At the CMTS, the total number of DOCSIS data bytes transported
by this downstream channel since CMTS initialization. The number
of data bytes is defined as the total number of bytes transported
in DOCSIS payloads minus the number of stuff bytes transported in
DOCSIS payloads.
This is the 64 bit version of docsIfCmtsDownChnlCtrUsedBytes, and
will not be accessible to SNMPv1 managers.