Going over your cap is going to hurt your wallet — just like using too much data on your cellphone. Blow past the limit, and you'll be charged another $10 that month. That gives you another 50 GB to use, but exhaust that too and you'll be hit again with a second $10 overage fee, and so on. (AT&T says that on average, its residential Internet subscribers use 100 GB of data per month, while based on their current usage habits, roughly 4 percent of customers today would run afoul of the new caps.)

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There are two main ways you can escape the caps: Either pay an extra $30 a month for an "unlimited" plan, or make sure you're paying for a double-play consisting of U-verse Internet service and one of AT&T's two television services, DirecTV or U-verse TV.

This gets us to AT&T's underlying strategy: Drive more customers to its pay-TV products as the company tries to execute a shift to digital and mobile video (and beyond that, to targeted advertising).

"AT&T’s announcement is another sign of the ways in which it intends to leverage its DirecTV asset to cross-sell services," said Jackdaw Research analyst Jan Dawson.

We've seen a similar story play out with AT&T's cellular plans. In January, the company said it was bringing back its famous unlimited data plans that let customers consume as much mobile data in a month as they wanted — so long as they also agreed to buy DirecTV or U-verse TV.

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AT&T operates other programs that hint at its long-term strategy, as well. One experiment offers consumers a discount on home Internet service if they agree to hand over their Web browsing history and other usage information, which AT&T uses to provide you with relevant advertising.

The future of that program may be in question as federal regulators seek to determine how to apply new privacy rules to Internet providers. But setting that aside, AT&T is a case study in how a traditional telephone company is trying to adapt to a video- and data-driven world. And the company's announcement on data caps is one more piece of that puzzle.