Sorry for my delayed response.
The counter DLSTRVOL gives the data volume for streaming. However, if you’ve not activated QOS, then this counter will be zero, which means DLINTBGVOL will give you the correct data volume.
On the protocol stack, LLC is between the SGSN and the MS and the RLC is between BTS and MS.
Now why LLC and not RLC to measure throughput? Ideally, throughput should be measured at the higher layers of the protocol stack(eg IP layer). This will give a better indication of the user perception. However, the BSS cannot collect statistics at layers higher than LLC.
Going by this argument, it’s better to measure throughput on the LLC layer since it’s higher than the RLC layer.