While AT&T quickly moves to usage-based billing (see "AT&T: Usage-Based Pricing is a Success" - here), had "More than 15 million subscribers on tiered data plans" - here) and a nice ARPU increase in Q2 (see chart), it still sees the need for implementing a new throttling policy.
The carrier announced it will do so starting October 1, for "a very small minority of smartphone customers who are on unlimited plans - those whose extraordinary level of data usage puts them in the top 5 percent of our heaviest data users in a billing period"
Currently AT&T's mobile broadband service plan, DataConnect, "is not an unlimited plan" and charges for overage ($10/GB or more, see chart below for 3G/4G plans - other plans exist for tablets).
"Starting October 1, smartphone customers with unlimited data plans may experience reduced speeds once their usage in a billing cycle reaches the level that puts them among the top 5 percent of heaviest data users. These customers can still use unlimited data and their speeds will be restored with the start of the next billing cycle. Before you are affected, we will provide multiple notices, including a grace period .. The amount of data usage of our top 5 percent of heaviest users varies from month to month, based on the usage of others and the ever-increasing demand for mobile broadband services .. Using Wi-Fi doesn't create wireless network congestion or count toward your wireless data usage".
"But even as we pursue this additional measure, it will not solve our spectrum shortage and network capacity issues. Nothing short of completing the T-Mobile merger will provide additional spectrum capacity to address these near term challenges"
See "An Update for Our Smartphone Customers With Unlimited Data Plans" - here.
See also "Sprint Will Throttle Virgin Mobile Users Exceeding 2.5GB" (also in October) - here and "Verizon's UBB Starts July 7"- here.