X.21 specifies a simple character-oriented procedure used to establish a transparent connection through a synchronous Public Data Network. The architecture of call establishment for X.21 is the same as that for call (virtual) establishment of X.25; in this regard, their functionality is nearly identical. This paper describes the background that went into the development of the Recommendation, discusses the architectural relationships, and presents the details of the X.21 circuit-switching protocol.
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Harold C. Folts (1980) studied this question.