Abstract:
A distributed packet-based audio conferencing system, method for packet-based audio conferencing, and a transceiver for use in such conferencing are disclosed. The system uses a collection of transceivers, with each conference participant connected to a local transceiver. When a participant speaks, the local transceiver is responsible for relaying the speaker&#39;s voice over a packet network by multicast transmission to transceivers local to each other conference participant. If multiple participants speak simultaneously, a multicast talk stream may originate from each speaker&#39;s local transceiver. The total number of simultaneous speakers, however, is limited by an arbitration function resident in each transceiver. 
     The system reduces the costs associated with an always-up communication system. Compared to a data network solution employing a centralized bridge, the present conferencing system enjoys lower delay, lower bandwidth requirements, the ability to utilize voice compression throughout, and ease of reconfiguration.

Description:
FIELD OF THE INVENTION 
     This invention pertains generally to multi-point remote audio conferencing, and more particularly to such conferencing utilizing packet data networks. 
     BACKGROUND OF THE INVENTION 
     A hoot n&#39; holler network is a multi-point four-wire audio conference network that is always ‘up’. When someone wants to communicate over the network, they push a button and speak either through a microphone, handset, or squawk box. The button does not cause any signaling to occur. It simply enables the audio that is normally disabled to prevent noise from being injected into the network when the person is not speaking. 
     Hoot n&#39; holler networks are used throughout the brokerage industry to communicate “The Morning Report” as well as to advise the trading community within a brokerage firm on market movements, trade executions, and so on. A typical brokerage firm will have several of these networks for equity, retail, bonds, etc., with size and the degree of interactivity varying depending on the application. 
     The hoot n&#39; holler system is not specific to the brokerage industry. Many other industries require collaboration on a regular basis. Some of these collaborations can be done via a scheduled conference call, but in problem-solving situations, an ad-hoc conference over a permanent hoot n&#39; holler network would be more efficient. 
     Hoot n&#39; holler networks are typically spread over four to eight sites, although retail networks may have as many as 500 sites interconnected. Within a site, bridging is done locally with a standard audio or digital bridge circuit. Between sites, the bridging is often provided by a phone carrier. The carrier provides dedicated (either analog or digital) four-wire connections into a central bridge. The customer is charged on a monthly basis for the service, with most of that cost attributed to leased-line point-to-point connections between the various sites and the bridge. 
     FIG. 1 shows an example hoot n&#39; holler network  20 . Conference sets  22  at each remote location are connected to a central bridge  24  via leased lines  26 ,  28 ,  30 , and  32 . Four wire connections and N−1 bridges are used to avoid echo problems (an N−1 bridge mixes up to N possible input signals, but subtracts out each speaker&#39;s contribution individually for the mixed signal being transmitting back to him). 
     SUMMARY OF THE INVENTION 
     Hoot n&#39; holler networks are costly, particularly because they require dedicated leased lines for their operation. The present invention provides a packet data solution to an audio conferencing system that replaces the leased lines with data network connections. The typical hoot n&#39; holler network has less than 15% overall utilization—as the present invention utilizes data network bandwidth primarily when someone is actually speaking, the present invention can provides significant cost savings, compared to dedicated connections, for the typical hoot n&#39; holler user. 
     The present invention does more than merely replace the leased lines of FIG. 1 with packet data connections. The present invention removes the requirement for a central bridging site altogether. Instead, direct connections between conferencing system endpoints are provided “in-the-cloud” of the data network, e.g., using multicast packet transmission. When a conference participant speaks, a packetized version of their voice is multicast to all other participants. Transceivers local to each conferencing system endpoint provide speaker arbitration and/or mixing necessary for the system to operate. This system provides several advantages: it allows data network bandwidth reduction by avoiding multiple unicast signals, allows voice compression to be used throughout the conference to further reduce bandwidth, is easily rescaled and reconfigured, and eliminates a portion of the delay inherent in a system requiring communications to-and-from a central site. 
