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 32 bit version of docsIfCmtsDownChnlCtrExtUsedBytes,
included to provide back compatibility with SNMPv1 managers.