X.21 is a protocol that runs over subscriber networks (i.e. leased line connections). It is an ITU-T data interface standard for connecting a DTE (i.e. Tieline codec) to a DCE (terminal adapter) and transferring synchronous serial data. It can operate as a full duplex connection with data rates from between 9,600 bps to 2 megabits.
The properties of X.21 define the physical, electrical and call control procedures for interfacing using this standard. The basic interface signals performed in most X.21 signalling include transmit, receive, control, indicate and timing. X.21 connections use balanced data, clock and control signal lines and have been fully integrated into Tieline codecs.
64 - 384 kbps connections are most common and Tieline codecs support X.21 leased line connections from 64 - 2,048 kbps. The physical interface to a Tieline codec module is via a straight-through DB15 cable and the connection profiles supported are mono, stereo, dual mono and mono/IFB. Choose your connection algorithm from G.711, G.722, MPEG 2 Layer 2 (MP2), and Tieline Music or MusicPLUS.