Abstract:
A distributed multipoint conferencing system ( 3 ) comprises a plurality of participating terminals ( 31;32;33 ) for sending and receiving media streams in a conferencing interchange; the system further includes at least one multipoint distributor ( 30 ) (MD) connected to one terminal ( 31 ) for receiving at least one or all media streams from that terminal ( 31 ) but not from any other of the participating ( 32;33 ); the multipoint distributor ( 30 ) is connected to the other participating terminals ( 32;33 ) for sending the media stream or streams received from the MD-associated terminal ( 31 ) to the other participating terminals ( 32;33 ).

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to the exchange of electronic data between a plurality of terminals and specifically to distributed multipoint conferencing. 
     BACKGROUND OF THE INVENTION 
     According to the present industrywide accepted standard for multimedia conferencing over packetbased networks, i.e. the ITUT Recommendation H.323 (also termed H.323 standard for short) a distinction is made between differing types of multipoint conferencing, depending upon the way in which media streams are distributed, namely either centralized multipoint conferencing or distributed multipoint conferencing. The term “media stream” as used herein is intended to refer to audio streams, video streams, text streams, shared object streams (e.g. shared window streams), shared remote pointer streams and the like bit streams that are connected with conferencing. 
     In centralized multipoint conferencing all participating terminals send their media streams to a multipoint processor (MP) via unicast (pointtopoint) deliveries. The multipoint processor processes, (i.e. mixes or switches) all media streams received from the terminals and sends the resulting streams back to the terminals. In the case of audio media streams, signals sent from a terminal cannot be sent back to that source because of echo problems. That is one reason why MP implementations tend to be complex and expensive. 
     In distributed multipoint conferencing, on the other hand, all participating terminals send their media streams to all other participating terminals, and each terminal does its own mixing and/or switching of the media streams. Thus, there is no need for a multipoint processor. The terminals can send their media streams to the other terminals either via a multicast delivery (assuming that the network supports multicast) or via multiple unicast deliveries (multiunicast) if the network does not support multicasting. 
     In addition to these two methods, the H.323 standard specifies a hybrid-type multipoint conferencing in which the centralized approach applies to some media streams while the distributed approach applies to other streams. For example, audio streams are distributed directly to the terminals while video data streams are processed by a central video multipoint processor. 
     In addition to the specifications for media streams, the H.323 Recommendation also specifies the control architecture of a multipoint conference. Participating terminals communicate only via a multipoint controller (MC) which serves, inter alia, to invite and add new participants, to select the appropriate media codecs, to determine media distribution methods, and to inform the terminals of the addresses to be used for sending and receiving media streams. 
     When comparing the two multipoint conferencing methods just mentioned, the distributed approach has the advantage of not requiring an expensive MP. Furthermore, it scales well for large numbers of conferences because of its distributed structure. However, the number of participants within a conference is limited by the number of media streams which a terminal can process in parallel, and by the total network capacity consumed by the distribution of the media streams. The second limiting factor becomes critical when the network is accessed by a terminal via a low capacity link, and when multiunicast is to be used, e.g. because multicast is not supported by the network; this latter condition currently is the case with the majority of IPbased networks. 
     For a distributed multiunicast conference with N participating terminals, a terminal is required to send its media streams to the other (N−1) terminals and also to receive the streams sent by those (N−1) terminals. Consequently, the capacity required for a specific media by a terminal on its access link is B=2*(N1)*b, with b representing the capacity required by a single stream. 
     Assuming that a terminal accesses the network via a standard-type 28.8 kbit/s modem and G.723 is used for audio: with a G.723 audio stream consuming 6 kbit/s, such a terminal would be able to participate in an audio conference with only three parties. 
     Accordingly, a primary object of the present invention is to provide for a new distributed conferencing system and method capable of significantly reducing the link capacity required for one or more of the participating terminals. 
     SUMMARY OF THE INVENTION 
     This object and further advantages that will become apparent as this specification proceeds are achieved, in a first general embodiment of the present invention by a distributed multipoint conferencing system formed by a plurality of participating terminals for sending and receiving media streams in a conferencing interchange; the system further including at least one multipoint distributor (MD) connected to one of the terminals for receiving at least one or all of the media streams from that terminal but not from any other of the participating terminals; the multipoint distributor, on the other hand, is connected to the other participating terminals only for sending to them the media stream, or media streams, that it receives from its associated terminal; the other participating terminals receive additional media streams directly from any of the other participating terminals. In other words, any terminal that is associated with an MD, sends at least one or all of its media streams to the MD while it receives media streams either directly from the other terminals or from other MDs. By the same token, any terminal that is not associated with an MD in this manner sends its media streams directly, i.e. not via an MD, to all other terminals. Typically, up to ten terminals can participate in a conferencing system according to the invention and one, or more, or all of the terminals, can be associated with an MD of their own. As a matter of practice, an MD could be implemented in any general-purpose computer that includes a networking sub-system. 
