I\'m not on the run yet. But I\'ve been warned. AT&T doesn\'t like what I\'m doing.

The mobile carrier sent me an email out of the blue last week. Apparently I had reached a milestone: I\'m in the top 5% of the carrier\'s heaviest data users.

But there were no prizes. Repeat the feat, and I might be punished with slower service, the email said. Just in case I didn\'t get the message, AT&T followed up with a text reminding me to use Wi-Fi to help avoid pokey download speeds.

How did I get to this digital penalty box? In 2010, AT&T announced new tiered data plans, with prices escalating based on the amount of a customer\'s monthly data use.

In July, it warned that data hogs might suffer network slowdowns. I stuck with the $30 a month unlimited plan, figuring I wouldn\'t be one of them.

I was wrong. I had been using my iPhone for nearly four years, doing nearly the same things every day—checking email, listening to Pandora Internet radio, using Google Maps, browsing the Web at night and shopping online. I averaged about 1.76 gigabytes of data per month in 2011.

That changed this month. I\'m not sure what did it. Perhaps it was one 84-minute movie on Netflix, or a bunch of shorter clips that came to total 200 minutes of video. Maybe it was the 24 photos I uploaded to Facebook or the extra hours on Pandora as I took bus rides between New York and Washington, D.C.

By Jan. 18, about a week before my billing cycle ended, I had already used 2.05 gigabytes.

There isn\'t much agreement on what counts as normal data use. The average smartphone owner used 606 megabytes of data per month in the third quarter of 2011, according to Nielsen.

An NPD Connected Intelligence study of 700 Android smartphone users found they used 724 megabytes per month on AT&T\'s network. The average was 1.7 gigabytes on T-Mobile, 902 megabytes on Verizon Wireless and 1.2 gigabytes on Sprint.

But there is agreement that data consumption is on the rise. NPD group expects video streaming to double over the next 12 months.

Oddly, this isn\'t great news for AT&T, which over the years signed up millions of users with the promise of unlimited data access.

When data use was relatively low, this made business sense for the carrier. Now, voracious data use is making those unlimited plans less profitable—and harmful to AT&T\'s wireless network.

\"If you look around at competitors you\'ll find similar approaches,\" said AT&T spokesman Mark Siegel. \"We all have to find ways to find efficient uses of that spectrum available.\"

AT&T\'s network has had trouble handling the crush of traffic set off by the iPhone, particularly in big cities like New York and San Francisco. The carrier\'s ill-fated $39 billion deal to buy T-Mobile USA, along with its spectrum and thousands of cell towers, was intended in part as a fix for those problems.

In the meantime, AT&T is trying to get customers into tiered plans, so those who use more will pay more.

The problem is that I\'m not alone in using my cellphone as my primary path to the Internet. About 25% of smartphone owners said they mostly use their phones, rather than a computer, to go online, according to a Pew Internet Project survey conductedlast year.

AT&T\'s email had suggestions including that I: \"Consider using Wi-Fi when possible for applications that use the highest amounts of data, such as streaming video apps, remote web camera apps, large data-file transfers (like video) and some online gaming.\"

I shut off my iPhone\'s Wi-Fi option long ago, because constantly searching for a signal can eat up battery life. But after AT&T\'s gentle suggestion, I came back to Wi-Fi instead of accessing AT&T\'s network.

While this works at home, it\'s still a hassle when I\'m on the move. Coverage cuts in and out, and I can\'t listen to Pandora on the train anymore. I have to lug my work laptop home more often. So much for mobile.

The warning notices have had another effect: I\'m starting to get paranoid. Was that sluggish service when I was trying to find a restaurant online last Wednesday?

AT&T\'s Mr. Siegel said slowing down heavy data consumers is a way to ensure that all customers can use the company\'s network.

On Oct. 1, the carrier started slowing network speeds for the top 5% of unlimited-plan users in each market, a group that varies by market and by month. I asked how much data the average AT&T customer uses, but was told that wouldn\'t be a helpful comparison.

AT&T subscribers on tiered plans don\'t get throttled. They just have to pay for more data if they go over their limit. AT&T\'s letter suggested that I look at these tiered plans.

It may be a coincidence, but it arrived the same week the carrier said it would raise the price and size of its data tiers. The new data plans offer 300 megabytes for $20 a month, three gigabytes for $30 and five gigabytes for $50.

I used just over two gigabytes, so why would I switch to a capped plan for the same price as my $30 unlimited plan? Oh yes, the throttling threat.

New customers can\'t sign up for unlimited plans anymore. When asked if slowing down the network is to encourage unlimited customers to switch, Mr. Siegel said: \"We offer people the variety of choices. They can pick the plan that is right for them.\"

Indeed, the right plan for me may be with another carrier. Lately I\'ve been looking at Sprint. It offers \"unlimited data,\" too.

http://online.wsj.com/article/SB10001424052970204624204577183032028581306.html