The top 5 percent of AT&T's DSL customers use 46 percent of the total bandwidth, Coe said. Overall bandwidth use on the network is surging, doubling every year and a half.

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AT&T doesn't have any specific plans or fees to announce yet, Coe said.

Most cable companies have official or secret caps on the amount of data they allow subscribers to download every month. Time Warner Cable started a trial earlier this month in Beaumont, Texas, under which it will charge subscribers who go over their monthly bandwidth cap $1 per gigabyte.

Cable companies are at the forefront of usage-based pricing because neighbors share capacity on the local cable lines, and bandwidth hogs can slow down traffic for others. Phone companies have been less concerned about congestion because the phone lines they use to provide Internet service using DSL, or Digital Subscriber Line technology, aren't shared between neighbors, but AT&T is evidently concerned about congestion higher up in the network.

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Those who mainly do Web surfing or e-mail use little data and have scant reason to pay attention to traffic caps. But those who download movies or TV, particularly in high definition, can hit the caps imposed by cable companies.

Download caps could put a crimp in the plans of services like Apple Inc.'s iTunes that use the Internet to deliver video. DVD-by-mail pioneer Netflix Inc. just launched a TV set-top box that receives an unlimited stream of Internet video to a TV set for as little as $8.99 per month.