X.21 is a physical-layer protocol used in X.25 that defines the electrical and mechanical procedures for using the physical medium. The physical X.25 connector is 15 pins with some pins used as NULL (unconnected). Please refer to the chart below:
| Line | NAME | From DTE |
From DCE |
| G | GROUND | - | - |
| Ga | DTE common ground | Y | - |
| T | Transmit | Y | - |
| R | Receive | - | Y |
| C | Control | Y | - |
| I | Indication | - | Y |
| S | Signal Element Timing | - | Y |
| B | Byte Timing | - | Y |
DTE INTERFACE
The DTE uses the Tx and Ctrl circuits to transmit data and control information, and the common ground for electrical purposes.
DCE INTERFACE
The DCE uses the Rx and Indication circuits for data and control. The DCE emits a timing signal stream on S to indicate bit-level timing to the DTE end. The DCE can also use the B wire to indicate byte-level framing. If the B circuit isn't used, two synchronization bytes (SYN) are transmitted from DCE to DTE to allow the DTE to synchronize and calculate the byte-frame boundary.
As explained above, X.21bis handles the activation and deactivation of the physical medium connecting DTE and DCE devices. It supports point-to-point connections, speeds up to 19.2 kbps, and synchronous, full-duplex transmission over four-wire media. (See below).