Computer Networking
X.21 is

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

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Computer Networking
The data-link layer, in reference to the OSI model, specifies

data link procedures that provide for the exchange of data via frames that can be sent and received
None of these
the interface between the X.25 network and packet mode device
the virtual circuit interface to packet-switched service
All of these

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