port number and coding method, which are to be used for the subsequent exchange of useful data (not shown). This exchange of useful data and associated media processing is completely separate from the exchange of signaling data.  
         [0048]     Communication connections between different logical levels are described in more detail below with further reference to the functional units in  FIG. 1 .  
         [0049]      FIG. 2  shows an exchange of messages or function calls between different functional units known to an extent from  FIG. 1  and the logical levels LYR 1 ,LYR 2 ,LYR 3 . For the sake of clarity the logical levels LYR 1 ,LYR 2 ,LYR 3  are only symbolized by direction arrows unlike the diagram in  FIG. 1 .  
         [0050]     A further communication unit TERM is arranged in the third logical level TERM in addition to the service users SC 1 ,SC 2 .  
         [0051]     Unlike  FIG. 1  it is assumed in the exemplary embodiment to be described below that two service provider units PRX 1 ,PRX 2  are arranged in the second logical level of the communication system CSY and only one master computer SRV is arranged in the first logical level LYR 1 . The master computer SRV is for example in the form of a central SIP administration entity.  
         [0052]     The communication unit TERM is correspondingly in the form of a SIP communication terminal and exchanges with the master computer SRV on the basis of signaling messages SGN configured according to the SIP standard.  
         [0053]     The communication users in the method to be described below are in particular the communication unit TERM, the first service user SC 1  and the second service user SC 2 .  
         [0054]     In the case of an existing communication connection between the communication unit TERM and the first service user SC 1 , useful data PLD 1  is exchanged directly between the communication unit TERM and the first service provider SC 1 .  
         [0055]     A similar direct exchange of useful data PLD 2  also takes place in the case of an existing communication connection between the first and the second service user unit SC 1 ,SC 2 .  
         [0056]     The first service user unit SC 1  uses a first service call PR 1  via the first interface IF 2 . 1  of the first master computer PRX 1  to call up the latter&#39;s communication services or web services.  
         [0057]     The second service user unit SC 2  correspondingly uses a second service call PR 2  to call up communication services of the second master computer PRX 2 .  
         [0058]     An exchange of data also takes place between communication services of the first master computer PRX 1  and communication services of the second master computer PRX 2  via a third service call PR 3 .  
         [0059]     The functional units located in the first logical level LYR 1  are only used in the method provided according to the invention, if a calling or called communication partner can only be addressed via one of the supported communication protocols—SIP in the exemplary embodiment.  
         [0060]     A request to establish a connection initiated from the third logical level LYR 3 , for example by the first connection taker SC 1  using the communication unit TERM is routed in such a case via the second logical level LYR 2  to the first logical level LYR 1 , whereby necessary parameters within the signaling information—e.g. call number and characteristics of the calling user—are forwarded in corresponding messages or communication function calls PR 1 ,PR 2 .  
         [0061]     Signaling messages SGN are then exchanged between the called communication unit TERM and the master computer SRV in the first logical level LYR 1 . The signaling messages SGN are used at the time of establishment of the communication connection for example to agree which port and codec should be used for the communication to be set up. After the end of this signaling phase, confirmation of a successfully established connection and result parameters accompanying this are returned via the second logical level LYR 2  and via the third logical level LYR 3  to the calling communication unit TERM for voice transmission and monitoring of the communication connection.  
         [0062]     From the point of view of the called communication unit TERM the exchange of messages in the logical level LYR 1  corresponds to communication with a SIP proxy server or in an alternative embodiment with an H.323 gatekeeper, depending on which of the two communication protocols was used to exchange signaling messages with the communication unit TERM.  
         [0063]     From the point of view of the calling first service user SC 1  in the third logical level LYR 3  this message exchange remains hidden. In this level LYR 3  only abstracted communication services PR 1  are called up and parameters returned relating to said services PR 1 .  
         [0064]     If however the called communication partner is arranged in the third logical level LYR 3  like the calling communication partner, in other words for example in the case of a request for a connection between the first and second service users SC 1 ,SC 2 , the connection is established directly on the second logical level LYR 2 .  
