Computer Networking
X.21 is

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

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Computer Networking
A front-end processor is

None of these
a processor in a large-scale computer that executes operating system instructions
preliminary processor of batch jobs.
a user computer system
a minicomputer that relieves main-frame computers at a computer centre of communications control functions

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