AT&T is cracking down on unauthorized smartphone tethering.

This week the company has been sending out e-mails and text messages to wireless subscribers who have been using their smartphones as modems to connect laptops and other devices to AT&T's wireless data network without paying the additional $20 tethering fee. Letters and text messages that have been sent to offending subscribers have been posted on various Web sites.

In short, AT&T is telling these customers to pay up or stop using their devices as modems.

"Our records show that you use this capability (tethering), but are not subscribed to our tethering plan," the e-mail reads. "If you would like to continue tethering, please log into your account online at www.wireless.att.com, or call us...If you discontinue tethering, no changes to your current plan will be required. "

If AT&T doesn't hear from the offending subscriber and he or she continues to use tethering without paying for it, the company will automatically begin charging customers for the usage, the letters indicate.

AT&T requires smartphone customers who use the tethering feature to subscribe to its 2GB data service for $25 a month and pay a $20 a month tethering fee. Customers who tether are given 4GB of data to use during the month. Customers who exceed that limit are charged $10 a GB thereafter.

"This is all part of ensuring that we can manage our network," said Mark Siegel, a spokesman for AT&T. "We are simply letting customers know that we have a tethering plan in place, and we're inviting them to get on the plan."

Siegel said that AT&T is able to detect how devices are being used. But he wouldn't explain how the company determines whether a smartphone is being used to connect to the Internet or whether it's used to provide Internet connectivity to other devices.

Siegel said the company has always been able to monitor subscribers' activity, but it has just recently begun sending notifications to a small number of customers.

"We have to be able to tell what is happening on the network to manage it properly," he said. "So if someone is tethering, then they need to be on a tethering plan."