WO2001005077A1

WO2001005077A1 - Data transmission system from plural sources

Info

Publication number: WO2001005077A1
Application number: PCT/GB2000/002614
Authority: WO
Inventors: Ross Galvin; Steven Mcbirnie
Original assignee: Simoco International Limited
Priority date: 1999-07-09
Filing date: 2000-07-07
Publication date: 2001-01-18
Also published as: GB9916179D0; GB2351883A; GB0016794D0; AU5699600A

Abstract

A first application (1) receives continuously in real time information relating to, for example, a speech conversation, a video signal, or telemetry information, and includes a variable rate data generator (3) which generates from that information a continuous data stream for transmission. A second application (2) comprises a device which receives other data for transmission, such as encryption synchronisation data, channel coding information, a text message, or position data (e.g. GPS (Global Positioning System) data), e.g. relating to the location of a mobile transmitter. A data combiner (5) selects a rate at which data should be generated for transmission by the variable rate data generator (3) of the application (1) on the basis of whether or not the other application (2) is producing data for transmission and preferably also on the basis of the maximum data transmission capacity, e.g. maximum bit rate, of the transmission channel to be used.

Description

DATA TRANSMISSION SYSTEM FROM PLURAL SOURCES

This invention relates to the field of data transmission and, more particularly, to a method of and an apparatus for transmitting more than one set of data or information over a single transmission channel, where one of the sets of data is generated from information received continuously in real-time. In some circumstances, it is necessary to transmit to a receiver data relating to events that are happening in real-time. This could for example be a video or audio broadcast of an event , or the relaying of telemetry data from e.g. a vehicle engine or medical equipment that is being monitored. In such arrangements, a data generator will receive information in real-time relating to the event and generate from that information a continuous stream of data (e.g. video data) for transmission to the receiver. Data transmission can usually be thought of as occurring over a transmission channel linking the data source and the receiver. As is known in the art, transmission or communication channels can be defined in a number of ways. For example, in a radio communications system each channel could comprise a different carrier frequency. Alternatively, more complex ways of separating transmission channels can be used. For example, in time division multiple access (TDMA) communication systems, such as the GSM (General System for Mobile Communications) and TETRA (TErrestrial

Trunked RAdio) mobile radio communications systems, multiple transmission channels on the same carrier frequency are established by allotting different slots in time (time slots) to different transmission channels. It is usual for the maximum rate at which data can be transmitted on a transmission channel to be limited. For example, the maximum data transmission rate will be restricted by the permitted bandwidth of the channel and, in a TDMA system, also by the limited time available on a given frequency to the particular channel . Generally a given transmission channel will be devoted to transmitting information or data from a single source, such as acoustic data or video data, relating, for example, to a telephone conversation or a media broadcast, at any given time. However, it is sometimes desirable to transmit additional data over the same channel whilst there is an ongoing transmission from another primary data source. Such additional data may, for example, be other acoustic or video data from another source, or encryption information for decrypting an encrypted data stream. Alternatively, the additional data could be text data unrelated to the normal data being transmitted over the channel, or position information relating to a mobile transmitter's position. This additional data must therefore be fitted into the transmission channel together with the data from the primary data source. There therefore needs to be some mechanism for including it in the transmission, but without needing to exceed the maximum data rate for the transmission channe1. Where the data from the primary source can be transmitted over a flexible time frame, e.g. it is data such as text information that can be transmitted discontinuously as there is no requirement for the text to arrive at a particular time, i.e. gaps or breaks in the transmission can be tolerated, the generation of primary data for transmission can simply be temporarily stopped while the additional data is transmitted.

In another example, it may be possible to selectively stop producing data for given sections of information. For example, for voice data, voice data need not be produced in silent portions of speech, and additional data can be inserted when voice data is not produced .

However, the Applicants have recognised that where the primary data source is generating data for transmission from information received relating to a continuous real-time event, then stopping entirely the primary data generation while the additional data is transmitted can be undesirable, since primary data is then lost from the transmission and this may be undesirably apparent to a user. Such stopping entirely primary data generation will also result in a break in data transmission is also undesirable since any break in continuous real time data transmission can be more difficult to compensate for in the receiver and may make successful reception of subsequent primary data more difficult.

It may also be undesirable to try to solve these problems by continuing to generate the primary data at the same rate and to store it for later transmission while the additional data is transmitted, because this may introduce a delay relative to the real-time event in the signal reproduced by the receiver. Also, once the data store was full, one would still have to stop the generation of primary data to allow the transmission of any additional data thereafter. In an alternative method, known as stealing, primary data may be continuously produced but sections of the primary data to be transmitted are replaced with the additional data. This method is used, for example, to transmit encryption information in GSM networks (for example by replacing speech data relating to silences with encryption information) . Although stealing does not stop the generation of the primary data, it inevitably causes information relating to the replaced sections of primary data to be lost. Although this lost information may be trivial in some applications (e.g. silent portions of a speech signal) or compensated for, for example by error correction processes, these gaps in the primary •'data and the resultant signal quality degradation are generally undesirable. Stealing is especially disadvantageous for real time continuous data transmission, as again data can be irrevocably lost causing gaps in received data apparent to a user and general signal quality degradation. In some encoding schemes typically used for continuous real time data transmission, such as differential encoding, the loss of one frame or section of a differentially encoded bit stream, such as that of MPEG video transmissions, can also prevent the decoding of numerous subsequent frames in the receiver, accentuating such problems.

