the answer-back code sent by teleprinter equipment in the gentex service should provide as much useful information as possible for the operational services. Procedures for checking answer-back codes should be simple and speedy because the average time taken to transmit the text of a telegram in the gentex system is about one minute and this means that roughly every minute three answer-back codes have to be checked by the operator (two at the beginning of the telegram, the answer-back code of the station obtained and the answer-back code of the calling station, and one at the end: the answer-back code of the station obtained)
ITU-T F.21-1993 Referenced Document
ITU-T F.21 COMPOSITION OF ANSWER - BACK CODES FOR THE INTERNATIONAL GENTEX SERVICE
ITU-T F.21-1993 history
1993ITU-T F.21-1993 COMPOSITION OF ANSWER - BACK CODES FOR THE INTERNATIONAL GENTEX SERVICE
1988ITU-T F.21-1988 Composition of Answer-Back Codes for the International Gentex Service - Telegraph and Mobile Services Operations and Quality of Service Study Group I; 3 pp