     In one aspect of the present invention, a distributed packet data network conferencing system is disclosed. This system comprises multiple packet data conferencing transceivers interconnected via a packet data network. Each transceiver is also connected to at least one conferencing endpoint local to that transceiver. The transceiver comprises two data flow paths—a first path from the local conferencing endpoint to the network, and a second path from the network to the local conferencing endpoint. Each transceiver implements a switchable connection in the first path controlled by an arbitrator within the transceiver. 
     The conferencing transceivers utilized in the system above comprise a further aspect of the present invention. In particular, the arbitrator contained in each transceiver enables distributed conferencing by providing control over the number of simultaneous talk streams present in the system. Preferably, the arbitrator monitors source attributes of conference data appearing on both data paths. The arbitrator opens the switch to prevent transmission of the local signal when the number of sources of conference data exceeds a preset maximum and the local signal loses an arbitration against the remote sources. The arbitrator may preferably also provide guidance to an input selector inserted in the second data path, allowing it to discard packets from remote sources that have lost arbitration. 
     In yet another aspect of the present invention, a method for packet data conferencing between multiple remote conference participants is disclosed. The method comprises transmitting each remote conference participant&#39;s voice to other remote conference participants by transmitting a transmit packet data talk stream from a transceiver local to that conference participant to a multicast group address. The method further comprises receiving, at each transceiver local to a remote conference participant, receive packet data talk streams from other remote conference participants sent to the multicast group address. At each transceiver, that transceiver&#39;s transmit packet data talk stream source is ranked against each receive packet data talk stream source. When the total number of packet data talk streams sensed by that transceiver exceeds a preset maximum and the transmit packet data talk stream source ranks lowest, that transceiver inhibits transmission of that transceiver&#39;s transmit packet data talk stream. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention may be best understood by reading the disclosure with reference to the drawing, wherein: 
     FIG. 1 illustrates a prior art hoot n&#39; holler network; 
     FIG. 2 illustrates a packet data conferencing system employing a centralized bridge; 
     FIGS. 3 and 4 show a distributed packet data conferencing system according to one embodiment of the present invention, with one and with two speakers respectively; 
     FIG. 5 shows a distributed packet data conferencing system, illustrating several connection options for a system according to an embodiment of the invention; 
     FIGS. 6 and 7 contain speaker arbitration timing diagrams; and 
     FIGS. 8-12 contain block diagrams of conferencing transceiver embodiments according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention generally applies to data network audio conferencing systems (although audio-plus conferencing systems, such as audio-plus-video, may also utilize the invention). Systems according to the invention require at least one transceiver for each remote site in the conferencing system. The transceiver may take several physical forms: it may be contained in a network-connected conference set sitting on a participant&#39;s desk; it may be contained in a personal computer; it may be a stand-alone unit connected between a phone terminal and a network bridge or router or connected only to a network bridge or router; or it may be partially or totally subsumed in a network voice gateway or router. 
     The following terms have the following meanings for purposes of this disclosure. A transceiver relays data bi-directionally between two interfaces. Of course, if both interfaces are packet data interfaces, they may be implemented using the same physical interface. A transceiver interface is “remote” when communication over the interface utilizes group addressing. Conversely, a transceiver interface is “local” when the connection utilizes addressing (or hard-wiring) to a specific conference point or points. A conferencing point may, e.g., comprise a single conferencing set (e.g., a speakerphone) or a local bridge connected to multiple conferencing sets. From the standpoint of a transceiver, both local conferencing points and other transceivers are “sources” of conference data. 
     A Packet Network Centralized Solution 
     A data network adaptation of a conferencing system as shown in FIG. 1 is not trivial. In what might appear at first blush to be a straightforward approach, leased lines  26 ,  28 ,  30 , and  32  to and from central site N−1 voice bridge  24  could be replaced as shown in FIG.  2 : conference sets  22  would instead connect to voice gateways  42 , and an N−1 voice bridge  44  would connect to a separate voice gateway  46 . Gateways  42  and gateway  46  connect to IP (Internet Protocol) network  48 . Such a system  40  could use voice activity detection (VAD) to reduce traffic into the central site, e.g., by only transmitting packets for those speaking. But creating separate traffic out of the voice bridge for each remote location could swamp network  48  during conferencing. Furthermore, gateway  46  and voice bridge  44  hardware preferably would be required to scale as the number of users of a conferencing network increased. 
