However, doing the calculations and subtracting a value from its previous value (since in the parameter description, it is an accumulation), the results are negative.
Is there any way to measure this?
Or some parameter that brings this internet consumption?
What exactly are you trying to measure? Total consumption over time? Doing this with the CPE is a bad idea. TotalBytes[Sent|Received] are not persisted on reboot. Some CPEs don’t track the bits at all either:
The accurate way of doing this is to track the usage upstream. We track usage using PPPoE and radius. This is the usage chart since midnight for the user above: