X.21
Encyclopedia
X.21 is an interface specification for differential communications introduced in the mid 1970s by the ITU-T
. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customers' equipment. This includes specifications for DTE
/DCE
physical interface elements, alignment of call control
characters and error checking, elements of the call control phase for circuit switching services, and test loops.
When X.21 is used with V.11, it provides synchronous data transmission at rates from 600 bit/s to 10 Mbit/s. There is also a variant of X.21 that is only used in select legacy applications, “circuit switched X.21”. 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 (your telephone company), and is responsible for correct clocking of the data. X.21 is primarily used in Europe and Japan, for example in the Scandinavian DATEX and German DATEX-L circuit switched networks during the 1980s.
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customers' equipment. This includes specifications for DTE
Data terminal equipment
Data Terminal Equipment is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment...
/DCE
Data circuit-terminating equipment
A data circuit-terminating equipment is a device that sits between the data terminal equipment and a data transmission circuit. It is also called data communications equipment and data carrier equipment...
physical interface elements, alignment of call control
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 can be used to adapt standard switch operation to...
characters and error checking, elements of the call control phase for circuit switching services, and test loops.
When X.21 is used with V.11, it provides synchronous data transmission at rates from 600 bit/s to 10 Mbit/s. There is also a variant of X.21 that is only used in select legacy applications, “circuit switched X.21”. 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 (your telephone company), and is responsible for correct clocking of the data. X.21 is primarily used in Europe and Japan, for example in the Scandinavian DATEX and German DATEX-L circuit switched networks during the 1980s.