According to a first aspect of the present invention, there is provided a method of transmitting data from two or more data sources over a transmission channel to a receiver, in which a primary data source during the transmission generates from information provided continuously to it a continuous stream of data for transmission to the receiver, and one or more secondary data sources generate additional data for transmission to the receiver on the transmission channel during the transmission of the stream of data from the primary data source, the method comprising: controlling the primary data source to generate its continuous data stream at a rate selected in accordance with the rate that the additional data from the secondary data source or sources is provided to the channel for transmission.

According to a second aspect of the present invention, there is provided a data transmission system, comprising: a primary data source for generating a continuous stream of data for transmission to a receiver on a transmission channel from information provided to it continuously; one or more secondary data sources for generating additional data for transmission to the receiver on the transmission channel during the transmission of the stream of darta from the primary data source; and a controller for controlling the primary data source to generate its continuous data stream from the information it receives at a rate selected in accordance with the rate that the additional data from the secondary data source or sources is provided to the transmission channel for transmission.

In these aspects of the present invention, the rate at which a primary data source generates data for transmission from the information it receives is selected in accordance with the rate at which data from other data sources is to be transmitted on the same transmission channel during the continuous data stream transmission from the primary data source. Thus, for example, when it is desired to transmit additional data from another secondary data source, the primary data source can be controlled to generate its data stream at a lower data rate so that the additional data can fit on the transmission channel. In other words, space is provided on the transmission channel for the transmission of additional data from a secondary data source during the continuous transmission from the primary source by reducing the rate that data from the primary data source is generated. This allows transmission of the additional data, while maintaining data generation by the primary data source, and avoids the need to stop generation of data by the primary data source and the need to steal generated primary data which is to be transmitted, when the additional data is to be transmitted.

This should be contrasted with prior art methods in which secondary data can be transmitted in gaps created in the primary data in response to the nature of the information from which the primary data is encoded, e.g. in silent portions of speech. Such methods rely on the nature or content of the information from which the primary data is generated to allow gaps in the primary data in which secondary data can be inserted, whereas the present invention controls the generation of primary data in accordance with the rate of the additional data to be transmitted on the channel . Reducing the rate at which data is generated from the information received by the primary data source will also have a less perceptible effect on the data reproduced by the receiver than either interrupting data generation or stealing data to be transmitted. For example, by reducing the rate at which primary data is generated rather than producing data and then discarding a portion of it as is the case with data stealing, signal quality degradation is reduced and any perceptible gaps in the received data can be eliminated. As data generation is not interrupted, no discontinuity in the reproduced primary data is experienced.

These advantages are particularly useful for data encoded such that decoding of a data portion is reliant not only on proper reception of that data portion, but also on proper reception of one or more other data portions on which the decoding of the data portion depends, e.g. differentially encoded data, as improper reception of one data portion (due to data stealing or interruption of data production) may prevent proper decoding of several portions of data. In a particularly preferred embodiment, the primary data source may therefore comprise differentially encoded data (such as video data for example) .

The primary data source generates its data for transmission from information that it receives continuously in real-time relating to some event or events that are occurring in real-time. As discussed above, this information could be digital data, a video signal relating to an event that is being videoed, an audio signal relating a speech conversation, or telemetry information from, for example, a vehicle or medical equipment. The way that the data is generated from the real-time information can be selected as desired and will depend on the information being received. For example, where video is to be broadcast, the video information can be appropriately encoded, e.g. differentially encoded, to generate the data stream for transmission.

The secondary data could comprise any data that it may be desirable to transmit simultaneously with the primary data. Thus it could comprise another audio broadcast or a video broadcast, or other data generated from information received in real time (i.e. be similar to the primary data source) . However, it will typically be a source more of one-off or less frequent data, such as synchronisation, e.g. encryption synchronisation, information to be transmitted at intervals, channel coding information such as information relating to the size of data packets or time slots, a text message, or a position update from a mobile transmitter.

The primary data generation rate can be varied as desired. Preferably the volume of data generated for transmission by the primary data source from the information that it receives is varied (as this has the net effect of varying the rate of data generated) . For example, the relative amount by which the information received is compressed for transmission could be varied. Thus, for example, in a video transmission the video frames could be more compressed to reduce the data generation rate. Alternatively or additionally the content of the data generated for transmission could be varied (e.g. to reduce the data generation rate by omitting from the data generated for transmission some of the information received) . For example, in a telemetry application the update rate of one or more parameters could be reduced, or one or more parameters could be omitted from the transmission, when it is desired to reduce the volume of data generated, for video data, the rate at which frames of video data are made or encoded may be reduced, and for acoustic data, the sampling rate may be reduced.

Thus, according to a third aspect of the present invention there is provided a method of transmitting data from two or more data sources simultaneously over a transmission channel, at least a first of the data sources during the transmission generating from information provided to it continuously in real time a continuous stream of data for transmission to the receiver, the method comprising varying the volume of data generated by said first data source from the information it receives in order to provide space on the transmission channel for transmitting data from another of the data sources . According to a fourth aspect of the present invention, there is provided an apparatus for transmitting data from two or more data sources simultaneously over a transmission channel, comprising: a first data source for generating from information provided to it continuously in real time a continuous stream of data for transmission to a receiver on the transmission channel; and a control means for varying the volume of data generated by the first data source from the information it receives in order to provide space on the transmission channel for transmitting data from another of the data sources .

