Abstract:
The invention concerns a conference bridge server ( 30 ) for establishing a conference telephone call, and a corresponding method and a computer program product. The conference bridge server ( 30 ) comprises input and output means ( 31, 33 ) for a plurality of communication connections connecting conference terminals ( 11  to  16, 20 ) with the conference bridge server ( 30 ) and a conference core ( 32 ) for distributing input signals from conference terminals ( 11  to  16, 20 ) to at least some of the remaining conference terminals ( 11  to  16, 20 ). The conference bridge server ( 30 ) comprises means for issuing a special address to be utilized, by one or more participants ( 111  to  116 ) at a first location ( 1 ) using a common conference phone ( 10 ), to connect one or more laptop computers ( 11  to  16 ) as additional conference terminals ( 11  to  16 ). Said conference core ( 32 ) is programmed to distribute, in said conference call, input signals from conference terminals ( 11  to  16 ) connected via said special address to conference terminals ( 20 ) assigned to other participants ( 200 ) at one or more other locations ( 2 ). At the same time, said conference core ( 32 ) is programmed to redirect output signals destined for conference terminals ( 11  to  16 ) connected via said special address to said common conference phone ( 10 ).

Description:
The invention is based on a priority application EP 06 291 250.6 which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a method of establishing telephone conference calls, and a conference bridge server and a computer program product to execute said method. 
     BACKGROUND OF THE INVENTION 
     Conference calls play a significant role in today&#39;s business life. In conference calls, where some participants share a common conference room while other participants are at remote locations, the voice quality from participants speaking in the common conference room is often poor when using a standard conference phone with handsfree feature. 
     Professional conference rooms exist where each participant has their own input device, e.g., their own microphone, while audio output goes to a common room loudspeaker. In addition, conference systems are known that use desktop or laptop computers as audio and/or video terminals and that have a bridge server that can be configured to route output to a common output terminal such as a common conference phone. 
     WO 94/24803 describes a telecommunications network architecture which facilitates the handling of multimedia communications between any number of conference participants, who may have varying media communications capabilities involving one or more of audio, video, and data. A calling party and a called party meet each other in a virtual meeting room generated by the system. The telecommunications network architecture further comprises bridging resources to effectuate a mediation between different types of media. 
     EP 0 898 424 A2 describes a teleconferencing system for conducting a teleconference among a plurality of participants having workstations with monitors for displaying visual images and with associated audio-visual (AV) capture and reproduction capabilities for capturing and reproducing video images and spoken audio of participants. Furthermore, a common collaboration initiator exists for initiating a plurality of types of collaboration, e.g., data conferencing, telephone conferencing, video conferencing, etc. among the participants. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide an improved conference call. 
     The object of the present invention is achieved by a conference bridge server for establishing a conference telephone call, the conference bridge server comprising input and output means for a plurality of communication connections connecting conference terminals with the conference bridge server and a conference core for distributing input signals from any of the conference terminals to at least some of the remaining conference terminals, wherein the conference bridge server comprises means for issuing a special address to be utilized by one or more participants at a first location using a common conference phone to connect one or more laptop computers as additional conference terminals, and wherein said conference core is adapted to distribute, in said conference call, input signals from conference terminals connected via said special address to conference terminals assigned to other participants at one or more other locations, but redirect output signals for conference terminals connected via said special address to said common conference phone. Moreover, the object of the present invention is achieved by method of providing a conference telephone call wherein said method comprises the steps of establishing a plurality of communication connections connecting conference terminals with a conference bridge server, and distributing input signals from one or more conference terminals to at least some of the remaining conference terminals, whereby the method comprises the further steps of utilizing a special address to connect one or more laptop computers as additional conference terminals, the one or more laptop computers assigned to one or more participants at a first location using a common conference phone, distributing, in said conference call, input signals from conference terminals connected via said special address to conference terminals assigned to other participants at one or more other locations, and redirecting output signals destined for conference terminals connected via said special address to said common conference phone. And the object of the present invention is further achieved by computer program product for establishing a conference telephone call with a plurality of communication connections connecting conference terminals with a conference bridge server and distributing input signals from one or more conference terminals to at least some of the remaining conference terminals, wherein the computer program product, when executed, performs the steps of issuing a special address to be utilized, by one or more participants at a first location using a common conference phone, to connect one or more laptop computers as additional conference terminals, distributing, in said conference call, input signals from conference terminals connected via said special address to conference terminals assigned to other participants at one or more other locations, and redirecting output signals destined for conference terminals connected via said special address to said common conference phone. 
