AT&T has changed how the company throttles the connections of customers in congested areas, depending on how much data they consume. The US carrier had a limit of 5GB in place for unlimited data plans, and passing this bar would cause connectivity to the internet to be limited altogether. This limit has been increased to 22GB, enabling subscribers to download more data before having their connections throttled.
This limit is configured to run within billing periods, so you'll be able to use 22GB of data in a month before AT&T would take action and limit your connection while in congested areas only, such as densely populated cities. From the AT&T website:
"In line with common industry standards, our network management practices assure that our network resources are used for the benefit of all our mobile broadband customers especially during periods when network demand exceeds available network resources (also known as "congestion").
As you would expect, these network management practices have continued to evolve over time to benefit our customers and take advantage of the billions we have spent to expand and augment our networks. As a result of this evolution, we recently revised our practices such that Unlimited Data Plan smartphone customers can now use 22GB of high-speed data during a billing period before becoming subject to network management practices that might result in reduced data speeds and increased latency."
It's worth noting that this change only affects those on the unlimited data plans, which are no longer available for new lines. See the official website for more details.