With the new policy, AT&T customers who exceed a monthly limit of 150GB of data in three separate months will be charged $10 extra for every additional 50GB of data they consume.
Even AT&T's faster U-verse broadband service will have caps. Those customers will have a limit of 250GB.
The new policy only affects users who exceed the new usage cap three times. AT&T then plans to alert customers several times if they are near the limit or if they exceed their limit. These notifications will occur when usage hits 65 percent, 90 percent and 100 percent of total usage.
AT&T, which plans to provide online tools to help DSL customers track their usage, already caps data usage on its wireless service.
Another broadband provider, Comcast, imposed usage caps on its broadband service in 2008. The company limits residential customers to 250GB of data per month.
Other providers, including Charter Communications, Cox Communications and Time Warner Cable, are expected to follow AT&T's and Comcast's data-cap policy.