INTERNATIONAL TELECOMMUNICATION UNION )45 4 & TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU /0%2!4)/.3 !.$ 15!,)49 /& 3%26)#% 4%,%'2!0( 3%26)#%3 #/-0/3)4)/. /& !.37%2 "!#+ #/$%3 &/2 4(% ).4%2.!4)/.!, '%.4%8 3%26)#% )45 4 Recommendation & (Extract from the "LUE "OOK) NOTES 1 ITU-T Recommendation F.21 was published in Fascicle II.4 of the Blue Book. This file is an extract from the Blue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, the contents of the file are identical to the Blue Book version and copyright conditions remain unchanged (see below). 2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. © ITU 1988, 1993 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. Recommendation F.21 Fascicle II.4 - Rec. F.21 COMPOSITION OF ANSWER-BACK CODES FOR THE INTERNATIONAL GENTEX SERVICE The CCITT, considering (a) that 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); (b) that the answer-back code in the international gentex service should therefore include the call-number of the office and as much of the name of this office as possible; (c) that it is also essential to show in the answer-back code one or two characteristic letters of the country in which the teleprinter equipment is situated, for the worst routing mistake is that of sending a telegram to the wrong country; (d) that Administrations may identify, by additional letters in the answer-back code, not only the office but also the nature of the position in the office (outgoing position, incoming position), or the identity of the position among all similarly specialized positions, so as to facilitate the location of any faults in the equipment of the tracing of any telegrams in disupute; (e) that the initial letters of the alphabet: A, B, C, etc., are to be used for identifying specialized outgoing positions and one of the final letters of the alphabet Z, Y, X, etc. for identifying specialized incoming positions. For very large offices, where groups of machines having the same specialized function, outgoing or incoming, may comprise more than 12 machines, it will be necessary to use additional specialization letters: T to indicate a position specializing in transmission; R to indicate a position specializing in reception; (f) that if an office that uses letters denoting specialization is also equipped with combined incoming/outgoing positions, such positions will be identified by the same specialization letter as the incoming positions; (g) that should outgoing or incoming groups comprise more than 26 machines, the letters S and Q, denoting outgoing or incoming specialization respectively, may be used in conjunction with the letters T and R, thus increasing the possibility of identification in a group of machines to 52; (h) that in the case of overflow positions, they must indicate very clearly the name of the office obtained, for this name belongs to an office other than the one called. For this purpose, the call-number of the overflow office will not appear in the answer-back code of such a position, so as to leave space for the name of the office as complete as possible and the characteristic indication DEB, which has been chosen to denote overflow; (i) that because machines in the gentex service can be page-printing machines, it is necessary to provide carriage-return and line-feed signals at the beginning of the answer-back code and for technical reasons the last character of the answer-back code must be the letter-shift signal, unanimously declares (1) that answer-back codes of machines used in the international gentex service should be made up of 20 signals; (2) that, for machines other than those used on positions specialized for receiving overflow traffic, the series of 20 signals in the answer-back code should, in principle, be as follows: − carriage-return, − line-feed, − figure-shift, Fascicle II.4 - Rec.