         [0065]     For this purpose the service provider units PRX 1 ,PRX 2  arranged in the second logical level LYR 2  are responsible for all the steps required to establish the connection and the associated parameters, such as the request for establishment of the connection, the capacity interrogation, request confirmation, etc.  
         [0066]      FIG. 3  shows a chronological flow diagram with further reference to the functional units in  FIG. 2  to illustrate schematically an exchange of signaling messages and control messages during the establishment of a connection between the calling first service user unit SC 1  and the called communication unit TERM.  
         [0067]     The time lines  1 ,  2 ,  3 ,  4  and  5  are assigned in this order to the first service provider unit SC 1 , the second interface IF 2 . 1  of the first service provider unit PRX 1 , the first interface IF 1  of the communication master computer SRV, the first processing unit of the communication master computer SRV, and the communication unit TERM. The time lines  1 , 2 , 3 , 4  and  5  run from top to bottom, so that later times t are lower down than earlier times t.  
         [0068]     At a time t0 the first service user unit SC 1  calls up the communication service CALL to establish a connection to a communication partner to be specified in parameters. This call is sent via a prompt  310  containing further parameter from the service user unit SC 1  to the assigned first service provider unit PRX 1  and received by the first interface IF 2 . 1  of the first service provider unit PRX 1 . As it is already known in the first service provider unit PRX 1  on the second logical level LYR 2  due to a previous registration of the service user unit SC 1  which coding methods or codecs the service user unit SC 1  uses or a preferred sequence for their use, it is not necessary for the service user unit SC 1  to specify these parameters in the previous prompt  310  and they do not have to be specified additionally in every method call.  
         [0069]     The service provider unit PRX 1  receives the request for a service call sent with the prompt  310 , processes the parameters contained in the prompt  310  and at a time t1 generates a further prompt  312  directed to the first logical level LYR 1 , which is received at the first interface IF 1  of the SIP master computer SRV. This prompt  312  contains a connection identification number identifying the current connection request, also referred to as Call ID, which is allocated by the service provider unit PRX 1 .  
         [0070]     At a time t2 a confirmation message  312  is transmitted from the first interface IF 2 . 1  of the first service provider unit PRX 1  to the first service user unit, generally referred to in specialist circles as an acknowledge message  314 . This confirmation message  314  sent from the first service provider unit PRX 1  via the first interface IF 2 . 1  reports the ongoing processing of the connection set-up initiated with the prompt  310 .  
         [0071]     During subsequent processing in the first logical level LYR 1  all the parameters received with the prompt  312  are extracted from said message and transformed into a standard SIP invite request. SIP session set-up is then started.  
         [0072]     At a time t3 this SIP invite request is generated in the form of an invite message  316  by the first interface IF 1  of the master computer SRV based on the prompt  312  and using the parameters contained therein and transmitted to the first processing unit BL 1  of the master computer SRV. As with the previous comments on the distributed nature of the SIP master computer SRV, it should be noted here that in an alternative embodiment the first processing unit BL 1 , the converting first interface IF 1  and the SIP master computer SRV are implemented in distributed systems, which communicate with each other via the packet-oriented network (not shown).  
         [0073]     At a time t4 SIP session set-up is started by the first processing unit BL 1  and an invite message  318  is sent to the communication unit TERM. The first processing unit BL 1  takes over the role of a SIP proxy in respect of the communication unit. In the present exemplary embodiment the communication unit TERM is in the form of a so-called SIP user agent.  
         [0074]     At a time t5 a signaling message  320  is sent from the first communication unit TERM to the first processing unit BL 1 . After successful verification of the readiness of the communication unit TERM to receive the connection request, the signaling message  320  reports successful signaling of the connection request, which is generally effected with call signaling in conventional communication units. In the case of a preferred implementation of the communication unit in the form of a software application in the exemplary embodiment, a number of other signaling forms can be implemented in addition to acoustic signaling to indicate an incoming connection request. The signaling message  320  is also referred to as ringing in SIP terminology, based on conventional signaling forms. According to SIP conventions this signaling message is categorized with code  180 .  
