X.21, also referred to as X21, is an interface specification for
differential communications introduced in the mid-1970s by the
CCITT
The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunicat ...
, now ITU-T. X.21 was the first
digital signaling interface developed. The connector specification is defined by the ISO document 4903.
At the time, most physical layer protocols such as
RS-232-C and
RS-449
The RS-449 specification, also known as EIA-449 or TIA-449, defines the functional and mechanical characteristics of the interface between data terminal equipment, typically a computer, and data communications equipment, typically a modem or te ...
use
analog signal
An analog signal (American English) or analogue signal (British and Commonwealth English) is any continuous-time signal representing some other quantity, i.e., ''analogous'' to another quantity. For example, in an analog audio signal, the ins ...
ing. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications. This includes specifications for
DTE/
DCE physical interface elements, alignment of
call control In telephony, call control refers to the software within a telephone switch that supplies its central function. Call control decodes addressing information and routes telephone calls from one end point to another. It also creates the features that c ...
characters and
error checking
In information theory and coding theory with applications in computer science and telecommunications, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communic ...
, elements of the call control phase for
circuit switching
Circuit switching is a method of implementing a telecommunications network in which two network nodes establish a dedicated communications channel ( circuit) through the network before the nodes may communicate. The circuit guarantees the full ...
services, and test loops.
X.21 normally is found on a 15-pin
D-sub connector and is capable of running full-duplex data transmissions. The Signal Element Timing, or clock, is provided by the carrier (the telephone company), and is responsible for correct clocking of the data. X.21 was primarily used in Europe and Japan, for example in the Scandinavian
DATEX and
German circuit-switched networks during the 1980s.
A variant of X.21 can be found in select legacy applications as “circuit switched X.21”.
Electrical characteristics
Electrical characteristics for
V.10 and
V.11 specifies voltage levels. V.10 is for
unbalanced circuits, whereas V.11 is for
balanced circuits.
With electrical characteristics
V.11, it provides synchronous data transmission at rates from 600 bit/s to 10 Mbit/s.
With electrical characteristics
V.10, it provides transmission speeds of up to 100Kb/s.
External links
An X.21 brief Tutorial and overviewX.21 pinout and some explanations
References
''Article based o
X.21
a
FOLDOC
an
used with permission.''
Physical layer protocols
ITU-T recommendations
ITU-T X Series Recommendations
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