GB2196516A

GB2196516A - Data transmission

Info

Publication number: GB2196516A
Application number: GB08705615A
Authority: GB
Inventors: Derek Thomas Wright
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1987-03-10
Filing date: 1987-03-10
Publication date: 1988-04-27
Also published as: GB8705615D0

Abstract

Digital data is transmitted in packets in accordance with an error protection code which is capable of correcting errors up to a predetermined error frequency. The code is capable of application at several different levels. For each packet an appropriate one of the plurality of levels is used enabling correction of errors at respective different error frequencies for different packets. The system may be used in MAC television transmissions to provide better error protection of authorisation keys for receivers at the fringe of the service area.

Description

SPECIFICATION Data transmission This invention relates to the transmission of digital data in packets. The invention is particularly suitable for use in a conditional-access broadcast television system using over-air addressing to enable authorised receivers.

This specification will assume a knowledge of document Tech. 3258 published by the European Broadcasting Union Technical Centre, Brussels, October 1986, entitled "Specification of the Systems of the MAC/packet family".

This document will be referred to as the "MAC specification".

Attention is particularly drawn to Part 6 of the MAC specification which concerns the conditional access system. This system is designed so that programmes or data services are only available to selected viewers, e.g.

those who have paid the necessary subscription. To this end the television signal is scrambled before being transmitted in accordance with a key. The key is transmitted in encrypted form. To decrypt the scrambling key the receiver needs an authorisation key which is in turn encrypted by the user's specific distribution key. The resulting cryptogram comprising the authorisation key and distribution key is specific to an individual user or group of users and is termed the entitlement management message (EMM). One way of transmitting the EMM to the user in accordance with the MAC specification is to transmit it in the same channel as the programme and this is called over-air addressing.

Typically the EMM may comprise a 36 bit unique user address for an individual user or a 24 bit shared address for a group of users.

Clearly error protection for this conditional access data is important as it is necessary to ensure that a receiver at the fringes of the service area can reliably receive the conditional access data even during adverse weather conditions. This means that a relativeiy high level of error protection has to be provided to this data and this increases the amount of conditional access data to be transmitted. As it will generally be necessary to send an individual EMM to every receiver this increase in data rate results in a noticeable increase in the time it takes for any individual receiver to be authenticated or re-authenticated.

We have appreciated, however, that the system just described is unduly wasteful of channel capacity. This is because the receivers which are well within the service area, and not at the fringes, will have an unnecessarily high level of error protection applied to their EMMs, and similarly in good weather conditions all the receivers will receive EMMs which suffer a level of protection which is higher than that actually required.

We have thus appreciated that the effective data rate can be substantially increased by transmitting the EMM in accordance with an error protection code which is capable of correcting errors up to a predetermined error frequency, and selectively applying the code at one of a plurality of levels enabling correction of errors at respective different error frequencies. The level of error protection can then be varied for all EMMs to reflect the current transmission conditions generally and can be varied for individual EMMs to reflect the signal strength received at the location where the receiver is situated.

The receiver will need to know the level of error protection which is being applied and this will normally be done by detecting an indicator byte transmitted in the message, but in principle could be done by other means such as detecting the spacing between field separation for example.

When using the group addressing method, if the groups are arranged so that all users in any particular group are also within a reasonably limited geographical area (say 40 km diameter), then the reception conditions for that particular group at any particular time will be fairly uniform. It now becomes feasible for EMM data addressed to good reception areas to be given a lower level of error protection than than intended for poor reception areas. It is also possible to increase the error protection for users in areas where reception is often affected by bad weather, but to apply this extra protection only while the bad weather persists.

The invention is defined in the appended claims to which reference should now be made.

The invention can be implemented for example in a system of the type described in Part 6 of the MAC specification, see particularly Fig. 1 thereof. A detailed example will not, therefore, be given here. It will be appreciated that provision for dynamic control of the level of error protection has to be designed into the system as a whole from the beginning even if not used by any particular channel now or in the future.

