Computer Networking
Bulletin board system

None of these
converts digital signals to analog signals
is a modem capable of accepting commands
is a public access message system
converts analog signals to digital signals

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Computer Networking
X.21 is

a method access control technique for multiple-access transmission media
None of these
a very common bit-oriented data link protocol issued by ISO
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

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