AT&T will throttle heaviest unlimited smartphone data users starting Oct. 1

AT&T Mobility (NYSE:T) confirmed that starting Oct. 1 it may throttle the data speeds of smartphone users with unlimited data plans who are among the heaviest top 5 percent of data users in a given billing period.

AT&T said that those customers who are affected can still use unlimited data and their speeds will be restored with the start of the next billing cycle. Before customers have their speeds throttled, likely down to 2G EDGE data speeds, AT&T said it will provide multiple notices, including a grace period. (AT&T began offering usage-based smartphone data pricing in June 2010, but customers with existing contracts could be grandfathered into unlimited data plans.)

The carrier said the new policy will affect a "very small minority" of subscribers who use an "extraordinary" amount of data. "In fact, these customers on average use 12 times more data than the average of all other smartphone data customers," the company said in a statement, explaining that users in the top 5 percent are streaming very large amounts of video and music daily. "This step will not apply to our 15 million smartphone customers on a tiered data plan or the vast majority of smartphone customers who still have unlimited data plans."

AT&T did not indicate what amount of data usage would qualify users to be in the top 5 percent of heaviest users, but noted that as demand for mobile data increases, the top 5 percent threshold will change from month to month. AT&T spokesman Mark Siegel told FierceWireless that the announcement is not tied to any specific product launch or change at AT&T. 

The blog 9to5 Mac first reported that AT&T would throttle users' data speeds.

AT&T isn't the first to throttle users' data speeds. T-Mobile USA announced in May it would not charge customers for data overage fees but would instead throttle users' down to 2G EDGE speeds if they went over their data allotments. Verizon Wireless (NYSE:VZ) instituted usage-based smartphone data plans in early July and charges a $10/GB overage fee. However, Verizon said in February, ahead of the launch of Apple's (NASDAQ:AAPL) iPhone 4, that it would begin throttling the data speeds of its the top 5 percent of its heaviest unlimited data users.

Sprint Nextel (NYSE:S) does not charge overage fees or throttle its smartphone data users, but its Virgin Mobile brand will begin throttling the speeds of smartphone users who use more than 2.5 GB of data per month. The company said less than 3 percent of its customers currently use that much data monthly.

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