Computer Networking
Modulation is the process of

Multiplexing various signals into high speed line signals
performing data encryption.
converting digital signals to analog signals
converting analog signals to digital signals

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Computer Networking
X.21 is

None of these
a method of determining which device has access to the transmission medium at any time
a very common bit-oriented data link protocol issued by ISO
network access standard for connecting stations to a circuit-switched network
a method access control technique for multiple-access transmission media

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