Computer Networking
X.21 is

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
a method access control technique for multiple-access transmission media
network access standard for connecting stations to a circuit-switched network

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Computer Networking
Satellite-Switched Time-Division Multiple Access (SS/TDMA) is

the method of determining which device has access to the transmission medium at any time.
All of these
a form of TDMA in which circuit switching is used to dynamically change the channel assignments
a medium access control technique for multiple access transmission media
None of these

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