A new study is putting U.S. wireless carriers on blast for slowing down online video traffic, even during periods where they claim they are not.

Research conducted by Northeastern University and the University of Massachusetts found that wireless companies — including the main four U.S. carriers: Verizon, AT&T, T-Mobile, and Sprint, all slowed down video traffic from early 2018 to 2019.

The resulting effect for consumers is delayed load times, buffering, and degradation in quality when streaming video content.

These carriers have long maintained that this practice, known as throttling , is done to avoid congestion. However, the study found that online video throttling was occurring around the clock, regardless of congestion.

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According to Bloomberg , for example, the study found that AT&T had been slowing down network traffic for Netflix and YouTube videos more than 70 percent of the time. However, the researchers did not find any throttling from AT&T when it came to Amazon Prime Video. T-Mobile, on the other hand, throttled the ecommerce giant’s video service in about half of the tests.

The tests were conducted via a mobile application called Wehe, which tests users’ internet connections. More than 126,000 smartphone users from around the world took part in the research. In addition, researchers purchased SIM cards from Verizon, AT&T, T-Mobile, and Sprint in order to carry out their own tests. Over 650,000 tests in total were conducted in the U.S. for the study.

Users taking part in the study were able to test online connections with YouTube, Netflix, Amazon Prime Video, NBCSports, Skype, Spotify, and Vimeo.

YouTube and Netflix traffic was slowed down by all four major U.S. wireless carriers: Verizon, AT&T, T-Mobile, and Sprint. Just one of those streaming services, Spotify, were not being throttled by any of the providers. Of the major four U.S. carriers, only Sprint was found to be throttling Skype video calls.

Since the repeal of net neutrality under the Trump administration, advocates of the rule have worried that internet service providers would no longer treat all web traffic equal. A main concern is that internet providers will create a tiered system that prices out competition and providers faster service to only those that can afford it.

While policy changes in response to the demise of net neutrality are still very much an issue, it should be noted that the study found this throttling behavior was occurring even while net neutrality was in effect.