GB2358997A

GB2358997A - Group communications system with assignable transmission authority

Info

Publication number: GB2358997A
Application number: GB0002529A
Authority: GB
Inventors: John Hughes; Jon Alastair Gibbs
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 2000-02-04
Filing date: 2000-02-04
Publication date: 2001-08-08
Anticipated expiration: 2020-02-04
Also published as: GB0002529D0; GB2358997B

Abstract

In a group communications system a method of arbitrating over which group member has permission to transmit comprises the steps of (1) a member detecting that a transmission channel has become free, (2) introducing a time stamp into a signal to be transmitted, the time stamp indicating the time delay between detection that the channel is free and initiation of transmission, (3) comparing time stamps in incoming signals with the time stamp in the signal being transmitted and (4) ceasing transmission if an incoming signal has a time stamp indicating a smaller delay than that of the signal being transmitted. When a plurality of signals are received only the one whose time stamp indicates the shortest delay is processed, all others are discarded. The system may be a teleconferencing system and the method avoids the confusion resulting from users attempting to speak at the same time. Users may be connected via the Internet or other wide area network (WAN).

Description

2358997 Group Communications System With Assignable Transmission Authorit

Field of the Invention

The present invention relates to methods of assigning transmission authority in group communications systems, methods of operating communications apparatuses for use in a group communications system, as well as group communications systems and communications apparatus for use in group communications systems.

Background of the Invention

Group communications systems are systems in which a number of users are able to communicate with one another via a communications network. A typical and simple example is a telephone conference. It is now technically feasible and desirable to allow group communications using a number of different communication technologies. For example, traditional wireless communications systems such as private mobile radio (PMR) and public access mobile radio (PAMR) and cellular services such as GSM may be used.

A further area where group communications are of particular interest is that of computer networks and in particular the Internet. Internet protocol (IP) systems exist which allow information to be streamed to multiple receivers in a very efficient way. For example the ITU (International Telecommunications Union) H.323 framework provides a means by which voice, video and data can be carried simultaneously over an IP network. Moreover recent developments in low rate video and speech encoders such as H.263 and G.729 mean that even with the low bandwidth which is typically available over the Internet, acceptable video and speech can be achieved.

However, a particular problem which needs to be addressed in all of the above cases is the provision of a practical system for arbitrating over which member of the group has permission to transmit. This problem is particularly acute in the case of IP networks. In Internet terminology this arbitration is termed 'floor control'.

With group communications many users may attempt to transmit simultaneously. If the system is a teleconferencing system, for example, then users may attempt to speak at the same time. If this is allowed, it can result in confusion and any system which offers the 2 ability for simultaneous transmission between a group of users will inevitably occupy a larger bandwidth than one in which the ability to transmit is restricted. In general terms there are two existing ways in which permission to transmit is controlled within group communications, The first is the use of a centralised controller and the second is the use of a token passing technique.

The use of a centralised controller has disadvantages in that a single entity controls who can transmit, and users' systems must request permission from the centralised controller and wait for approval before they are able to transmit. This means that there is an inevitable delay between requesting permission to transmit and being able to transmit. This can become a serious problem in communication systems where significant delays in the transmission and receipt of data can occur. The problem is exacerbated where some users may suffer from greater delays than others. Thus, such a technique can work in PMR and PAMR systems but is highly unsatisfactory for use with the Intemet.

Further, in the case of all communications systems the centralised controller technique has a disadvantage that if the centralised controller fails then all corranunications must cease unless redundancy is built in. In an alternative approach, rather than providing a dedicated centralised arbitrator, one of the group may be appointed as arbitrator. However, this can cause problems if that member of the group wishes to leave the session early.

A 'token' passing scheme is a distributed technique which contrasts with the centralised scheme described above. However, significant delays can still occur when a user wishes to transmit because that user must wait to receive or actively seek the token. The problems associated with this become particularly acute when the number of group members increases.

