Computer Networking
X.21 is

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

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Computer Networking
The main difference between synchronous and asynchronous transmission is

the bandwidth required is different
None of these
the clocking is mixed with the data in asynchronous transmission
the clocking is derived from the data in synchronous transmission
the pulse height is different.

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