         [0075]     At a time t6 the signaling message  320  received by the first processing unit BL 1  is forwarded in the form of a further signaling message  322  to the first interface IF 1 . Optionally some of the parameters contained in the previous signaling message  320  are modified by the first processing unit BL 1  before it is forwarded in the form of the signaling message  322 .  
         [0076]     At a time t7 a signaling message  324  is sent by the first interface IF 1  to the second interface IF 2 . 1  located on the second logical level LYR 2 , in which data from the previous signaling message  322  contained in a SIP format is transformed to a format anticipated by the second interface IF 2 . 1 , e.g. XML. The signaling message  324  is now processed in the service provider unit PRX and assigned to the previously started communication service using the determined call ID.  
         [0077]     Finally at a time t8 acknowledgement of the called communication service is given to the calling first service user unit SC 1  in the form of a signaling message  326  by means of signaling to the called communication unit TERM.  
         [0078]     If a user of the called communication unit TERM receives the connection request, at times t9, . . . ,t12 respectively confirmation messages  328 ,  330 , . . . ,  334  are sent from the communication unit TERM to the first service user unit SC 1 . The first confirmation message  328 , structured in a SIP format—in SIP terminology also referred to as OK and categorized as code  200  according to SIP conventions, is similarly received finally as acknowledgment of the called communication service in the form of the confirmation message  334  at the first service user unit SC 1 . If connection set-up is rejected by the called communication unit, a reason for the rejection is contained in the—in this instance negative—confirmation message  334 , which is transmitted in an XML format to the first service user SC 1 .  
         [0079]     If a positive confirmation message  334  reaches the first service user SC 1 , at times t13, . . . ,t16 respectively backward confirmation messages  336 ,  338 , . . . ,  342  are similarly sent from the first service user unit SC 1  to the communication unit TERM. The first confirmation message  336  in an XML format is sent by the first service user SC 1 , if this latter agrees to the communication connection.  
         [0080]     A direct useful data connection PLD 1  is then established between the first service user SC 1  and the called communication unit TERM.  
         [0081]      FIG. 4  shows a chronological flow diagram with further reference to the functional units in  FIG. 2  to illustrate schematically an exchange of signaling messages and control messages during the establishment of a connection between the calling first service user unit SC 1  and the called second service user unit SC 2 .  
         [0082]     The time lines  1 , 2 , 3 , 4  and  5  are assigned in this order to the first service provider unit SC 1 , the second interface IF 2 . 1  of the first service provider unit PRX 1 , the second interface IF 2 . 2  of the second service provider unit PRX 2  and the communication unit TERM. The time lines  1 , 2 , 3  and  4  run from top to bottom, so that later times t are lower down than earlier times t.  
         [0083]     If as in the present exemplary embodiment, a communication connection is to be established between two service user units SC 1 ,SC 2 , functional units of the first logical level LYR 1  are not required. From the point of view of the calling service user unit SC 1  communication with the second logical level LYR 2  is identical to the exemplary embodiment in  FIG. 4 .  
         [0084]     At a time t0 the first service user unit SC 1  calls up a communication service CALL to establish a connection to a communication partner to be specified in parameters. This call is sent from the service user unit SC 1  to the assigned first service provider unit PRX 1  via a prompt  410  containing further parameters and received by the first interface IF 2 . 1  of the first service provider unit PRX 1 . As it is already known in the first service provider unit PRX 1  on the second logical level LYR 2  due to a previous registration of the service user unit SC 1  which coding methods or codecs the service user unit SC 1  uses or a preferred sequence for their use, it is not necessary for the service user unit SC 1  to specify these parameters in the previous prompt  310  and they do not have to be specified additionally in every method call.  
