We describe the function of X.25 but ignore interface details here. The designer can learn from this section the function available from an X.25 circuit. We believe that the X25 circuit keys provide the same function. In this regard we believe that this section is complete and precise. Other sections are required, however, to find out how to code these functions. Section (x25map) relates this terminology to that of the official X.25 definition.

The role of X.25 is to transport data both ways between a pair of "DTE"s (data terminal equipment) by X.25. One DTE may belong to many pairs at once.

The Signals Between the DTEs