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
None of these
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
Error rate is

a check bit appended to an array of binary digits to make the sum of all the binary digits.
an error-detecting code based on a summation operation performed on the bits to be checked.
a code in which each expression conforms to specific rules of construction, so that if certain errors occur in an expression, the resulting expression will not conform to the rules of construction and
None of these
the ratio of the number of data units in error to the total number of data units

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