ISO/IEC 13818-4：The ITU-T (the ITU Telecommunication Standardisation Sector) is a permanent organ of the International Telecommunications Union (ITU). The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to developing telecommunication standards on a world-wide basis-The ITU-T (the ITU Telecommunication Standardisation Sector) is a permanent organ of the International Telecommunications Union (ITU). The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to developing telecommunication standards on a world-wide basis