     According to a preferred embodiment of the invention, the system includes high capacity linked terminals (HCL-terminals) mutually interconnected for sending and receiving media streams while the at least one terminal that is connected with the multipoint distributor is a low capacity linked terminal (LCL-terminal). However, connection of the multipoint distributor to an LCL-terminal is not a critical requirement. On the other hand, a conferencing system according to the invention may include more than one multipoint distributor in which case each multipoint distributor receives at least one media stream from its associated terminal; by the same token, all terminals of the system could be connected to, or supplied with, an associated multipoint distributor. 
     According to a further embodiment, the present invention provides for a distributed multipoint conferencing method between a plurality of participating terminals for sending and receiving media streams in a conferencing interchange; the method further including providing at least one multipoint distributor connected to one of the terminals for receiving at least one media stream from its associated terminal but not from any other of the participating terminals; the multipoint distributor being connected to the other participating terminals for sending to them the media stream or media streams received from its MD-associated terminal. The other participating terminals may also receive additional media streams directly from any of the other participating terminals. 
     Again, the conferencing method according to the invention may include one or more HCL-terminals and one or more LCL-terminals, wherein any LCL terminal is associated with its multipoint distributor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be discussed in more detail with reference to the enclosed drawings intended to illustrate the invention without limiting it. In the drawings: 
     FIG. 1 is a diagrammatic view of a prior art centralized multipoint conferencing system including a number of terminals linked via modem/telephone connections to a common multipoint processor; 
     FIG. 2 is a diagrammatic view of another prior art system for distributed multipoint conferencing by way of multi-unicast; and 
     FIG. 3 is a diagrammatic view of a distributed multipoint conferencing system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventive system and method will now be discussed in more detail with reference to the figures. Therein, FIG. 1 is a diagrammatic illustration of centralized multipoint conference system  1  according to the art as defined by the H.323 standard. It includes a number of terminals  11 ,  12 ,  13  etc. each of which is linked to a multipoint processor  10 . It is to be noted that the number of terminals could be smaller or greater than shown in FIG.  1 . While only one pair of arrows  10 ′, 11 ′;  10 ″, 12 ′; and  10 ′″,  13 ′ is shown between multipoint processor  10  and terminals  11 ,  12 , and  13 , respectively, this does by no means imply that only one media stream is sent in each direction but that any number of media streams, in line with conferencing requirements and system capacities, is exchanged. It is to be noted however, that currents for transmitting media streams  10 ′,  10 ″, and  10 ′″ emanating from multipoint processor  10  are not identical but different for each participating terminal  11 ,  12 , and  13 . 
     FIG. 2 illustrates another prior art system according to the H.323 standard, i.e. a distributed multipoint conference system (multi-unicast)  2  formed of a number of terminals  21 ,  22 ,  23  which send their media streams  21 ′,  21 ″,  22 ′,  22 ″,  23 ′,  23 ″ directly to each other. The exact number of terminals again is not limited to three terminals as shown in FIG. 2 and, again, each arrow pair  21 ′, 22 ′;  22 ″, 23 ′; and  21 ″, 23 ″ may represent any number of media streams in line with conferencing requirements and system capacities. 
     FIG. 3 is a diagrammatic illustration of a distributed multipoint conferencing system  3  according to the invention formed by a plurality of terminals  31 ,  32 , 33  with terminal  31  being associated with a multipoint distributor  30 . Again, the exact number of terminals is not limited to three terminals as shown in FIG. 3 and, here again, each arrow  30 ′,  30 ″,  31 ′,  31 ″,  32 ′,  32 ″, 33 ′, and  33 ″ may represent any number of media streams in line with conferencing requirements and system capacities. It is to be noted however, that the currents or media streams  30 ′,  30 ″ emanating from multipoint distributor  30  are the same as data stream  31 ′ that is sent from terminal  31  to multipoint distributor  30 . 
     As in the conventional distributed conferencing system, all participating terminals  32 ,  33 , except terminal  31  associated with the multipoint distributor  30 , send their media streams  32 ′,  32 ″,  33 ′,  33 ″ directly to the other terminals. Terminal  31  gets support from the multipoint distributor  30 . Instead of sending (N−1) media streams, terminal  31  sends only a single stream  31 ′ to its MD  30 . MD  30  will then redistribute the stream  31 ′ further to the other terminals  32 ,  33  via the streams  30 ′,  30 ″. 
     The multipoint distributor  30  distributes a media stream in the following manner: it receives a piece or portion of media data from a terminal, puts the data into a buffer and sends the buffered data to each other terminal. It repeats the procedure for each piece of received media data. The multipoint distributor  30  could, for example, be implemented as a piece of hardware, e.g. a chip, or as a piece of software in a general-purpose computer with a networking sub-system. 
