Some people use the Internet simply to check e-mail and look up phone numbers. Others are online all day, downloading big video and music files.
For years, both kinds of Web surfers have paid the same price for access. But now three of the country’s largest Internet service providers are threatening to clamp down on their most active subscribers by placing monthly limits on their online activity.
One of them, Time Warner Cable, began a trial of “Internet metering” in one Texas city early this month, asking customers to select a monthly plan and pay surcharges when they exceed their bandwidth limit. The idea is that people who use the network more heavily should pay more, the way they do for water, electricity, or, in many cases, cellphone minutes.