     Besides bandwidth and scaling, this centralized data network conferencing solution has several additional difficulties related to the IP implementation. One difficulty is the added delay caused by buffering both at central gateway  46  and at the endpoint gateways  42 . At both locations, arriving packets must be buffered for a sufficient time to encompass packet jitter, i.e., variances in sequential packet arrival rate, and ensure smooth playout. These dual buffer delays, as well as the additional algorithmic delay associated with dual compression and decompression, add to create an overall delay that may make interactive conferencing difficult. 
     A second additional difficulty precludes the use of voice compression, a common network bandwidth reduction tool, within the conference. Gateways  42  may compress, or encode, voice traffic before sending it to gateway  46 . Gateway  46  must decode this traffic before passing it to bridge  44  for mixing. After bridge  44  mixes the conference traffic, it passes it back to gateway  46 . If gateway  46  encodes the mixed traffic to reduce bandwidth, this produces a “tandem encoding” of the signals that were just decoded and mixed. Many low-bit-rate encoders produce particularly poor audio quality for tandem encoded data—poor enough that conferencing system  40  of FIG. 2 may be precluded from using compression algorithms to reduce bandwidth. 
     Some, but not all, of the problems of this centralized approach may be mitigated by adding complexity to gateways  42 , gateway  46 , and voice bridge  44 . For example, asymmetric transport may be used to reduce IP traffic and the load on gateway  46  and bridge  44 . With asymmetric transport, sending gateways  42  unicast their outgoing conference traffic to central gateway  46 . However, central gateway  46  may multicast a mixed conference signal to all endpoint gateways  42 . For best performance, this system would require creation and transmission of, in addition to a multicast mixed signal, several N−1 signals—one for each sending gateway  42 . Receiving gateways  42  would also be responsible for selecting the unicast, rather than the multicast, return signal when they were transmitting. 
     The problem of tandem encodings may be solved, at some bandwidth expense, by asymmetric coding. In a preferred asymmetric coding approach, sending gateways  42  send packets of uncompressed PCM samples to central gateway  46  and on to bridge  44 . Gateway  46  compresses the returned mixed signal from bridge  44  before multicasting it back to endpoint gateways  42 . Since a typical scenario comprises many more listeners than talkers, compression of the mixed signal decreases bandwidth greatly, while the additional bandwidth borne by the sending channels results in greatly increased conference voice quality. 
     Finally, the problem of resource scaling may be at least partially addressed at central gateway  46  or at gateways  42  by limiting the number of possible simultaneous talk streams. Gateway  46  may arbitrarily, or according to some rule, select talk streams up to a maximum fixed number of incoming talk streams to pass to bridge  44 . Alternatively, or additionally, bridge  44  could affix the number of current contributing sources to outgoing mixed signal packets. Gateways  42  could then suppress outgoing talk streams if the mixed signal count was at or near its limit. Note that the Real-time Transport Protocol (described below) provides a mechanism for identifying both the number of sources and the identity of each source contributing to a stream. 
     Distributed Conference Operation 
     A centralized packet conferencing system as described above contains several undesirable features. Talkers must be connected to the packet network through a connection capable of supporting an uncompressed voice data rate. Listeners must endure two separate jitter-induced delays. Someone is responsible for setting up and maintaining a smart bridge, and configuring smart gateways as well. The present invention overcomes these difficulties by removing central gateway  46  and bridge  44  and replacing them with a distributed conferencing system. 
