In court papers filed earlier this week, Microsoft denied infringing Lucent's patent and argued that it is invalid anyway, in part because Lucent failed to disclose "prior art" when it made its application to the U.S. Patent and Trademark Office. Prior art refers to existing inventions in the field where a patent is being sought. Microsoft also accused Lucent of infringing several Microsoft patents.

Lucent filed its suit against Microsoft at the end of March, arguing that technology used in the Xbox 360 for decoding MPEG-2 video files infringes on one of its patents. It requested a trial and monetary damages but stopped short of seeking an injunction to prevent Microsoft from selling the console. The patent in question is Number 5,227,878, "Adaptive Coding and Decoding of Frames and Fields of Video," and can be viewed by searching at the USPTO Web site. The suits were filed in the U.S. District Court, Southern District of California.

The dispute dates back to 2003, when Lucent filed other lawsuits against Dell and Gateway over the same patent. Microsoft stepped in because of an indemnity agreement it had with the computer makers, suing Lucent in an effort to gain a judgment of noninfringement of the patent. Microsoft was successful – a judge granted it summary judgment in the case last year on the grounds that Lucent's patent contained a typographical error. Lucent had the patent corrected by the USPTO, however, and filed its new lawsuit against Microsoft this year.

In a statement this week, Microsoft Deputy General Counsel Tom Burt said the company stands by its intellectual property and its partners. He accused Lucent of choosing to litigate rather than engaging in "meaningful license negotiations." Lucent representatives couldn't immediately be reached for comment.

If the case comes to trial, some legal experts see Lucent seeking an injunction to block Microsoft's sale of the Xbox 360. Others say that because the case has a complicated history, such a measure would probably be years away.