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Want to dial 911 from a mobile phone? Better go outside. Chances are an operator cannot automatically find the location of an emergency call made indoors.

The Federal Communications Commission wants to fix that, proposing rules on Thursday that would require wireless phone companies to transmit specific information for 911 calls, down to the longitude and latitude on a floor in a multistory building, within five years.

It all sounds reasonable, especially because 70 percent of all calls to 911 are now made from mobile phones. And wireless companies say they want to comply. The trouble is, they don’t have the technology to do so.

That raises questions about whether the F.C.C.’s proposed deadlines can be met. AT&T, for one, says probably not. “The commission has tentatively proposed unrealistic targets for location accuracy indoors,” the company said in a statement. A report on the most recent tests by the F.C.C.’s Communications Security, Reliability and Interoperability Council “shows that no vendor currently has proven technology that can meet the proposed standards,” AT&T said.

But Tom Wheeler, the F.C.C. chairman, wants to set the bar high. “We’re setting off on a path to improve public safety and to save human lives,” he said during the F.C.C.’s meeting on Thursday. “It’s never wrong to overreach on those goals.”

The technology isn’t the only challenge to meeting the deadline, said Michael O’Rielly, an F.C.C. commissioner, who voted to move ahead with the proposed rules but not with the mandated deadlines. “Vendors will have to test their technology and go through the standards setting process,” Mr. O’Rielly explained. “Location systems will have to be built. Hardware will have to be added to handsets. New handsets will have to be introduced to consumers and achieve sufficient market penetration. This all takes time.”

Take the location finders. While most smartphones already have them, they base their information on where a user is in relation to a known Wi-Fi hot spot. Hot spot databases are private, and public safety officials cannot rely on them for emergency information without rigorous testing of their reliability.

Commissioner Ajit Pai, who also voted against the timeline, noted that the last round of F.C.C. requirements for 911 calling data was established in 1996. But it will not be fully carried out until 2019. “I’m doubtful that this deployment can be completed in two to three years,” Mr. Pai said.

Accuracy is an issue, too.

The newly proposed rules would require location data to be correct in 80 percent of the cases within five years. In the most recent round of testing, the single technology tested found the correct floor in urban, multistory buildings in only about 67 percent of cases.

When tests were run to see whether the devices could locate a wireless caller within a specific building in an urban area, the devices were off target by 200 to 750 feet. In most cities, that is a distance that would encompass several buildings — meaning that emergency medical workers might go to a wrong address multiple times.

“We strongly encourage the F.C.C. to consider location accuracy requirements that are grounded in verified data, not aspirational target setting,” Scott Bergmann, vice president of regulatory affairs for CTIA-The Wireless Association, a trade group for mobile phone companies.

Mr. Wheeler says the commission must forge ahead. “Our job,” he said, “is to ensure that as network providers and their customers upgrade to new technologies, there is no downgrade in reliability, availability, or public safety.”