This object represents the mode used to transport DS0
signalling information for T1 byteSynchronous mapping (GR253).
In signallingTransferMode(2), the robbed-bit signalling is
transferred to the VT header. In clearMode(3), only the
framing bit is transferred to the VT header.
Default is signallingTransferMode(2) if csTributaryMappingType
is byteSynchronous. For asynchronous mapping, it is
notApplicable(1).
The value notApplicable(1) can not be set.