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Search for ITU-T Recommendations and Supplements: ITU-T Recommendations by series Select a series of Recommendations A series: Organization of the work of ITU-T D series: General tariff principles E series: Overall network operation, telephone service, service operation and human factors F series: Non-telephone telecommunication services G series: Transmission systems and media, digital systems and networks H series: Audiovisual and multimedia systems I series: Integrated services digital network J series: Cable networks and transmission of television, sound programme and other multimedia signals K series: Protection against interference L series: Construction, installation and protection of cables and other elements of outside plant M series: Telecommunication management, including TMN and network maintenance N series: Maintenance: international sound programme and television transmission circuits O series: Specifications of measuring equipment P series: Terminals and subjective and objective assessment methods Q series: Switching and signalling R series: Telegraph transmission S series: Telegraph services terminal equipment T series: Terminals for telematic services U series: Telegraph switching V series: Data communication over the telephone network X series: Data networks, open system communications and security Y series: Global information infrastructure, Internet protocol aspects and next-generation networks Z series: Languages and general software aspects for telecommunication systems

X series: Data networks, open system communications and security X.1-X.199: Public data networks X.200-X.299: Open Systems Interconnection X.300-X.399: Interworking between networks X.400-X.499: Message Handling Systems X.500-X.599: Directory X.600-X.699: OSI networking and system aspects X.700-X.799: OSI management X.800-X.849: Security X.850-X.899: OSI applications X.900-X.999: Open distributed processing X.1000-X.1099: Information and network security X.1100-X.1199: Secure applications and services X.1200-X.1299: Cyberspace security X.1300-X.1399: Secure applications and services X.1500-X.1599: Cybersecurity information exchange X supplements: Supplements to ITU-T X-series Recommendations