The secondary modulation profile for the upstream channel.
This should be a QPSK modulation profile if the primary
profile is QAM-16. The CMTS will switch from primary
profile (QAM16) to secondary profile (QPSK) depending on
the noise level of a particular spectrum band.
This is 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 does not exist or is empty.