In data communications, a specification of the CCITT that defines the connection of data terminal equipment to an X.21 (public data) network.

An ITU standard governing the serial interface between DCE and DTE, generally synchronous data on a DB15. Includes connector, electrical, and dialing specifications

A CCITT recommendation which specifies a general purpose interface between data circuit-terminating equipment for synchronous operation on public data networks.

Is the CCITT description for the Physical Layer.

A cable type that can handle data rates up to 2.038 Mbit/s, as well as lower rates . It has a two-row connector and is the most popular in Europe and South Africa, as it allows connections to be set up and removed very quickly.

A CCITT recommendation that defines a protocol for communication between a circuit-switched network and user devices.

Synchronous communications interface definitions for use over the PSTN.

In data communications, the 21st CCITT recommendation in the X series, defining the connection of data terminal equipment to an X.21 public data network for digital leased and circuit-switched services. In X.25 communications, X.21 is available at the physical level only.

A technical specification recommended by the ITU-T that describes the interface used in the ITU-T packet-switching protocol & in some types of circuit switched data transmission.

An International Telecommunication Union (ITU) specification that describes the interface used in the ITU X.25 packet-switching protocol, and in some types of circuit-switched data transmissions. See X.25.

Defines the interface between a computer and a public data network where the access to the network is made over synchronous digital lines.

ITU-T standard for serial communications over synchronous digital lines. The X.21 protocol is used primarily in Europe and Japan.

(1) See specification X.21.(2) See Recommendation X.21.

An interface for synchronous operation on public data networks, a successor for V.24 and V.25 definitions, from ITU-T. Data rates can be up to 2Mbits/s.

An ITU-T recommendation that defines a general purpose physical interface between a DTE and DCE for full duplex synchronous operation on circuit switched or packet-switched data networks.