Computer Networking
X.21 is

None of these
a very common bit-oriented data link protocol issued by ISO
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
a method access control technique for multiple-access transmission media

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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
the virtual circuit interface to packet-switched service
None of these
the interface between the X.25 network and packet mode device
All of these

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