The four largest U.S. cellphone carriers are deploying portable towers to step up coverage for the massive crowds expected to gather during Monday's solar eclipse.

Verizon, AT&T, Sprint and T-Mobile will be sending mobile cellphone units to towns, many of them in rural areas, where they expect networks to be at greatest risk of strain.

"We used NASA information to identify the places where the full eclipse would be the longest, and then used that information to look at cities that were planning events," said Scott Mair, AT&T senior vice president of technology, planning and engineering.

The three other carriers are taking similar approaches, with some slight differences among them.

Verizon will not be deploying as many temporary units, in part because recent "advancements in our network and a focus on building permanent solutions have made the deployment of temporary network assets such as COWS and COLTS to supplement coverage and capacity at events, concerts and festivals largely unnecessary," a Verizon representative said in an email to CNBC. COWs refers to Cell on Wheels units and COLTs refer to Cell on Light Truck units.

The company will send two to separate locations in Oregon (one to Solarfest in Madras), and a third to Hopkinsville, Kentucky, a small town expecting a big influx of tourists, due to the fact that the eclipse will be at its longest duration of 3 minutes.

Sprint will add capacity to its existing towers, in addition to deploying temporary cell towers. The company also said it is adding its high-band spectrum to locations, which Sprint said is "good at moving large amounts of data traffic and supporting large crowds."

The company also said customers in remote locations can improve service by managing data usage, disabling app updates on their devices temporarily and using text messaging when in a crowded or congested location.

T-Mobile told CNBC the company is taking a number of measures, but said it will be either adding capacity or dispatching temporary towers to certain "key markets" listed below.

AT&T's own COWs and COLTs will can increase network capacity by 150 to 300 percent. Mair said the carrier uses them to boost coverage at one event or another just about every weekend. The company sends units to fairs and festivals, large concerts and events such as the Coachella music festival or South by Southwest and the PGA championship golf tournament. The company even deploys them during natural disasters.

In each case, Mair said his team looks at the number of people at the event and then uses forecasting models to estimate the amount of coverage that will be needed, Mair said.

The eclipse presents a twist in that it will be a single event taking place across several geographic locations across the country on the same day, he said.

Mair also said the company has invested deeply in stepping up infrastructure in recent years because video is an ever-larger slice of the total amount of wireless capacity used.

Video traffic on AT&T's wireless network rose 75 percent over 2016 and now accounts for just over half of the wireless network's traffic. AT&T expects video usage to be particularly high around the time of the eclipse, as people attempt to capture the moment.

With the augmentations, users should not have any connection problems, he said. If they do, they will likely be momentary. In the event of an emergency, text messaging is a reliable form of communication, he added.

Here is a full list of the towns all four carriers will be boosting coverage either through network upgrades or through adding mobile towers: