An entry identical to the docsIfModIndex in the
docsIfCmtsModulationTable that describes this channel.
This channel is further instantiated there by a grouping
of interval usage codes which together fully describe the
channel modulation. This object returns 0 if the
docsIfCmtsModulationTable entry does not exist or
docsIfCmtsModulationTable is empty. See
the associated conformance object for write conditions
and limitations.