Verizon (VZ), AT&T (T), Sprint (S) and T-Mobile (TMUS), the four largest wireless companies, scrapped unlimited data plans a few years ago because they couldn’t handle the demand. But now they’re bringing back the all-you-can-use offerings after consumers clamored for them. And that has some telecom observers wondering if the companies are going to repeat their earlier problems.

According to a new report from Cisco (CSCO), mobile data traffic reached 7.2 exabytes (equal to 1 billion megabytes) in 2016, a 62 percent increase from the previous year. In North America, traffic grew 44 percent. Fourth-generation (4G) traffic accounted for 69 percent of mobile usage, even though it accounted for just 26 percent of mobile connections.

Cisco now forecasts that “The average smartphone will generate 6.8 GB of traffic per month by 2021, a fourfold increase over the 2016 average of 1.6 GB per month.”

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On a global basis, Cisco expects mobile data traffic to increase sevenfold between 2016 and 2021, reaching 49 exabytes per month by 2021. At that time, the company forecasts that 11.6 billion mobile connected devices will be connected, which will easily exceed the world’s projected population at that time (7.8 billion).

“There are concerns about that,” said Colin Gibbs, editor of the trade website FierceWireless.com. “Not only do we see the reemergence of the unlimited plans but video consumption is going way up. And as we have seen in the past few days, Verizon and T-Mobile are both packaging HD video in their unlimited data plans, and that requires a whole lot of data.”

Carriers can accommodate the growth thanks to Wi-Fi networks deployed in urban areas and hotspots, where carriers currently offload about 64 percent of all North American data. Because of the data-usage costs involved, many users make sure they stream video on Wi-FI because it’s cheaper and not subject to caps. In fact, the average Wi-Fi connection at current standards is speedier than 4G technology, according to Thomas Barnett, a senior analyst at Cisco.

“Traffic volumes are always a concern for global carriers,” he said. “We have been a little bit spoiled with mobile networks where we expect mobile networks to give us the same quality of experience and reliability as fixed networks. For the most part, they do a very good job.”

About 70 percent of U.S. mobile traffic comes from video content, which presents a challenge for carriers. And an increasing amount of that video is HD,which takes up even more space on the network. However, 4K, or ultra-high-definition video, uses more than three times the bandwidth as HD. Now, wireless companies are developing 5G technology, which will generate about 4.7 times more traffic per month than 4G and 10 times the level of 3G, according to Barrett.

Unlimited data plans were first offered about a decade ago when the Apple (AAPL) iPhone and Android smartphones first entered the market. They were immediate hits with consumers as mobile app use skyrocketed, according to Jeff Kagan, an independent telecom analyst.

One reason the networks experienced technical problems was that they were using 3G technology designed for voice traffic to move data, said Gunther Ottendorger, Sprint’s chief operating officer for technology.

Verizon, the largest wireless company, yesterday announced an unlimited data plan for $80 a month with paperless billing and AutoPay. Customers can get up to four additional lines for their devices and tablets for $45 a month. That’s a big change from not even a month ago, when Verizon Chief Financial Officer Matt Ellis said the company didn’t feel it needed to offer unlimited data, resisting pressure from consumers to do so for years.

The New York-based company’s earnings lagged Wall Street’s expectations as consumers dropped the service for rivals with unlimited data plans. Fourth-quarter 2016 profit fell more than 16 percent to $4.5 billion, or $1.10 per share. Revenue dropped 5.6 percent to $32.4 billion.

T-Mobile responded to Verizon with a new deal for its T-Mobile ONE service, offering new and existing customers two lines for $100 per month, including monthly taxes and fees. Sprint has a limited-time offer for as many as five phone lines for $90 a month. AT&T charges $100 a month but only for customers who also subscribe to its DirecTV satellite service.

As the wireless market becomes saturated, carriers need to steal customers from one another as the only way to fuel their growth, which is why they’re also expanding into the content business. Verizon has acquired AOL and plans to purchase Yahoo. And AT&T is awaiting regulatory approval for its planned $80 billion acquisition of Time Warner (TWX).

With so much content at their command, the carriers will have little choice but to ensure their wireless networks can handle it.