The Physical Layer - Example : X.21

A digital signaling interface called X.21 was recommended by the CCITT in 1976. The recommendation specifies how the customer's computer, the DTE, sets up and clears calls by exchanging signals with the carrier's equipment, the DCE.

The names and functions of the eight wires defined by X.21 are given in the following figure. The physical connector has 15 pins, but not all of them are used. the DTE uses the T and C lines to transmit data and control information, respectively. The DCE uses the R and I lines for data and control. The S line contains a signal stream emitted by the DCE to provide timing information, so the DTE knows when each bit interval starts and stops. At the carrier's option, a B line may also be provided to group the bits into 8-bit frames. If this option is provided, the DTE must begin each character on a frame boundary. If the option is not provided, both DTE and DCE must begin every control sequence with at least two SYN characters, to enable the other one to deduce the implied frame boundaries.

[PHOTO 2]

Although X.21 is a long and complicated document, the simple example of the next figure illustrates the main features. In this example it is shown how the DTE places a call to a remote DTE, and how the originating DTE clears the call when it is finished. To make the explanation clearer, the calling and clearing procedures is described in terms of an analogy with the telephone system.

[PHOTO 3]