Last June, AT&T implemented a policy of slowing its unlimited data users' speeds only when they fell into the top 5% of users in terms of consumption. Previously, AT&T didn't slow speeds for unlimited data users.
The policy change will not affect users who have AT&T service and are on one of the company's tiered data plans.
AT&T said on its website the changes are all a response to "soaring mobile broadband usage and the limited availability of wireless spectrum."
The company said its seen a 20,000% increase in data consumption by its users over the last five years, and has gone from 7 million smartphones on its network in 2006 to 39.4 million in 2011.
AT&T's biggest rival, Verizon Wireless, also slows down service speeds for its top 5 percent of data users, but only in areas where the network is crowded. And unlike AT&T, once its customers leave the congested area their speeds return to normal, according to the company.