The rate of generation (or volume generated) of primary data should be selected such that the rate of primary data and secondary data for transmission is less than or equal to the predetermined overall or maximum transmission rate of the channel . It should also be selected such that the generation of primary data does not stop and is not interrupted, since, as discussed above, that is undesirable. In other words, the data selection should be such that space is provided on the transmission channel for the data from the secondary data source ✓or sources, but without stopping data generation by the primary data source.

Preferably the rate that data is generated (the volume of data generated) by the primary data source is selected so that the data to be transmitted from the primary and secondary data sources utilises as much of the data transmission capacity of the transmission channel as possible, as this increases the efficiency of use of the data transmission capacity of the channel . Preferably, therefore, the data is generated by the primary source such that the overall data rate of the primary and secondary sources equals the maximum, or predetermined, overall data transmission rate (e.g. bit rate) of the transmission channel . Thus, when there is no secondary data to be transmitted, the rate that data is produced from the primary data source is preferably set to be the same as the maximum data transmission rate (i.e. bit rate) of the transmission channel. Correspondingly, when it is known that a secondary source or sources will produce data for transmission at a given overall data rate, such as may the case where, e.g., encryption synchronisation information is to be inserted into the primary data stream at intervals, the rate that data is generated by the primary data source is preferably selected to be equal to the predetermined overall transmission data rate of the transmission channel minus the (known) rate that data will be produced from the secondary data source or sources . Thus for example, where the secondary data is produced steadily over a relatively long time period for transmission, such as might be the case for synchronisation, e.g. encryption synchronisation, information, which may comprise short packets of data transmitted at intervals in the primary data stream, the transmission rate of the secondary data can be estimated and the rate at which primary data is generated is reduced below the overall or maximum transmission rate of the transmission channel appropriately throughout the transmission, or during the period in which secondary data is produced, to allow the extra data to be transmitted but not to waste transmission space on the channel .

In a particularly preferred embodiment the rate of data generation or volume of data generated by the primary data source is varied in accordance with whether or not there is data from secondary data sources to be transmitted. Thus the data generation rate is preferably reduced when there is secondary data to transmit, and increased again when there is no secondary data to transmit . This avoids permanently reducing the rate of primary data generation and thus transmission, even where there is no secondary data to transmit, which might not then use the full data transmission capacity of the channel at all times. Preferably this is done so as to always use the maximum overall data capacity of the transmission channel. Thus, preferably, primary data is generated at a rate the same as or close to the maximum transmission rate of the channel when secondary data is not being transmitted, so as to better exploit the full transmission capacity of the channel . Thus, according to a fifth aspect of the present invention, there is provided a method of transmitting data from two or more data sources to a receiver on a transmission channel, wherein during the transmission a first data source generates a continuous stream of data for transmission from information received continuously by it, the method comprising: determining if a second data source has data to transmit during the data transmission from the first data source ; if a second data source has data to transmit, reducing the rate at which the first data source generates data for transmission from the information it receives to ✓provide space on the transmission channel for the data from the secondary data source; and, thereafter, determining when the secondary data source transmission has finished; and, when it is determined that the secondary data source transmission has finished, increasing the rate at which the first data source generates data for transmission from the information it receives. According to a sixth aspect of the present invention, there is provided an apparatus for transmitting data from two or more data sources to a receiver on a transmission channel, in which a first of the data sources during the transmission generates a continuous stream of data for transmission from information received continuously by it, the apparatus comprising : means for determining if a second data source has data to transmit during the data transmission from the first data source; if a second data source has data to transmit, means for reducing the rate at which the first data source generates data for transmission from the information it receives to provide space on the transmission channel for the data from the secondary data source; means for determining when the secondary data source transmission has finished; and, means for increasing the rate at which the first data source generates data for transmission from the information it receives when it is determined that the secondary data source transmission has finished.

Reducing the primary data generation rate while the secondary data is being transmitted and then increasing it again after the secondary data transmission has finished is more suited to the situation where the secondary data is more of a one-off transmission, such as a text message, particularly if it is relatively long compared to sections of the primary data .

One way of transmitting such 'one-off' secondary data would be to reduce, when the secondary data is to be transmitted, the rate at which primary data is generated to be less than the data transmission rate of the channel and to stop transmitting the primary data but instead store it temporarily while the secondary data is transmitted at the transmission rate of the transmission channel. After the secondary data has been transmitted the primary data continues to be produced at a reduced rate, but the previously stored primary data and subsequently the newly generated primary data is now transmitted at the faster transmission rate of the channel. The excess primary data that accumulated whilst the secondary data was transmitted is therefore gradually transmitted until the transmission catches up in time with the data generation, at which point the rate of generation of primary data can be increased to match the rate at which it is transmitted, i.e the transmission rate of the transmission channel .

Alternatively, the secondary data could be divided into smaller portions to be sent at intervals at a thereby allocated data rate, and the primary data generation rate reduced accordingly until the secondary data has all been sent, at which point the primary data generation rate can be increased again. This operation is similar, while the secondary data is being transmitted, to the above method for sending intermittent secondary data over a longer time period.

