{{Short description|None}} The '''ITU-T V-Series Recommendations''' on '''Data communication over the telephone network''' specify the protocols that govern approved [[modem]] communication standards and interfaces.<ref name="Vseries">{{cite web |url=https://www.itu.int/rec/T-REC-V/en |title=Data communication over the telephone network |access-date=2008-03-02 |author=ITU-T |author-link=ITU-T |date=2006-09-29 |publisher=ITU }}</ref>
'''Note:''' the ''[[wikt:bis#Latin|bis]]'' and ''[[wikt:ter#Latin|ter]]'' suffixes are [[ITU-T]] standard designators of successive iterations of a standard (''bis'' and ''ter'' are the [[Latin]] words for "twice" and "thrice").
== General standards ==
''Applies to V.1–V.9''
* {{Anchor|V.1}} '''V.1''' is an ITU-T recommendation, entitled ''Equivalence between binary notation symbols and the significant conditions of a two-condition code.'' * {{Anchor|V.2}} '''V.2''' is an ITU-T recommendation, approved in November 1988, titled ''Power levels for data transmission over telephone lines.'' * {{Anchor|V.4}} '''V.4''' is an ITU-T recommendation, approved in November 1988, titled ''General structure of signals of International Alphabet No. 5 code for character oriented data transmission over public telephone networks.'' * {{Anchor|V.5}} '''V.5''' was an ITU-T recommendation, approved in November 1988, titled ''Standardization of data signalling rates for synchronous data transmission in the general switched telephone network.'' It has been withdrawn since. * {{Anchor|V.6}} '''V.6''' was an ITU-T recommendation, approved in November 1988, titled ''Standardization of data signalling rates for synchronous data transmission on leased telephone-type circuits.'' It has been withdrawn since. * {{Anchor|V.7}} '''V.7''' is an ITU-T recommendation, approved in November 1988, titled ''Definitions of terms concerning data communication over the telephone network.'' * {{Anchor|V.8}} '''V.8''' is an ITU-T recommendation, first approved in September 1994, titled ''Procedures for starting sessions of data transmission over the public switched telephone network.'' It has been superseded three times. The current version was approved in November 2000. ** '''V.8bis''' is an ITU-T recommendation, first approved in August 1996, titled ''Procedures for the identification and selection of common modes of operation between data circuit-terminating equipments (DCEs) and between data terminal equipments (DTEs) over the public switched telephone network and on leased point-to-point telephone-type circuits.'' It has been superseded twice. The current version was approved in November 2000.
== Interfaces and voiceband modems ==
''Applies to V.10–V.34''
* '''[[ITU-T V.10|V.10]]''' is an ITU-T recommendation, first agreed in 1976, for unbalanced electrical circuits for data communication at up to 100 [[kilobit|kbit]]/s. It can interwork with V.28, provided it is not exposed to signals greater than 12 volts. Used with the 37-pin ISO 4902 connector, it is compatible with EIA [[RS-423]]. * '''[[ITU-T V.11|V.11]]''' is an ITU-T recommendation, first agreed in 1976, for balanced electrical circuits for data communication at up to 10 [[megabit|Mbit]]/s. Used with the 37-pin ISO 4902 connector ("[[D-subminiature|DC-37]]"), it is compatible with [[EIA-422]]. The 15-pin ISO 4903 connector ("[[D-subminiature|DA-15]]") is recommended for data network interface. * {{Anchor|V.17}} '''V.17''' is an ITU-T recommendation for a [[fax]] modem using [[Trellis coded modulation|TCM]] at 12 and 14.4 [[kbit/s]]. * '''[[ITU-T V.18|V.18]]''' is an ITU-T recommendation for operation and interworking of text telephone devices * {{Anchor|V.21}} '''V.21''' is an ITU-T recommendation for [[duplex (telecommunications)|full-duplex]] communication between two [[analog signal|analogue]] dial-up [[modem]]s using [[audio frequency-shift keying]] [[modulation]] at 300 [[baud]] to carry [[Digital data|digital]] [[data]] at 300 [[bit/s]]. It is a variant of the original [[Bell 103]] modulation format. * {{Anchor|V.22}} '''V.22''' is an ITU-T recommendation for [[duplex (telecommunications)|full-duplex]] communication between two [[analog signal|analogue]] dial-up [[modem]]s using [[Phase-shift keying|PSK]] [[modulation]] at 600 [[baud]] to carry data at 1200 or 600 [[bit/s]]. It is a variant of the [[Bell 212A]] modulation format. ** '''V.22bis''' is an ITU-T recommendation extending V.22 with a faster rate using [[Quadrature amplitude modulation|QAM]] at 600 [[baud]] to carry [[Digital data|digital]] [[data]] at 