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.

The latest proposed revisions to CCITT Recommendation X.25 for packet-switched service in public data networks now include two new capabilities suitable for transport of a small amount of data. The first provides datagram service for the transport of independent "message type" packets. The other new feature is the fast select facility which provides for the inclusion of 128 octets of user data in the call establishment packets for virtual call service. Both these new provisions greatly enhance the capability X.25 to efficiently support the broadest range of user applications.

Recent developments in data communications standards have been patterned on a hierarchical approach to communications system architecture. A number of independent levels have been identified, and standards development has proceeded at its own pace within each level. These levels are identified and recent progress at standardization is discussed for each.