Computer Networking
X.21 is

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

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Computer Networking
Modulation is the process of

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

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