AT&T's home Internet data caps got an overhaul yesterday when the company implemented a recently announced plan to strictly enforce the caps and collect overage fees from more customers. Customers stuck on AT&T's older DSL architecture will be facing lower caps and potentially higher overage fees than customers with more modern Internet service.

AT&T put a positive spin on the changes when it announced them in March, saying that it was increasing the monthly data limits imposed on most home Internet customers. This was technically true as AT&T already had caps for most Internet users. But previously, the caps were only enforced in DSL areas, so the limits had no financial impact on most customers. Now, a huge swath of AT&T customers has effectively gone from unlimited plans to ones that are capped, with an extra $10 charge for each additional 50GB of data provided per month.

The only customers who aren't getting an increase in their monthly data allowance are the ones who have been dealing with caps the past few years, according to AT&T's data usage website:

U-verse plans that previously had an unenforced 250GB-per-month cap will now have enforced caps of 300GB or 600GB, while 500GB plans were raised to 1TB. The DSL cap, meanwhile, is staying at 150GB per month. DSL users can also be hit with overage fees of up to $200 a month, while U-verse customers face maximum overage charges of $100 a month. For customers trying to avoid overage fees, AT&T offers such tips as choosing low-definition video and limiting use of torrents and other file transfer programs. Overage charges are waived the first two months in which customers exceed their caps.

But where is the cutoff between DSL plans with the lower caps and U-verse plans with the new, higher limits? The AT&T chart above shows that customers with speeds as low as 768kbps will be getting at least 300GB a month. But that doesn't necessarily mean that only customers with fewer than 768kbps are hit with the lower caps, as the chart leaves out the speeds for DSL.

This other AT&T webpage shows that DSL plans—or "High Speed Internet, AT&T's euphemism for DSL—range from 200kbps to 6Mbps. From the 1.56Mbps to 6Mbps range, there are separate plans that have identical speeds but are branded as "U-verse":

This seems to indicate that in the 6Mbps and less range, two customers living in different areas could have the same speeds but different data caps. If the distinction isn't being made on speed, it might be made based on architecture. The "U-verse" brand is typically used in areas where AT&T has built fiber closer to homes, but some of the U-verse speeds are still pretty low. We've asked AT&T for clarification on this point and will provide an update if we get one.

Back in 2013, AT&T told DSLReports that cap enforcement wasn't necessary in U-verse areas because of "the greater capacity of the U-verse architecture as compared to legacy DSL." Data caps, particularly on wireline Internet services, are imposed due to business reasons rather than technical ones, as one Comcast executive acknowledged.

If there's a "positive" for AT&T DSL users, it's that their slow speeds might make it hard to exceed the 150GB cap. But for those who do go over the cap, it could be a big problem, as customers in AT&T DSL areas often have no other choice for wireline Internet.

These are often the same areas where AT&T would like to shut off the traditional copper-based landline phone network and shift customers to more profitable wireless networks. That won't happen without oversight from the Federal Communications Commission, but in some areas the network has already deteriorated to the point where AT&T won't hook up new customers requesting DSL Internet service.