2400 or 1200 [[bit/s]]. The 1200 bit/s mode is compatible with V.22. * '''[[ITU-T V.23|V.23]]''' is an ITU-T recommendation for [[Duplex (telecommunications)|half-duplex]] communication between two [[analog signal|analogue]] dial-up [[modem]]s using [[Frequency-shift keying|FSK]] [[modulation]] at up to 600 or 1200 [[baud]] to carry [[Digital data|digital]] [[data]] at up to 600 or 1200 [[bit/s]] respectively. An optional 75 baud reverse channel carries 75 [[bit]]/s. * '''[[ITU-T V.24|V.24]]''' is referenced as [[RS-232]] which also includes V.28. * '''V.25ter''' see [[#V.250|V.250]] * {{Anchor|V.27ter}} '''V.27ter''' is an ITU-T recommendation for a [[half-duplex]] [[modem]], allowing 2400 and 4800 bit/s ([[Phase-shift keying|PSK]] modulation). * {{Anchor|V.28}} '''V.28''' is an ITU-T recommendation defining the electrical characteristics for unbalanced double-current interchange circuits. * {{Anchor|V.29}} '''V.29''' is an ITU-T recommendation for a [[modem]], allowing 4.8 kbit/s, 7.2 kbit/s and 9.6 kbit/s transfer modes ([[Phase-shift keying|PSK]] and [[QAM]] modulations). * {{Anchor|V.32}} '''V.32 (11/88)''' is an ITU-T recommendation for a [[modem]] operating as [[full-duplex]] on a four-wire circuit, or [[half-duplex]] on a two-wire circuit, allowing bidirectional data transfer at either 9.6 kbit/s or 4.8 kbit/s at a [[symbol rate]] of 2,400 [[baud]] instead of the 600 [[baud]] of the V.22 standards.<ref>{{Cite web|title=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|url=https://www.itu.int/rec/T-REC-V.32-198811-S/en|access-date=2020-08-20|website=www.itu.int}}</ref> ** '''V.32bis (02/91)''' is an ITU-T recommendation for a [[modem]], allowing up to 14.4 kbit/s bidirectional data transfer. Other additional defined data transfer rates are 12.0 kbit/s, 9.6 kbit/s, 7.2 kbit/s, and 4.8 kbit/s.<ref>{{Cite web|title=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|url=https://www.itu.int/rec/T-REC-V.32bis-199102-I/en|access-date=2020-08-20|website=www.itu.int}}</ref> The standard was extended by several modem manufacturers to allow bidirectional data transfer rates of 19.2 kbit/s, but never ratified as a V.32''ter'' standard. These non-ITU-T standard modems were often referred to as "[[#V.32terbo|V.32terbo]]" modems.<ref>[https://books.google.com/books?id=Sx0EAAAAMBAJ&dq=v.32terbo&pg=PA4 Vendors rallying around high-speed modem scheme]</ref> * {{Anchor|V.33}} '''V.33''' is an ITU-T recommendation for a [[modem]] operating as [[full-duplex]] on a four-wire point-to-point leased line allowing bidirectional data transfer at either 14.4 kbit/s. * {{Anchor|V.34}} '''V.34 (09/94)''' is an ITU-T recommendation (superseded) for a [[modem]], allowing up to 28.8 kbit/s bidirectional data transfer using [[Trellis modulation|TCM]] [[modulation]]. Other additional defined data transfer rates are 24.0 kbit/s and 19.2 kbit/s as well as all the permitted V.32 and V.32bis rates. Additionally, V.34 modems employ shell mapping as [[shaping codes|shaping code]] to reduce the transmit power.<ref>{{Cite web|title=V.34: a modem operating at data signalling rates of up to 28 800 bit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits|url=https://www.itu.int/rec/T-REC-V.34-199409-S/en|access-date=2020-07-05|website=www.itu.int}}</ref> ** '''V.34 (10/96)''' is an updated ITU-T recommendation for a modem, building on the V.34 standard but allowing up to 33.6 kbit/s bidirectional data transfer. Other additional defined data transfer rates are 31.2 kbit/s, as well as all the permitted V.34 rates. Modems implementing this standard were often marketed under the moniker '''V.34+'''.<ref>{{Cite web|title=V.34: A modem operating at data signalling rates of up to 33 600 bit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits|url=https://www.itu.int/rec/T-REC-V.34-199610-S/en|access-date=2020-07-05|website=www.itu.int}}</ref> ** '''V.34 (02/98)''' commonly rendered as V.34bis, is a further update to V.34 which corrected some errata in the original 1996 document.<ref>{{Cite web|title=V.34: A modem operating at data signalling rates of up to 33 600 bit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits|url=https://www.itu.int/rec/T-REC-V.34-199802-I/en|access-date=2020-07-05|website=www.itu.int}}</ref>
===Ad hoc standards=== In order to gain [[first-mover advantage]], many modem companies introduced models based on upcoming V-series standards before they reached final ratification. In other cases, companies introduced non-standard systems but gave them ITU-like names.
