This attribute identifies the combination of ABCD bits used for
channel associated signalling.
If casSignalling is transparent, signalBits must have the same value
at both ends of a Vs. If casSignalling is transparent and signalBits
does not have the same value at both ends of a Vs, the Vs will be out
of service. In this case, Vs serviceFailureReason is
mismatchedSignalBits.