X interfaces are the most common standards for digitalinterfaces. They are standardized by the International Telecommunication Union (ITU).
The ITU-T uses letters and numbers to identify its recommendations. The ITU X-series recommendations describe protocols and interfaces in data networks. The X interfaces refer to pure data networks. These networks are digital in design and, except for ISDN, are not intended for voice transmissions, which has allowed the definition to be kept simple. The signaling for connection control is performed depending on the states of two control lines via the data lines. The X interfaces are defined for start-stop procedures as well as for synchronous procedures. The best known recommendations are X.20, X. 21, X. 21bis and X.25 .