Airlines have voiced concern that the rollout of 5G mobile technology in the United States (US) could cause potentially dangerous interference with aircraft systems.
Verizon and AT&T were activating their networks on Wednesday but agreed to delay their rollout near airports.
Here are some key questions about the issue:
Why Are Airlines Afraid?
The C-Band frequencies used for 5G networks are close to those used by the altimeters of airplanes.
US telecom operators were allocated frequencies between 3.7 and 3.98 gigahertz for their 5G networks – a right that cost the companies tens of billions of dollars.
But the aviation sector is concerned that the frequencies used by the mobile companies are too close for comfort to those used by aircraft's radio altimeters, which operate between 4.2 and 4.4 GHz.
Altimeters measure the distance between an aircraft and the ground, a vital instrument when landing at night or during bad weather.
"The concern is that the smaller gap between the upper 5G frequency in the US and these frequencies is small enough to potentially cause interference as aircraft are on approach to land," Nigel Linge, telecommunications professor at the University of Salford in the UK, told AFP.
What Have US Authorities Decided?
Aircraft makers Airbus and Boeing voiced concerns about the potential risks of interferences to the US transportation secretary in December.
The 5G rollout was then delayed until 19 January following a request from the Federal Aviation Administration (FAA) to ensure the system is safe.
"If there's the possibility of a risk to the flying public, we are obligated to restrict the relevant flight activity until we can prove it is safe," the FAA says on its website.
The FAA approved two models of radio altimeters and gave the greenlight to 48 out of 88 US airports most directly linked to 5G risks.
Other Countries At Risk?
The concerns are confined to the US.
"It's not a global or European issue. It's really an issue specific to the use of 5G and its rollout in the United States in terms of frequency bands and power," Airbus chief Guillaume Faury said earlier this month.
In Europe, 5G networks were given C-Band frequencies ranging between 3.4 and 3.8 GHz, thus providing more distance to those used by aircraft.
"Europe has been transmitting 5G on their frequencies for many months and no reports of problems have been made in respect of airlines," Linge said.
South Korea, a leader in the new-generation mobile technology, gave 5G networks a ceiling of 3.7 GHz.
But the US telecom industry's lobby group, the Cellular Telecommunications Industry Association (CTIA), has pointed to another example: Japan, where 5G networks have been given a frequency spectrum as high as 4.1 GHz.
"There have been no claims of interference" in Japan, the CTIA has said.
What Measures To Mitigate Risks?
In France, buffer zones were created around 17 large airports, restricting the strength of 5G signals and ensuring mobile base station antennas do not broadcast upwards, according to French authorities.
The areas extend several kilometres from landing trips.
The FAA says the mitigation measures in the US are partly similar to those in France, with "significant differences".
The buffer zones for US airports only protect the last 20 seconds of flight, compared to 96 seconds in France, according to the FAA.
The temporary lower power levels in the US are 2.5 times higher than in France. And in the US, mobile base station antennas are not required to be tilted downward, the FAA says.