The majority of patent lawsuits today are brought by "patent trolls" that do nothing but sue—but suits between actual competitors do still happen.

Case in point: AT&T has sued Cox Communications, saying that Cox has infringed seven AT&T patents covering everything from DVRs to methods for hiding "packet loss or frame erasure" over a network.

In its complaint (PDF), AT&T claims it "provided a detailed explanation" of how Cox's products infringe its patents during meetings that took place in 2009 and 2010.

"Despite years of protracted negotiations, Cox has sought to avoid payment for its infringement by repeatedly delaying and rescheduling negotiations," wrote AT&T lawyers in the complaint, filed in federal court in Delaware. "Given every opportunity, Cox has failed to provide substantial arguments for either non-infringement or invalidity of AT&T's patents. Cox’s conduct constitutes a steadfast refusal to take a license, even though Cox generates billions of dollars in revenue every year through its use of AT&T’s technologies."

Sued over other companies’ hardware

AT&T claims that Cox DVRs and set-top boxes, including the Cisco-made Explorer 8000 and 8300 lines, infringe two AT&T patents: No. 5,809,492, which covers the DVR's ability to "create, organize, identify, and resolve potentially conflicting program recording requests," as well as No. 6,118,976, which "allow[s] a single device to select between and tune both analog and digital content streams."

Cox's digital telephone system is said to infringe AT&T's patent No. 6,487,200. AT&T patents numbered 6,952,668, 7,233,897, and 7,908,140 also get lobbed at Cox's "packet telephone system network components," including all of Cox's "eMTAs [cable modems that can interface with home phones] and other network components implementing... packet loss concealment or an equivalent, including at least [modems such as] the Motorola 5220 and Arris TM502G, TM202A, and TM402A models."

The "packet loss" patents are related to an AT&T licensing program that has published rates, ranging from six cents to 20 cents per device for large-scale providers.

Another patent covers transmitting data using "multiple RF channels" and is said to be infringed by Cox cable modems implementing the DOCSIS 3.0 standard.

Finally, Patent No. 7,907,714 discloses a system for allowing users to maintain subscriber profiles. "This allows subscribers to obtain a higher level of service while simultaneously lowering cost to the service provider by reducing the need for customer service representatives," says the patent.

An AT&T spokesperson told Ars, "After trying for more than five years to reach a reasonable license agreement with Cox for its use of our patented technology, we had no alternative but to seek assistance from the court."

Representatives from Cox did not immediately respond to requests for comment.

Tough target

AT&T is more often on the defense side of IP disputes, at least in public. But it has enforced its patents before. In 2001, AT&T sued Microsoft in a patent suit that went to the Supreme Court, which found Microsoft not liable for infringement.

In December 2005, two months after Apple introduced new iPods that included video capabilities, AT&T started sending out licensing letters saying it owned patents underlying the MPEG-4 digital video standard. According to a report in PC World, the targets included Apple, CyberLink, DivX, InterVideo, and Sonic Solutions, as well as retailers.

The new lawsuit, which was filed by two AT&T intellectual property holding companies, isn't seeking an injunction that would shut down any Cox services. Instead, it asks for a "compulsory ongoing royalty" for future infringement.

It could be a bruising dispute. It's impossible at this stage to know what kind of numbers are being talked about behind the scenes, but Cox has a recent history of being willing to see through patent fights. In 2008, Verizon used its VoIP patents against Cox, fresh off a punishing $117M win against Vonage. A Virginia jury rebuffed Verizon's $404 million case and invalidated two of its patents.