AT&T said Friday that it's going to start limiting speeds for the 5 percent of its customers with "unlimited" data smartphone plans who clog the airwaves the most. "We're taking additional, more immediate measures to help address network congestion, " the company said in a statement.

One new measure is a step that may reduce the data throughput speed experienced by "a very small minority" of smartphone customers who are on unlimited plans, AT&T said. The company added that these customers on average use 12 times more data than the average of all other smartphone data customers. This step will not apply to the company's 15 million smartphone customers on a tiered data plan or the vast majority of smartphone customers who still have unlimited data plans.

"Starting October 1, AT&T's 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," AT&T said.

Typically what puts someone in the top 5 percent is streaming very large amounts of video and music daily over the wireless network, not Wi-Fi. Streaming video apps, remote web camera apps, sending large data files (like video) and some online gaming are examples of applications that can use data quickly. Using Wi-Fi doesn't create wireless network congestion or count toward your wireless data usage.

AT&T also said that even this additional measure will not solve its spectrum shortage and network capacity issues, at least before the company merge with T-Mobile.

Verizon Wireless also recently stopped signing up new customers for unlimited service.