Broadly speaking, at the transmitter the coding circuitry, which codes the data packets in accordance with an error protection code of a type which is capable of correcting errors up to a predetermined error frequency, is supplemented by control circuitry which causes it selectively to apply coding at one of a plurality of levels enabling correction of errors at respective different error rates. The control circuitry is in part operator controlled in accordance with transmission (i.e. weather) conditions, and in part is computer controlled to vary the coding level in dependence upon the particular EMMs being transmitted.A receiver includes a decoding circuit for decoding the received packets in accordance with the same error protection code, and also control cir cuitry which correspondingly causes the decoding means selectively to apply decoding at one of the plurality of levels enabling correction of errors at respective different error frequencies. The transmitter will normally transmit an indicator byte with the data packets indicating the level of coding being applied and so the control circuitry in the receiver is connected to receive this indicator from the incoming received data packets.

Various examples of the type of coding can be considered. For example, the extra control parameters for any particular packet could be defined by the use of extra 8,4 Hamming coded bytes included as part of the packet header. The parameters available to dynamic error control might be as follows: (a) Whether or not the packet includes a cyclic redundancy check on the useful data content (a packet suffix).

(b) Whether the packet will be repeated and how many further repeats can be expected.

(c) Whether the useful data within the packet uses Hamming coding (8,4), Golay coding with parity (24,12), or does not use an error correcting code.

Features (a) and (b) above are already contemplated for system indicator (SI) packets in the MAC specification, see Section 3,2 of Part 5. The variations possible under (c) can be defined by one further Hamming coded byte.

Thus it is seen that the use of dynamic error control on EMM data packets can permit more efficient use of those data channels of the MAC/packet system which carry addressed data packets. This would be achieved by avoiding wasteful over-protection of data destined for areas of good reception, and by increasing the error protection to cope with poor weather conditions only while and where such conditions exist.

There would be no particular disadvantage in defining the error protection level of packets carrying the entitlement checking message (ECM) data as defined in Part 6 of the MAC specification in a similar way. It would be possible to specify a structure for general-purpose byte-organised transparent data channels carried by the MAC/packet system, to provide these with dynamically definable error control and then to use them as the transport medium for the ECM and EMM data of the Conditional Access service. Such general purpose data channels have already been defined for terrestrial services as a sub-set of the telextext specification. If similar general purpose data channels were used to carry conditional access data for DBS services, there could be a convenient common standard for interfacing to the Conditional Access sub-system regardless the source of the data.

While the invention has been described in the context of conditional-access television broadcasting, it is applicable more generally to digital data transmission in packets.

Claims

1. A method of transmitting digital data in packets, in which the data is transmitted in accordance with an error protection code which is capable of correcting errors up to a predetermined error frequency, characterised in that the code is selectively applied at one of the plurality of levels enabling correction of errors at respective different error frequencies.

2. A method according to claim 1, in which the transmitted data contains an indication of the level at which coding is being applied.

3. A method according to claim 1 or 2, in which the code is a code of the class comprising Hamming, Golay and the like codes.

4. A method according to claim 1 or 2, in which the transmitted data comprises a decoding key to enable a receiver to generate a descrambling key and is transmitted in association with a scrambled television signal, the descrambling key enabling the receiver to descramble the scrambled television signal.

5. A method according to claim 4, in which the transmitted data comprises data individual to specific receivers or groups of receivers and the level of coding is varied for different receivers or groups of receivers.

6. A method according to claim 4 or 5, in which the level of coding is varied in accordance with transmission conditions.

7. A method according to any preceding claim in which the transmission is a broadcast transmission.

8. A transmitter for transmitting digital data in packets, comprising input means for receiving data packets to be transmitted, coding means for coding the received packets in accordance with an error protection code which is capable of correcting errors up to a predetermined error frequency, and output means for transmitting the coded packets, characterised by control means coupled to the coding means so as to cause the coding means selectively to apply coding at one of a plurality of levels enabling correction of errors at respective different error frequencies.

9. A receiver for receiving coded digital data packets, comprising input means for receiving the transmitted data packets, decoding means for decoding the received packets in accordance with an error protection code which is capable of correcting errors up to a predetermined error frequency, and output means for supplying the decoded packets, characterised by control means coupled to the decoding means so as to cause the decoding means selectively to apply decoding at one of a plurality of levels enabling correction of errors at respective different error frequencies.

10. A receiver according to claim 9, in which the control means has an input connected to the input means to receive an indication of the level at which decoding is to be applied which is received with the received data packets.