The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU). It was formed in 1993 to replace the International Telegraph and Telephone Consultative Committee (CCITT, from the French name "Comité Consultatif International Téléphonique et Télégraphique").
The international standards that are produced by the ITU-T are referred to as "Recommendations" (with the word ordinarily capitalized to distinguish its meaning from the ordinary sense of the word "recommendation"). Since the ITU-T is part of the ITU, which is a United Nations Organization (UNO), its standards carry more formal international recognition than those of most other organizations that publish technical specifications of a similar form.
The sector divides its work into categories that are each identified by a single letter, referred to as the "series" (see below), and Recommendations are numbered within each series, for example "V.90".
Historically from 1960 until the formation of ITU-T in 1993, the Recommendations of the CCITT were presented to four-yearly "plenary assemblies" for endorsement, and the full set of Recommendations were published after each plenary assembly, in a set of volumes titled collectively for the colour of their covers. For example the publication after the 1980 plenary session was the Yellow Book while that after 1984 was the Red Book. These publications were divided into "fascicles" of several hundred pages that could be bought separately. The four-year approval cycle made the CCITT a rather slow and deliberate organization.
However, the rise of the personal computer industry in the early 1980s created a new common practice among both consumers and businesses of adopting "bleeding edge" communications technology even if it was not yet standardized. Thus, standards organizations had to put forth standards much faster, or find themselves ratifying de facto standards after the fact. Unfortunately, like ISO, CCITT was slow to adapt.
In some cases, a hopeless hodgepodge of proprietary standards resulted, with no clear winner; this was and still is the case with color fax technology. Another phenomenon was that the general public sought standards from organizations which it perceived as more responsive or inclusive; these included informal nongovernmental organizations like IETF or private consortia like W3C.
In response, the ITU-T now operates under much more streamlined processes. The time between an initial proposal of a draft document by a member company and the final approval of a full-status ITU-T Recommendation can now be as short as a few months (or less in some cases). This makes the standardization approval process in the ITU-T much more responsive to the needs of rapid technology development than in the ITU's historical past.
A standard that has been amended can (if desired) retain its designation so that, for example, in the mid-1980s, terminal equipment for connection to an X.25 (packet switched) network might need alternative modes of operation depending on whether the network implemented the 1980 (Yellow Book) or the 1984 (Red Book) version of the standard. However, it is now more common for older versions of a standard to simply be marked as "superseded" when a standard is revised, and features of prior versions are ordinarily kept unchanged within the specification as new enhancements are added in new versions.
A standard can be developed that extends or is complementary to an existing one rather than replacing it. Such a standard is sometimes designated by the suffix "bis" or "ter" added to the base standard name, for example "V.26bis" and "V.26ter".
Series and Recommendations
ITU-T issues Recommendations that have names like X.500, where X is the series and 500 is a serial number.
See Category:ITU-T recommendations.
Significant ITU-T series and Recommendations are:
- 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
E.123 Notation for national and international telephone numbers
E.163 Numbering plan for the international telephone service
E.164 The international public telecommunication numbering plan
- Supplement 2 - Number Portability
- F - Non-telephone telecommunication services
- G - Transmission systems and media, digital systems and networks
- H - Audiovisual and multimedia systems
- H.223 Multiplexing protocol for low bit rate multimedia communication
H.225.0 Also known as RTP
H.261 Video compression standard, circa 1991
H.262 Video compression standard (common text with part 2 of MPEG-2), circa 1994
H.263 Video compression standard, circa 1995
H.263v2 (a.k.a. H.263+) Video compression standard, circa 1998
H.264 Video compression standard (technically aligned with MPEG-4 part 10), circa 2003
H.323 Packet-based multimedia communications systems
- Annex D - Real-time facsimile over H.323 systems
- Annex G - Text conversation and Text SET
- Annex J - Security for H.323 Annex F
- Annex K - HTTP based service control transport channel in H.323
- Annex M.1 - Tunnelling of signalling protocol (Qsig) in H.323
- Annex M.2 - Tunnelling of signalling protocol (Isup) in H.323
- H.324 Terminal for low bit-rate multimedia communication
- H.332 H.323 extended for loosely coupled conferences
- I - Integrated services digital network (ISDN)
- J - 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 - TMN and network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile and leased circuits
- 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
- U - Telegraph switching
- V - Data communication over the telephone network
- V.1 Equivalence between binary notation symbols and the significant conditions of a two-condition code.
- V.5 Standard that synchronous data signalling rates should be 600, 1200, 2400, 4800 and 9600 bit/s, to within better than 0.01%.
V.10 A recommendation, first agreed in 1976, for unbalanced electrical circuits for data communication at up to 100 kbit/s.
V.11 A recommendation, first agreed in 1976, for balanced electrical circuits for data communication at up to 10 Mbit/s.
V.17 A fax protocol that uses TCM modulation at 12 and 14.4 kbit/s.
V.21 A recommendation for full-duplex communication between two analogue dial-up modems using AFSK modulation at 300 bauds to carry data at 300 bit/s.
V.22 A recommendation for full-duplex communication between two analogue dial-up modems using PSK modulation at 600 bauds to carry data at 1200 or 600 bit/s.
V.22bis An extension of V.22 using QAM modulation at 600 bauds to carry data at 2400 or 1200 bit/s, with fall-back to V.22 mode.
V.23 A recommendation for half-duplex communication between two analogue dial-up modems using FSK modulation at up to 600 or 1200 bauds to carry digital data at up to 600 or 1200 bit/s respectively. An optional 75 bauds reverse channel carries 75 bit/s.
V.24 A list of definitions for interchange circuits between data terminal equipment (DTE) and data circuit terminating equipment (DCE), first agreed in 1964. This is equivalent to a subset of EIA RS 232: for the electrical and physical details, see V.28 and others.
