AT&T iPhone unlimited data plan is not actually unlimited. It is a grandfathered plan. Meaning that the network will throttle speeds after the monthly usage limit is crossed. Most users have been surprised by the fact that AT&T is actually throttling speeds after 2GB of usage.
Back in October 2011 AT&T first announced their decision to throttle speeds on unlimited data plan for iPhone. They had originally said that only the top 5% data users will have speed throttled on their unlimited iPhone data plans. At that time 10GB was considered the limit after which the speeds were throttled. However it seems like things have taken a turn on the contrary.
A blogger called John Cozen was shocked when he found out that AT&T throttled his plan after only 2GB usage. He's currently paying $30/month for this AT&T iPhone grandfathered unlimited data plan. AT&T also offers a 3GB data plan for $30/month, so you can imagine how agonizing it must have been for John. AT&T confirms to him that this throttling wasn't a mistake, his usage of 2.1GB, which is now the new limit, resulted in it.
The iPhone is a seriously data hungry smartphone, given all the apps and services. Throttling after 2GB seems preposterous but believe it or not, AT&T iPhone unlimited data plan users are being subjected to this. John received the following response from the network after he inquired about this issue:
To give you a baseline – the average data use across the country by the top 5% of AT&T smartphone customers was 2GB per month, effective August 2011. The amount of data usage of our top 5% of heaviest users varies from month-to-month and by market, based on the usage of others and the ever-increasing demand for mobile broadband services. To rank among the top 5%, you must use an extraordinary amount of data in a single billing period. It’s not how much time you spend using your device, it’s what you do with it….You’ll see the biggest impact when you try to stream video or upload large files. Speeds will be reduced considerably with those types of data transactions, though you should still be able to check e-mail and browse the Web.
AT&T iPhone data plan users in some regions claim that they have not been subjected to throttling even after surpassing 2.1GB usage. It can only be assumed that this new limit is being gradually rolled out to all AT&T iPhone subscribers in the USA.