The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecommunication Union.The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis.The World Telecommunication Standardization Conference (WTSC),which meets every four years,established the topics for study by the ITU-T Study Groups which, in their turn,produce Recommendations on these topics.ITU-T Recommendation I.256.2b was prepared by the ITU-T Study Group I (1988-1993) and was approved by the WTSC (Helsinki, March 1-12, 1993).