AOL Time Warner announced an agreement Tuesday with IBM's Lotus division to test ways for their instant messaging services to communicate with each other. The test signals the first time that America Online, a unit of AOL Time Warner, has attempted to open its IM network to communicate with an outside company. AOL has come under considerable fire by competitors such as Microsoft and legislators for refusing to allow others to communicate with its popular services. AOL owns the two most popular IM services: AOL Instant Messenger (AIM) and ICQ. The trial will be conducted between AIM and Lotus' Sametime. The companies will test how the services communicate between their servers through SIMPLE (Session Initiation Protocol for Instant Messaging and Presence Leveraging). SIMPLE is one of three IM protocol standards being considered by the Internet Engineering Task Force, a standards body. "Lotus' market-leading expertise in real-time collaboration and messaging software will be of great benefit as we examine how we might enable interoperability between IM systems using this new protocol," Donn Davis, AOL's president of interactive properties, said in a statement. Tuesday's announcement comes less than a month after AOL told regulators in a filing that it would begin testing IM interoperability with an undisclosed technology partner. The protocol that AOL has adopted for its test with Lotus is based on SIP (Session Initiation Protocol), which is co-authored by Microsoft. For Microsoft, instant messaging has become a central element in its attempt to remain nimble in the future. The company is planning to launch its controversial Windows XP operating system in October, which comes bundled with an advanced IM service. Called Windows Messenger, the product lets XP users exchange and collaborate on documents or use voice and video to conference. Windows Messenger will be based on SIP. Microsoft and AOL have sparred many times in the past over instant messaging, and Tuesday was no different. Microsoft came out swinging on Tuesday, condemning the test as bad for consumers. "This doesn't give consumers any choices and any opportunities for interoperability," said Bob Visse, Microsoft Network group product manager. "It's not surprising that they wouldn't interoperate with any competitor, and I don't anticipate they'll show any willingness to do that going forward." Visse added it would be easy for AOL to evolve SIMPLE away from SIP, making it more difficult to interoperate. However, Visse acknowledged the common protocol is endorsed by the standards body and both companies. When asked whether AOL plans to offer interoperability with Microsoft, AOL spokeswoman Kathy McKiernan raised concerns about the "anti-competitive elements" of XP, such as the embedding of "the messaging product into their monopoly operating system." "These issues would have to be looked at seriously before we can consider anything in this (interoperability) area," McKiernan said. AOL and IBM have been partners since 1998, under a deal whereby AOL has licensed the ability for Sametime users to communicate with AIM users. The agreement lets Sametime users manage their buddy lists on the same service, but people are required to register two separate screen names. Because the services run on separate protocols, the two are technically not interoperable, according to AOL and Lotus. AIM and Sametime also differ in the markets they target. AIM is focused on general Internet consumers, while Sametime is geared for use in businesses. AOL has also begun testing the waters for corporate IM products in its development of Project RAC (Real-time Asynchronous Communications) with iPlanet, its partnership with Sun Microsystems. The company that spawned Sametime, a start-up called Ubique, was once owned by AOL. It was purchased by Lotus after being spun off. AOL's filing last month was part of a government-imposed order requiring AOL Time Warner to file periodic updates in its IM efforts. As a condition for approving the AOL-Time Warner merger, the Federal Communications Commission required the company to file "progress reports" every 180 days after the closing of the deal. The FCC-imposed requirement stemmed from a flurry of criticism raised by legislators and technology competitors alleging AOL had turned its back on its promises to support interoperability. Companies such as Microsoft used the review as an opportunity to draw regulatory attention to AOL's dominance in instant messaging and AOL's alleged broken promises. In July 1999, when Microsoft launched MSN Messenger, the software giant allowed its consumers to communicate with AIM users. AOL blocked Microsoft's attempts and sparked a war of words, claiming Microsoft was hacking into its servers. Microsoft eventually backed down, but AOL's actions attracted attention from the high-tech industry and inside the Washington Beltway. Instant messaging has become an incendiary issue because it pits technology giants against each other. Instant messaging lets people exchange text messages in real-time and has attracted millions of Internet users. However, separate products segmented by companies cannot communicate. Instead, people can only chat with others on the same IM service. Microsoft has become the most vocal proponent for AOL to allow interoperability for AIM. Other companies, including Excite@Home, Yahoo, Odigo and AT&T have also joined the rallying cry by participating in IMUnified, a coalition trying to develop a common interoperability standard. But to date, IMUnified remains a stalled effort. IMUnified member Yahoo downplayed the importance of Tuesday's announcement. "Although we're pleased that AOL has taken a small step towards interoperability, until and unless they become serious about interoperating with the leading IM services, such as Yahoo Messenger, it really won't have a positive impact on users -- which is Yahoo Messenger's end-goal," a Yahoo representative said.