As the popularity of IP multimedia subsystems races ahead of the development of the technical standards upon which the technology is based, service providers have grown increasingly concerned about how well IMS equipment will interoperate, as vendors apply their own implementations of those standards. And the sector is scrambling to make interop testing catch up to market demand.

“We're looking for people who are willing to live up to a true vision of interoperability, so you don't have to have proprietary extensions and pieces that tie you to specific vendors,” said Dorene Weiland, director of business development for Sprint Nextel. “There's this fear, and rightly so, that if it doesn't interoperate, I could be investing a lot of money for naught.”

Although there's been relatively little IMS interop testing, various groups are making early inroads on that front. The Multiservice Forum is preparing a vast global interop test focused largely on IMS for October 2006, marshalling a mix of 40 vendors and major carriers. System integrator TekVizion announced new IMS conformance and interoperability testing services last week.

Some vendors are assembling their own gangs of interoperable partners. IBM and Nortel Networks at the Fall VON 2005 show in Boston last week trumpeted six new additions to the stable of SIP equipment certified to interoperate with its MCS 5200 media server, bringing the group's total ranks to more than 50. Though such clubs give service providers a degree of choice, they're still restrictive. “It's better than nothing,” said Chad Hart, Venture Development Corp. analyst. “It's not an ideal solution, but it's a pragmatic solution for now.”

Some softswitch vendors, pressured by customer demand, are offering migration paths to IMS through software upgrades to installed gear, adding call control functions, for example — just one part of IMS. This approach, which Hart calls “a marketing band-aid,” falls short of the full IMS architecture, and some say it doesn't scale well.

“IMS in a box doesn't work,” said Greg Carter, solutions manager for Ericsson, one of the firms exploiting IMS interop issues for system integration revenue.

However, the incremental migration approach doesn't carry with it as many interop fears, and it may appeal to some big carriers for whom IMS's separation of subscriber data, application servers and call control functions into separate devices creates operational headaches.

“If the goal of IMS is to separate three layers and have the ability to substitute one box for another vendors', that's hard if you don't specify everything,” said Paul Perry, Verizon's executive director. “In a telco environment, the operations side has a lot to say about what gets plugged into that infrastructure. If the OSS specifications aren't defined, it takes a lot of work just to plug into that box. That makes this model of substitutability very complicated.”