The primary data and secondary data can be combined as desired for transmission. Preferably at least the primary data is provided to a buffer for temporarily storing the data, from which buffer the data is then removed for transmission. The primary data can then be accumulated in the buffer while the secondary data is transmitted. The buffer can then be emptied by continuing to produce the primary data at a reduced rate after the secondary data has been transmitted, until the transmission has caught up with the data generation. If desired, data rate information indicating the rate at which data from the primary source is being generated can be transmitted on the transmission channel to the receiver, which can then vary the rate at which the primary data is processed accordingly. However, this may not always be necessary. For example, some video decoders will be able to decode a signal at whatever rate the video frames arrive, or however much detail they contain. This data generation rate information may be inserted in the primary data or transmitted as secondary data. According to a seventh aspect of the present invention, there is provided a method of receiving data comprising varying the rate at which data is decoded according to data rate generation information received from a transmitter. According to an eighth aspect of the present invention, there is provided a receiver comprising means for varying the rate at which received data is decoded according to data rate generation information received from a transmitter. Thus, the receiver is alerted to changes in the rate at which the data is generated and is then able to successfully decode, demodulate or decrypt the data without losing synchronisation when the rate of data generated for transmission changes. The means for carrying out methods in accordance with the present invention may comprise pure hardware means such as discrete components or hard-wired logic gates. Alternatively, the methods may be implemented at least partially using software e.g. computer programs. It will thus be seen that when viewed from a further aspect the present invention provides computer software specifically adapted to carry out the methods herein above described when installed on data processing means and a computer program element comprising computer software code portions for performing the methods hereinabove described when the program element is run on a computer. The invention also extends to a computer software carrier comprising such software which when used to operate a data transmitting apparatus comprising a digital computer causes in conjunction with said computer said system to carry out the steps of the method of the present invention. Such a computer software carrier could be physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like. It will further be appreciated that not all steps of the method of the invention need be carried out by computer software and thus from a further broad aspect the present invention provides computer software and such software installed on a computer software carrier for carrying out at least one of the steps of the methods set out herein above.

The invention is particularly applicable to radio transmissions and in a further aspect the invention therefore provides a radio system, transmitter or receiver for carrying out the methods, or incorporating the apparatus, herein above described.

A number of preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of a data transmitting device according to a first embodiment of present invention;

Figure 2 is a message sequence chart showing the messaging between components of the data transmitter of Figure 1 when transmitting data in accordance with the present invention; Figure ^3 is a message sequence chart showing messaging between components when transmitting an encrypted message in accordance with the present invention; and Figure 4 is a message sequence chart showing messaging between components of a receiver when receiving a data signal having embedded synchronisation portions .

Figure 1 shows schematically a data transmitting device that can be operated in accordance with the present invention and, in particular, how data generators 3 , 4 or variable rate devices of the present invention can be interfaced with a transmission system. The data transmitting device includes a first application 1 and a second application 2 which act to provide data for transmission to a receiver via a network device 6. The data transmitted can be transmitted on a radio network, for example, a mobile radio communications system such as a TETRA system. At least the first application 1 in this embodiment, when a transmission is to be made, receives continuously in real time information relating to an event, and includes one of the data generators 3 which generates from that information a continuous data stream for transmission. The first application 1 could, for example, receive information relating to a speech conversation or a video signal, or telemetry information relating, for example, to a vehicle engine or a medical function being monitored. The variable rate data generator 3 takes the information and generates a continuous data stream suitable for transmission. Thus, in the case of a video signal, the variable rate data generator 3 can comprise a video encoder, such as a differential encoder. The second application 2 can also comprise a device receiving information relating to a continuous real time event, but more probably comprises a device which receives other data for transmission, such as encryption synchronisation data, channel coding information such as information relating to the size of data packets or time slots, a text message, or position data (e.g. GPS (Global Positioning System) data) , e.g. relating to the location of a mobile transmitter. In this example, the second application also includes a variable rate data generator 4 which again generates data for transmission from the information provided to it by the second application, although it is not essential for the second application to include a variable rate generator, and one would not necessarily be required, where, for example, the second application is a text message source . The transmitting device includes a data combiner 5 which communicates with the variable rate data generators 3, 4 and receives the data generated by the variable rate generators 3 , 4 and combines that data into a single data stream which it then passes to the network device 6 for transmission over the transmission channel to the receiver as a single data stream. The data combiner 5 can also control the variable rate data generators 3 , 4 to vary the rate at which they generate the data provided to the data combiner for transmission. The data combiner 5 also includes a data buffer which receives and temporarily stores the data generated by the first application 1 and, if appropriate, the data generated by the second application 2, so as to facilitate the transmission of the data as the data generation rates are varied. The data combiner 5 provides data from the data buffer at an appropriate rate to the network device 6 for transmission on the transmission channel .

The network device 6 acts to transmit the data stream over a transmission channel. Thus it could for example comprise a radio transmitter where the invention is being applied in a radio communications system. Figure ^2 exemplifies a messaging sequence between the various transmission device components for controlling the transmission of data from the applications 1, 2 over a transmission channel via the network device 6. In particular, Figure 2 shows the setting up of staggered data calls by separate applications 1, 2 over a single transmission channel.