     Participants in the common conference room use their laptop computers as input source, i.e., for providing audio input to the conference call. However, audio output is not directed to each participant&#39;s laptop computer but is directed to the conference phone in the common conference room. The idea is that the conference bridge server has some means to inform a participant of an address to connect his laptop as additional voice input means to the conference call system. 
     Each laptop computer is used as a microphone of a virtual conferencing phone wherein the detection of a speaking participant can be done as in a state-of-the-art audio conference. A conference call is created on the conference bridge server. A VoIP client on a laptop computer calls the conference bridge server (VoIP=Voice over Internet Protocol). However, only the microphone input from the participants is used by the conference bridge server and the audio output is sent to only one device, e.g., a speaker of the conference phone, in the common conference room shared by the participants. The participants in the common conference room call a specific number or use a specific access code so that the conferencing bridge is able to differentiate the microphones in the common conference room from the microphones in other locations. 
     With this method, each of the participants of a telephone conference is provided with an input means, e.g., a microphone, of their own. Each participant may use their laptop computer, which may be anyway used to take notes or review documents, as input means. Each laptop is used as a microphone of a virtual conferencing phone. Therefore, each participant can be heard equally well in a conference call. 
     The invention provides an elegant solution for the problem if there are more participants than microphones. By using laptop computers as individual microphones, the participants at a remote location can hear properly what is said in the common conference room because each participant is very close to his own microphone. Besides, there is no need that microphones of the hardware conferencing phone are passed around in the common conference room so that each speaker can be heard correctly. 
     A conferencing tool is able to differentiate input signals originating from microphones at a specific location from input signals originating from microphones at another location. Thus, parameters of echo cancellation may be optimized for the microphones at a specific location to prevent echo due to the mixed audio input and output. The availability of a separate microphone for each participant and the improved echo cancellation result in a better media quality at remote locations. 
     Moreover, the invention provides means to inform the participants of the conference call about who is currently speaking. Therefore, the participants may be provided with a list of all participants and an identification of a speaking participant. 
     In addition to that, the invention provides a solution for the case that there are more participants than audio input channels at the conferencing bridge or microphones. For this case the laptops may be provided with local mixing capabilities to act as a local audio mixing node. 
     Further advantages are achieved by the embodiments of the invention indicated by the dependent claims. 
     According to a preferred embodiment of the invention, the special address that is utilized by a participant to connect his laptop to the conference call is an address assigned to the conference bridge server. Preferably, the conference bridge server provides separate dial-in URLs for people at a first location, for the conference phone and people at a second location so that separate processing can be applied to the audio streams from and to each of the locations and the conference phone (URL=Universal Resource Locator). All participants willing to add their laptop to the virtual conference bridge dial the received URL as they would do for a “standard” dial-in conference. 
     According to another preferred embodiment of the invention, different echo cancellation parameters are applied to each participant in a common conference room in dependence on his/her microphone&#39;s relative position with regard to the common conference phone. In addition, also the distance of a participant to his microphone, i.e., laptop computer, may be taken into account for the choice of the echo cancellation parameters. That means that different echo cancellation parameters are applied to the input/output signal channels related to the conference call whereby the used echo cancellation parameters depend on the location of the participant or device associated to the signal channel. For example, if the common conference room is a very spacious room with a capacity of twenty conference participants and resounding acoustics, it is advantageous to apply different echo cancellation parameters to each of the participants in the common conference room because each of the participants will be located at a different distance from the common conference phone. 