     No media processing is done by the MD and every terminal  31 , 32 , 33  still receives all (N−1) streams sent by the other and process them locally. As a consequence, the advantage of the distributed system will be preserved, because no complex multiprocessor is required as in system  1  (FIG.  1 ). The system and method according to the invention are scaleable for a large number of conferences because the MD  30  is quite simple to implement since it redistributes only those media streams which it receives from the associated terminal  31  to the rest of the terminals  32 ,  33  etc. 
     Because any terminal  31  sends only its streams to its associated MD  30  (instead of to all other terminals), the capacity required by a terminal for a single media is reduced from 2*(N−1)*b to N*b. 
     Taking as an example a terminal  31  with a 28.8 kbit/s modem, this terminal would be able to participate in a four party audio conference. It is to be noted, however, that if it participates in a three party conference using the distributed multipoint conferencing system  3  according to the invention or the corresponding method embodiment, some capacity would remain available for other applications. In contrast, a conventional distributed system  2  would consume the entire capacity of the access link. 
     The distributed multipoint conferencing system according to the invention and method is very attractive for asymmetric access configurations, with little capacity on the up-link to the network and large capacity in the opposite direction, e.g. ADSL (asynchronous digital subscriber), or for hybrid accesses with normal modem for up-link and cable modem for down-link. In these situations, the limited up-link capacity is only used by the terminal to send its streams to its MD while it receives the (N−1) streams sent by the other terminals via its down-link. 
     It is to be noted that a MD is only required for those terminals that require support, i.e. the LCL-terminals and not the HCL-terminals. 
     With the H.323 standard with its centralized control architecture, a terminal can inform the MC about its need for an MD during the H.245 capability exchange phase. The MC will then create the required MD accordingly and instruct the requesting terminal to send its media to the created MD. For example, the MD can be hosted by the same device which contains the MC of the conference, or as close as possible to the LCL-terminal that requires it, or somewhere in the network based on conventional optimization rules, e.g. as close as possible to the receivers. 
     The following specific, but not limiting, example illustrates how the invention can be used in a conference by means of a conferencing system with three participating terminals T 1 , T 2 , T 3  and a multipoint distributor MD. While terminals T 1  and T 2  send their media streams directly to the other terminals, terminal T 3  uses the multipoint distributor to distribute its media streams to terminals T 1  and T 2 . 
     Assuming the terminals use the procedures defined in H.323 to set up a conference call on an IP network, one possibility to create a conference is to start with a point-to-point call between T 1  and T 2  and expand it into a conference, the so-called ad-hoc conference expansion. Assuming T 1  invites T 2 , and assuming T 1  has been elected Master, following the H.323 Master/Slave determination procedure, T 1  creates an MC which has a signaling connection to itself and T 2 . T 1  then starts the H.323 invitation procedure by tunneling a signaling connection through the MC to T 3 . T 3  follows the H.323 procedure to accept the invitation. Assuming the H.245 messages for capability description have been extended to describe the capability of a terminal to use a multipoint distributor and, assuming T 3  advertises such a capability in its TerminalCapabilitySet message, the MC learns about that capability. When the H.323 invitation procedure completes successfully, T 3  starts the H.323 procedure to open a logical channel for a media stream by sending an OpenLogicalChannel message to the MC. The MC then sends an OpenLogicalChannel message to T 1  and T 2 . T 1  prepares itself to receive a media stream and sends the transport address where it is listening for the media stream to the MC in an OpenLogicalChannelAck message. The same happens for T 2 . The MC then instructs the multipoint distributor with a signaling message to receive the media stream from T 3  and to distribute it to T 1  and T 2 . The multipoint distributor acknowledges that request and informs the MC about the transport address where it listens for media data with a signaling message. The MC sends an OpenLogicalChannelAck message to T 3  to inform it that it needs to send the media stream to the multipoint distributor. Then T 3  starts to send media data to the multipoint distributor which distributes it to T 1  and T 2  as previously described. 
     Generally, the invention provides for a distributed conferencing system and method in which the media streams (e.g. audio, video) are mixed or switched by the participating terminals themselves. This is of particular advantage in the case of conferences with a small number of participants (typically up to 5, 6 or 7) because it does not require the assistance of an expensive multipoint processor. The trade-off consists in the fact that a distributed conferencing system according to the art requires substantially more network capacity than a centralized system, which becomes a problem whenever a conferencing participant accesses the network via a low capacity link. This disadvantage of prior art distributed conferencing systems can be substantially reduced with the system and method according to the invention which reduces the required capacity to roughly half of the capacity required by distributed conferencing systems according to prior art. 
     Many modifications of the system and method disclosed herein will be apparent to those experienced in the art. For example, the telephone connection could be effected by wire or by wireless transmission. Generally, any type of interconnection between the terminals that provides the required data transmission capacity is applicable and the scope of the invention is to be construed on the basis of the following claims.