In order to initiate a transmission, the relevant application 1, 2 sends a request to the network device 6 to allocate a transmission channel, or otherwise set up a call. In this example, application 1 sends a signal ("S_setup") to the network device 6 requesting a call be set up. When the network device 6 is ready to begin transmission it returns an acknowledgement signal ( "P_acknowledge" ) to the application 1. Thus, a call is established in a conventional manner.

However, before the first application 1 begins its data generation, it must also establish at what rate to generate data for transmission from the information being supplied to it. Thus, on receipt of the acknowledgement signal from the network device 6, the application 1, or more specifically, its variable rate data generator 3, sends a signal ( "S_report_request " ) to the data combiner 5 to ask at what rate data should be generated.

The data combiner 5 decides the rate at which data should be generated, and informs the variable rate data generator 3 of the application 1 of the selected rate and confirms to the network device 6 that transmission is about to begin. The network device 6 may also confirm to the receiver that transmission is about to begin and/or inform the receiver of the selected rate. The rate of data generation is selected on the basis of whether or not the other application 2 is producing data for transmission and preferably also on the basis of the maximum data transmission capacity, e.g. maximum bit rate, of the transmission channel to be used. The case where when the transmission is initiated, only the first application has data to transmit, will first be considered. In that case the data combiner 5 would select the maximum bit rate of the channel for the data generation rate for the first application as only the application 1 is attempting to transmit data. The data combiner 5 sends a signal ( "S_report_indication" ) to the application 1, indicating that data should be provided at that rate. The data combiner 5 also sends a signal ( "T_UNITDATA_request " ) to the network device 6, indicating that transmission is about to start. This ensures that the full transmission capacity of the channel is used.

The application 1, or more specifically the variable rate data generator 3, then begins to generate data at the specified rate and sends it (as signals "S_UNITDATA_request" ) to the data combiner 5. The data combiner 5 passes the data as signals ("T_UNITDATA_request" ) to the network device 6 for transmission.

Consider now the situation where, at a later stage, after transmission of data from application 1 has begun, application 2 wants to transmit data on the same transmission channel. The data from application 2 may be, for example, GPS (Global Positioning System) position data relating to the location of a mobile transmitter, or a text message unrelated to the data being transmitted by application 1.

Application 2 follows a similar procedure to application 1 to request permission to send its data.

It therefore sends a signal ("S_setup") to the- network device 6 requesting permission to transmit its data. As the network device 6 is already sending data from application 1, the network device sends a signal ( "P_acknowledge" ) to the application 2, indicating that transmission is already in progress. In view of this, the application 2, or more specifically its variable rate data generator 4, sends a signal

( "S_report_request " ) to the data combiner 5 indicating that it wishes to transmit data.

The data combiner must then decide at what rate both the applications should generate data. In this example it is not possible to simply add the data from application 2 onto the transmission channel, as application 1 is already producing data at the maximum data rate of the channel and there is therefore no spare space on the channel for the data of application 2 to also be transmitted.

The data combiner 5 must therefore control the data generation by the first and second applications 1, 2 such that their combined data rate does not exceed the maximum data transmission rate of the transmission channel . It does this by instructing the variable rate data generator 3 of the first application to reduce its data generation rate to provide space on the transmission channel for the data from the second application. It therefore determines the rate at which the second application will generate data and selects a reduced data generation rate for the first application accordingly. Preferably the data generation rate is also selected such that the combined data rate equals or is close to the maximum transmission channel data transmission rate, so as to use the transmission channel as efficiently as possible.

The application 1, 2, or variable rate generator 3, 4 of the application 1, 2 may inform the data combiner 5 of the maximum rate that it can provide data, or the quantity of data that it wants to transmit. This enables the data combiner to decide the best rate to request the variable rate generators 3 , 4 to provide data. For example, if the data combiner 5 needs to transmit data from the first application 1 consisting of x bits, at a rate of n_x times per second, and y bits from the second application 2 at a rate of n_y times per second, then it commands each application 1, 2 to supply it data so that :

maximum transmission channel data rate > (n_xx + n_yy)

In this way the data combiner decides how it will allocate the available channel data transmission capacity and informs each application accordingly of the rate at which it should generate data for transmission. When the data combiner 5 has decided the rate at which data should be generated by the applications 1, 2, the data combiner sends a signal ( "S_report_indication" ) to each of the applications 1, 2, indicating the rate at which they are to generate data, and a signal ("T_UNITDATA_request" ) to the network device 6 indicating that two sets of data are going to be present in the data stream and, if desired a signal for transmission to the receiving device to alert it to the change in the data transmission contents.

The data combiner 5 can also, if desired, send a signal to the network device 6 indicating the respective rates that data will be sent from each of the applications 1, 2, and when to expect each set of data in the data stream, for transmission to the receiver to assist it in processing the received signal. This also indicates to the receiver that two data streams are present and timing information, for example, may be sent to enable the receiver to separate the data streams . Transmission of a data steam containing data from both the applications then commences.

When the second application finishes its data transmission, it can again inform the data combiner and network device accordingly. In response to this, the data combiner will then instruct the first application to increase its data generation rate to equal the maximum transmission capacity of the transmission channel, so_,as to ensure that the maximum transmission capacity of the transmission channel is used as far as possible .

The actual mechanism of inserting the data from the second application and combining it into the transmission can be performed in various ways.