* {{Anchor|V.32terbo}} '''V.32terbo''', or '''V.32ter''' for short, was a 19.2 kbit/s standard introduced by [[AT&T Paradyne]]. It was based on V.32bis and did little other than increase the data rate. V.32ter is compatible with V.32bis at speeds of 14.4 kbit/s and lower, but it is not compatible with V.34 at 19.2. * {{Anchor|V.FC}} '''V.FC''', short for '''V.Fast Class''' and sometimes referred to as '''V.FAST''', was developed by [[Hayes Microcomputer Products|Hayes]] and [[Conexant|Rockwell]] to introduce a 28.8 kbit/s standard while the V.34 efforts dragged on. V.FC was not compatible with V.34, although most V.34 modems could support V.FC, notably, those using Rockwell chipsets.<ref>[https://encyclopedia2.thefreedictionary.com/V.Fast+Class "V.FC"]</ref>
== Wideband modems ==
''Applies to V.35–V.39''
* {{Anchor|V.35}} '''V.35''' is an ITU-T standard located on layer 1 on the [[OSI model]]. Max speed is 2 Mbit/s and synchronous only. Withdrawn ITU-T recommendation for 48 kbit/s data transmission over wideband circuits. The physical and electrical characteristics of this interface are now specified in ITU-T recommendation V.11. * {{Anchor|V.36}} '''V.36''' is an ITU-T standard located on layer 2 on the [[OSI model]] called Modems For Synchronous Data Transmission Using 60–108 kHz Group Band Circuits.
== Error control and data compression == ''Applies to V.40–V.49'' * {{Anchor|V.41}} '''V.41''' is a code-independent error control system. It defines the [[ITU-T]] standard [[Cyclic redundancy check|CRC-16]] polynomial as x<sup>16</sup> + x<sup>12</sup> + x<sup>5</sup> + 1 * {{Anchor|V.42}} '''V.42''' is an error correction protocol. The receiver uses it to immediately request re-transmission of any lost data packets. However, it does not guarantee how quickly the error-free data will be delivered to the receiving end. V.42 is generally included in dialup modems. It also defines the ITU-T standard CRC-32 polynomial as x<sup>32</sup>+x<sup>26</sup>+x<sup>23</sup>+x<sup>22</sup>+x<sup>16</sup>+x<sup>12</sup>+x<sup>11</sup>+x<sup>10</sup>+x<sup>8</sup>+x<sup>7</sup>+x<sup>5</sup>+x<sup>4</sup>+x<sup>2</sup>+x+1, and includes the [[LAPM]] framing protocol. ** '''V.42bis''', also an adaptive data compression standard, is based on the [[Lempel Ziv]] dynamic dictionary approach, and may go to "transparent mode," in which data is transmitted uncompressed. The specific algorithm is "[[BTLZ]]" (British Telecom Lempel Ziv), which was developed by Alan Clark (then with BT). * {{Anchor|V.44}} '''V.44''' is based on [[LZJH]] (Lempel–Ziv–Jeff–Heath) adaptive data compression developed by Hughes Electronics for its DirecPC satellite Internet, and incorporated into the v.92 dial-up modem standard. V.44 offers somewhat better compression performance for certain types of data than the V.42bis standard. On average, it achieves 15% greater throughput. For instance, a 53.3 kbit/s connection with V.44 can transmit up to 53.3×6 = 320 kbit/s using pure text. In reality, the overall data rate rarely exceeds 3:1 compression (~150 kbit/s). The compression tends to get better and worse over time due to noise on the line, or due to the transfer of already-compressed files (ZIP files, JPEG images, MP3 audio, MPEG video). At some points the modem will be sending compressed files at 53 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s, or any value in between.<ref>{{cite web|url=http://fndcg0.fnal.gov/Net/modm8-94.txt |title=What You Need to Know about Modems |access-date=2008-03-02 |last=Willdig |first=Karl |author2=Patrik Chen |date=August 1994 |url-status=dead |archive-url=https://web.archive.org/web/20070104124418/http://fndcg0.fnal.gov/Net/modm8-94.txt |archive-date=2007-01-04 }}</ref><ref>{{cite web |url=https://www.pricenfees.com/digit-life-archives/modem-compression-v-44-v-42bis |title=Modem compression: V.44 against V.42bis |access-date=2008-03-02 |last=Mitronov |first=Pavel |date=2001-06-29 |publisher=Pricenfees.com |url-status=dead |archive-url=https://web.archive.org/web/20170202062749/https://www.pricenfees.com/digit-life-archives/modem-compression-v-44-v-42bis |archive-date=2017-02-02 }}</ref>
