The CCITT,considering(a) Recommendations F.60 [4] and F.21 [5] concerning the telex and gentex services respectively;(b) that start-stop equipment is capable of receiving communications without the help of an operator;(c) that this advantage is useful to users of the international Telegraph Alphabet No.2 (ITA2);(d) that it is therefore desirable that the identity of either the calling or the called party should be capable of being checked;(e) that it may be necessary to verify the correct functioning of the line and of the distant terminal equipment;(f) that it is desirable to give confidence to the calling party that the reception of the called station’s answerback code is related to the proper working of that station as a whole.
ITU-T S.6-1988 Referenced Document
ITU-T F.130 MARITIME ANSWER - BACK CODES*， 1993-01-01 Update
ITU-T F.21 COMPOSITION OF ANSWER - BACK CODES FOR THE INTERNATIONAL GENTEX SERVICE*， 1993-01-01 Update
ITU-T F.60 Operational Provisions for the International Telex Service (Study Group I) 34 pp*， 1992-01-01 Update
ITU-T S.6 Characteristics of Answerback Units (ITA2) - Telegraph Services Terminal Equipment (Study Group IX) 2 pp*， 1989-01-01 Update
ITU-T S.6-1988 history
1988ITU-T S.6-1988 CHARACTERISTICS OF ANSWERBACK UNITS (ITA2)