AT&T is beginning trials for metered internet access in Reno, Nevada, where it plans to tighten the clamps on the “small minority” of customers who use a “disproportionately large amount” of its bandwidth capacity.

In a letter filed electronically with the FCC (PDF), AT&T attorney Jack Zimmerman says the company will provide customers in the trial with a written notice of their service limitations, along with another written warning when they reach the 80 percent mark. For customers that exceed their bandwidth allotment more than once – the size of which varies based on the service level, starting with a 20 GB limit for 768kbps customers and topping out at 150 GB for 6 mbps lines – they will find an additional charge of $1 per gigabyte on their monthly bill.

Customers unwilling to participate have the option of cancelling their service, and those that choose to do so will have their early termination fee waived.

AT&T says its trial “underscores [its] commitment to bring bigger, faster and smarter broadband networks to more and more communities at affordable rates.” According to DSLReports, the company has been hinting about a shift since last summer, if not earlier.

With more than 14.7 million subscribers, AT&T is the largest ISP in the United States.  It joins Time Warner and Comcast, among others, in a seemingly growing list of companies experimenting with – or already implementing – bandwidth caps in order to curb what they consider to be “excessive” internet usage.

The U.S. ISPs’ seemingly gradual descent from the realm of unlimited bandwidth, a number of observers – including this writer – wonder if ISPs are placing themselves on a collision course with the next generation of internet applications, of which high-quality video is expected to feature prominently.