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Foreword

ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of
telecommunications. The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent
organ of the ITU. Some 166 member countries, 68 telecom operating entities, 163 scientific and industrial organizations
and 39 international organizations participate in CCITT which is the body which sets world telecommunications
standards (Recommendations).

The approval of Recommendations by the members of CCITT is covered by the procedure laid down in CCITT Resolution
No. 2 (Melbourne, 1988). In addition, the Plenary Assembly of CCITT, which meets every four years, approves
Recommendations submitted to it and establishes the study programme for the following period.

In some areas of information technology, which fall within CCITT's purview, the necessary standards are prepared on a
collaborative basis with ISO and IEC. The text of CCITT Recommendation T.81 was approved on 18th September 1992.
The identical text is also published as ISO/IEC International Standard 10918-1.

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CCITT NOTE

In this Recommendation, the expression "Administration" is used for conciseness to indicate both a telecommunication
administration and a recognized private operating agency.

ITU 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.