     According to another preferred embodiment of the invention, the signalling and session initiation traffic is transmitted using the SIP protocol, and the control traffic associated with the transmission of media data is encoded in RTCP, while the media data are transmitted using RTP (SIP=Session Initiation Protocol; RTP=Realtime Transmission Protocol; RTCP=Realtime Transmission Control Protocol). 
     In a preferred embodiment, the conference bridge server is enabled to indicate a currently speaking participant. From the source fields of the SIP protocol, the conference bridge server can extract the names of the participants, and by means of voice detection such as VAD (=Voice Activity Detection), the conference bridge server can allocate to the speaking participant or group the extracted names and display them to the other participants. Preferably, the conference bridge server also holds a picture of each participant in a data base and displays the picture of a speaking participant to the other participants. 
     According to another preferred embodiment of the invention, the conference bridge server provides a participant with a special address in dependence on the location of the participant. The participant uses the received address to contact the conference bridge server. The conference bridge server recognizes the origin of an incoming signal by the address used to send the signal to the conference bridge server. Thus, the incoming signals and the related participants can be distinguished according to their location. 
     Preferably, all participants at a common location are provided with a common special address. The conference bridge server may send a conference form to the participants applying to join a conference call and require them to indicate their location. Thus, the conference bridge server has the information needed for building location-dependent groups. It is also possible that a participant receives the address of a website and that the participant retrieves the common special address from the website. 
     Preferably, the laptop computers comprise VoIP client software which enables the laptop computers to act as a local mixing entity. For instance, if there are more participants than can be accepted by the conference bridge server, an already connected laptop computer may accept one or more other laptop computers and act as a relay station to the conference bridge server. Thus, also the indirectly connected laptop computers can join the virtual conference phone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These as well as further features and advantages of the invention will be better appreciated by reading the following detailed description of presently preferred exemplary embodiments taken in conjunction with accompanying drawings of which: 
         FIG. 1  is a block diagram of a conference call system according to an embodiment of the invention. 
         FIG. 2  is a message flow diagram related to a conference call according to an embodiment of the invention. 
         FIG. 3  is a message flow diagram related to a conference call according to another embodiment of the invention. 
         FIG. 4  is a block diagram of a conference call system according to still another embodiment of the invention. 
         FIG. 5  is a message flow diagram related to a conference call according to still another embodiment of the invention. 
         FIG. 6  is a schematic diagram specifying the data flows and related signalling and control traffic flows according to a preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a conference call system comprising a hardware conference phone  10  with multiple microphones  101  and a loudspeaker  102  at a first location  1 , a telecommunications network  3  with a conference bridge server  30 , a virtual conference phone unit  40  accessible via another network  4 , and a remote telephone  20  at a second location  2 . The virtual conference phone unit  40  may be provided and/or installed by a web server where web server here means web technology and not necessarily public internet location. The virtual conference phone unit  40  may be in the Internet but it may be found in a corporate network as well. Thus, the other network  4  may be the public Internet but it may be a corporate network. 
     The hardware conference phone  10  is used for a conference call by participants  111  to  116  at the first location  1 . The participants  111  to  116  wish to communicate with another participant  200  using the remote telephone  20  at the second location  2 . Each of the participants  111  to  116  has available their own respective laptop computer  11  to  16  which may be connected to the telecommunications network  3 . 
     The multiple microphones  101  of the hardware conference phone  10  record the sounds from the participants  111  to  116 . The loudspeaker  102  of the hardware conference phone  10  outputs the sounds from the remote telephone  20 . 
     The conference bridge server  30  comprises a first port module  31  with seven ports  310  to  316  to provide connections to the hardware conference phone  10  and the laptop computers  11  to  16 , a conference core unit  32 , and a second port module  33  with a single port  330  providing a connection to the remote phone  20 . The telecommunications network  3  is connected to the Internet  4  so that the conference bridge server  30  can communicate with the virtual conference phone unit  40 . In the embodiment shown, the virtual conference phone unit  40  is implemented as a web application accessible by means of a web access. 
