For the second time, a device submitted by Microsoft for U.S. Federal Communications Commission testing of wireless "white spaces" technology has stopped working and been taken out of the process.

The wireless prototype, which was not made by Microsoft, unexpectedly stopped working on Wednesday, Microsoft said Friday. In February, the FCC took another Microsoft-submitted device out of testing because it had power problems.

The White Spaces Coalition, which includes Google, Philips and Dell as well as Microsoft, has asked the FCC to let wireless devices use vacant frequencies, referred to as white spaces, in the spectrum band allocated to television. They say this would give consumers more wireless broadband options. Opponents, including the US National Association of Broadcasters (NAB), say such devices would interfere with TV broadcasts. Microsoft and others voluntarily submitted prototype devices for testing, a step that usually isn't included in this type of FCC process, said Microsoft spokeswoman Ginny Terzano.

The NAB used Wednesday's unexpected shutdown to attack the white-spaces backers.

"In baseball, it's three strikes and you're out. How many strikes does Microsoft get? If they can't get the device to work in the lab, how are they going to get it to work in the real world?" NAB Executive Vice President Dennis Wharton said in a statement sent to reporters.

Microsoft said the problems that took the two devices out of testing were unrelated to interference and that the units were experimental, not production devices. The FCC gathered valuable information by testing them, and there are three other devices submitted by other parties that can still be used, Terzano said. If Microsoft had tried to do anything to the two devices to get them working again, they probably would have been considered new devices and the FCC would have had to redo its tests on them, she said.

"Considering where we are in the lab testing process ... we think that it's more prudent to continue with the other non-Microsoft devices that are currently going through testing," Terzano said.

Based on observations from Microsoft engineers observing the tests, "the data the FCC has gotten so far has found that there is no interference," she said.