- V.25 A recommendation, first agreed in 1968, for automatic calling and/or answering equipment on dial-up lines, using interchange circuits defined in V.24 specifically for parallel automatic calling. It includes procedures for disabling network echo cancellers and suppressors.
- V.25bis An extension of V.25 using the serial interchange circuits defined in V.24 for normal data transfer. Command formats are defined for asynchronous, and synchronous character- or bit-oriented (HDLC) operation.
- V.26 A recommendation, first agreed in 1968, for full-duplex communication between two analogue fixed-line modems using PSK modulation at 1200 bauds to carry synchronous data at 2400 bit/s. An optional 75 bauds reverse channel carries 75 bit/s in either direction.
- V.26bis An extension of V.26 , first agreed in 1972, for half-duplex operation of dial-up modems, adding a fall-back rate of 1200 bit/s (still at 1200 bauds).
- V.26ter An extension of V.26 , first agreed in 1984, for full-duplex fixed-line or dial-up operation to carry synchronous or asynchronous data with a fall-back rate of 1200 bit/s (at 1200 bauds), separating channels by echo cancellation.
- V.27 A recommendation, first agreed in 1972, for full- or half-duplex communication between two analogue fixed-line modems using PSK modulation at 1600 bauds to carry synchronous data at 4800 bit/s. An optional 75 bauds reverse channel carries 75 bit/s.
- V.27bis An extension of V.27 , first agreed in 1976, adding a fall-back modulation rate, compatible with V.26 , of 1200 bauds to carry data at 2400 bit/s. An adaptive equalizer is included to handle lower grade lines.
- V.27ter An extension of V.27bis for use on dial-up lines.
- V.28 A recommendation, first agreed in 1972, for unbalanced electrical circuits for data communication. Together with the circuit specifications of V.24 and the 25-pin connector and pin assignments of IS 2110, this is compatible with EIA RS 232.
- V.29 A recommendation, first agreed in 1976, for full-duplex communication between two analogue fixed-line modems using QAM modulation at 2400 bauds to carry synchronous data at 9600 bit/s. Fallback rates of 7200 and 4800 bit/s use 2400 bauds at reduced modulations. Multiplexing of 7200, 4800 and 2400 bit/s subchannels up to an aggregate rate of 9600 bit/s is optional. An adaptation of this standard is used for facsimile (fax) transmission.
- V.32 A recommendation, first agreed in 1984, for a family of duplex analogue dial-up or fixed-line modems using QAM modulation at 2400 bauds to carry data at 9600/4800/2400 bit/s.
- V.32bis A duplex modem operating at data signalling rates of up to 14 400 bit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits, with fallback to 12000 bits/s. This standard was improved on by modem manufacturers to create the V.terbo adhoc standard, signalling at 19,2 kb/s, as suggestive of a V.32ter standard that never materialized.
- V.34 is the ITU-T standard for full-duplex data communications up to 28,800 bit/s with fallback to lower speeds depending on the remote modem and the conditions of the phone line. This standard was known informally as V.Fast, hence pre-standard modems called V.FC (V.FastClass).
- [[V.34+]] (aka V.34bis) A communications protocol for full-duplex datacommunications up to 33,6 kb/s between two analog modems on dial-up lines.
V.42 Error correction protocol
V.42bis Data compression procedures for data circuit terminating equipment (DCE) using error correction procedures to try to ensure the transfer of error-free data, even over the noisiest telephone lines. Ratified by CCITT in January 1990.
- V.44 Data compression protocol
V.90 A digital modem and analogue modem pair for use on the Public Switched Telephone Network (PSTN) at data signalling rates of up to 56 000 bit/s downstream and up to 33 600 bit/s upstream, using PCM encoding downstream, and QAM encoding upstream. V.90 Mode 2 used PCM upstream as well. This standard was known informally as V.Last and some pre-standard modems indicate V.Last compatibility or upgradeability. Prior to the arrival of the standard, there were two competing industry standards for 56 kbit/s downstream signalling, X2 and K56flex. K56flex itself is a merged standard of K56 and 56flex.
V.92 A digital modem and analog modem pair for use on POTS at data signalling rates of up to 56 kbit/s downstream and up to 48 kbit/s upstream, using PCM encoding both ways, supporting Modem-on-Hold technology. This is a development of V.90 Mode 2.
- X - Data networks and open system communication
- X.1 International user classes of service in, and categories of access to, public data networks and Integrated Services Digital Networks (ISDNs)
X.21 Interface between Data Terminal Equipment and Data Circuit-terminating Equipment for synchronous operation on public data networks
X.25 Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and connected to public data networks by dedicated circuit
- X.110 International routing principles and routing plan for public data networks
X.121 International numbering plan for public data networks
- X.200 Information technology - Open Systems Interconnection - Basic Reference Model: The basic model
X.208 Specification of Abstract Syntax Notation One (ASN.1)
X.209 Specification of Basic Encoding Rules for Abstract Syntax Notation One (ASN.1)
X.400 Message handling services: Message handling system and service overview
X.500 Information technology - Open Systems Interconnection - The Directory: Overview of concepts, models and services Common text with ISO/IEC
X.509 Information technology - Open Systems Interconnection - The Directory: Public-key and attribute certificate frameworks
- X.520 Information technology – Open Systems Interconnection – The Directory - Selected Attribute Types
- X.521 Information technology – Open Systems Interconnection – The Directory - Selected Object Classes
- Y - Global information infrastructure and Internet protocol aspects
- Z - Languages and general software aspects for telecommunication systems
- Other specifications which are numbered similar to ITU-T Recommendations, but are not products of the ITU-T, include:
Last updated: 06-02-2005 13:21:29