     The conference bridge server  30  may be composed of one or several interlinked computers, i.e., a hardware platform, a software platform basing on the hardware platform and several application programs executed by the system platform formed by the software and hardware platform. The functionalities of the server are provided by the execution of these application programs. The application programs or a selected part of these application programs constitute a computer software product providing a conference bridge service as described in the following, when executed on the system platform. Further, such computer software product is constituted by a storage medium storing these application programs or said selected part of application programs. 
     The virtual conference phone unit  40  comprises an interface  41  for exchanging data with external devices, a processor  42  for data processing and control of functions of the virtual conference phone unit  40 , and a memory  43  for storing data. In another embodiment, the virtual conference phone unit  40  may be installed in the telecommunications network  3  and/or integrated into the conference bridge server  30 . 
     The first location  1  may be a spacious conference room while the second location  2  may be a small office room. The terminals  11  to  16  may be situated at different distances to the loudspeaker  102  of the common conference phone  10 . In addition to that, the participants  111  to  116  at the first location  1  may be situated at different positions to their terminals  11  to  16  and the recording quality of each microphone of the terminals  11  to  16  may vary, too. The conference bridge server  30 , since it identifies each participant  111  to  116  at the first location  1  as a separate, individual participant, is able to supply an echo cancellation software with different echo cancellation parameters for each of the participating terminal  11  to  16 ,  20 . 
       FIG. 2  shows data flows for establishing a conference call according to a preferred embodiment of the invention. A user  111  called Alice situated at location  1  wishes to call in a telephone call another user  200  called Charles situated at location  2 . In a first step  201 , Alice  111  dials at the conference phone  10  the telecommunication address assigned to Charles  200  or assigned to the phone  20  used by Charles  200 . Preferably, a VoIP client is installed on the conference phone  10  and on Charles&#39; phone  20 . 
     If the telecommunications devices utilized for the conference call support SIP, the telecommunication address dialed by Alice  111  may be a SIP address such as “charles@example.net”. The steps  202  to  207  represent the usual SIP call setup messages. First, the conference phone  10  sends an INVITE message  202  comprising the dialed address to the conference bridge server  30  which represents a SIP proxy. The conference bridge server  30 , knowing the location of the phone  20  of Charles  200  from a registrar, forwards the INVITE message  203  to the phone  20  of Charles  200 . When Charles  200  reacts to the indicated waiting call in step  204 , e.g., by lifting the handset, his SIP user agent, i.e., the phone  20 , sends a  200  OK message back to the conference phone  10  used by Alice  111 . This is indicated by messages  205  and  206 . Then, the conference phone  10  sends an ACK (=acknowledge) message  207  to the phone  20 . 
     After completion of the SIP call setup messages  202  to  207 , a communication connection  208  between Alice  111  and Charles  200  is established and media streams can be exchanged via RTP/RTCP. Therefore, Charles  200  is enabled to propose to Alice  111  in step  209  to use a virtual conference phone. In step  210 , Alice  111  agrees with this proposal. 
     In step  211 , Alice  111  uses her laptop computer  11  to log in at the virtual conference phone unit  40 . A VoIP client software is installed on her laptop computer  11 . In a preferred embodiment, the virtual conference phone unit  40  is implemented as a web application which may be contacted, e.g., via an HTTP address (HTTP=Hypertext Transfer Protocol). Therefore, Alice  111  may use a browser on her laptop  11  to reach the virtual conference phone unit  40  web application and apply for a virtual conference. In step  212 , the virtual conference phone unit  40  presents a conference form to Alice  111 . In step  213 , Alice  111  fills in the conference form and, in particular, indicates that she sits in location  1 , e.g., a specific conference room called “shared”. 