         [0085]     The service provider unit PRX 1  receives the request for a service call sent with the prompt  410  and processes the parameters contained in the prompt  410 . From a parameter sent to identify the called communication user, the second service user unit SC 2 , it is particularly clear to the service provider unit PRX 1  that the called user like the calling user is associated with the third logical level LYR 3 .  
         [0086]     Then at time t1 a prompt  412  is sent to call up a communication service on the service provider unit PRX 2  associated with the second service user unit SC 2 . This prompt  412  contains parameters required for the communication connection and is sent from the first interface IF 2 . 1  of the first service provider unit PRX 1  to the second interface IF 2 . 2  of the second service provider unit PRX 2 .  
         [0087]     At a time t2 a confirmation message  414  is transmitted from the first interface IF 2 . 1  of the first service provider unit PRX 1  to the first service user unit SC 1 , which is also referred to in specialist circles as an acknowledge message  414 . This confirmation message  414  sent from the first service provider unit PRX 1  via the first interface IF 2 . 1  reports the ongoing processing of the connection set-up initiated with the prompt  410 .  
         [0088]     At a time t3 a prompt  416  is sent from the second interface IF 2 . 2  of the second service provider unit PRX 2  to the second service user unit SC 2 . This prompt  416  has an XML structure and forwards the request initiated with the prompt  410  to establish a communication connection or call request directly to the second service user unit SC 2 , optionally with the addition of further administrative parameters by the service provider units PRX 1 ,PRX 2 .  
         [0089]     After successful verification of the readiness of the second service user unit SC 2  to receive the connection request, signaling messages  418 , 420 , 422  sent at times t4,t5,t6 report successful signaling of the connection request, which is effected with call signaling in conventional communication units. The signaling messages  418 , 420 , 422  are therefore also referred to as ringing.  
         [0090]     If a user of the called second service user unit SC 2  receives the connection request, at times t7,t8,t9 respectively confirmation messages  424 , 426 , 428  are sent from the second service user unit SC 2  to the first service user unit SC 1 . The first confirmation message  424 , structured in an XML format—also referred to as OK—is similarly received finally as acknowledgment of the called communication service in the form of the confirmation message  428  at the first service user unit SC 1 . If connection set-up is rejected by the called second service user unit SC 2 , a reason for rejection is contained in the—in this instance negative—confirmation message  428 .  
         [0091]     If a positive confirmation message  428  reaches the first service user SC 1 , at times t10,t11,t12 respectively backward confirmation messages  430 ,  432 ,  434  are similarly sent from the first service user unit SC 1  to the second service user unit SC 2 .  
         [0092]      FIG. 3  shows a chronological flow diagram with further reference to the functional units of  FIG. 2  to illustrate schematically an exchange of signaling messages and control messages during the establishment of a connection between the calling communication unit TERM and the called first service user unit SC 1 .  
         [0093]     It is however assumed with reference to the exemplary embodiment shown according to  FIG. 5  that the calling communication unit TERM is provided for an exchange of messages based on the SIP communication protocol. The communication protocol present in the communication system CSY, particularly SIP or H.323, however essentially has no influence on the successful implementation of the inventive method. In particular the inventive method can also be implemented even in the case of a plurality of different communication protocols, as already set out clearly with reference to the diagram in  FIG. 1 .  
         [0094]     The time lines  1 , 2 , 3 , 4  and  5  are assigned in this order to the communication unit TERM, the first execution unit BL 1  assigned to the SIP communication master computer SRV, the first interface IF 2 . 1  assigned to the first service provider unit PRX 1 , the first interface IF 1  assigned to the SIP communication master computer SRV and the first service provider unit SC 1 . The time lines  1 , 2 , 3 , 4  and  5  run from top to bottom, so that later times t are lower down than earlier times t.  
