ITU-T 

It issues recommendations that have names like X.500, where X is the series and 500 is a serial number.

See also:

• ITU-R

See ITU-T's own website, http://www.itu.int/ITU-T/

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
  • G.711 Audio compression (mu-law)
  • G.723.1 Audio compression
  • G.729[?] Audio compression
• H - Audiovisual and multimedia systems
  • H.225.0[?] Also known as RTP
  • H.261[?] Video compression standard, circa 1991
  • H.263 Video compression standard, circa 1995
  • H.263v2 (a.k.a. H.263+) Video compression standard, circa 1998
  • H.264 Video compression standard (also known as MPEG-4 part 10)
  • 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 - Tunneling of signalling protocol (Isup) in H.323
  • 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.24[?] List of definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) - equivalent to EIA[?] RS 232[?]
  • V.32[?] A family of 2-wire, duplex modems operating at data signalling rates of up to 9600 bit/s for use on the general switched telephone network and on leased telephone-type circuits
  • 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
  • V.42bis[?] Data compression procedures for data circuit-terminating equipment (DCE) using error correction procedures
  • 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
• 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
• Y - Global information infrastructure and Internet protocol aspects
• Z - Languages and general software aspects for telecommunication systems