Summary of the Invention

It is an object of the invention to provide a system for assigning transmission authority in group communications systems which alleviates at least some of the problems associated with the prior art.

According to one aspect of the present invention, there is provided a method of operating communications apparatus for use in a group communications system as claimed in claim 1.

3 According to a second aspect of the invention, there is provided a communications apparatus for use in a group communications system, as claimed in claim 10.

According to a third aspect of the invention, there is provided a method of assigning 5 transmission authority in a group communications system, as claimed in claim 5.

In such systems, transmission from each apparatus is controlled locally. Thus the system is a distributed scheme and no centralised arbitrator is required. Moreover, the system can be arranged so that only a single member in a group can transmit at any one time.

This minimizes the bandwidth occupied by the group and avoids confusion which may result if, for example, two members attempt to speak simultaneously.

Further aspects of the invention are as claimed in the dependent claims.

Additional specific advantages are apparent from the following description and figures.

Brief Description of the Drawings

FIG. 1 is a schematic view of a group communications system; FIG. 2 is a schematic view of a communications apparatus for use in the group communications system shown in Figure 1; FIG. 3 is a functional diagram showing the interaction between an application and a 25 transmit control entity embodied in the communications apparatus shown in FIG. 2; FIG. 4 shows a channel protocol state machine; FIG. 5 shows a transmit control entity state machine; and FIG. 6 is a flow chart showing a protocol procedure followed when a communications apparatus is transmitting and another signal is received.

Description of an Embodiment of the Invention

A group communications system generally comprises a plurality of communication apparatuses 1, three of which are shown in Figure 1, a first belonging to a first group 4 member A, a second belonging to a second group member B and a third belonging to a third group member C. Each of these communication apparatuses 1 are connected via suitable communication networks 2 to one another in such a way as to allow group communications between all of the group members. Thus, in the present case, group members A, B and C can, for example, conduct a three-way conversation. It will of course be appreciated that the number of group members is not limited to three and this merely serves as an example.

In the present example, group member A and group member B are both connected to the same wide area network and therefore the delay in the transmission of signals between A and B is relatively small. On the other hand, group member C is connected to the wide area network by a relatively slow link, for example via the Intemet. In the present embodiment group members A, B and C are able to communicate with one another by means of IP networks. Group members A, B and C can in principle transmit to one another any one of, or any combination of, audio signals, video signals and data signal s. However, for the sake of simplicity, in the remainder of the description it is assumed that the group communication system is being used as a teleconferencing facility wherein the group members A, B and C communicate by voice alone.

Figure 2 shows schematically in more detail one of the communications apparatuses 1 shown in Figure 1. Each of the other communications apparatuses has a similar structure. Each communications apparatus 1 comprises a microphone 3 into which the respective group member may speak. Each communications apparatus 1 also comprises transmitting means 4 arranged to process the speech for transmission over the network 2 via a multiplexer/demultiplexer 5. The communications apparatus 1 further comprises receiving means 6 for receiving signals ftom the network 2 via the multiplexer/demultiplexer 5 and for outputting audible signals via a speaker 7 to the respective group member. The transmission means 4 and receiving means 6 are connected to transmission control means 8. Transmission control means 8 is arranged to control the circumstances under which the respective communications apparatus 1 is able to transmit and receive signals as discussed in greater detail below.

It will be appreciated that FIG 2 only shows the communications apparatus 1 schematically, and that the specific functions may be implemented in a variety of different ways using appropriate hardware and software.

In operation, each of the communications apparatuses 1 is used to determine which communications apparatus and hence which group member is able to transmit. As will be described in more detail below, each group member is only permitted to begin transmitting when the transmission control means 8, in the respective communications apparatus 1, determines that the communications channel is free. Once the transmission control means 8 determines that the communications channel is ftee, it starts a timer (not shown) to record how long the channel has been free. If and when the respective group member initiates transmission, for example by beginning to speak, the first packet output by the multiplexer 5 onto the network 2 contains a time stamp reference indicating the delay between the channel becoming ftee and the transmission being initiated.