         [0095]     At a time t0 the first communication unit TERM sends a prompt  510  in a SIP format to establish a connection with a communication partner, also referred to as a SIP INVITE. This prompt is sent from the communication unit TERM to its assigned SIP master computer SRV and received by the latter&#39;s processing unit BL 1 . The processing unit takes over the role of a SIP proxy. The communication partner specified using parameters in the prompt  510 , namely the first service user unit SC 1 , is not controlled by the processing unit BL 1  of the SIP communication master computer SRV, so the prompt  510  is forwarded by this latter at time t1 in the form of the largely unmodified processing message  512  to the second logical level LYR 2 , more specifically to the first interface IF 2 . 1  of the service provider unit PRX 1  there.  
         [0096]     From the point of view of the first service user unit SC 1  functional units in the second logical level LYR 2  interact with the first interface IF 2 . 1  like a proxy, i.e. all the bi-directional signaling messages described below operate via this level LYR 2 .  
         [0097]     At a time t2 the previously received prompt  512  is forwarded via the first interface IF 2 . 1  of the communication master computer PRX 1  to the interface IF 1  of the SIP communication master computer SRV. This forwarding is effected as an appendix to a message, which is otherwise effected in a format provided for communication between these two interfaces IF 2 . 1 ,IF 1 , e.g. in an XML format.  
         [0098]     In the first logical level LYR 1  a SIP INVITE request, i.e. an invitation in the communication protocol SIP, is identified from this appendix to the message  514  and sends a prompt  516  in XML format at time t3 via the interface IF 1  to the first interface IF 2 . 1  of the communication master computer PRX 1  to call up a communication service CALL.  
         [0099]     In the service provider unit PRX 1  on the second logical level LYR 2  a communication service call—or call request —is identified for the controlled service user unit SC 1 , whereupon at time t4 a corresponding prompt  518  in an XML format is sent from the first interface IF 2 . 1  of the communication master computer PRX 1  to the first service user unit SC 1 .  
         [0100]     Before an operator or a software application decides to accept the communication connection requested by means of the prompt  518 , at time t5 a signaling message  520 —also referred to as ringing—is sent to the first interface IF 2 . 1  of the communication master computer PRX 1 .  
         [0101]     At time t6 this signaling message  520  is passed on in the form of a forwarded signaling message  522  to the first logical level LYR 1  or to the interface IF 1 .  
         [0102]     The signaling message  522  received at the first interface IF 1  is evaluated and a corresponding SIP signaling message  524 —categorized as code  180  according to SIP conventions—is transmitted to the processing unit BL 1  acting as the SIP proxy.  
         [0103]     At a time t8 this SIP signaling message  524  in the form of a further SIP signaling message  526  is forwarded from the processing unit BL 1  to the SIP communication unit TERM.  
         [0104]     If a user of the called first service user unit SC 1  receives the connection request, at times t9, . . . ,t12 respectively confirmation messages  528 , 530 , . . . , 534  are then sent from the first service user unit SC 1  to the communication unit TERM. The confirmation messages  528 , . . . , 534  exchanged at times t9, . . . ,t12 correspond in the counter direction to the exchanged confirmation messages  328 , . . . ,  334  known from  FIG. 3  except that the messages are only routed via the interface IF 2 . 1  acting as a proxy in the second logical level LYR 2 .  
         [0105]     At time t9 a first call of a communication service confirming acceptance of the connection—Response or ok—is sent in the form of the confirmation message  528  first to the interface IF 2 . 1  acting as proxy in the second logical level LYR 2 , which transmits said confirmation message  528  at time t10 in the form of confirmation message  530  to the interface IF 1 . The confirmation message  530  received at the first interface IF 1  is evaluated and a corresponding SIP confirmation message  532 —in SIP terminology also referred to as OK and categorized according to SIP conventions as code  200 —is transmitted to the processing unit BL 1  acting as the SIP proxy, from whence it is finally transmitted at time t12 as a SIP confirmation message  534  of the SIP communication unit TERM.  
         [0106]     If a positive confirmation message  534  reaches the SIP communication unit TERM, at times t13, . . . ,t17 respectively backward confirmation messages  536 ,  538 , . . . ,  544  are similarly sent from the SIP communication unit TERM to the first service user unit SC 1 .