X.21 is a
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Answer: A) Physical layer protocol
X.21 is an ITU-T standard for digital signaling at the physical layer, used with circuit-switched networks.
Source: Scan #55 page 3 Q7
X.21 is an ITU-T standard for digital signaling at the physical layer, used with circuit-switched networks.
Source: Scan #55 page 3 Q7