A to the FCC from and New America Foundation's urges Sharon Gillett, chief of the FCC's Wireline Competition Bureau, to "exercise its statutory authority to fully investigate the nature, purpose, and impact of those caps upon consumers."

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The strongly-worded letter accuses AT&T of breaking past industry practice by attempting to "convert its data cap into a revenue source."

Just this week, AT&T slapped data caps on its home broadband customers. The ISP's DSL users now have a monthly cap of 150 gigabytes a month, while its subscribers get 250GB. A user who exceeds the new usage caps three times will pay an additional $10 for every 50GB of data. By comparison, Comcast limits monthly data usage to 250GB.