International Telecommunication Union Telecommunication Standardization Sector
Based in Geneva, Switzerland, The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) coordinates development of international standards in the field of telecommunications within the International Telecommunication Union (ITU).
The ITU dates back to 1865, when the International Telegraph Union (ITU) was formed. Two consultative committees were created in 1925 to deal with the complexities of international telephone services (International Telephone Advisory Committee, or CCIF) and long-distance telegraphy (International Telegraphic Advisory Committee, or CCIT). In 1947, it became a part of the United Nations. Because the issues faced by the two committees were so similar, in 1956 the International Telegraph and Telephone Consultative Committee (CCITT) was formed, combining the two previously separate organizations.
The observant reader will note the mismatch between the names of the various committees and their acronyms. This is because the acronyms were based on the French names for these organizations. For example, the CCITT was the Comité Consultatif International Téléphonique et Télégraphique.
In 1992, the ITU underwent significant reform to reflect the increasingly complex telecommunications world. The CCITT was renamed the Telecommunication Standardization Sector (ITU-T) and became one of three Sectors within the ITU, the other two being the Radiocommunication Sector (ITU-R) and the Telecommunication Development Sector (ITU-D).
Technically, the standards produced by the ITU-T are called Recommendations. This reflects the ITU-T's role as an international organization. The ITU-T has no authority to impose a standard on a sovereign nation. The ITU-T recommendations become standards within a given nation when they have been adopted by the appropriate authority in that nation. However, because the ITU-T is part of a United Nations specialized agency, the Recommendations it produces carry significant international weight.
In the past, CCITT Recommendations were published every four years in a set of books with a designated color (e.g., the Blue Books in 1988, the Red Books in 1984, the Yellow Books in 1980, etc.). However, this model proved to be unresponsive to the increasingly fast-paced telecommunications industry. The rise of the personal computer made real-time standardization far more practical, and the practice was discontinued with the 1988 Blue Books.
The ITU-T Recommendations are designated by a letter.number format (e.g., V.92). The letter identifies the series and the number identifies the particular Recommendation within that series. When a Recommendation is updated, it typically keeps the same number. Often, Recommendations with a common first number are related to one another. For example, X.25, X.28, X.29, and X.21 are all related Recommendations. In some cases, a collection of such Recommendations will be referenced by the higher order number (e.g., X.500 refers to both a particular Recommendation, as well as the entire set of Recommendations that fall in the 500's in the X series). The defined ITU-T series include:
- A - Organization of the work of ITU-T
- B - Means of expression: definitions, symbols, classification
- C - General telecommunication statistics
- D - General tariff principles
- E - Overall network operation, telephone service, service operation and human factors
- F - Non-telephone telecommunication services
- G - Transmission systems and media, digital systems and networks
- H - Audiovisual and multimedia systems
- I - Integrated services digital network
- J - Cable networks and transmission of television, sound programme and other multimedia signals
- K - Protection against interference
- L - Construction, installation and protection of cables and other elements of outside plant
- M - Telecommunication management, including TMN and network maintenance
- N - Maintenance: international sound programme and television transmission circuits
- O - Specifications of measuring equipment
- P - Telephone transmission quality, telephone installations, local line networks
- Q - Switching and signalling
- R - Telegraph transmission
- S - Telegraph services terminal equipment
- T - Terminals for telematic services (i.e. fax)
- U - Telegraph switching
- V - Data communication over the telephone network
- X - Data networks, open system communications and security
- Y - Global information infrastructure, Internet protocol aspects and next-generation networks
- Z - Languages and general software aspects for telecommunication systems
- American National Standards Institute (ANSI)
- International Organization for Standardization (ISO)
- International Telecommunication Union (ITU)
- ITU Standardization Sector (ITU-T)
- ITU-T Guide for beginners (PDF)
- 50 years of CCITT/ITU-T (PDF)
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