Where the second application wishes to transmit a single piece of data of known length, such as a text message or a position message, one suitable technique would be for the data combiner to calculate how long it will take to send the secondary data in one piece, and to then determine how much space remains in its data buffer. The data combiner 5 then instructs the first application to adjust its data generation rate so that it will just fail to fill the data buffer during the known transmission time of the secondary application data and controls the data buffer to stop providing data for transmission during this time. It then instructs the second application to transmit its data in one piece and once that transmission is finished, controls the data buffer to restart providing data for transmission at the maximum channel transmission rate, but with the first application still generating its data for the data buffer at the reduced rate. This causes the primary data accumulated in the data buffer while the secondary data was being transmitted to empty from the data buffer, such that eventually the data transmission will catch up again with the data transmission. When the data combiner 5 determines the data buffer is almost empty, it adjusts the data generation rate of the first application back to the full channel rate so as to match the rate at which the data buffer is emptying. This method will be useful for relatively urgent data or for short data insertions at unpredictable times. An alternative method would be to split the secondary data into smaller portions to be inserted periodically at an average data rate selected by the data combiner. The first application, i.e. primary data source, is then controlled to reduce its data generation rate accordingly. The data buffer is still emptied at the full transmission rate and thus it gradually empties (as the primary data is supplied at a lower rate) so as to provide space for the secondary data to be inserted in the buffer at the calculated intervals for transmission. When the last piece of secondary data has been sent, and the data input buffer has almost emptied, the primary data source can then be instructed to return to the full channel rate, as above. This method will be more useful for longer pieces of secondary data, or for prolonged low rate insertions at predictable times, such as the periodic insertion of synchronisation information.

The way that the data generation rate of the first application is varied can be selected as desired, in accordance with the real time event that it relates to. For example, the compression ratio of the generated data relative to the information provided to the data generator can be varied. This would be suitable in the case of a video transmission, where the video signal is compressed for transmission, although this may have the effect of altering the resolution of the received video signal. Alternatively the update rate of the signal could be reduced, for example in a video transmission by reducing the frame rate, or in an telemetry application by reducing the update rate of one or more parameters (for example in medical telemetry, a less detailed ECG waveform could be delivered, or in vehicle engine telemetry the update rate of particular, e.g. - temperature, readings could be reduced) . Alternatively, certain information could be omitted from the signal . In a telemetry application, for example, one or more of the parameters could be omitted from the signal when it is desired to reduce the data generation rate. For example, in vehicle engine telemetry, oil temperature readings could be omitted. In all these cases the volume of data generated from the information relating to the real time event is varied and thus accordingly is the data generation rate. It would also be possible, if desired, to provide for a primary data application to be able to indicate a high priority for its data transmission which would instruct the data combiner not to reduce its data generation rate even if there is additional data to be transmitted. Instead, the additional data transmission would be blocked.

Another embodiment of the present invention is illustrated m Figures 3 and 4. In this case, the primary data relating to the continuous real time event is to be transmitted m an encrypted form.

As is known m the art, encryption is often achieved using a time varying algorithm or code. For such an encrypted signal to be properly received, the encryption at the transmission end and decryption at the receiving end must be synchronised. To achieve this, encryption synchronisation information must be inserted in the transmission. For example, a random Initialisation Vector (IV) is sent at the beginning of a transmission to allow encryption synchronisation buffers m the transmitter and receiver to be synchronised. Furthermore, it may be preferable to send the Initialisation Vector, or other encryption synchronisation information, at intervals during the transmission to allow a receiver which loses synchronisation, for example due to poor signal quality, to achieve synchronisation again. Similarly, -m communication systems which transmit a signal to more than one receiver, or user, it may be desirable to transmit the Initialisation Vector, or other encryption synchronisation information, at intervals to allow receivers to achieve synchronisation, i.e. to begin receiving and decoding the signal, during the transmission rather than just at the beginning of the transmission (i.e. to permit so-called "late-entry").

In this arrangement the first or primary application 1 would generate a continuous data stream relating to the continuous real time event that it is monitoring, and provide that data stream to an encryption device 7. The encryption device will encrypt the primary data and also provide at intervals encryption synchronisation information for inclusion in the data transmission to enable a receiver to maintain its synchronisation and, where appropriate, to enable receivers to achieve "late-entry" to the transmission. The encryption device 7 can also conveniently include or carry out the functions of the data combiner 5 referred to in the earlier embodiments of the invention.

In this arrangement it would be possible for the data combiner or encryption device to control the first data application such that it only reduces its data generation rate at the time when there is encryption synchronisation information to transmit, and then increases its rate to the maximum transmission rate of the channel once the encryption synchronisation information has been transmitted, and repeats this process at intervals whenever it is necessary to transmit encryption synchronisation information.

