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

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

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Computer Networking
X.21 is

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
a method access control technique for multiple-access transmission media
None of these

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