Trying to download the new Frank Ocean album onto your iPhone before you have to board your flight and turn on the dreaded airplane mode? Better hope you’re not in Los Angeles, Miami, or Austin, Texas.

Those three airports have the worst mobile connectivity among the 50 busiest U.S. airports, according to testing by Rootmetrics.

LA’s Los Angeles International Airport, known by its moniker LAX, was deemed worst of all with slow phone download speeds and iffy reliability. While data download speeds hit more than 40 megabits per second at top-ranked airports, none of the big four wireless carriers—Verizon, AT&T, T-Mobile or Sprint—reached even three megabits per second at LAX.

“Despite being the second-busiest airport in the country, mobile network performance at Los Angeles International has often struggled,” Rootmetrics noted in its report. “These findings were almost identical to what we found in the second half of 2015, suggesting carriers are still facing challenges in attempts at delivering good coverage at LAX.”

Downloading a typical 45-minute, high-definition TV episode onto a smartphone would take almost half an hour at LAX, compared with two minutes at Atlanta’s Hartsfield-Jackson Atlanta International Airport, which ranked sixth overall and is the busiest in the country.

At the top of the charts, phones at Metropolitan Oakland International recorded download speeds as high as 91 megabits per second for a Verizon vz phone. Download speeds averaged almost 63 Mbps across all four carriers. General Mitchell International in Milwaukee, Wis., ranked second overall and Southwest Florida International in Fort Myers ranked third. But all three of those airports serve a relatively paltry number of passengers.

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The busiest airports in the country after LAX and Atlanta are Chicago O’Hare International, Dallas-Fort Worth International, and John F. Kennedy International in New York City. Among those three, O’Hare ranked 14th in the Rootmetrics tests, Dallas ranked 25th, and JFK ranked 29th. Rootmetrics also posted the entire top 50 list.

Among the carriers, AT&T t and Verizon were rated “excellent” for getting connected at all 50 airports. T-Mobile tmus achieved that rating at 42 airports, and Sprint s at 40.

Rootmetrics said it visited each of the top 50 airports twice a year. It tested download speeds on all four major wireless networks and at locations around the facilities including ticketing, baggage claim, and within terminals.