GB2248750A - Suppressing errors in videotex transmission via radio channels to mobile receivers - Google Patents

Description

2 2 4 3 7 1 - Method of suppressing errors in a videotex transmission via

radio channels to mobile receivers The invention relates to a method of suppressing errors in a videotex transmission via radio channels to mobile receivers.

As long as a videotex transmission takes place via fixedly wired lines, for example private or public line networks, as a rule errors occur very rarely and consequently need not necessarily be additionally corrected. If, however, the videotex transmission is via radio to mobile subscribers, for example receivers in motor vehicles, additional disturbances occur which are not to be expected in the videotex transmission over fixedly wired networks and in the hitherto usual decoding methods and consequently are also not recognised and in particular not eliminated. However, the interferences occurring in a videotex radio transmission in some cases disturb and mutilate the reproduction on the screen of the videotex terminal to such an extent that a screen page can no longer be read.

In transmissions to mobile subscribers interferences due to fading on the high-frequency path are not detected as disturbances in the receiver but as additional characters. These additional characters are also inserted as disturbances into the transmission pauses of varying length of the asynchronous transmission. Since this, however, hardly occurs in line transmission, a videotex decoder is programmed so that it does not respond at all to said characters. The reason for this is, in particular, that a videotex decoder is always open to the reception of characters so that, for example, the echoes, which normally with entry by means of the keyboard of the - videotex terminal are intended as control for the entering videotex user, appear directly on the screen.

Echoes here mean characters which correspond to a character transmitted from a videotex terminal to the videotex computer centre and are to be expected at the videotex terminal after a certain delay. These characters can be checked with regard to character identity and are passed on to the screen of the videotex terminal only after confirmation of such coincidence, whereas in the event of defective identity they are suppressed. Apart from the echoes, character sequences are also sent from a videotex computer centre to a videotex terminal which represent known exceptions, for example error and disturbance reports, but which are not expected in the videotex terminal. In addition, the videotex central computer transmits characters of a videotex protocol sequence which for example belong to a block which starts with an "STX character", followed by information characters, and ends with a "CRU (Cyclic Redundance Check) character.

All the other characters received are undesired additional signals which are passed directly and unchanged onto the screen, the screen content thereby being altered to a greater or lesser extent.

The number of additional characters which arise here depends on the varying degree of fading, the varying quality of the signal-noise performance ratio (C/No) and the transmitting pauses of varying length. A large fading and a small signal-noise ratio permit more additional characters.

When disturbances of this type occur within a videotex protocol block they are, however, in the great majority of cases discovered by the CRC (Cyclic Redundance Check) tests of the decoder, i.e. by characters which contain check sums and permit an assessment (erroneous or not) of the preceding block; disturbances of this type therefore result only in a block repetition and normally do not lead to impairment of the reproduction on the screen of the videotex terminal.

It would thus be desirable to provide a method of suppressing errors in a videotex transmission via radio channels to mobile receivers which prevents undesired additional characters reaching the screen and leading there to considerable interferences.

According to the present invention there is provided a method of suppressing errors in a videotex transmission vi.4 radio channels to mobile receivers, characters being transmitted by a videotex central computer to a videotex terminal, received in the terminal and then processed in the terminal by means of a videotex decoder, wherein the method comprises the steps, prior to processing in the videotex decoder, of investigating all received characters to determine whether they are echoes, known exceptions or characters of a videotex protocol sequence, classifying as disturbances all those received and investigated characters which have not been recognised as echoes, known exceptions or characters or a videotex protocol sequence, and suppressing such disturbances.

In such a method, received characters in the videotex terminal, i.e. via the videotex decoder thereof, do not reach the following screen unchecked and consequently a screen page cannot be altered randomly in dependence upon the particular data content of the 5" undesired character or characters. For this purpose prior to further processing in the videotex decoder all the characters received are investigated to determine whether they are echoes, known exceptions or characters of a videotex protocol sequence. Then, of the received and investigated characters all the caracters which have been detected as not properly transmitted characters are classified as disturbances and accordingly suppressed. Thus, if received characters cannot be assigned either to the echoes or to the known exceptions or characters of a videotex protocol block, they are disturbances which have occurred or been added during a radio transmission and which hitherto could not be detected as such and would therefore have been mistakenly further processed by the videotex decoder.

However, with a method exemplifying the present invention such undesired characters can be recognised as such and thus dealt with as interferences, which are suppressed and are therefore not further processed in a videotex decoder and cannot reach the screen thereof.

A method exemplifying the present invention will now be more particularly described with reference to the accompanying drawing, the single figure of which is a flow chart illustrating the steps of such a method.

In the illustrated flow chart, for clarification the step sequence for carrying out the method is framed in dashed line. To enable the method to be carried out with the aid of a videotex decoder, as usual always open to reception, the function of the videotex decoder must be modified by providing a corresponding firmware, software or hardware addition so that the checking and 5" transmitting of the received characters can be carried out. For this amplification of a videotex decoder the person skilled in the art need only take account of the specific construction thereof with the aid of suitably detailed documents; from said documents the specific mode of operation and function of the videotex decoder provided for the user must be apparent. In other words, every person skilled in the art who has hitherto already developed videotex decoders, or is only generally familiar with the construction or mode of operation of videotex decoders or even of only one specific videotex decoder type, can implement the method in videotex decoders and thus arrive at the result which can be obtained by the method, i.e. that the undesired additional characters are suppressed and no longer appear on a screen following the videotex decoder.

Firstly, in a step S1 a terminal keyboard key on a videotex terminal is depressed, an echo flag thereby being set at a step S2.

In a step S3, as hitherto, an initialisation of the videotex decoder is carried out, whereupon in the step S4 both a protocol and an echo flag are then cancelled. In the step S5 characters are then expected from the videotex central computer and as soon as characters have been received in the step S6 it is determined in a step S7 whether or not an echo flag is set. If in the step S7 an echo flag is set, in a step S8 echoes are checked and investigated for errors in a step S9.

If no error is detected in the step S9 the procedure moves to a step S11 in which the set echo flags are cancelled. If on the other hand errors are detected in the step S9 they are indicated in the step S10, whereupon the echor flags are again cancelled, and 5' thereafter in a step S19 further (already present hitherto) tasks are carried out by the videotex decoder.

If it is determined in the step S7 that no echo flag is set, it is checked in a step S12 whether known exceptions are present or not. If they are, the next step is the step S19, i.e. the hitherto already present tasks are performed by the videotex decoder, whereas if in the check in the step S12 it is found that no known exceptions are present the next step is a step S13 in which the received characters are checked for the presence of a protocol sequence or its "start", for example in the form of the character "STV. If such a start of a protocol sequence has been detected in the step S13, in a step S14 a protocol flag is set and thereafter a move again made to the step S19.

If on the other hand in the step S13 the start of a protocol sequence is not determined, in a step S15 a protocol sequence "end" is checked, for example in the form of a "ENQ" character. If this is the case, thereafter in a step S16 the protocol flag is cancelled and the sequence moves to the step S19 so that further tasks can be performed by the videotex decoder. If the check in the step S15 provides negative, in a step S17 it is checked whether a protocol flag has been set or not. If it has, the sequence then also moves in this case to the step S19.

- 7 If, however, in the step S17 it was not possible to determine that a protocol flag is set, this is a sure sign that the characters received are undesirable characters. All these characters are then suppressed in a step S18, the sequence then returning to the step S5 where characters are expected from the videotex central computer.

If, therefore, the check made in each of the steps S7, S12, S13, S15 and S17 is negative, this is a substantially clear indication that the characters received are unexpected and thus undesired characters which are also completely suppressed in the step S18. It is ensured in this manner that such undesired characters can in no way reach the screen where they would disadvantageously influence a screen page or make it incomprehensible.

Fundamentally, the method exemplifying the invention can also be used in a modified videotex decoder having a line-link transmission. When using the method in such a videotex decoder with line-linked transmission in general no improvement at all should be detected. In contrast, when using the method exemplifying the invention in videotex or similar transmission protocols or in their decoder, successful use and thus improvement of the reproduction can be expected.

Claims (2)

1. A method of suppressing errors in a videotex transmission via radio channels to mobile receives, characters being transmitted by a videotex central computer to a videotex terminal, received in the terminal and then processed in the terminal by means of a videotex decoder, wherein the method comprises the steps, prior to processing in the videotex decoder, of investigating all received characters to determine whether they are echoes, known exceptions or characters of a videotex protocol sequence, classifying as disturbances all those received and investigated characters which have not been recognised as echoes, known exceptions or characters or a videotex protocol sequence, and suppressing such disturbances.

2. A method as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.