AT&T is capping the monthly bandwidth use of new DSL (Digital Subscriber Line) users in Reno, Nevada, to try out a system for easing the impact of heavy network users.

In a trial that began Nov. 1, the carrier is limiting subscribers of each speed tier of DSL to uploading and downloading a certain amount of data, the company said. The limits would range from 20G bytes for subscribers of the 768M bps (bit-per-second) tier to 150G bytes for users of the 10M bps offering.

The trial may be expanded to one other market before the end of this year, according to an AT&T statement. It's part of an effort to deal with a few very heavy users who hurt the broadband experience for average subscribers, the carrier said in a letter to the U.S. Federal Communications Commission. Almost 50 percent of total bandwidth is used by just 5 percent of users, according to AT&T's statement. They characterized these users as the kind who upload and download the equivalent of 40,000 YouTube videos or 40 million e-mail messages per month.

The largest U.S. wireline carrier, serving 14 million households, wouldn't be the first to cap downloads. Comcast, Time Warner Cable and other broadband providers have already trialed or imposed limits. Earlier this year, the FCC ordered Comcast to stop slowing traffic from BitTorrent and other peer-to-peer applications. Advocates of network neutrality slammed the idea of a broadband provider discriminating against certain online services or applications, especially those that might compete with the provider's own offerings.

But contractual download limits have faced less criticism, because they aren't related to what subscribers are using their connections for.

AT&T will give customers involved with the trial a bandwidth measuring tool and tell them when they have reached 80 percent of their download limit. Customers will get a warning the first time they exceed the monthly cap, and on the second occasion they will be charged $1 per gigabyte for all usage over the limit. The carrier won't terminate anyone's service because of their usage, the statement said. Customers who don't want to participate in the trial will be allowed to cancel their subscriptions without an early termination fee, according to AT&T.

Existing users in the trial area will receive a usage amount of 150G bytes per month.

The caps appear to be high enough that the average user is unlikely to hit them, said Yankee Group analyst Andy Castonguay. He applauded AT&T giving customers some information and warning. But he still thinks consumers may feel they are getting less for their money.