The inclusion of this time stamp reference in all normal transmissions generated by each of the communications apparatuses 1 is fundamental to the operation of the invention.

If, for example, a transmission is initiated by group member A and then its associated communication apparatus 1 receives an incoming signal from either group member B or group member Q then the time stamp in the incoming signal is compared with that of the signal being transmitted. If the time stamp reference indicates a delay smaller than that in the signal currently being transmitted, the transmission control means 8 of A's communication apparatus 1 will immediately cause the respective transmission means 4 to stop transmitting and allow the respective receiver means 6 to start receiving the incoming signal. Group member A will then become aware that his transmission has been overridden as he begins to hear group member B or group member C's voice. This should prompt group member A to stop talking but even if he does not, the action of the transmission control means 8 in his communications apparatus is such that his transmission is prevented.

This is the first function of each communications apparatus, namely to prevent the respective group member transmitting signals when he receives a signal having an earlier time stamp. A second important function of the communications apparatus is that if, for example, signals are received both from group member B and group member Q then the transmission control means 8 in A's communications apparatus will compare the time stamps in both of the incoming signals and only allow reception of the signal having the time stamp indicating the smallest delay. Thus when two or more group members start to transmit, i.e. talk, at approximately the same time, then that group member who responded most quickly to the channel becoming free at his end is the one who prevails.

The fact that the channel is free at a group member's end will become obvious when the group member hears the end of the previous speaker's message.

6 1 1) An important advantage of the system described above is that it introduces an inherent fairness. That is to say, a group member such as C whose signals are subject to a high delay because of having to pass over a slow link is not significantly disadvantaged in gaining permission to transmit. This is because the factor which determines who is able to transmit is the response time to the channel becoming ftee at their end. Thus the fact that member C may not become aware that the channel is free until significantly later than either member A or B, does not prevent member C being judged the first to respond.

There is a further advantage that there is no delay generated by having to seek permission and wait for approval. This is because in the present system, a user simply begins to transmit and if he fails to obtain transmission permission his transmission will be cut off, whereas the successful party will be able to continue.

It will also be appreciated that the present system is a distributed scheme in which the communications apparatus 1 of each of the group members A, B, C is used in determining who obtains transmission permission and the continuity of the communications session is not dependent on any one device.

The scheme is also completely transparent to the user. For example in the case of voice communications, the system manifests itself in the fact that only one group member A, B, C will ever be heard at any one time and a group member's inability to transmit will be evident by the fact that he is receiving a transmission from another. The system also therefore ensures that there is never any interference caused by a number of voices being heard at the same time.

The operation of an embodiment of the present invention, and in particular the protocols used in its implementation, is given in more detail below.

FIG 3 is a functional diagram showing the interaction between an application 10 1 and a transmission control entity 102 provided at each communications apparatus 1 and the protocol data units (PDUs) which are sent over the communications channel 103. As is well known to those in the art, the application 10 1 receives and deals with information such as video, audio and data packets to be sent. Application 10 1 passes these to the transmission control entity 102, to be transmitted via a channel 103 provided across the network 2. As will become clear from the description below, the transmission control

7 entity 102, amongst other things, performs the functions of the transmission control means 8 described above. Various 'primitives' are used by the application 101 and the transmission control entity 102 to pass data and control signals therebetween. A list of the primitives used is given below with a brief explanation of each.

Primitives Descriptio: Appl_SDU request: This primitive is used by the application to pass application data (e.g. video, audio, data packets) to the Transmit Control Entity for transmission over the channel.

End-Tx request: This primitive is used by the application to indicate that it has ceased transmission (equivalent to releasing the PTT(Press to talk) button).

Emergencyjx request: This primitive is used by the application to indicate that it has a high priority transmission pending which should be allowed to interrupt any existing communication.

