Circuit-Switched Network Layer
CCITT Recommendation X.21 has been developed as “The General Purpose Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for Synchronous Operation on Public Data Networks” , The only “general purpose” part, however, is the designation of the physical elements which include the electrical (X.26/X.27), functional (X.24), and mechanical (ISO 4903) characteristics described in Chapter 3 (Bertine). Additionally, the basic family of quiescent signals and states for the interface is specified. These provide the fundamental components of X.21 which will apply to all modes of operation in new data communications applications for circuit-switched, packet-switched, and general purpose integrated services .
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