These standards and recommendations are issued by the International Telecommunications
Union (ITU-T), formerly the International Telegraph and Telephone
Consultative Committee (CCITT), which is a specialist agency
of the United Nations.
There are four series of protocol and standard references
that are commonly encountered in the area of communications
covered by this document. These are
| I Series |
Standards referring to ISDN and related
topics |
| Q Series |
Standards relevant to European ISDN networks |
| V Series
|
Recommendations covering data
transmission over telephone lines |
| X Series |
Recommendations covering the interfacing
of data transmission and terminal devices |
The following list is for reference.
| X Series |
| Number |
Areas Covered |
| X.1 |
International user classes of service |
| X.2 |
International data services and optional
facilities |
| X.3 |
Packet assembly and disassembly (PAD) |
| X.4 |
International alphabet No.5 |
| X.20 |
Asynchronous communications over PSTN |
| X.20 bis |
V series modems - asynchronous communication
over PSTN |
| X.21 |
Synchronous communications over the PSTN |
| X.21 bis |
V series modems - synchronous communication
over PSTN |
| X.22 |
DTE/DCE multiplex interface for user classes
5 and 6 |
| X.24 |
DTE/DCE interchange circuits on public data
networks (PDNs) |
| X.25 |
DTE/DCE packet mode operation across public
data networks |
| X.28 |
DTE/DCE interface for asynchronous DTE accessing
a PAD |
| X.29 |
DTE/PAD and PAD/PAD handshake protocol |
| X.30 |
ISDN support for X.20 bis, X.21 and X.21
bis devices |
| X.31 |
ISDN support for packet mode DTE |
| X.32 |
ISDN/PSTN/PDN access using packet mode interfacing
DTE/DCE |
| X.35 |
PDN packet switched X.25 interface - PSPDN
gateway |
| X.50 |
International interface between synchronous
data networks |
| X.50 bis |
48Kbps for international interface between
synchronous data networks |
| X.51 |
International interface between synchronous
data networks -
10 bit envelope structure |
| X.51 bis |
48Kbps for international interface between
synchronous data networks -
10bit envelope |
| X.58 |
International interface between synchronous
data networks |
| X.60 |
Common channel signalling for circuit switched
data |
| X.61 |
Signalling system No.7 |
| X.70 |
Terminal & transit signalling
of asynchronous services on international circuits |
| X.71 |
Decentralised terminal & transit signals
on international circuits - sync. networks |
| X.75 |
PSS between public networks with data transmission
services |
| X.80 |
Inter-exchange signals for circuit switched
data |
| X.92 |
PDN reference model for synchronous connection |
| X.96 |
PDN call progress signals |
| X.110 |
PDN international routing principles |
| X.121 |
PDN international numbering plan |
| X.130 |
PDN international connection call set up
and clear down times |
| X.132 |
PDN international connections - grade of
service |
| X.400 |
Message handling for electronic data exchange |
| X.402 |
X.400 architecture |
| X.403 |
X.400 conformance testing |
| X.407 |
X.400 service definition conversions |
| X.408 |
X.400 encoded information conversion rules |
| X.411 |
X.400 message transfer system |
| X.413 |
X.400 message storing |
| X.419 |
X.400 protocols |
| X.420 |
X.400 interpersonal messaging system |
| X.500 |
Directory services standard |