Channel - Free indication: This primitive is used by the Transmit Control Entity to inform the application that the communication channel is now free. The application on receiving this message may attempt to send application data using the Appl._Tx request primitive.

Channel - Busy indication: This primitive is used by the Transmit Control Entity to inform the application that the communication channel is now busy. The application on receiving this message may not attempt to send application data using the Appl-Tx request primitive.

Cease-Tx indication: This primitive is used by the Transmit Control Entity to force the application into stopping any transmission, i.e. to stop the application from sending further Appl_SDU request primitives.

AppLSDU indication: This primitive is used by the Transmit Control Entity to pass application data (e.g. video, audio, data packets) which has been received from the channel to the application.

Similarly, various PDUs are used by the transmit control entity 102 to pass signals to the channel 103. A list of these is given below with brief explanations.

8 PDUs Descriptio: Begin-Transmit PDU: This PDU is used by the Transmit Control Entity to pass the first packet of a transmission over the channel. The Begin - Transmit PDU contains application data (e.g. video, audio, data) as well as a time reference which indicates the time relative to a reference value at which the Transmit Control Entity placed the packet on the channel.

Appl_Data PDU: This PDU is used by Transmit Control Entity to pass all application data with the exception of the first and last packets of the transmission over the channel.

End-Transmit PDU: This PDU is used by Transmit Control Entity to pass the last packet of a transmission over the channel.

Emergency_Transmit PDU: This PDU is used by Transmit Control Entity to interrupt an ongoing transmission and thus allow high priority application data to be sent over the channel.

FIGS 4 and 5 respectively show a channel protocol state machine and a transmit control entity protocol state machine.

As shown in FIG 4 the channel has two states, that is, idle and busy. The channel busy state implies that one or more transmission attempts are in progress, whereas the channel idle state implies that there are no transmissions currently in progress. The transmission of an End Transmit PDU over the channel causes the channel state to change to channel idle, whereas the transmission of a Begin Transmit PDU over the channel causes the channel state to change to channel busy.

As shown in FIG 5 the transmit control entity 102 has three different states, channel idle, transmission (Tx) and receive only (Rx_ only). Considering these states:

(i) The 'channel idle' state implies that no transmission is currently in progress over the channel as far as that transmit control entity 102 is concerned and hence the application is neither receiving nor sending application data PDUs (Appl. - data PDUs).

(ii) The 'receive only' state implies that the respective transmit control entity is in receive only mode. In this state, the transmit control entity 102 may not attempt to begin a transmission unless that transmission is of high priority, that is to say it is an emergency transmission.

9 (iii) The 'transmit state' implies that the transmit control entity 102 is currently transmitting application data across the channel 103.

FIG 5 also illustrates how the states of the transmit control entity 102 change as various actions are undertaken. Thus with the transmit control entity in channel idle state it changes to transmit state when a Begin Transmit PDU is sent and changes to receive only state when a Begin-Transmit PDU is received. When the transmit control entity 102 is in receive only state, it changes to the channel idle state upon receipt of an End - Transmit PDU and changes to the transmit state if an Emergency_Transmit PDU is sent. When the transmit control entity 102 is in the transmit state and it receives an Emergency_Transmit PDU, the transmit control entity 102 changes to the receive only state. This change of state also occurs if a Begin - Transmit PDU is received which has an earlier reference time stamp than that in the signal currently being transmitted. When the transmit control entity 102 is in the transmit state and a EndTransmit PDU is sent, the transmit control entity reverts to the channel idle state.

Details of the protocol procedures are now described.

Each transmit control entity 102 comprises a timer (not shown) which, as discussed above, is used in the reference time stamping operation. In the present embodiment the timer has a resolution of 1 x 10' seconds. However, this may be varied depending on the applications being supported.

The timer in a particular transmit control entity 102 is reset to zero on the receipt of an End Transmit PDU. Thus the reception of an End Transmit PDU causes the transmit control entity 102 to reset its timer, enter an idle state and pass a Channel Free indication primitive to the application 101.

