U.S. cable and fiber Internet providers generally deliver the download speeds they advertise, though not consistently, while DSL connections increasingly fail to meet promised speeds, the Federal Communications Commission said on Wednesday.
The agency has been testing download and upload speeds of top Internet service providers (ISPs) yearly since 2011, but this year for the first time also assessed how likely consumers were to get speeds as advertised.
The report found Cablevision and Verizon Fiber to be two providers to deliver faster-than-promised download speeds to 80 percent of consumers 80 percent of the time, with Comcast and Frontier Fiber delivering just short of promised speeds. (Disclosure: Comcast is the owner of NBCUniversal, the parent company of CNBC and CNBC.com.)
Windstream, Verizon DSL and Frontier DSL delivered download speeds less than 60 percent of what they promised to 80 percent of consumers 80 percent of the time. AT&T and Cox performed at about 80 percent speed consistency.
The results of cable and fiber ISPs improved on average from 2013, when the FCC found they largely delivered on the promised speeds, with Quest/CenturyLink showing a 16 percent improvement.
DSL connectivity has begun lagging behind, with Verizon DSL showing the only result that was worse than last year.
FCC senior officials said they could not determine whether that was because DSL was lagging in capacity or investments in upgrades or because DSL providers overpromised.
The FCC planned to write to the CEOs of laggard companies, demanding explanations and improvements, FCC officials said, though they did not name specific companies.
Broadly, nearly all Verizon Fiber, Cablevision and ViaSat/Exede customers received faster-than-advertised speeds, while Comcast, Frontier Fiber, Time Warner Cable and Cox delivered more than 90 percent of the speed to almost all their customers.
Conducting the speed tests, the FCC also discovered serious web traffic congestion at certain points where networks interconnect. The agency is working on tools to measure and analyze how such congestion may affect consumers but planned to make the raw data public on Wednesday, FCC officials said.
Consumers have complained to the FCC about slower download speeds with popular video streaming by Netflix.
The FCC last week said it will review closely held agreements between Netflix, Comcast, Verizon and other content and Internet companies to see whether they play a role. Netflix and ISPs have accused each other of causing a slowdown in Internet speeds by the way they route traffic.
The regulators are also collecting comments until Sept. 10 on new proposed rules that would regulate how ISPs manage the traffic crossing their networks, though the FCC has in the past only regulated "net neutrality" on the part of the network that goes from the Internet service providers to the consumer, and has not delved into what happens before that.