     In step  214 , the virtual conference phone unit  40  provides Alice  111  with an address, e.g., “sip:shared@example.net”, to be used by Alice  111  to contact the virtual conference phone unit  40 . For example, the address may be displayed on a display of her laptop  11  and Alice  111  may type the displayed address on her laptop keyboard. It is also possible that the virtual conference phone unit  40  provides Alice  111  also with an address, e.g., “sip:remote@example.net”, to be used by Charles  200  to contact the virtual conference phone unit  40 . 
     In another embodiment, the address not displayed to Alice  111  but is used internally in by an application, e.g., running on the laptop  11  of Alive  111 , to dial the received address. Thus, Alice  111  is not required to type the address herself on the keyboard. This solution provides more comfort for the user. 
     In still another embodiment, the connection to the conference bridge  30  is not set up in the direction subscriber—conference bridge  30  but in the opposite direction conference bridge  30 —subscriber. For example, the virtual conference phone unit  40  manages one or more different conference calls. A subscriber may log in into the virtual conference phone unit  40 , select a conference call he wants to participate in and apply for participation, e.g., by clicking a pre-defined button. Then, the virtual conference phone unit  40  or another dedicated web/network application, knowing the address of the applying subscriber from his application messages, establishes a connection to the applying subscriber. Once the connection has been set up, the subscriber is registered as a participant of the selected conference call. 
     Generally, the virtual conference phone unit  40  provides separate addresses, e.g., dial-in URLs, for people at different locations. For example, the virtual conference phone unit  40  provides a separate dial-in URL for the participants  111  to  116  in the conference room  1 , a separate dial-in URL for the conference phone  10 , and a separate dial-in URL for the participant  200  in the remote location  2  so that separate processing can be applied to the audio streams from and to the conference room  1 . This may be because of more strict echo cancellation processing. Then, all participants willing to add their laptop to the virtual conference phone dial the allocated URL as they would do for a “standard” dial-in conference. 
     Thus, a mixing of conference data takes place both in the conference phone  10  and in the conference bridge server  30 . 
     In a preferred embodiment, the VoIP client software on each laptop can be set up to provide the name of the speaking participant or group of speakers to the conference bridge server. For instance, the display name of the “From” header field in a SIP INVITE message can transport the various names. Thus, the conference bridge server can provide each participant of the conference call with information on who is currently speaking. 
     Preferably, the conference bridge server performs voice detection and can display who or which group is speaking if the participants have a man machine interface to the conference bridge server. 
     The conference bridge server does not send back audio to each of the laptop computers in order to prevent various delay issues, e.g., that the laptops are not identical, that each laptop processes sound differently, such that the combined result may not be satisfactory, etc. Instead, the conference bridge server sends back audio only to the speaker of the hardware conferencing phone or to a single dedicated softphone. 
       FIG. 3  shows data flows for establishing a conference call according to another preferred embodiment of the invention. The shown data flow start from a situation where, as described with reference to  FIG. 2 , Alice has been provided with an address to use for contacting the virtual conference phone. In the embodiment of  FIG. 3 , the virtual conference phone unit  40  is implemented within the conference bridge server  30 . This means that the participants, when contacting the conference bridge server  30 , at the same time can communicate with the virtual conference phone unit  40  without the need to address them separately. 
       FIG. 3  describes a situation where another participant  112  called Bob wants to join a conference call and is informed by Alice  111  about the relevant address of the virtual conference phone unit  40  installed in the conference bridge server  30 . In step  350 , Alice  111  tells Bob  112  the address of the virtual conference phone unit  40 , e.g., “shared@example.net”, to be used by the participants situated at the location  1 , i.e., the conference room. In step  351 , Bob  112  dials the received address at his laptop  12  to contact the virtual conference phone unit  40  installed in the conference bridge server  30 . Preferably, Bob&#39;s laptop  12  comprises a SIP softphone which runs a virtual conference phone plug-in. 
