Root Wireless, a Bellevue startup making online maps that show how the four major wireless networks perform in major cities, has raised more than $3 million in new funding, according to a filing with the Securities and Exchange Commission.

Root Wireless spokesman Mike Brennan confirmed the new funding but declined to specify the amount, saying it would be announced Monday. He described it as an oversubscribed Series B round.

The company makes color-coded online maps that show how AT&T, Sprint Nextel, T-Mobile and Verizon networks perform down to the granular street level. It's meant to give consumers a way to compare the strength and reliability of network plans where they live and work. That could be useful information, at a time of competing claims about wireless coverage and high-profile outages.

Root Wireless, which came out of stealth mode last month, has been testing the service in New York, Los Angeles, the San Francisco Bay Area, Seattle, Chicago, Washington D.C., Dallas and Orange County.

My colleague John Cook profiled Root Wireless last month when the startup came out of stealth mode and announced a media partnership with CNET. The company aims to "crowdsource" network testing with a free application that runs in the background on GPS-enabled smartphones, collecting network data.

Root Wireless said last month it was planning to release a public beta app for Windows Mobile, RIM and Android phones, and was working on an iPhone app as well.

The startup was founded by former Bsquare executive Paul Griff. It raised $1 million earlier this year from angel investors including former T-Mobile exec John Stanton, former McCaw Cellular exec Scott Anderson and former Telephia CEO Jack Roberts.