Your Internet access probably isn't bad, but it may not be consistent either. And in a few ugly cases, it could be much less than what you pay for. That's a compressed download of a wide-reaching study by the Federal Communications Commission of wired broadband Internet service in the United States.

The FCC's 36-page report (PDF) released Tuesday goes into more detail and covers more ground than  earlier, more subjective studies, like the reader surveys published by Consumer Reports in May (subscription required) and PC Magazine in June. The commission had some 9,000 volunteers install special measuring boxes on their digital subscriber line, cable or fiber-optic connections, then set up similar test units at the 13 Internet providers they used. The FCC plans to conduct a similar study of wireless-broadband services.

Not all of these companies (AT&T, Cablevision, CenturyLink, Charter, Comcast, Cox, Frontier, Mediacom, Insight, Qwest, TimeWarner, Verizon, and Windstream) should brag about the FCC's findings.

The commission's research, conducted by the U.K.-based firm SamKnows throughout March, found that most broadband services offered downloads within 20 percent of advertised speeds, but some slowed further during peak periods. Uploads were better: Nearly all providers "reached 90 percent or above of their advertised rate, even during peak periods."

But your ISP's technology could skew things further. The survey reported that phone line-based DSL fell short of marketed speeds by the biggest margin, cable averaged about even and fiber (in most markets, Verizon's FiOS) beat advertised speeds by about 10 percent.

The "burst" downloading offered by cable providers, however, can even the score against fiber. The survey clocked Comcast as briefly accelerating downloads to 152 percent of advertised speeds (although other cable ISPs didn't offer the same boost).

Then again, for Web use there is apparently such a thing as too much speed. The report concludes that connections faster than 10 million bits per second (Mbps) don't deliver "a significant performance increase" in everyday text-and-images browsing.

Things look fairly even on another crucial score — "latency," the lag time it takes for your data to get through your provider and onto the open Internet. Although gamers and other intensive users will want to stick to cable or fiber, the survey didn't find any broadband service incapable of adequately supporting Internet calling and other common interactive applications.

But if you look at Internet access like a traffic reporter would — how congested do things get at busy times of the day — the biggest gap opens up. While Verizon's Fios and Comcast's cable service offered almost the same speed from midnight to midnight, cable services Cox, Insight and Mediacom slowed notably during the evening. Cablevision did worst of all, slowing to only 50 percent of its touted speed around 8 p.m.

To understand why that could be a problem, consider an aspect of Internet access in America unmentioned in this survey: the lack of competition. As the FCC documented in a separate report last year, 78 percent of housing units are in areas with only two wired providers and 13 percent have only one. If you can't fire your broadband service or if the only alternative is slower DSL, your leverage as a customer sinks: They're the shark, you're the plankton.

Credit: Chart images from broadband.gov.