T-Mobile has asked the Federal Communications Commission to investigate AT&T’s “artificially high roaming rates” charged when its customers travel outside of T-Mobile’s home service area.

T-Mobile is heavily reliant on AT&T for roaming service outside of major cities and the country’s smallest national wireless carrier complains AT&T is using their market power to put it at a major disadvantage, which could force new limits on roaming access in some areas.

T-Mobile provided examples of the damage already done by AT&T’s roaming rates:

“Limitless Mobile has severely restricted its customers’ access to AT&T’s network ‘for the sole reason that AT&T’s data roaming rates are too high and by continuing roaming access, Limitless could not maintain a commercially competitive retail wireless data offering to the general public,’” T-Mobile told the FCC.

The Rural Wireless Association noted that competing carriers “cannot sustain the provision of data roaming services if [they] must provide that service at a loss.”

The problem of data roaming rates is getting larger as carrier agreements are due for renewal at many mobile providers. Independent cellular companies are finding AT&T unwilling to renew at prices and terms comparable to their existing contracts. Instead, they face renewal rates that average a minimum of 10 and as much as 33 times higher than the national carriers’ retail rates.

For example, T-Mobile’s agreement with AT&T includes a data roaming rate that is now 150 percent higher than the average domestic rate that T-Mobile pays for data roaming.

This is one thousand percent higher than the data roaming rate negotiated between Leap Wireless and MetroPCS prior to their respective acquisitions, wrote T-Mobile.

With the stark price increases, carriers have begun imposing limits, including speed throttling and data caps, on customers when roaming on AT&T’s network.

Because of AT&T’s artificially high roaming rates, T-Mobile wireless customers roaming in South Africa have a better user experience than customers roaming on AT&T’s network in South Dakota, argues T-Mobile. Their speed is twice as fast, and their data usage is unlimited.

T-Mobile is asking the FCC to intervene by establishing some type of standard about what constitutes “commercially reasonable” roaming rates as part of its 2011 Data Roaming Order, designed to protect competition.

This year, carriers dependent on Verizon Wireless or AT&T to help deliver “nationwide coverage” are negotiating roaming access to the companies’ 4G LTE networks for the first time. Most roaming agreements used to only cover 3G service, delivered at a slower speed.

If carriers like Sprint and T-Mobile are unable to negotiate fair terms, both companies will be at a major competitive disadvantage, relegated to providing only regional coverage or charging higher prices for roaming service.

AT&T vice president of regulatory affairs Joan Marsh said T-Mobile’s request bordered on being illegal, in direct violation of the Telecommunications Act. Marsh argued T-Mobile and other carriers should be incentivized to build their own networks instead of relying on cheap roaming access from companies like AT&T. Marsh added any move by the FCC to set rates or benchmarks would be beyond the FCC’s mandate. Wireless carrier rates are deregulated and not subject to common carrier regulation.