This attribute specifies the value to be substituted into the ABCD
signaling bits for the timeslots defined by the Chan component
under fault conditions. The value will be substituted after the
duration provisioned in signalOneDuration has expired and when a
lineType that uses CAS has been defined.
For a lineType of d4Cas (Super Frame), signaling bits C and D are
required to equal bits A and B respectively.