NEW YORK — Just in time for the release of a new iPhone, AT&T will stop letting new customers sign up for its unlimited Internet data plan for smart phones and iPads and charge more for users who hog the most bandwidth.

AT&T hopes to ease congestion on its network, which has drawn complaints, particularly in big cities. But the approach could confuse customers unfamiliar with how much data it takes to watch a YouTube video or fire up a favorite app.

Current subscribers will be able to keep their $30-per-month unlimited plans, even if they renew their contracts. But starting Monday, new customers will have to choose one of two new data plans for all smart phones, including iPhones and BlackBerrys.

Subscribers who use little data – such as those who may get dozens of e-mails a day but don’t watch much video – will pay slightly less every month than they do now, while heavy users will be dinged with higher bills.

The move takes effect in time for the expected unveiling of Apple’s new iPhone next week. Analysts said they expect other phone companies to follow. With no caps on consummption, data use could swamp wireless networks while revenue for the operators remains flat.

Verizon Wireless, the largest wireless carrier and AT&T’s chief rival, had no immediate comment on AT&T’s move. There has been much speculation about Verizon getting to sell its own version of the iPhone, but that prospect still appears distant.

One of the new AT&T plans will cost $25 per month and offer two gigabytes of data per month, which AT&T says will be enough for 98 percent of its smart phone customers. Additional gigabytes will cost $10 each.

A second plan will cost $15 per month for 200 megabytes of data, which AT&T says is enough for 65 percent of its smart phone customers. If they go over, they’ll pay another $15 for 200 more megabytes.

A gigabyte is enough for hundreds of e-mails and Web pages, but it’s quickly eaten up by Internet video and videoconferencing. The 200 megabytes offered under the $15 plan is enough for about 20 minutes of streaming video, AT&T says.

With the smaller plan and voice service, a smart phone could cost as little as $55 per month before taxes and add-on fees, down from $70 now.

Figuring out which plan to choose may not be easy, because many people have only a hazy notion of the size of a gigabyte and how many they use now.

A minute spent talking on the phone is easy to understand, and many people have learned roughly how many minutes they use every month.

The limits will apply only on AT&T’s cellular networks. Data usage over Wi-Fi networks, including AT&T’s public Wi-Fi “hot spots,” will not count toward the limits.

For the iPad, the tablet computer Apple released a few months ago, the new $25-per-month plan will replace the $30 unlimited plan. IPad owners can keep the old unlimited plan as long as they keep paying $30 per month, AT&T said.

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