Visit almost any coffee shop in America today, and chances are it's got a sign advertising free WiFi. Outside, though, you won't get very far before you lose the cafe's connection. That's when your smartphone knows it's time to pick up a data signal from the nearest cell tower, so that you can surf the Web and watch YouTube wherever you go next.

Cellular data is one of the great innovations of the 21st century. Now it's about to take its next big leap, one its supporters say will improve service on carriers such as Verizon and T-Mobile. Its critics, however, say it could undermine WiFi, a free, time-tested technology that handles roughly half of the Internet's mobile traffic today.

It's called LTE-U, and it's being developed by Verizon, the chipmaker Qualcomm and a number of other telecom companies. The innovation represents the next evolution in mobile data — and a looming flashpoint for two gigantic industries.

If you use any of the four major national cell carriers, you're probably familiar with 4G LTE, the current cutting edge of mobile data technology. Under ideal conditions, it provides download speeds that rival what you can get on a wired connection — fast enough to download a song in less than a minute.

LTE-U is virtually identical to LTE, but with one key difference: It runs on the same frequencies that WiFi does. Unlike regular LTE, which piggybacks on airwaves owned exclusively by your carrier, LTE-U travels on public airwaves that are free to anyone. Garage door openers, cordless phones, WiFi routers — all also transmit over these open channels.

Industry engineers promise that LTE-U will deliver faster speeds and downloads, and that it will make the most difference in crowded areas, such as cities and universities, where networks tend to get congested.

Under LTE-U, your device will likely report being on "LTE" just as it always has. But its introduction reflects an unprecedented move by wireless carriers onto open airwaves — known in the industry as unlicensed spectrum — and that's going to have important repercussions.

To cellular providers, WiFi represents a huge missed opportunity. Internet consumption on cellular data networks — your 3G or 4G connection - could've grown by a whopping 84 percent last year, according to Cisco. But because consumers shunted so much traffic to WiFi, that figure was much lower, at 69 percent.

Carriers could charge you for all that extra access to the mobile data network. Instead they're losing out when you hop onto WiFi at your home or office. And LTE-U is the industry's solution.

The cable industry, on the other hand, wants to keep you on WiFi as much as possible.

This is the math they fear: By 2019, Americans are expected to consume nearly 10 times more mobile data than they did in 2014. By then, 77 percent of all Internet traffic will be sent and received over mobile devices rather than stationary PCs. That's not good for cable, an industry that built its reputation on running fast (but fixed) Internet service into people's homes and businesses.

"Wireless is king," said Robert Pepper, Cisco's vice president of global technology policy.

To keep up, cable companies will need to offer a solution that lets customers use their services outside the home.

The workaround: Instead of keeping subscribers chained to their own cable boxes, companies are letting them take advantage of other people's routers, too. This means that if you're a Comcast customer in Philadelphia, you can log on to any eligible Xfinity hotspot in, say, Washington. Thanks to a roaming agreement for cable-powered WiFi hotspots, Comcast customers can even use routers belonging to Time Warner Cable in New York.

In 2012, the cable industry's WiFi consortium had 100,000 public, out-of-home WiFi hotspots to its name. Today, it's more like 400,000, creating a rudimentary if patchy equivalent of a cellular network.

Firms such as Cablevision have experimented with providing a kind of cellular service over WiFi in recent months. Non-cable companies such as Google and Republic Wireless also believe that WiFi can cheaply support voice calls with less reliance on the traditional cellular network run by companies such as Verizon and AT&T.

Both industries need each other. Cellular providers rely on the WiFi network to ease congestion on their proprietary networks, and the cable industry probably won't be able to provide a compelling cellular experience without partnering with a cell provider in areas lacking coverage.

But there's no denying that as each becomes more like the other, the cable and cellular industries increasingly compete for customers.

Can LTE-U coexist with WiFi?

Tension stems from something else, too: A fundamental difference in technical design that could strangle WiFi, giving cellular providers the upper hand over cable companies, according to LTE-U's critics.

Because it has to share the airwaves with so many other wireless devices such as Bluetooth headsets and wireless mice, WiFi antennas follow a "politeness" protocol that controls when they transmit and receive data. When your smartphone is on WiFi and it senses other devices communicating over the same frequencies, it backs off until the channel is clear. It's a lot like the automotive principle of yielding to pedestrians in a crosswalk.

That's not the case with today's LTE technologies, in part because LTE was designed with proprietary networks in mind. So if left unchecked it could prevent WiFi devices from uploading or downloading content from the Internet — the equivalent of running through the crosswalk, the next five stoplights and over whoever might be in the way.

Interference between the two technologies can slash WiFi transmission rates by 75 percent, according to a Google white paper filed last month to the federal government. The cable industry's top trade group, the National Cable and Telecommunications Association, argued the technology could be "disastrous" without further protections and "will severely degrade consumers' Wi-Fi experience, rendering unusable many services that are widespread today, to say nothing of the innovative new uses currently on the horizon."

Whether it's voice calls, video conferencing or online games, services that rely on a steady WiFi connection could be slowed or interrupted by LTE-U. It could affect schools, libraries, homeowners and small businesses.

LTE-U's designers say they've implemented safeguards to ensure that it co-exists peacefully with WiFi. For instance, LTE-U turns on only when the cellular network is full and needs extra capacity. Even then, it can only help a device download data — it doesn't have the ability to upload it.

"We're very excited about LTE-U," said Dean Brenner, Qualcomm's senior vice president of government affairs. "It's been designed from the ground up to be a very good neighbor to WiFi, and in many cases, actually improves WiFi throughput."

A device running LTE-U will first look for the least occupied WiFi channel to minimize the chances of interrupting WiFi. And then it calculates, based on the number of other devices sharing the channel, what proportion of the time it's allowed to be active on it. If there are three WiFi-based tablets and an LTE-U smartphone on the same channel, the smartphone will adjust itself to talk only a quarter of the time.

Even this may not help much when WiFi is experiencing heavy use, said Clint Brown, a director of wireless initiatives at Broadcom and the vice chair for the WiFi Alliance.

"It may look for a clear channel," Brown said. "But once it's on a clear channel, odds are the way it provides fairness may be about how much time you use the channel rather than whether there's traffic."

Telecom industry officials say their efforts at designing a sharing mechanism that works is evidence of their goodwill toward WiFi. After all, cellular providers rely on it, too.

One official for a large telecom company said that under the current rules, his firm could roll out the existing LTE on unlicensed airwaves today, but the resulting interference would make it a horrible experience — hence all of the work to develop LTE-U, which will be more polite.

This fight is already settled — overseas, anyway

In places like Europe and Japan, strict rules on mobile data require carriers' technology to listen before talking, just like WiFi does by default. To meet that standard, a version of LTE-U called LAA, or License-Assisted Access, is being designed. An added benefit is that LAA is capable of uploads and downloads, unlike LTE-U.

So why doesn't the U.S. cellular industry adopt that standard? The companies say they don't want to wait around for LAA to be finalized when the demand for cellular data is growing so quickly.