Math is Hard
If you're AT&T, it's so hard that it's a proprietary trade secret:
As many of you know, AT&T has implemented caps on DSL usage. When this was implemented, I started getting emails letting me know my usage as likely to exceed the cap. After consulting their Internet Usage web page, I felt the numbers just weren't right. With the help of Tomato on my router, I started measuring my usage, and ended up with numbers substantially below what AT&T was reporting on a day-to-day basis. Typically around 20-30% less. By the way, this usage is the sum of inbound and outbound. At this point, I decided to contact AT&T support to determine what exactly they were defining as usage, as their web pages never really define it. Boy, did I get a surprise. After several calls, they finally told me they consider the methodology by which they calculate bandwidth usage to be proprietary. Yes, you read that right; it's a secret.
I've noticed the same differential on my "usage" tab on my iPhone and the text messages AT&T sends out.