In an alternative arrangement where it is known that there are going to be intermittent encryption synchronisation information insertions for the duration of the transmission, rather than continually varying the data generation rate of the first application before and after each individual encryption synchronisation information insertion, the system could instead determine the data rate of the encryption synchronisation information and set the data rate for the first application to be equal to the maximum data transmission rate of the transmission channel less the known data rate of the encryption synchronisation inforraation/ (i . e . such that the primary application data rate and the encryption synchronisation information data rate together equal the maximum data transmission rate of the channel) . The first application would then supply its data at a more or less steady, reduced rate to the data buffer of the data combiner (e.g. encryption device) which stores the data until it is required by the faster constant rate transmission channel . The data is removed from the buffer at the higher transmission rate of the channel and thus because the rate of supply to the buffer from the first application is a little lower than the rate at which the data is removed from the buffer, the buffer gradually empties. However, at intervals, the secondary encryption synchronisation information is inserted by the encryption device into the buffer which effectively fills in the space in the buffer and so ensures that the buffer never actually empties and that data can be continuously transmitted at the maximum data transmission rate of the transmission channel. Thus the data generation rate from the primary application is set to give space to insert say, one synchronisation information frame per second (e.g., if video, per I- frame period) . This arrangement would normally continue for the duration of the encrypted transmission, but if for some reason it is decided to no longer insert encryption synchronisation information into the transmission, the data combiner or encryption device could then instruct the first application to increase its data generation rate to the full channel rate.

Figures 3 and 4 illustrate suitable messaging sequences at the transmission end (Figure 3) and receiving end (Figure 4) for this arrangement. Referring to Figure 3, the first or primary application 1 sends a signal ( "S_report_request) to an encryption device 7 to enquire whether or not its signal is to be encrypted. ^The encryption device sends a reply signal (" S_UNITDATA_indication" ) confirming that it is present and will encrypt the signal .

The application 1 then requests a call to be set up by sending a signal ("S_setup") to the network device 6. The network device 6 confirms that a call has been set up or that transmission can begin by sending a signal ( "P_acknowledge" ) to the application 1.

The application 1 then sends a signal ( "S_report_request " ) to the encryption device 7 (which acts also as the data combiner of the previous embodiments) indicating that encrypted transmission is about to begin. The encryption device 7 decides the rate at which data should be produced by the primary application and sends a signal ( "S_report_indication" ) to the application indicating the rate that data should be generated. Data is then generated by the application 1 and sent to the encryption device 7 for encryption and then transmission. At the same time, the encryption device generates at appropriate intervals encryption synchronisation information for transmission.

As before, the encryption device includes a buffer for temporarily storing the data produced. After, or as, the encryption device 7 begins to receive data from the primary application 1, the encryption device 7 first sends data ( "T_UNITDATA_request " ) to the network device relating to encryption synchronisation, such as a random Initialisation Vector. Whilst this encryption synchronisation information is sent, the encryption device 7 buffers the data received from the primary application 1.

After the encryption synchronisation data has been sent, the encryption device 7 removes from the buffer the data produced by the primary application 1 (which it has encrypted) and transmits it. This data is removed at a rate slightly higher than the rate that it is generated by the primary application 1, with the result that the buffer gradually empties and the data transmission catches up with the data generation by the primary application 1. The data rates are selected such that when the encryption device 7 is ready to send further encryption synchronisation information, the buffer is sufficiently empty to be able to store all the data generated by the primary application 1 during the time when the encryption synchronisation information is being transmitted (and no primary data is being removed from the buffer) . Transmission continues in the above manner until the end of the transmission, or some other change .

Figure 4, illustrates the operation of a receiver that receives the encrypted data transmitted in the above manner. It comprises a network device 12, application 10 and an encryption device 11.

When the network device 12 first receives a transmitted signal it sends a signal ("S_setup") to the application 10 indicating that a data transmission is about to be received. Information relating to the rate at which data is to be received is then passed to the encryption device 11 and the signal begins to be received at the prescribed rate. The encryption device sends a signal ( "S_report_indication" ) to the application 10 indicating the rate at which it will pass data to the application 10 and proceeds to extract encryption synchronisation information from the received signal, decrypt the primary data and then pass it to the application 10 at the set rate. The application 10 can then reproduce the primary data appropriately. Although, as in the above embodiments, the secondary data would normally be one-off or less frequent data to be transmitted, it could also be data generated continuously from information relating to a real-time event, i.e. be similar to the primary data.

In that case the rate of generation of both the primary and secondary data could be varied and controlled as described above with reference to the primary data to allow the data to fit onto the transmission channel.

As will be appreciated from the above, the present invention is applicable to any data transmission system, such as mobile radio communications systems, such as the TETRA system.

Claims

1. A method of transmitting data from two or more data sources over a transmission channel to a receiver, in which a primary data source during the transmission generates from information provided continuously to it a continuous stream of data for transmission to the receiver, and one or more secondary data sources generate additional data for transmission to the receiver on the transmission channel during the transmission of the stream of data from the primary data source, the method comprising: controlling the primary data source to generate its continuous data stream at a rate selected in accordance with the rate that the additional data from the secondary data source or sources is provided to the channel for transmission.

2. The method of claim 1, wherein the primary data source data generation rate is controlled by varying the volume of data the primary data source generates from the information that it receives.

3. The method of claim 1 or 2 , wherein the primary data source data generation rate is controlled by varying the relative amount by which the primary data source compresses for transmission the information it receives .

4. The method of claim 1, 2 or 3 , wherein the primary data source data generation rate is controlled by varying the amount of the information received that is transmitted.

5. The method of claim 1, 2 or 3 , wherein the primary data source is a telemetry application and the rate of generation of its continuous data stream is controlled by varying the update rate of one or more telemetry parameters .

6. The method of claim 1, 2 or 3, wherein the continuous data stream represents video data and the rate of generation of the video data is controlled by varying the rate at which frames of video data are made or encoded.