     The steps  352  to  354  describe the usual SIP call setup messages. First, Bob&#39;s laptop  12  sends an INVITE message  352  comprising the dialed address to the conference bridge server  30  which represents the final destination of the INVITE message  352 . The conference bridge server  30  reacts to the INVITE message  352  by sending a  200  OK message  353  back to Bob&#39;s laptop  12 . Then, Bob&#39;s laptop  12  sends an ACK message  354  to the conference bridge server  30 . 
     After completion of the SIP call setup messages  352  to  354 , a communication connection  355  between Bob  112  and the conference bridge server  30  is established and media streams can be exchanged via RTP/RTCP. 
     In the following steps  356  and  357 , the conference bridge server  30  sends an INVITE message  356  to the conference phone  10  and an INVITE message  357  to Charles&#39; phone  20 . The conference phone  10  replies a  200  OK message  358  to the conference bridge server  30 , and Charles&#39; phone  20  replies a  200  OK message  359  to the conference bridge server  30 . Then, the conference bridge server  30  sends an ACK message  360  to the conference phone  10  and an ACK message  361  to Charles&#39; phone  20 . After reception of the ACK messages  360  and  361 , a communication connection  362  between the conference phone  10  and the conference bridge server  30  and a communication connection  363  between Charles&#39; phone  20  and the conference bridge server  30  are established. 
     In another embodiment, it is also possible that the steps  352  to  355  and the steps  356  to  363  are executed contemporaneously. 
     In the steps  364  to  371 , the participants Bob  112  and Charles  200  subscribe to the conference call. This is done by sending a SUBSCRIBE conference message  364  from Bob&#39;s laptop  12  to the conference bridge server  30  and a SUBSCRIBE conference message  365  from Charles&#39; telephone  20  to the conference bridge server  30 . Usually, the SIP SUBSCRIBE message is used to subscribe to resource or call state for various resources or calls in a network. The conference bridge server  30  answers by sending  200  OK messages  366  and  367  to Bob&#39;s laptop  12  and to Charles&#39; telephone  20 . After that, SIP NOTIFY messages  368  and  369  are sent from the conference bridge server  30  to Bob&#39;s laptop  12  and to Charles&#39; telephone  20 . The NOTIFY messages  368  and  369  return current state information and comprise identifiers of the participants of the conference call, i.e., of the conference phone  10 , of Charles  200  and his telephone  20 , respectively, and of Bob  112  and his laptop  12 , respectively. Bob&#39;s laptop  12  and Charles&#39; phone  20  answer by sending  200  OK messages  370  and  371 . 
       FIG. 4  shows a constellation where all conference ports of the conference bridge server  30  are used by the participants  111  to  116  using the computer laptop devices  11  to  16  and by the conference phone  10 . If an additional participant  117  using a computer laptop  17  also wants to join the conference call at the conference room location  1 , a suitable mechanism is used where the VoIP client software is extended with local mixing capabilities on the laptop. This mechanism, which can be implemented with basic SIP if the VoIP client software is a standard SIP softphone and if the conference bridge server supports SIP, is explained in the following. 
       FIG. 5  shows a situation where participant Bob&#39;s SIP softphone  12  runs a virtual conference phone plug-in and is in a conference with the conference phone  10  and Charles&#39; device  20 . This is indicated by the messages  501  and  502 . Then an additional participant called Daniel  117  using the computer laptop  17  walks in location  1  and connects the computer laptop  17  utilizing a SIP softphone software to “shared@example.net”, i.e., the address to be utilized by participants at location  1  to join the conference call. Daniel  117  dials the address “shared@example.net” on his laptop  17  and Daniel&#39;s laptop  17  sends an INVITE message  503  comprising the dialed address “shared@example.net” to the conference bridge server  30 . 
     The conference bridge server  30  sends an INVITE message  504  addressed to Bob&#39;s address, e.g., “bob@example.net” to Bob&#39;s laptop  12 . The INVITE message  504  carries in the destination field “To:” the address of the on-going conference, i.e., “shared@example.net”. Bob&#39;s laptop  12  answers with a  200  OK message  505 . Then, the conference bridge server  30  sends a  200  OK message  506  to Daniel&#39;s laptop  17 . Daniel&#39;s laptop  17  answers with an ACK message  507  to the conference bridge server  30 . The conference bridge server  30  sends an ACK message  508  to Bob&#39;s laptop  12 . After the ACK message  508  has been received, a communication connection  509  between Daniel&#39;s laptop  17  and Bob&#39;s laptop is established. 