Where the application 101 detects that the channel is idle by virtue of the receipt of a Channel Free indication primitive, it may begin to transmit. When transmission is initiated by the application 10 1 it passes an Appl_SDU request primitive to the transmit control entity and the first application data packet is encapsulated within a Begin - Transmit PDU. The contents of the timer is then read and incorporated into the Begin Transmit PDU as a reference time stamp. The reference time stamp is also stored in storage means (not shown) provided at the transmit control entity for possible later use. When the Begin - Transmit PDU is transmitted, the transmit control entity 102 enters the transmit state as mentioned above.

When the present embodiment is used for real time communications over an engineered network with specified guarantees on quality of service, it may only be necessary to transmit the Begin-Transmit PDU once. However, to enhance reliability and in circumstances where no, or unsatisfactory, guarantees on quality of service are given, the Begin_Transmit PDU may be transmitted a number of times and on each occasion the same reference time stamp will be included.

After the Begin-Transmit PDU has been transmitted, the remainder of the application data is transmitted encapsulated in Appl_Data PDUs until the end of transmission is reached.

Once the application 10 1 has completed its transmission, it informs the transmit control entity 102 with an End - Tx Request primitive. The last application data package is then transported in or is followed by an End Transmit PDU. As the transmit control entity 102 sends the End-Transmit PDU it resets its timer, enters the idle state and passes the Channel-Free Indication primitive to the application. Again, if it is desired to enhance reliability, then the End - Transmit PDU may be transmitted multiple times by the transmit control entity 102.

Below is a description of the procedure used at each communications apparatus 1 to determine whether it has permission to continue transmission. The following procedure, which is shown in FIG 6, is used to determine which transmission should succeed when the transmit control entity 102 of a particular communications apparatus 1 is in the transmit state and it receives a Begin-Transmit PDU from a different communications apparatus 1.

In a first step 201, the transmit control entity 102 of a first communications apparatus 1, which is currently transmitting, receives a Begin-Transmit PDU from a second communications apparatus 1. The transmit control entity 102 then reads the reference time stamp contained in this received Begin - Transmit PDU at step 202. At step 203 it is determined whether the value of the time reference stamp of the incoming signal is less than the stored reference time stamp which corresponds to the reference time stamp included in the Begin-Transmit PDU of the outgoing signal. Two outcomes are possible:

(i) If it is determined that the value of the incoming time stamp is less than the value of the outgoing time stamp then, at step 204, the newly received reference time stamp is 11 stored in the transmit control entity 102 and transmission from the first communications apparatus 1 is terminated. This is implemented by the transmit control entity 102 sending the application 101 a Cease-Tx indication primitive. The transmit control entity 102 then enters receive only mode, and passes the incoming application data to the application 10 1 in Appl - SDU indications. The incoming application data continues to be passed in this way until the incoming signal ends. (ii) On the other hand, if it is determined at step 203 that the value of the incoming reference time stamp is greater than the outgoing reference time stamp, then the incoming Begin - Transmit PDU and all further Appl-Data PDUs from the second communications apparatus are discarded.

Below is described the procedure followed at each communications apparatus to determine which of several incoming signals should be received. When a particular transmit control entity 102 is in the receive only state because it has received a Begin - Transmit PDU which is acceptable the following process is initiated if another Begin-Transmit PDU is received. The transmit control entity 102 compares the reference time stamps contained in both of the Begin - Transmit PDUs, one of which is in the storage means of the transmit control entity 102 and the other of which is read as the second Begin - Transmit PDU is received. After this comparison has been carried out, the transmit control entity 102 only accepts Appl-Data PDUs from the source having the smallest reference time stamp.

