Calculating inound and outbound traffic

I have seen many graphs about traffic calulation to datacenter from big BW provder. How do they calcualte inound and outbound traffics?.

All that is done with MRTG's, which are the most common way of measuring traffic, is that a measurement is taken at the switch port every 5 minutes to determine how much incoming and outgoing traffic there is at that time. From these measurements you can generate numbers on how many GB of data were used or gauge usage on 95th percentile. Basically, the switch will tell them everything they need to know, and they just need a program such as RTG or MRTG to gather, display, and translate the data.