X.21 as a universal digital service access interface

X.21 is one of the most prominent candidates for a future standard for a universal digital service access interface in the sense of the OSI Reference Model. This paper provides a basis for discussing the suitability of X.21 as the UDSA interface by explaining the context of X.21 and the philosophy behind it, by giving a formal specification, by discussing the architectural issues related to X.21, to the UDSA interface and to the ISDN interface, by outlining the user requirements that the UDSA interface has to meet and by briefly mentioning the procedural simplifications achievable by agreeing on X.21 as the future UDSA interface.