Three US Senate Democrats today asked the four major wireless carriers about allegations they've been throttling video services and—in the case of Sprint—the senators asked about alleged throttling of Skype video calls.

Sens. Edward Markey (D-Mass.), Richard Blumenthal (D-Conn.), and Ron Wyden (D-Ore.) sent the letters to AT&T, Verizon, Sprint, and T-Mobile, noting that recent research using the Wehe testing platform found indications of throttling by all four carriers.

"All online traffic should be treated equally, and Internet service providers should not discriminate against particular content or applications for competitive advantage purposes or otherwise," the senators wrote.

Specifically, the Wehe tests "indicated throttling on AT&T for YouTube, Netflix, and NBC Sports... throttling on Verizon for Amazon Prime, YouTube, and Netflix... throttling on Sprint for YouTube, Netflix, Amazon Prime, and Skype Video calls... [and] delayed throttling, or boosting, on T-Mobile for Netflix, NBC Sports, and Amazon Prime by providing un-throttled streaming at the beginning of the connection, and then subsequently throttling the connection," the senators' letters said.

Much of the throttling can be explained by carriers' policies of limiting video throughput, particularly on lower-cost wireless plans, a practice that wouldn't necessarily have violated the net neutrality rules repealed by the Federal Communications Commission. Current FCC rules don't ban throttling or blocking, but the FCC still requires carriers to disclose throttling and other forms of network management.

We asked all four carriers for responses to the senators' letter today. Sprint confirmed to Ars that it received the senators' letter and said it will respond directly to the senators. AT&T questioned the accuracy of the research.

We'll update this story with any responses from the other carriers.

Sprint denied Skype throttling

Wehe, which we wrote about in January, was created by David Choffnes, a Northeastern University professor who researches distributed systems and networking. Wehe relies on an app for Android and iPhone; the throttling allegations are based on user tests run with these apps.

Sprint previously denied the allegation that it throttles Skype.

"We do not impose any restrictions on VoIP traffic or VoIP services," Sprint told Ars last week. "Sprint is not throttling Skype, and Sprint does not single out Skype or any individual content provider in this way."

Ars is working on a separate story about the Sprint/Skype allegations, which we expect to publish within the next week.

AT&T disputed the Wehe research, and pointed to an analysis made last month by mobile industry lobby group CTIA. "CTIA thoroughly debunked Wehe app last month," an AT&T spokesperson told Ars today.

But the CTIA did not dispute any specific Wehe data. The CTIA blog says, "The Wehe app is not measuring performance between users and content providers like Netflix or YouTube, but instead uses simulated data traffic to their own servers. This simulated traffic between artificial network end points is 1) not disclosed and 2) has nothing to do with actual network performance between mobile customers and actual content providers."

However, the CTIA blog did not say whether carriers treat the actual content providers differently than the Wehe app's simulation of those providers' traffic. CTIA acknowledged that the Wehe app detected "basic wireless network management... that reduce video resolution of data traffic flowing through their sites or apps depending on the consumer’s mobile device." CTIA argued that carriers put consumers "in the driver’s seat" by providing "the ability to alter video resolution settings or sign up for data plans that do or don’t use those features."

The senators' letters say that the throttling found by Wehe "would violate the principles of net neutrality and unfairly treat consumers who are unaware that their carriers are selecting which services receive faster or slower treatment."

The senators asked the carriers for written answers to a series of questions by December 6. The questions are as follows:

Please provide a list of all applications or services that are subject to traffic discrimination.

When did your company put into practice policies to throttle or prioritize Internet traffic for consumers? What is the purpose of these policies?

Do you inform customers about differentiation in the treatment of Internet traffic, particularly video or communications services? If so, how? If no, Why not?

Are consumers able to opt-in or opt-out of traffic differentiation? Does a customer's choice change the price or affect their service, such as data allocation or requiring a different plan?

Does your company implement traffic differentiation policies based on a consumers' contract or the brand of service? If so, please describe which plans experience throttling or prioritization, including prepaid and lower‐cost plans.

How do you determine which network traffic receives faster or slower treatment? Is it based on content, behavior, or IP address?

Are applications or services provided notice regarding the throttling of their customers using your network? Does your company provide such companies the ability to avoid traffic discrimination, and if so, under what financial and operational conditions?

Does your company engage in throttling or prioritization of services for subscribers of Mobile Virtual Network Operators (MVNOs) that use your company's network? Are these MVNOs aware of such throttling or prioritization?

FCC Chairman Ajit Pai was asked about the allegations at a press conference after today's commission meeting. "My understanding is that the data and analysis that went into those conclusions have not been made available to the extent that others independently can verify those results," Pai said.

Allegations of throttling can be submitted to the Federal Trade Commission, Pai said. The FTC has no rule against throttling, but it could try to punish carriers if they mislead consumers about the existence of throttling.