As always, Unlimited Data Plan customers have the comfort of knowing that, no matter how much data they use in a billing cycle, they will pay a single monthly flat rate
That won't change. That is the essential promise of the Unlimited Data Plan, and we are pleased to continue honoring that promise. As a result of AT&T's network management practices, customers on a smartphone with an Unlimited Data Plan who have exceeded 22GB of data in a billing period may experience reduced speeds for the remainder of that billing period when using data services at times and in areas that are experiencing network congestion. Their data usage threshold will return to 22GB with the start of the next billing cycle. Even when subject to potentially reduced speeds customers normally can still have a good experience surfing the Web, accessing email, and continuing to use an unlimited amount of data each month without incurring overage charges. Customers will likely notice the biggest difference while streaming video. Streaming video consumes the most data of all activities and may be slow or choppy when a customer has used more than 22GB of data in a month and is using data at times and in areas that are experiencing network congestion.

출처: http://www.att.com/esupport/article.jsp?sid=KB410284&cv=820

작년 말에 FTC 가 ATT 의 무제한 플랜 속도 제한에 대해 대해서 소송을 걸었었네요..   잠시 ATT를 대견스럽게 생각했었는데 기업이 스스로 불리한 결정을 내렸을리가 없지요. 

https://www.consumer.ftc.gov/blog/ftc-sues-att-limiting-unlimited-data