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The unlimited plan worked fine for AT&T a few years ago, when the iPhone was new. The company had ample capacity on its network, and wanted to lure customers with the peace of mind offered by unlimited plans. Now, a majority of AT&T subscribers on contract-based plans have smartphones, and the proportion is growing every month. That’s putting a big load on AT&T’s network.

The limited data plans force subscribers to keep an eye on their usage, so they don’t overwhelm AT&T’s network. Verizon Wireless has adopted similar plans. But the two companies differ in how they manage their remaining “unlimited” subscribers.

Verizon doesn’t slow down the “5 percent” unless the cell tower their phone is connected to is congested at that moment, and it slows them down by the minimum amount necessary. By contrast, once AT&T has decided to throttle your phone, it will be slow for the rest of the billing cycle, even if it’s 3 a.m. and there are no other cellphones competing for the capacity of that particular cell tower.

Verizon’s measures have drawn few complaints, and indeed, may have gone unnoticed even by the “5 percent.”

T-Mobile USA is up front about the level it starts throttling at: 5 gigabytes. AT&T subscribers have no idea if they might be among the top 5 percent until they get the warning, which is soon followed by throttled service. While Trang was throttled at 2.3 gigabytes, he knows other iPhone owners who are using 5 or 6 gigabytes per month with impunity.

“It seems very random,” Trang said.

Sprint Nextel Corp. is hanging on to unlimited data plans without throttling, alone among the “Big Four” national wireless carriers.

Tallman sees few prospects for a lawsuit against AT&T. The company is still providing unlimited data usage to throttled customers, even if the speeds are so low as to make the phone useless for anything but phone calls and text messages. The company made no promises that “unlimited” data would always be coupled with high speeds, he notes.

“They just guaranteed the highway. They didn’t guarantee the speed limit,” he said.

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Online:

AT&T’s July 29 letter on throttling: http://bit.ly/qddCeI

Verizon page on its version of throttling: http://bit.ly/pMMCfs

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Peter Svensson can be reached at http://www.twitter.com/petersvensson