     The present invention places all conferencing functions near the endpoints of the conferencing system, thus isolating them to the greatest extent possible from disadvantageous peculiarities of the IP network. The endpoint transceivers each decide who will talk and who will be heard. Preferably, talk streams cross the IP network only once—this both avoids tandem packet delay buffers and allows originating talk streams to be compressed. Conference participants all receive the same multicast signals, such that participants can be freely added and deleted from a conference without reconfiguring each transceiver. 
     In almost all conferencing situations, only one or a few participants speak at once. By using VAD, data packets need only be transmitted for those participants currently speaking. For instance, in conference system  50  of FIG. 3 a single speaker  62  is active. Gateway  64  multicasts speaker  62 &#39;s voice to gateways  66 ,  68 ,  70 , and  72  at other conference locations. No N−1 bridging is required; gateway  64  simply and locally performs the “N−1” function, as it does not receive its own transmissions. Gateways  66 ,  68 ,  70 , and  72  pick up the multicast signal, relay it to their respective endpoints  22 , and the conference communication path is complete. 
     FIG. 4 shows two speakers  62  and  84  utilizing conference system  50 . If both speakers are allowed to talk at once, gateways  64  and  66  will each multicast a talk stream from their respective speaker to each other gateway in system  50 . Each gateway receiving two talk streams (i.e., gateways  68 ,  70 , and  72 ) will either mix the two speakers&#39; voices or arbitrate and select one speaker&#39;s voice to pass to conference sets  22 . Mixing and arbitration are discussed in the following section. 
     FIG. 5 shows some of the endpoint options for a conferencing system according to the present invention. In FIG. 5, two conference sets  22  and a receive-only conference set  76  are illustrated separately connected to gateway  90  at one remote location. Also, a local bridge  74  (either analog or digital) is shown connected to gateway  92  at another remote location. Bridge  74  mixes its two local streams with each other and with the remote talk streams passed through gateway  92 . Generally, non-network local conferencing systems of different sizes may be connected as conferencing endpoints to a data network remote conferencing system according to the invention. Note that a local bridge appears as a single speaker to its network transceiver. 
     FIG. 5 illustrates one further endpoint configuration. Transceiver  112  performs conference set functions and conference data packet functions. This transceiver connects directly to IP network  52  and needs no voice gateway. It handles receive and transmit data packets and includes an audio interface for a speaker. Transceiver  112  may, e.g., be implemented on a typical desktop computer running VoIP software incorporating one or more of the conference-specific functions described below. 
     Mixing and Arbitration 
     A centralized bridge performs two desirable functions: it combines incoming talk streams to create a single outgoing talk stream for each endpoint, and it ensures that all participants hear the same conference. The distributed system of the present invention preferably either emulates these bridge functions or provides an acceptable alternative. Unless a distributed system provides some sort of distributed arbitration function, however, there is at least the possibility for a large number of talk streams to be occasionally directed to a given endpoint. 
     As long as the current number of speakers in a distributed system does not exceed a preset maximum, transceivers may allow mixing of all speakers. In this mode, transceivers “play out” packets received from each active speaker into a mixer, and the mixer output is conveyed to the local conference set. 
     When the number of speakers in a conference exceeds the intended playout capabilities of the system, the distributed transceivers must provide some type of arbitration. One type, receiver arbitration, has receivers decide whether to keep or throw away packets from each received source based on some decision criteria. Sender arbitration, on the other hand, has senders arbitrate and decide whether to send and/or whether to continue sending their particular talk stream. Mixed arbitration uses a combination of sender and receiver arbitration. 
     Real-time Transport Protocol 
     Arbitration with the present invention may advantageously be based on information gleaned from existing data transport protocols. One transport layer data protocol useful with the present invention is the Real-time Transport Protocol (RTP). RTP is a potential standard described in RFC 1889, a Request for Comments generated by the Network Working Group. RTP provides network transport functions suitable for real-time audio and video, including multicast functionality. RTP does not guarantee a quality of service or prevent out-of-order packet delivery. RTP also includes RTCP (Real-time Transport Control Protocol), which is generally used for monitoring a session and conveying information about its participants. 
