The ITU-T standard, approved by ISO, which specifies the basic functions and user-selectable capabilities of a PAD.

An ITU-T standard protocol suite for the DTE-DCE interface in a packet-switched network, approved by ISO. X.25 defines standard physical layer, data link layer and network layers (layers 1 through 3). It was developed to describe how data passes into and out of public data communications networks. X.25 networks are in use throughout the world.

The ITU-T standard specifying how to control a PAD from character-mode DTE, approved by ISO.

The ITU-T standard, approved by ISO, specifying procedures for the exchange of control information and user data between a PAD and a remote packet-mode DTE. Character-mode DTEs are often referred to as X.29 terminals.

The ITU-T standard specifying the protocols for communication between two PSDNs.

The set of ITU-T communications standards covering electronic mail services provided by data networks. It is widely used in Europe and Canada. x.400 addresses can be very long.

Extensible Markup Language

A pared-down version of SGML, designed especially for Web documents. It allows designers to create their own customized tags, enabling the definition, transmission, validation, and interpretation of data between applications and between organizations.