T-Mobile made a move this week that should be of interest to anyone considering the Comcast-Time Warner cable merger. While it lowered its rates for people who use less than 3GB of data per month, it hiked up rates for its unlimited-data customers.

That's because success is a perverse thing for wireless carriers. Wireless networks have a relatively fixed capacity, with less flexible choke points than wired networks do. So as a carrier like T-Mobile gets more popular and its unused airwaves fill up, it has to cut down on people using its network as a primary means of home Internet access.

One heavy, home-style user can use as much data as 20 average mobile users. T-Mobile told me last week that its average smartphone user consumes between 1 and 2GB of data, up 50 percent from last year. One big reason is that most people don't want to watch long-form video or download huge files on smartphones, which are the two things that eat up the most data.

But an average home AT&T cable user gobbled up 21GB of data two years ago; it has to be more than that by now. Much of that is Netflix, iTunes, and their ilk. This is a big part of why even carriers with "unlimited data" don't offer it for tethering onto your laptop or TV. They just wouldn't be able to handle the load.

So you see what T-Mobile is trying to do here: attract more of those 2GB customers, and make the 20GB customers pay for it.

Unlimited's Days are Numbered

All four major carriers once offered unlimited data, but the two most successful, AT&T and Verizon, cut it out years ago to prevent this kind of heavy use. Sprint and T-Mobile have kept the unlimited data flame alive because their comparative lack of customers (especially data-hungry iPhone customers) meant that their networks were more lightly used.

But now we see T-Mobile moving away from that model. Sprint has more headroom, with its huge array of ex-Clearwire airwaves. Clearwire had so much spectrum that for a while it didn't even have a problem offering unlimited data to home users.