7. The method of claim 1, 2 or 3, wherein the continuous data stream represents acoustic data and the rate of generation of the acoustic data is controlled by varying the sampling rate of the acoustic data.

8. The method of any one of claims 1 to 7, wherein the volume of data generated by the primary data source is selected such that the rate that primary data and secondary data are generated for transmission is equal to the predetermined overall or maximum transmission rate of the channel .

9. The method of any one of claims 1 to 8 , wherein the primary data source data generation rate is reduced when there is secondary data to transmit, and the rate is increased when there is no secondary data to transmit.

10. A method of transmitting data from two or more data sources to a receiver on a transmission channel, wherein during the transmission a first data source generates a continuous stream of data for transmission from information received continuously by it, the method comprising: determining if a second data source has data to transmit during the data transmission from the first data source; if a second data source has data to transmit, reducing the rate at which the first data source generates data for transmission from the information it

wherein at least the primary data is provided to a buffer for temporarily storing the data, from which buffer the data is then removed for transmission.

15. The method of any one of the preceding claims, wherein data rate information indicating the rate at which data from the primary source is being generated is transmitted on the transmission channel to the receiver.

16. A method of transmitting data from two or more data sources simultaneously over a transmission channel, at least a first of the data sources during the transmission generating from information provided to it continuously in real time a continuous stream of data for transmission to the receiver, the method comprising varying the volume of data generated by said first data source from the information it receives in order to provide space on the transmission channel for transmitting data from another of the data sources.

17. A method of receiving data comprising varying the rate at which data is decoded according to data rate generation information received from a transmitter.

18. An apparatus for transmitting data from two or more data sources over a transmission channel to a receiver, the apparatus comprising: a primary data source for generating a continuous stream of data for transmission to a receiver on a transmission channel from information provided to it continuously; one or more secondary data sources for generating additional data for transmission to the receiver on the transmission channel during the transmission of the stream of data from the primary data source; and a controller for controlling the primary data source to generate its continuous data stream from the information ✓it receives at a rate selected in accordance with the rate that the additional data from the secondary data source or sources is provided to the transmission channel for transmission.

19. The apparatus of claim 18, wherein the controller controls the primary data source data generation rate by varying the volume of data the primary data source generates from the information it receives.

20. The apparatus of claim 18 or claim 19, wherein the controller controls the primary data source data generation rate by varying the relative amount by which the primary data source compresses the information it receives.

21. The apparatus of any one of claims 18 to 20, wherein the controller controls the primary data source data generation rate by varying the amount of the information received that is transmitted.

22. The apparatus of any one of claims 18 to 21, wherein the controller controls the primary data source to generate primary data at a rate that is reduced when there is secondary data to transmit, and increased when there is no secondary data to transmit.

23. An apparatus for transmitting data from two or more data sources on a transmission channel to a receiver, in which a first of the data sources during the transmission generates a continuous stream of -data for transmission from information received continuously by it, the apparatus comprising: means for determining if a second data source has data to transmit during the data transmission from the first data source; means for reducing, if a second data source has data to transmit, the rate at which the first data source generates data for transmission from the information it receives to provide space on the transmission channel for the data from the secondary data source; means for determining when the secondary data source transmission has finished; and, means for increasing the rate at which the first data source generates data for transmission from the information it receives when it is determined that the secondary data source transmission has finished.

24. The apparatus of claim 22 or 23, further comprising means for reducing, when the secondary data is to be transmitted, the rate at which primary data is generated to be less than the data transmission rate of the channel, means for stopping transmitting the primary data but instead storing it temporarily while the secondary data is transmitted at the transmission rate of the transmission channel and means for, after the secondary data has been transmitted, continuing to produce the primary data at a reduced rate but transmitting the previously stored primary data and subsequently the newly generated primary data until the stored primary data has been transmitted.

25. The apparatus of claim 22 or 23, further comprising means for dividing the secondary data into smaller portions to be sent at intervals at an allocated data rate, and means for reducing the primary data generation rate accordingly until the secondary data has -all been sent, at which point the primary data generation rate is increased again.

26. The apparatus of any one of claims 18 to 25, further comprising a buffer for temporarily storing the primary data, and means for removing the primary data from the buffer for transmission.

27. The apparatus of any one of claims 18 to 26, further comprising means for transmitting on the transmission channel to the receiver data rate information indicating the rate at which data from the primary source is being generated.

28. An apparatus for transmitting data from two or more data sources simultaneously over a transmission channel, to a receiver, the apparatus comprising: a first data source for generating from information provided to it continuously in real time a continuous stream of data for transmission to a receiver on the transmission channel; and a control means for varying the volume of data generated by the first data source from the information it receives in order to provide space on the transmission channel for transmitting data from another of the data sources.

29. A receiver comprising means for varying the rate at which received data is decoded according to data rate generation information received from a transmitter.

30. Computer software comprising computer software code portions for performing the method of any one of claims 1 to 17 when said software is run on a computer.

31. An apparatus for transmitting data from two or more data sources on a transmission channel to a receiver, the apparatus being substantially as hereinbefore described with reference to any one of the accompanying drawings .

32. A receiver substantially as hereinbefore described with reference to any one of the accompanying drawings.

33. A method of transmitting data from two or more data sources on a transmission channel to a receiver, substantially as hereinbefore described with reference to any one of the accompanying drawings .