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Line Signals
For the digital version of MFC/R2 (defined in ITU-T Q.421), the digital line signals are the ABCD bits of the channel associated signaling (CAS) in timeslot 16 of an E1.
They represent the states of the line, and are similar to the states of an analog line. In general, only bits A and B are used. In most systems bits C and D are set to fixed values, and never change. There are some national variants where bit C or D may be used for metering pulses.
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