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The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. Prior to 1992, it was known as the International Telegraph and Telephone Consultative Committee (CCITT, from the French name "Comité consultatif international téléphonique et télégraphique").
- 1 Primary function
- 2 History
- 3 ITU reorganization 1970s-1990s
- 4 ITU's "real time" standardization: 2000-Present
- 5 Changes in ITU-T compliance practices
- 6 Series and Recommendations
- 6.1 A - Organization of the work of ITU-T
- 6.2 B - Means of expression: definitions, symbols, classification
- 6.3 C - General telecommunication statistics
- 6.4 D - General tariff principles
- 6.5 E - Overall network operation, telephone service, service operation and human factors
- 6.6 F - Non-telephone telecommunication services
- 6.7 G - Transmission systems and media, digital systems and networks
- 6.8 H - Audiovisual and multimedia systems
- 6.9 I - Integrated services digital network (ISDN)
- 6.10 J - Transmission of television, sound programme and other multimedia signals
- 6.11 K - Protection against interference
- 6.12 L - Construction, installation and protection of cables and other elements of outside plant
- 6.13 M - TMN and network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile and leased circuits
- 6.14 N - Maintenance: international sound programme and television transmission circuits
- 6.15 O - Specifications of measuring equipment
- 6.16 P - Telephone transmission quality, telephone installations, local line networks
- 6.17 Q - Switching and signalling
- 6.18 R - Telegraph transmission
- 6.19 S - Telegraph services terminal equipment
- 6.20 T - Terminals for telematic services
- 6.21 U - Telegraph switching
- 6.22 V - Data communication over the telephone network
- 6.23 X - Data networks and open system communication
- 6.24 Y - Global information infrastructure and Internet protocol aspects
- 6.25 Z - Languages and general software aspects for telecommunication systems
- 7 See also
- 8 External links
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 1992, 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 and that after 1988 was the Blue 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.
ITU reorganization 1970s-1990s
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 the International Organization for Standardization (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 non-governmental organizations like the Internet Engineering Task Force (IETF) or private consortia like the World Wide Web Consortium (W3C).
ITU's "real time" standardization: 2000-Present
In response to the mess that previous ITU practices had created, 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, but also means that the standards organization's classical functions of quality control and public review have far less time to be effective.
Changes in ITU-T compliance practices
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 an identifying number. When a Recommendation is updated, it will (mostly) keep the same number, so the year of issue may be necessary to identify a specific version of a recommendation. The term "X.500" is used both to refer to the specific X.500 recommendation, and to the entire family of recommendations named X.5xx, where the specific X.500 recommendation forms the introduction and overview to the set.
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
Often just called the G-standards.
- G.165 Echo cancellers
- G.703 Physical/electrical characteristics of hierarchical digital interfaces
- G.707 Network node interface for the synchronous digital hierarchy (SDH)
- G.711 Audio compression (mu-law)
- G.721 Audio compression ADPCM (superseded by G.726)
- G.722 Audio compression (wideband)
- G.722.1 Audio compression (wideband, lower bit rates)
- G.722.2 Speech compression AMR-WB (wideband, lower bit rates)
- G.723.1 Speech compression CELP
- G.726 Audio compression ADPCM
- G.727 Audio compression ADPCM (variant of G.726)
- G.728 Speech compression LD-CELP
- G.729 Speech compression ACELP
- G.991.2 Symmetric High-speed Digital Subscriber Line (G.SHDSL)
- G.992.1 ADSL (G.DMT)
- G.992.2 ADSL (G.Lite)
- G.992.3/4 ADSL2
- G.992.5 ADSL2+
H - Audiovisual and multimedia systems
- H.223 Multiplexing protocol for low bit rate multimedia communication
- H.225.0 protocol for call signaling, the media (audio and video), the stream packetization, media stream synchronization and control message formats. Formerly also included RTP
- H.235 describes security in H.323
- H.239 describes dual stream use in videoconferencing, usually one for live video, the other for presentation
- H.245 control protocol for multimedia communication
- 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.320 Narrow-band visual telephone systems and terminal equipment
- 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
- H.450 describes the Supplementary Services in H.323
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
- Q.23 Dual tone multi frequency access signalling
