Computer Networking
The data communication support should include, but is not restricted to

file and database access
All of these
terminal support, electronic mail and voice grams.
file transfer and transaction processing

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Computer Networking
X.21 is

a method of determining which device has access to the transmission medium at any time
a method access control technique for multiple-access transmission media
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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