Computer Networking
Carrier is

a collection of interconnected functional units that provides a data communications service among stations attached to the network
a continuous frequency capable of being modulated or impressed with a second signal
None of these
One or more conductors that serve as a common connection for a related group of devices
the condition when two or more sections attempt to use the same channel at the same time

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Computer Networking
X.21 is

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

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