- Q.700 Introduction to CCITT Signalling System 7
- Q.701 Functional description of the Message Transfer Part (MTP) of Signalling System 7
- Q.702 Signalling data link
- Q.703 Signalling link
- Q.704 Signalling network functions and messages
- Q.705 Signalling network structure
- Q.706 Message Transfer Part signalling performance
- Q.707 Testing and maintenance
- Q.708 Assignment procedures for international signalling point codes
- Q.709 Hypothetical signalling reference connection
- Q.710 Simplified MTP for small systems
- Q.711 Functional description of the Signalling Connection and Control Part
- Q.712 Definition and function of signalling connection control part messages
- Q.713 Signalling connection control part formats and codes
- Q.714 Signalling connection control part procedures
- Q.715 Signalling connection control part user guide
- Q.716 Signalling connection control part performance
- Q.931 Is the layer 3 standard for ISDN signalling
- Q.922-A Link Access Procedure Frame Bearer Services (LAPF)
- Q.933 A frame relay interoperatability standard that adopted most of RFC 1490 recommendation
R - Telegraph transmission
S - Telegraph services terminal equipment
T - Terminals for telematic services
- T.4 Group 3 Apparatus for Transmission (aka Group 3 faxes)
- T.6 Facsimile Coding Schemes and Coding Control Functions for Group 4 Facsimile Apparatus. (aka Group 4 Compression)
- T.30 Procedures for Document Facsimile Transmission. (T.30e defines colour faxing).
- T.31 and T.32 Provide an interface between fax machines and data terminals.
- T.120 - Data protocols for multimedia conferencing
- T.411 - T.424 Comprise the Open Document Architecture (ODA and ODIF), a standardized document file format
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 baud 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 baud to carry data at 1200 or 600 bit/s.
- V.22bis An extension of V.22 using QAM modulation at 600 baud 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 baud to carry digital data at up to 600 or 1200 bit/s respectively. An optional 75 baud 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 1 200 baud to carry synchronous data at 2400 bit/s. An optional 75 baud 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 baud).
- 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 baud), separating channels by echo cancellation.
- V.27 A recommendation, first agreed in 1972, for full-duplex or half-duplex communication between two analogue fixed-line modems using PSK modulation at 1600 baud to carry synchronous data at 4800 bit/s. An optional 75 baud 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 baud to carry synchronous data at 9600 bit/s. Fallback rates of 7200 and 4800 bit/s use 2400 baud 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 baud to carry data at 9600/4800/2400 bit/s.
- V.32bis A duplex modem operating at data rates of up to 14.4 kbit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits, with fallback to 12 kbit/s. This standard was improved on by modem manufacturers to create the V.terbo adhoc standard, signalling at 19.2 kbit/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 28.8 kbit/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.34bis A communications protocol for full-duplex datacommunications up to 33.6 kbit/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 rates of up to 56 kbit/s downstream and up to 33.6 kbit/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.3 This is essentially a numbered list of terminal and comms setting attributes.
- 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.29 Defines the level 4 (or the Session Layer, in the OSI model) for X.25 communications. Call establishment and X.3 parameter negotiation occurs via the X.29 protocol.
- X.75 Interface between DCE units in a network. Essentially transport of X.25 messaging.
- X.110 International routing principles and routing plan for public data networks
- X.121 International numbering plan for public data networks. This can be seen as the specification for X.25 Addresses.
- X.200 Information technology - Open Systems Interconnection - Basic Reference Model: The basic model
- X.208 Specification of Abstract Syntax Notation One (ASN.1) (superseded by X.680)
- X.209 Specification of Basic Encoding Rules for Abstract Syntax Notation One (ASN.1) (superseded by X.690)
- 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
- X.680 series - ASN.1 syntax specification
- X.690 series - ASN.1 encoding rules
- X.700 Management framework for Open Systems Interconnection for CCITT applications
- X.701 Information technology — Open Systems Interconnection — Systems management overview
- X.800 Security architecture for Open Systems Interconnection for CCITT applications
Y - Global information infrastructure and Internet protocol aspects
Z - Languages and general software aspects for telecommunication systems
- Z.200 The Programming Language CHILL
- Other specifications which are numbered similarly to ITU-T Recommendations, but are not products of the ITU-T, include: