It's been a fascinating six months for the U.S. mobile industry. After years of retreating from all-you-can-eat data services, both Verizon and AT&T reintroduced unlimited plans this year to counter the increasing threat of T-Mobile and Sprint. Those new plans not only had a big impact on the competitive landscape in the U.S. but also on OpenSignal's metrics. Our measured average speeds on Verizon and AT&T's networks have clearly dropped, almost certainly a result of new unlimited customers ramping up their data usage. Conversely, T-Mobile and Sprint's 4G and overall speeds are steadily increasing in our measurements. Those shifting speed results were one of the main reasons T-Mobile swept our six awards categories for this reporting period. Despite T-Mobile's wins, the Un-carrier and Verizon are still engaged in a very close fight in our 4G metrics in the urban battlegrounds of the U.S.

For this report, OpenSignal analyzed 5.07 billion datapoints collected from 172,919 OpenSignal users between March 31 and June 29, 2017. We examined 3G and 4G performance for all four of the major operators nationally, and we drilled down to the city level, looking at 4G speeds and 4G availability in 32 of the largest U.S. markets.

Unlimited takes a bite

Five years ago, you would be excused for thinking the unlimited data plan was an endangered species in the U.S. Three of the Big 4 operators had stopped offering unlimited options to new customers and were aggressively moving their old customers to new tiered plans. But in 2012, T-Mobile made unlimited a key pillar of its new Un-carrier strategy , which triggered a massive revival of T-Mobile fortunes. After four years of unparalleled growth at the expense of its competitors, T-Mobile last year made unlimited its standard postpaid data offer , doing away with other data tiers. Verizon and AT&T were forced to react , which they did in February, making unlimited plans available to the two-thirds of U.S. subscribers they serve. The move not only had enormous market implications, it also produced a big shift in the mobile user experience of U.S. subscribers.

In the six months since our last State of Mobile Networks: USA report, our measured average 4G speeds and overall speeds for Verizon and AT&T have dropped. The impact of unlimited was particularly evident for Verizon, which saw its average LTE download connection fall 12%, from 16.9 Mbps to 14.9 Mbps, in our speed tests. AT&T's decline was less pronounced. We measured average 4G download speeds on AT&T at 12.9 Mbps, down from 13.9 Mbps in our February report. The trend has been consistent as well. OpenSignal has recorded steady decreases in average 4G speed each month for both operators since they unveiled their unlimited plans in February.

What's causing this fall-off in experienced speeds? The answer is most likely congestion. Cellular networks are shared networks, meaning individual connection speeds are dictated by the total amount of demand for data at a particular cell site. If that demand increases — either through more customers or more data usage from existing customers — then average connection speed for all users starts dropping. We may well see this trend continue as more Verizon and AT&T customers take advantage of these new unlimited plans. (For more details on congestion's impact on data speeds, see the accompanying analysis in the OpenSignal blog.)

While T-Mobile and Sprint offer unlimited data options as well, neither took a hit in our speed measurements. As their unlimited plans have been available to consumers for years, their networks have been absorbing the high data volumes of such plans for some time. In fact, we measured increases in average 4G speeds for both operators. Sprint's average LTE download test improved from 9 Mbps to 9.8 Mbps between our reports, though that increase wasn't enough to lift it out of last place in our 4G speed rankings. Our average 4G speed test for T-Mobile was 17.5 Mbps, up from 16.7 Mbps. In our last report, T-Mobile and Verizon were tied for first place in both 4G speed and overall speed, which factors in both 3G and 4G results. But T-Mobile handily won both awards in this report due to the reversal of Verizon's momentum.

T-Mobile's LTE reach keeps growing

Speed wasn't the only area in which T-Mobile excelled during this test period. T-Mobile became the first U.S. operator to pass the 90% 4G availability barrier in our tests, allowing it to jump over Verizon to the top of our rankings. Our availability metric measures the proportion of time a user can connect to a particular network, so in this case our T-Mobile testers were able to find an LTE signal 90.9% of the time. Verizon, however, was just a hair shy of matching its rival signal for signal. Our users were able to latch onto a Verizon 4G connection 89.8% of the time.

T-Mobile's first-place finish may come as a surprise considering Verizon's long-deserved reputation for providing the best mobile coverage in the U.S. It's important to remember, though, that OpenSignal's 4G availability metric doesn't measure geographic coverage; rather it's a gauge of how often current subscribers are able to access an operator's 4G service no matter where those subscribers happen to be. T-Mobile has made a lot of progress in expanding its mobile footprint outside of cities in recent years, but T-Mobile's traditional focus has been on urban areas where it's much easier to provide a consistent mobile signal. It will be interesting to see if T-Mobile can maintain its high level of 4G availability as it continues to grow and its subscriber base expands into more rural areas.

There was good news for the other three major US operators in the 4G availability category as well. AT&T, Sprint and Verizon all saw their availability scores increase since our February report, and for the first time subscribers on all four operators were able to access LTE services more than 80% of the time. Sprint by far saw the biggest improvement in this metric. We measured Sprint's 4G availability at 81.6%, up from 76.8% six months ago. Sprint is now less than 2 percentage points away from matching AT&T in 4G availability. Sprint has done an impressive job in closing the once yawning gap between itself and its competitors in 4G availability. A year ago, our Sprint testers were able to connect to its LTE network less than 70% of the time.

The final category we looked at, latency, resulted in two more wins for the Un-carrier. Our latency metric measures the time it takes data to make a round-trip through the network and its a good measure of a network's responsiveness. T-Mobile's high-powered HSPA+ network has consistently ranked highest in 3G latency as well as speed, but in this test period it took the 4G latency award from Verizon. It's not that T-Mobile's latency score improved since our last report; rather Verizon's response time suffered in our latest tests.

Battle in the urban trenches

T-Mobile's clean sweep of awards in this report doesn't paint the full picture of the competitive situation in the U.S. 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. 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.

Verizon won our 4G speed award outright in 6 markets and shared the award in 24 markets, while T-Mobile won 4G speed outright in 4 markets and shared the top spot in 25 markets. In 4G availability, Verizon pulled off the direct win in 14 markets, but in every other market except Cincinnati, we recorded a two-way tie between T-Mobile and Verizon. In our national metrics T-Mobile may have the awards, but Verizon is clearly going head to head with T-Mobile in the big cities and in many cases gaining the upper hand.

We certainly don't expect either operator to let up in the coming year. Verizon has encountered a setback with the introduction of unlimited plans last spring, but ultimately the shift away from tiered data to unlimited data could have a lasting impact on all operators. The more demand created for data, the more network resources get taxed. Unless operators make up the difference by adding more capacity, data speeds will slow for everyone. Right now T-Mobile and Sprint benefit from their smaller size. Verizon and AT&T have twice the number of subscribers of their smaller competitors, which means they have twice as many devices competing for limited capacity. But T-Mobile is growing rapidly, and it continues to push all of its postpaid customers toward unlimited plans. If operators keep opening the data spigot wider, falling speeds could become a problem for the entire U.S. mobile industry, not just Verizon and AT&T.