     Each RTP packet has a header that is at least three 32-bit words in length, following this format:                           
     The first header word contains a version field (V), a padding bit (P), a header extension bit (X), a CSRC count field (CC), a marker bit (M), a payload type field (PT), and a sequence number field. The second word contains a packet timestamp, and the third word contains a synchronization source (SSRC) identifier. If data from several RTP sources is present in the packet (e.g., because the sources were mixed), the CC field specifies the number of contributing sources, and an additional word is attached to the header for each contributing source—each of these words contains a contributing source (CSRC) identifier, which is essentially the SSRC of the contributing source. 
     Several of these RTP header fields are potential candidates for use with source arbitration. For instance, both the sequence number and the timestamp are randomly initialized for each source. The sequence number increments by one for each RTP data packet sent, while the timestamp reflects the sampling instant of the first octet in that RTP data packet, and generally increments at the sampling frequency of the data. The SSRC identifier may also be useful; it is randomly generated, is guaranteed to be unique for each source, and is generally fixed for the duration of an RTP session. Finally, the header extension bit may be enabled and arbitration information may be placed in a valid RTP header extension. 
     SSRC Arbitration 
     The preferred method of performing RTP source arbitration in an embodiment of the present invention is to apply arbitration rules to the source-unique received-packet SSRC identifiers. SSRC arbitration generally would break ties caused when several sources desire to simultaneously transmit and all cannot be received by the endpoints. For instance, the source(s) with either the lowest or highest SSRC(s) may always be declared the winner(s). 
     A simple case of an SSRC arbitration embodiment of the present invention is one using a single multicast group with no mixing. In this case, if two or more persons attempt to speak simultaneously an SSRC-based arbitration function will choose the one speaker who will be heard by all transceivers. If the arbitration is done by the sending transceivers, each may compare its own SSRC to that of its competitors and sever broadcasting if it loses arbitration. If the arbitration is done by the receiving transceivers, they compare SSRCs from all received talk streams and select a winner. Preferably, and as shown in the following example, each transceiver employs a combination of sender and receiver arbitration. 
     Each endpoint transceiver may employ the following arbitration rules: 
     1. A local sending endpoint will be arbitrated out if its transceiver has already begun receiving a remote talk stream. 
     2. Sending transceivers perform SSRC arbitration if they receive an incoming RTP talk stream during an initial “arbitration time” after they begin transmitting; this time period in some embodiments equals an estimated worst case round-trip delay, and in others may be indefinite in length (i.e. continuous arbitration). 
     3. Sending transceivers who lose arbitration because they have a lower SSRC will stop sending, at least until the end of the winner&#39;s current talk spurt. 
     4. Receiving transceivers that receive multiple RTP talk streams will pick the one with the highest SSRC and ignore the rest. 
     FIG. 6 shows respective timelines  120 ,  122 ,  124 , and  128  for each of four transceivers A, B, C, and D in a distributed conferencing system. FIG. 6 illustrates the timing realities of a distributed conference with finite transport delays—although each speaker begins speaking at approximately the same “real” time, each transceiver has a distinctly different view of when each speaker began speaking. The arbitration takes place over a finite time; at the end of this time, 1) all receivers should have chosen the same talk stream as the winner, despite their different views of the world, and 2) senders should know whether they have won or lost arbitration. If losing senders turn off their transmission, traffic will quickly reduce to a single multicast talk stream. 
     In the example of FIG. 6, the vertical bars show when each speaker begins speaking, from each listener&#39;s viewpoint. The horizontal bars show which talk stream is chosen by a listener&#39;s transceiver as the arbitration winner. SSRC priority ranks from “A” to “D”—sender “A” should lose all arbitrations, and sender “D” should win all arbitrations. As viewed in a common timeframe, A speaks first, followed in order by B, C, and D. From the viewpoint of transceivers A, B, and C, each speaks first, and so each begins transmitting. Although D has the highest SSRC, it actually receives packets from A before it begins transmitting, and so D&#39;s voice is never transmitted. 
