Devices measure it in bytes sent/received, it doesn't "guess" your usage by assuming you are downloading at a certain speed.
Originally Posted by kodiak799
My guess is that AT&T just rounds up. Say you download 2.59GB one day, they may round that up to 3GB at the end of the day.
Actually since data uploads and downloads comes in bursts/packets, I would think that the rounding up happens at much smaller increments (i.e. you click on a hyperlink in a web page, it downloads the new web page and the size of that new web page downloaded is rounded up and counts as one "session"). If they are rounding up to the nearest MB, you could get screwed if you are just downloading 15 kb and they "charge" you 1 MB.
Originally Posted by geoff5093