It is possible that two group members initiate transmission at times such that their respective Begin - Transmit PDUs include the same time stamp reference. This will only occur if two users begin speaking or press their respective press to talk (M) buttons after exactly (within the resolution of the timers) the same delay following their respective communications apparatus 1 determining that the channel was free. Although such an occurrence is unlikely, the protocol may be enhanced so that if such a collision occurs there is a simple mechanism to determine who obtains transmission permission. The details of such a mechanism are unimportant but can, for example, be implemented by deciding between the two users on the basis of which of the sources has the lower IP address, 12

Claims

Claims

1. A method of operating a communications apparatus for use in a group communications system, the method comprising the steps of detecting that a transmission channel has become free; introducing a time stamp into a signal to be transmitted, which time stamp indicates the time delay between detection that the channel is free and initiation of transmission; comparing time stamps in incoming signals with the time stamp in the signal being transmitted; and controlling the transmission of signals from the apparatus on the basis of said comparison.

2. A method according to claim 1 including the step of ceasing transmission upon receipt of an incoming signal having a time stamp indicating a smaller delay than that of the signal being transmitted.

3. A method according to claim 1 or claim 2 including the further steps of comparing time stamps in incoming signals with one another and controlling processing of received signals on the basis of said comparison.

4. A method according to claim 3 wherein the step of controlling processing of received signals comprises the step of retaining a received signal whose time stamp indicates a shortest delay and discarding all other signals.

5. A method of assigning transmission authority in a group communications system comprising a plurality of communications apparatuses, the following steps being carried out at each communications apparatus: detecting that a transmission channel has become free; introducing a time stamp into a signal to be transmitted, which time stamp indicates the time delay between detection that the channel is free and initiation of transmission; comparing time stamps in incoming signals with the time stamp in the signal being transmitted; and controlling the transmission of signals from the apparatus on the basis of said comparison.

13 6. A method according to claim 5 including the step of ceasing transmission upon receipt of an incoming signal having a time stamp indicating a smaller delay than that of the signal being transmitted.

7. A method according to claim 5 or claim 6 including the further steps of comparing time stamps on incoming signals with one another and controlling processing of received signals on the basis of said comparison.

8. A method according to claim 7 wherein the step of controlling processing of received signals comprises the step of retaining a received signal whose time stamp indicates a shortest delay and discarding all other signals.

9. A method according to any preceding claim including the step of overriding all current transmitting and receiving operations on receipt of and/or on instruction to transmit an emergency signal.

10. A communications apparatus for use in a group communications system, the communications apparatus comprising:

transmitting means and receiving means; means for detecting that a transmission channel has become free; means for introducing a time stamp into a signal to be transmitted, the time stamp indicating the time delay between detection that the channel is free and initiation of transmission; and means for comparing time stamps in incoming signals with the time stamp in a signal being transmitted and means for controlling the transmission of signals from the apparatus on the basis of said comparison.

11. A communications apparatus according to claim 10 including means for disabling transmission upon receipt of an incoming signal having a time stamp indicating a smaller delay than that of the signal being transmitted.

12. A communications apparatus according to claim 10 or claim 11 including means for comparing time stamps on incoming signals with one another and means for controlling processing of received signals on the basis of said comparison.

14 13. A communications apparatus according to claim 12 wherein the means for controlling processing of received signals is arranged to retain the received signal whose time stamp indicates a shortest delay and discard all other signals.

14. A communications apparatus according to any one of claims 10 to 13 comprising means for overriding all current transmitting and receiving operations on receipt of and/or on instruction to transmit an emergency signal.

15. A group communications system compinsing a plurality of communications apparatuses according to any one of claims 10 to 14.

16. A communications apparatus or a group communications system substantially as hereinbefore described with reference to, or as illustrated by, any of figures 1 to 6.

17. A method of assigning transmission authority in a group communications system substantially as hereinbefore described with reference to, or as illustrated by, any of figures 1 to 6.

18. A method of operating a communications apparatus for use in a group communications system substantially as hereinbefore described with reference to, or as illustrated by, any of figures 1 to 6.

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