T-Mobile tops OpenSignal's latest network study while unlimited weighs on Verizon, AT&T

T-Mobile swept OpenSignal's performance results for both 4G and 3G networks.

Maybe the new unlimited plans from Verizon and AT&T are slowing their data speeds after all.

That’s the conclusion reached by OpenSignal, which released findings today from its latest biannual study of U.S. mobile networks. The network-measurement company determined T-Mobile operated the top-performing network in the nation from March 31 through June 29, beating its rivals across the board in categories including LTE download speed, LTE latency and LTE availability.

T-Mobile also ranked highest in 3G download speed, overall download speed and 3G latency.

“Six months after reintroducing unlimited plans, Verizon and AT&T experienced a marked decline in 4G speeds in our tests,” OpenSignal said in its report. “The impact appears to have hit Verizon the most. Its average LTE download test fell 2 Mbps to 14.9 Mbps in the six months between reports.

AT&T took issue with OpenSignal's findings, however, saying its data transmissions have been unaffected despite traffic that has presumably increased

"We constantly monitor our network performance," an AT&T spokesperson said via email. "The launch of our unlimited data plans has not impacted wireless data speeds on our network."

Verizon has also criticized similar survey data, saying last month that "Isolated speed peaks from a crowd-source test don't tell the whole story."

OpenSignal said competition between the top two networks it tested was particularly competitive in terms of accessibility.

“T-Mobile edged out Verizon in our 4G availability metric, but it was a very close call,” OpenSignal continued. “Our testers were able to find a 4G signal on T-Mobile 90.9% of the time compared to 89.8% of the time on Verizon.”

And while unlimited data plans may have taken a toll on the nation’s two largest network operators, the transmission speeds of both T-Mobile and Sprint actually accelerated over the last six months after they launched the first salvos in the unlimited war, OpenSignal said.

Interestingly, OpenSignal’s results come just a week after RootMetrics released data indicating Verizon managed the nation’s top wireless network, while T-Mobile’s ranked a distant fourth overall. But OpenSignal’s findings echo data from Ookla, which T-Mobile cited just two weeks ago when it claimed that the networks of both Verizon and AT&T “have caved” following the launch of unlimited plans from those carriers.

T-Mobile issued the obligatory press release to crow about OpenSignal’s findings.

“T-Mobile today announced its network has scored a clean sweep in the latest State of Mobile Networks report from OpenSignal, placing No. 1 in every category,” T-Mobile announced. “It is the first such sweep for any major wireless provider in the world. In the report, more than 5 billion tests from actual wireless customers in the second quarter show that T-Mobile’s network is not only fastest, but also more responsive, and T-Mobile customers get an LTE signal more often than customers of any other major wireless company do.”

Indeed, OpenSignal—like Ookla—uses crowdsourced data from consumers “under conditions of normal usage” through its mobile app. The latest report is based on nearly 5.1 billion measurements from 173,000 test devices across the country.

RootMetrics, by way of contrast, conducts its own tests, driving more than 276,000 miles across the country to take 4.7 million test samples for its latest report.

OpenSignal also noted that major metropolitan areas have become a key battleground for carriers when it comes to network performance. That trend is likely to ramp up as mobile data consumption continues to soar, forcing operators to densify their networks as they seek to increase capacity and improve coverage.

“A look at our city-by-city analysis of 4G availability and speed reveals just how close the war between Verizon and T-Mobile actually is,” OpenSignal wrote. “In each of the 32 cities we examined, either T-Mobile or Verizon ranked highest or were tied for first place in both our 4G availability and 4G speed metrics.”

T-Mobile CEO John Legere took to Twitter Wednesday morning to gloat about the findings.

This piece was updated August 2 to add comment from AT&T.