     In step  510 , the conference bridge server  30  sends a SUBSCRIBE conference message to Bob&#39;s laptop  12 . Bob&#39;s Laptop  12  answers by sending a  200  OK message  511  to the conference bridge server  30 . Then, a NOTIFY message  512  comprising identifiers of the conference bridge server  30 , of Daniel  117  and his laptop  17 , respectively, and of Bob  112  and his laptop  12 , respectively, is sent from from Bob&#39;s laptop  12  to the conference bridge server  30 . The conference bridge server  30  is the only entity which knows all participants of the conference phone. Thus, the conference bridge server  30  sends a NOTIFY message  513  comprising identifiers of the conference phone  10 , of Charles, Bob, and Daniel  117  and their laptops  20 ,  12 , and  17 , respectively, to Charles&#39;s laptop  20 . Additionally, the conference bridge server  30  sends a  200  OK message  514  to Bob&#39;s laptop  12 . Charles&#39; laptop  20  replies to the received notification  513  by sending a  200  OK message  515  to the conference bridge server  30 . 
     Then, Daniel&#39;s laptop  17  sends a SUBSCRIBE conference message  516  to the conference bridge server  30  which is answered by the conference bridge server  30  with a  200  OK message  517  sent to Daniel&#39;s laptop  17 . Next, the conference bridge server sends a notification message  518 , i.e., a NOTIFY message comprising identifiers of the conference phone  10 , of Charles, Bob, and Daniel  117  and their laptops  20 ,  12 , and  17 , respectively, to Daniel&#39;s laptop  17 . Daniel&#39;s laptop  17  answers by sending a  200  OK message  519  back to the conference bridge server  30 . 
     Thus, a mixing of conference data takes place both in Bob&#39;s laptop  12 , the conference phone  10  and in the conference bridge server  30 . 
       FIG. 6  shows that a laptop computer at a location can be used as a relay station for other laptop computers at the same location.  FIG. 6  shows laptop computers  11 ,  12 ,  18 ,  19  of participants using a common conference phone  10  at a first location  1  for conducting a conference call with a terminal  20  of another participant at a second location  2 , and a conference bridge server  30  comprising a virtual conference phone. In the diagram, stream data is drawn as a double-lined arrow, signalling and/or control traffic associated with and required for the transmission of said stream data is drawn as a single-lined arrow. Stream data in the sense of this invention denotes streaming data such as audio data, video data, etc. 
     The laptop computers  11  and  12  are directly logged in at the conference bridge server  30  for the transmission of both stream traffic  63 ,  64  and signalling traffic  73 ,  74 . Likewise, the terminal  20  is directly logged in at the conference bridge server  30  for the transmission of both stream traffic  65  and signalling traffic  75 . Output stream data  66  destined for the laptop computers  11  and  12  is not transmitted to the laptop computers  11  and  12  but is transmitted to the common conference phone  10  where it is delivered, by means of a loudspeaker  102 , to the participants associated with the laptop computers  11  and  12 . 
     The laptop computers  18  and  19 , however, are only logged in at the conference bridge server  30  in a limited manner. The stream data  61 ,  62  originating from the laptop computers  18  and  19  is not sent directly to the conference bridge server  30  but is sent to the conference bridge server  30  via the laptop computer  12  serving as a relay station. The reason may be, e.g., that there are not enough audio ports at the conference bridge server  30  to allow the direct connection of the laptops  18  and  19 . As far as signalling data traffic is concerned, the laptops  18  and  19  directly exchange signalling traffic  72 ,  73  with the conference bridge server  30  and do not use the laptop computer  12  as relay station.