A current G.Bond port operational Data Rate in the downstream
direction, in bps.
This object represents an estimation of the sum of individual
BCE downstream data rates, modified to compensate for
fragmentation and encapsulation overhead (e.g., for an
Ethernet service, the target data rate of 10Mbps SHALL allow
lossless transmission of full-duplex 10Mbps Ethernet frame
stream with minimal inter-frame gap).
Note that for symmetrical interfaces gBondPortStatUpDataRate ==
gBondPortStatDnDataRate == ifSpeed.
This object maps to the TR-159 attribute aGroupDownRate.