FOREWORD The formal decisions or agreements of the IEC on technical matters@ prepared by technical committees on which all the National Committees having a special interest therein are represented@ express@ as nearly as possible@ an international consensus of opinion on the subjects dealt with. They have the form of recommendations for international use and they are accepted by the National Committees in that sense. In order to promote international unification@ the IEC expresses the wish that all National Committees should adopt the text of the IEC recommendation for their national rules in so far as national conditions will permit. Any divergence between the IEC recommendation and the corresponding national rules should@ as far as possible@ be clearly indicated in the latter. Chapters of the International Electromechanical Vocabulary (IEV) relating to telecommunications (chapters of the 700 series) have been prepared by Joint Groups of experts from the technical committees of the International Telecommunication Union (UIT) - International Consultative Radiocommunications Committee (CCIR)@ International Consultative Telegraph and Telephone Committee (CCITT) - and from the International Electromechanical Commission (IEC)@ co-ordinated by the CCIRCCITT-IEC Joint Co-ordinating Group for vocabulary (JCG). The terms and definitions in these chapters are intended to promote a good understanding of telecommunications texts. They have been approved for publication by the IEC National Committees. They have not received formal approval by the CCIR or CCITT Plenary Assemblies and do not replace definitions contained in CCIR or CCITT recommendations (or in the Radio Regulations@ the International Telecommunication Regulations@ or the International Telecommunication Constitution or Convention) which are to be used in their respective fields of application. This International Standard has been prepared by a group of experts of the JCG under the responsibility of E C technical committee 1: Terminology. It forms Chapter 705 of the International Electromechanical Vocabulary (IEV).