     A switches talk streams two times. It begins with its own talk stream. Well into its arbitration period, A receives B&#39;s talk stream and turns off its own. Finally, near the end of the arbitration period, A receives C&#39;s talk stream and begins ignoring B&#39;s. 
     B only switches talk streams once. It also begins with its own talk stream. When it receives A&#39;s talk stream, it wins this arbitration, and thus ignores A. When it receives C&#39;s talk stream, B loses and turns off its own talk stream. 
     C never switches talk streams. It begins with its own talk stream, and never receives a talk stream from a source with a higher SSRC. C thus becomes the active speaker for the conference. 
     D, like A, first likes A, then B, then C. Note that if D did no receiver arbitration, it would still eventually end up receiving only C once A and B stopped sending. 
     Although such speaker collisions may be infrequent in practice, audible effects of these collisions may preferably be avoided by delaying playout of any signal until the end of the arbitration period. This delay avoids what may be disconcerting or annoying garble due to source switching on some transceivers during arbitration. FIG. 7 shows this concept for the same scenario as FIG.  6 . Although arbitration proceeds identically to that of FIG. 6, source playout is delayed such that each transceiver plays only C. One method of accomplishing this is to set the jitter buffer delay initially to be at least equal to the arbitration time, and then allow the buffer to be flushed if an arbitration is lost during this delay. In many circumstances, the arbitration time will pose a reasonable delay for the jitter buffer as both are related to worst-case transport time. 
     These arbitration methods scale naturally to conferencing systems allowing two, or more generally, “n” simultaneous speakers to be mixed. For example, in a two-speaker system no arbitration need be performed unless packets from a third speaker arrive within one arbitration time from the arrival of packets from a second speaker. 
     Other Forms of Arbitration 
     Speaker prioritization with the above system is random. RTP specifies that each source will create a random SSRC for its session—the highest random SSRC will always trump with the “highest SSRC rule.” If the conference is left “up” for an extended period, by the RTP definition this initial SSRC assignment would create a fixed prioritization. Prioritization may be varied by having transceivers periodically change SSRCs when they are not transmitting (although this may confuse systems doing monitoring). Or, a desired speaker prioritization could be hardcoded by forcing a nonrandom SSRC generation. But if an arbitration period is used, prioritization should preferably only come into play in unusual circumstances where speakers start speaking within a few hundred milliseconds of each other. 
     One other characteristic of an RTP multicast group is that each group member periodically transmits Real-time Transport Control Protocol (RTCP) packets. RTCP carries a persistent transport-level identifier for an RTP source called the canonical name or CNAME. Since the SSRC identifier may change if a conflict is discovered or a program is restarted, receivers require the CNAME to keep track of each participant. 
     Transceivers may utilize the CNAME RTCP field to implement arbitration. For instance, each transceiver may keep a ranked list of network subscribers by name. This name is transmitted in the CNAME field of RTCP packets sent from each local transceiver, and can be coordinated by each transceiver with the appropriate SSRC for that subscriber&#39;s RTP transmissions. After coordination, that SSRC is given a ranking according to the CNAME ranking. Each person&#39;s arbitration ranking is guaranteed, no matter what random SSRC they are assigned. 
     Other types of arbitration may work most of the time, although not as simply as SSRC arbitration. RTP sequence numbers and timestamps may not provide reliable arbitration: they are not guaranteed to be unique for every source (although duplication among sources is highly unlikely), and they change with every data packet. This latter issue becomes a problem when not all transceivers receive the first packet from a source (remember that RTP does not guarantee packet delivery). A transceiver receiving a second packet (thinking it to be the first) could conceivably produce a different arbitration result that other transceivers. 
     RTP header extension arbitration is another possibility. Instead of comparing SSRC values, arbitration could compare information stored in an RTP header extension. This information could be carried only for endpoints having override or “super priority”. It could also resemble a fixed SSRC scheme and be carried for all participants. 
