X.25

• X.25 layer 1 uses X.21 bis physical layer protocol

• Equivalent to EIA/TIA-232-C (formerly RS-232-C)

• Derived from ITU-T recommendations V.24 and V.28

• X.21 supports up 19.2 Kbps, synchronous, full-duplex transmission over four-wire media, max. distance between DTE and DCE is 15 meters

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