== Simultaneous transmission of data and other signals ==
''Applies to V.60–V.99''
* {{Anchor|V.61}} '''V.61''' Analog Simultaneous Voice and Data (ASVD) * {{Anchor|V.70}} '''V.70''' Digital Simultaneous Voice and Data (DSVD) * {{Anchor|V.80}} '''V.80''': [[videoconferencing]]. It is generally compatibilized with [[H.324]] standard point-to-point video telephony over regular phone lines. * {{Anchor|V.90}} '''V.90''' is an ITU-T recommendation for a [[modem]], allowing [[56 kbit/s]] digital download and 33.6 kbit/s analog upload. It replaced two vendor standards ([[56 kbit/s modem#K56flex|K56flex]] and [[56 kbit/s modem#X2|X2]]) and was designed to allow modems from both prior standards to be flash upgraded to support it. It is also known as ''V.Last'' as it was anticipated to be the last standard for modems operating near the channel capacity of [[Plain old telephone service|POTS]] lines to be developed. V.90 is generally used in concert with the V.42bis compression standard. * {{Anchor|V.91}} '''V.91''': ''A digital modem operating at data signalling rates of up to 64 000 bit/s for use on a four-wire circuit switched connection and on leased point-to-point four-wire digital circuits'', published in 1999 * '''[[ITU-T V.92|V.92]]''' is an [[ITU-T]] recommendation, titled ''Enhancements to Recommendation V.90'', that establishes a [[modem]] standard allowing [[56K|56 kbit/s]] download, 48 kbit/s upload rates, and the new V.44 compression method. It is intended to succeed the V.90 standards. With V.92, [[Pulse-code modulation|PCM]] is used for both the upstream and downstream connections; previously 56K modems only used PCM for downstream data.
== Interworking with other networks ==
''Applies to V.100–V.199''
* {{Anchor|V.110}} '''V.110''' is an ITU-T recommendation for using terminal adaptor functions for the connection of terminals having interfaces conforming to V-series to the [[ISDN]]. * {{Anchor|V.120}} '''V.120''' is an alternative to V.110 based on the LAPD protocol. * {{Anchor|V.150.1}} '''V.150.1''' is an ITU-T recommendation for using a [[modem]] over IP networks. * {{Anchor|V.152}} '''V.152''' is an ITU-T recommendation for using voice-band data over IP networks.
== Interface layer specifications for data communication ==
''Applies to V.200–V.249''
* {{Anchor|V.230}} '''V.230''' is an ITU-T recommendation, approved in November 1988, titled ''General data communications interface layer 1 specification.''
== Control procedures == ''Applies to V.250–V.299''
* {{Anchor|V.250}} '''V.250''', originally known as V.25ter, is an ITU-T recommendation for serial asynchronous automatic dialing and control.<ref>[https://www.itu.int/rec/T-REC-V.250-200203-S!Amd2/en Supplement (04/95) to Recommendation V.25ter - Serial asynchronous automatic dialling and control<!-- Bot generated title -->]</ref>
== Digital circuit modems == ''Applies to V.300–V.399''
* {{Anchor|V.300}} '''V.300''' is an ITU-T recommendation, first approved in January 1997, titled ''A 128 (144) kbit/s data circuit-terminating equipment standardized for use on digital point-to-point leased circuits''.
== See also == *[[Microcom Networking Protocol]]s (MNP1 to 10) *[[List of interface bit rates]]
== References == {{reflist}}
== External links == *[https://www.itu.int/itu-t/recommendations/index.aspx?ser=V ITU-T Recommendations: V Series] *[http://www.advice1.com/v35.htm V.35 Technical Info] {{Webarchive|url=https://web.archive.org/web/20060713201312/http://www.advice1.com/v35.htm |date=2006-07-13 }} – Includes pinouts and history *[https://web.archive.org/web/20060617021151/http://v92.com/faqs/ V.44 FAQ] *[https://archive.today/20130119200953/http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122limit/122x/122xa/122xa_2/ftv44mca.htm Technical information on the LZJH algorithm in use in the V.44 standard] *[https://3amSystems.com/wireline/hmo.htm Basic handshakes & modulations] – V.22, V.22bis & V.32 handshakes
{{ITU standards}} {{Modem standards}}
[[Category:Modems|ITU-T V-series]] [[Category:ITU-T recommendations|V-Series]] [[Category:Lists of standards|ITU-T V-series]]