AT&T is nearly exclusively focused on LTE Cat M for battery-powered cellular Internet of Things (IoT) applications now and in the foreseeable future, while the other three major carriers are pursuing a dual path of Cat M deployment first, with NB-IoT expected to follow in 2018.

So while no major US carrier is ruling out the NB-IoT version of 4G LTE, some are keener on it than others. That's the conclusion I drew from talking to people at the recent Mobile World Congress Americas show in San Francisco.

Verizon has deployed LTE Cat M now and plans to go ahead with narrowband IoT (NB-IoT) LTE trials in 2018.

That's what Tom Camp, who is in business development at the Verizon Innovation program, told me when I ran into him on the show floor. "It's just another variant, right?" he said of NB-IoT. (See What's the IoT Plan, Verizon?)

T-Mobile US Inc. , meanwhile, says it will light up an NB-IoT network nationwide in 2018 -- as well as support Cat M -- with the technology first arriving in Las Vegas next month.

Sprint Corp. (NYSE: S) announced in May, that it plans to begin deploying LTE Cat M in mid-2018, followed by NB-IoT.

AT&T Inc. (NYSE: T), though, is not so big on NB-IoT as a technology for machine communications. AT&T has deployed its Cat M nationwide in the US already, and is looking at Mexico for 2018.

David Allen, director of IoT advanced product development at AT&T, says that both Cat M and NB-IoT have similar battery-life profiles, while Cat M also supports voice communications, unlike NB-IoT. This means Cat M can be used in applications as diverse as smartwatches and home security systems.

"When there's a benefit of NB-IoT that can't be replicated by Cat M, then we'll be interested," he said. Neither of the AT&T executives I spoke to see a compelling reason to deploy NB-IoT yet. (See AT&T Pushes Cat M LTE for Cellular IoT in the Americas.)

Why is the interest in these two IoT technologies coming squarely into focus now for US carriers? Well, firstly, there is an opportunity for operators to open up new revenue streams with smart sensors, smart soda dispensers -- whatever device you can preface with the word "smart," really.

These new LTE technologies also allow sensors and asset trackers on the LTE network to have a battery life counted in years if they are not "chatty" devices. AT&T's Allen said that, in contrast, similar devices on 2G and 3G networks can run for "five to seven days... maybe two weeks" without a recharge.

US operators, in the meantime, either are starting to -- or want to -- switch off their 2G networks and start to re-use 3G spectrum. So you can see why having an LTE option in place is important.

Cat M also has a signal range seven times that of standard LTE. This is why AT&T and Verizon have been able to deploy nationwide so fast.

Last but not least is the silicon pricing. Cat M modules are now around $7.50 a pop, and AT&T's Allen expects them to go to $5. Pricing on NB-IoT modules will likely start higher as they become commercially available.

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— Dan Jones, Mobile Editor, Light Reading