Transcoding in data communications

A method of processing a data message, comprising a plurality of data fields and an authentication code, received from a source device wherein said data fields have been coded in accordance with a first coding system, whereby respective data field codes are generated for said data fields and a message code is derived from said data field codes, and wherein said message code has been coded in accordance with a second coding system to generate said authentication code, the method comprising:

• producing a transcoded message from the received data message by:
• determining for each data field of the received data message whether to maintain, modify or omit that field;
• for a field to be maintained, maintaining that field in said transcoded message;
• for a field to be omitted, coding the field in accordance with said first coding system to generate an omitted field code dependent upon the data field code for that field, and replacing that field by said omitted field code in the transcoded message;
• for a field to be modified, coding that field in accordance with said first coding system to generate a modified field code dependent upon the data field code for that field, and replacing that field by a modified field, comprising modified data and said modified field code, in the transcoded message;
• and including said authentication code in the transcoded message;
• transmitting the transcoded message to a destination device;
• and
• in said destination device:
• deriving a received message code from the transcoded message using maintained fields, modified fields and omitted field codes in said message in accordance with said first coding system;
• comparing the received message code with the message code encoded in said authentication code in accordance with said second coding system;
• and displaying a user message dependent upon the result of the message code comparison.

A transcoder for transcoding a data message, comprising a plurality of data fields and an authentication code, to produce a transcoded message for transmission to a destination device, the data message being received from a source device wherein said data fields have been coded in accordance with a first coding system, whereby respective data field codes are generated for said data fields and a message code is derived from said data field codes, and wherein said message code has been coded in accordance with a second coding system to generate said authentication code, the transcoder comprising:

• a memory for storing the received data message;
• transcoder logic configured to determine for each data field of the received data message whether to maintain, modify or omit that field, and to produce the transcoded message from the received data message;
• and
• means for transmitting the transcoded message to the destination device;
• wherein the transcoder logic is configured to produce the transcoded message from the received data message by:
• for a field to be maintained, maintaining that field in said transcoded message;
• for a field to be omitted, coding the field in accordance with said first coding system to generate an omitted field code dependent upon the data field code for that field, and replacing that field by said omitted field code in the transcoded message;
• for a field to be modified, coding that field in accordance with said first coding system to generate a modified field code dependent upon the data field code for that field, and replacing that field by a modified field, comprising modified data and said modified field code, in the transcoded message;
• and
• including said authentication code in the transcoded message.

A data communication system comprising:

• a transcoder for transcoding a data message, comprising a plurality of data fields and an authentication code, to produce a transcoded message for transmission to a destination device, the data message being received from a source device wherein said data fields have been coded in accordance with a first coding system, whereby respective data field codes are generated for said data fields and a message code is derived from said data field codes, and wherein said message code has been coded in accordance with a second coding system to generate said authentication code;
• and
• at least one destination device for receiving said transcoded message from the transcoder;
• wherein the transcoder comprises:
• a memory for storing the received data message;
• transcoder logic configured to determine for each data field of the received data message whether to maintain, modify or omit that field, and to produce the transcoded message from the received data message;
• and means for transmitting the transcoded message to said destination device;
• said transcoder logic being configured to produce the transcoded message from the received data message by:
• for a field to be maintained, maintaining that field in said transcoded message;
• for a field to be omitted, coding the field in accordance with said first coding system to generate an omitted field code dependent upon the data field code for that field, and replacing that field by said omitted field code in the transcoded message;
• for a field to be modified, coding that field in accordance with said first coding system to generate a modified field code dependent upon the data field code for that field, and replacing that field by a modified field, comprising modified data and said modified field code, in the transcoded message;
• and including said authentication code in the transcoded message;
• and wherein said destination device comprises a memory for storing a received transcoded message, a display, and control logic configured to derive a received message code from the transcoded message using maintained fields, modified fields and omitted field codes in said message in accordance with said first coding system, to compare the received message code with the message code encoded in said authentication code in accordance with said second coding system, and to supply a user message, dependent upon the result of the message code comparison, to said display for display to a user.