| Cost per GB isn't really even what you want to measure, and doing so is what causes so much confusion. Let's say you're an ISP and you have a 10Gbps connection to the outside world, and the routers etc. that can keep up with that speed. To a first approximation, your costs to serve up 10Gbps to all the customers in your area is 0 (you've got to pay for electricity and engineers and etc, but this doesn't vary with user activity). If your customers in your area all queued up nicely and downloaded stuff at full rate, never overlapping with each other, you could transfer 10Gbps * 1 month for basically nothing. However, your customers all want to do stuff at the same time, and thus contend with each other for a slice of that 10Gbps. If you want to serve all of their requests, you're going to have to spring to upgrade your total capacity. If you don't own your own fiber, you have to rent a link from someone else, or if you do and there's no more available you have to physically come up with more (and pay to dig up the road and etc.). Now your routers and etc. might not be able to keep up with your new backhaul speed, and you have to upgrade those... You really want to measure "Marginal cost of +x Kbps during the busiest typical hour." |