     Finally, arrival-time arbitration is a valid option for networks that allow a larger number of simultaneous speakers, generally three, four, or more. For a single-speaker system, a simple first-in-time-wins receiver arbitration scheme generally cannot work without source synchronization. First-in-time cannot guarantee consistent conference-wide arbitration results, as source arrival sequence may differ depending on each endpoint&#39;s view of the network. 
     Transceiver Hardware Configurations 
     A block diagram for a single-speaker conferencing transceiver  140  is shown in FIG. 8. A PCM interface  162  links the transceiver to a conference set (or local bridge). An IP interface  164  links the transceiver to an IP network. By definition, transceiver  140  operates as a “sender” when it is transferring data from PCM interface  162  to IP interface  164 , and operates as a “receiver” when it is transferring data in the opposite direction. 
     Transceiver  140  contains several well-understood blocks. Speech encoder  142  and decoder  156  preferably implement widely available compression algorithms, such as those described in ITU Recommendations G.726 (Adaptive Differential Pulse Code Modulation), G.728 (Low-Delay Code Excited Linear Prediction), G.729 and G.729 Annex A (Conjugate Structure Algebraic-Code-Excited Linear Prediction), and G.723.1 (Multi-Pulse Maximum Likelihood Quantizer). A simple encoder  142  may implement standard PCM (ITU Recommendation G.711), which results in no compression. A voice activity detector may also operate as part of encoder  142 . The operation of output packet processor  144  also is generally well understood—processor  144  takes one or more encoded frames of data from encoder  142 , places these in the payload of an RTP packet, and fills in RTP packet header values appropriately. Similarly, input packet processor  152  interprets RTP headers, extracts voice frames from received packets, and places these frames in the proper sequence in buffer  154 . 
     Buffer  154  operates according to well-understood VoIP jitter buffer principles. Buffer  154 &#39;s primary function is to prevent data starvation during playout of received packets. Buffer  154  delays playout of early-arriving packets, such that packets arriving relatively “late” do not miss their appointed playout time. The operation of buffer  154  may also be tied to arbitration, e.g., by a flush buffer signal path  158 . If data frames are buffered for a source that later loses arbitration, input selector  150  may flush those frames from buffer  154  before they are played out. And the minimum buffer size may also be controlled by input selector  150  in order to prevent switching garble, as discussed above in conjunction with FIG.  7 . 
     Arbitrator  148  implements conference arbitration rules. Arbitrator  148  receives outgoing packet header information from output packet processor  144 . Arbitrator  148  also receives incoming packet header information for all input talk streams from input selector  150 . Utilizing the conference-wide arbitration rules, arbitrator  148  decides whether output packet processor  144 &#39;s results should be relayed to IP interface  164  and activates switch  146  appropriately. 
     If transceiver  140  wins arbitration, arbitrator  148  may optionally signal input selector  150  to select no input. This allows input selector  150  to flush buffer  154  and avoid garble. Note that output packets need not be looped back to the sending transceiver, as a local side-tone path  160  may be provided within transceiver  140 . Side-tone may be added to speech decoder  156 &#39;s output at adder  166 —side-tone and speech decoder output may also be switched in a single-speaker system. 
     Besides operating as a sender arbitrator, arbitrator  148  may also provide receive arbitration for input selector  150 . In this mode, arbitrator  148  applies the conference-wide arbitration rules to choose one of n incoming packet data streams for further processing. Arbitrator  148  notifies selector  150  of the appropriate incoming stream. Selector  150  responds by forwarding only that stream to input packet processor  152 , and preferably by flushing buffer  154  if needed. 
     A block diagram for a multi-speaker conferencing transceiver  170  is shown in FIG.  9 . Overall operation of transceiver  170  is similar to that of transceiver  140 . But because transceiver  170  must handle multiple simultaneous speakers, it provides multiple speech-packet processing paths  182  and  184  (other such parallel paths may be added as needed). A buffer flush signal path is not shown; although it may be added to transceiver  170 , arbitration generally becomes less infrequent as the number of speakers is allowed to increase. In fact, receive arbitration may not be implemented at all in a multi-speaker system. 
