Bandwidth is not like gas. It's not something that you buy, and consume it, and then go out and buy more.

Instead, there's an infrastructure that need to be built. That infrastructure has a certain capacity; at any given time, only so many bits per second can be flowing over it.

Now, most people don't use all of their capacity at a continuous, constant rate. Rather, their usage is quite bursty. You use a lot of bandwidth for a short amount of time when loading a video, and then none while you're watching it. Or you use lots of small bursts while surfing the web.

If they sold 100 Mbps (no one sells 100 MB/s) and guaranteed that to you, it would be astronomically expensive. But selling you service that gets 100 Mbps in bursts if people have average personal-use utilization patterns is perfectly possible.

Now, how exactly should they advertise that? That's a tough question. If they were going to advertise the minimum guaranteed bandwidth, it would be laughably low (and of course, they could never really guarantee any minimum, since how much bandwidth you get for a given transfer depends not only on your connection, but the backbones in between and server's connection; there is no way to guarantee any particular amount of bandwidth in any meaningful way). And you would almost always get more than that, so that would be a fairly meaningless number.

Perhaps they could advertise an aggregate average bandwidth? That would be pretty hard to define and test (what sample period are you measuring over? How do you know when a transfer has stalled due to congestion somewhere vs being finished?), and wouldn't really give you much meaningful. Are you really going to go out an canvas your neighbors to determine if your neigbhorhood really is getting the promised average aggregate bandwidth?

The only really meaningful thing they can give you is what the actual maximimum bandwidth they will allow over your connection. That is a hard and fast number, and can be tested fairly easily. Of course, that doesn't say much about your bandwidth accessing any particular site, but there's nothing you can really do about that without control of the entire network.

Now, actually in practice, it appears that when I buy a certain level from Comcast, they don't actually use that as a hard maximum. I've seen bursts that were higher than the advertised bandwidth; and of course, I've seen transfers that were lower.

I know that everyone likes to criticize ISPs, and there is definitely some behavior that I find odious (blocking particular protocols, attempts to subvert net-neutrality). But over-selling bandwidth is not one of them. I can't really imagine a system that would work better than over-selling bandwidth.

I don't have any problem with an ISP saying "up to #Mbps" as that's what they allow each customer to get (though this is 2013 and I'm sure with the amount of metrics that CPE can gather, we could measure average thruput and advertise that too).

What I have a huge issue with is selling the services as unlimited and then smacking people down when they walk over some invisible line, or worse selling an "up to #Mbps" line when they know damn well based on basic math that nobody's ever going to get anywhere near that speed in that area.

It's absolutely ripe for abuse. You, as an ISP, can legally sell someone a 100 meg line you know in advance won't reach that speed.

If they has such hard time to measure limits, they can always use first hop router as test base.

But lets take the look from the buyer. You see a 8MB line for $80 amount, and 10MB line for say for $100. Knowing what you pay for is the minimum requirement for informed purchase, or you end up with a lemon market.

ISP are lemon markets, and you get poor quality for more expensive price. No amount of "The infrastructure has a certain capacity" argument is going to change that.