     If transceiver  170  expands to include three, four, or more possible simultaneous speakers, receive arbitration may alternately be based strictly on arrival time (i.e., in a four-simultaneous-speaker network, the first four speakers talk streams to arrive at transceiver  170  would be selected, and any further simultaneous streams would be thrown away). Although such a system does not absolutely guarantee that everyone always hears the same thing, a contrary result is unlikely, and large numbers of simultaneous speakers will generally be naturally limited to short durations. 
     A multi-speaker transceiver  170  may also include a mixer  192 . Mixer  192  mixes decoded speech samples from each speech-packet processing path  182  and  184 , and may also mix local side-tone obtained from path  198 . Mixer  192  may be a simple adder. It may perform more complicated functions such as selective attenuation (e.g., third and fourth speakers added at half-volume, or side-tone volume reduction). 
     FIG. 10 shows a multi-speaker transceiver  200  with a side-tone cutout switch  230 . If the local speaker loses arbitration, arbitrator  208  may activate cutout switch  230  to prevent side-tone  228  from contributing to mixer  222 &#39;s output. This advantageously provides a cue to a speaker that his voice is not being transmitted to the conference. An optional switch  232  may also operate to signal speech encoder  202  when no encoding is necessary due to a lost arbitration. 
     Although transceivers described above are shown as existing between a two-way IP interface and a two-way analog or digital pulse-code-modulation (PCM) interface, embodiments of the invention may include somewhat more or somewhat less. FIG. 11 shows one minimal implementation of a transceiver  240 , which provides little more than an arbitrator  246  and an input selector  248 . Arbitrator  246  reviews RTP headers from packets originating from both the conference set interface  242  and the network interface  244 . Transceiver  240  implements an outbound switch  250  to reduce network bandwidth when the outbound signal loses arbitration. The optional input selector  248  may also use arbitration results to select appropriate inputs for pass-through to the conference set. It is not necessary that both outbound and inbound arbitration fimctions be implemented, although outbound arbitration functions are preferable because they reduce network bandwidth requirements. 
     Transceiver  240  does not modify RTP packets passing in either direction—it merely decides whether or not to pass on the packets at all. Downstream hardware would be responsible for packetization, encode/decode, and mixing. 
     In a further minimal embodiment, a transceiver  252  maintains a single physical interface  258  connected to IP network  260 , e.g., by router  262 . Transceiver  252  subscribes to a multicast group address for a conference through router  262 . Transceiver  252  also communicates with conference set  22  through router  262  (this data path may optionally include a voice gateway between  262  and  22 , not shown). Within transceiver  252 , arbitrator  254  examines both incoming multicast remote conference packets and outgoing local conference packets, using source identifiers to monitor the origination point of each. By operating packet switch  256 , transceiver  252  can effectively remove both inbound and outbound packets that lose arbitration. 
     After reading this disclosure, one of ordinary skill in the art will recognize that many advantageous modifications to the disclosed embodiments are enabled by the concepts taught herein. For example, more complex transceivers according to the invention may encompass the entire conference set, including a microphone, a speaker, and an analog/digital interface. And although the described embodiments focus on audio conferencing, the present invention is also applicable to voice-controlled video conferencing. Arbitration may be performed on the talk streams of a packet video conference, with the results affecting both voice and video packets from the same source. 
     Even though the context of the disclosure is an always-up conferencing network, a large (but temporary or intermittent) packet network conference may also utilize the present invention. The packet network itself need not use either IP or RTP—such usage in this disclosure merely reflects the predominance of these protocols in data networking today. Other modifications to the disclosed embodiments will be obvious to those of ordinary skill in the art upon reading this disclosure, and are intended to fall within the scope of the invention as claimed.