With AT&T and Sprint's launches, the first IP multimedia subsystem applications are poking their heads above ground.

The hype around IMS may have toned down in the last year, but that doesn't mean operators have given up on the technology. In fact, the first IP multimedia subsystem applications have quietly made their way into the network. The first phones with full session initiation protocol stacks are now in the market, and both AT&T and Sprint have gone live with their first end-to-end IMS services.

AT&T's christening of IMS comes with its video share launch, which enables end users to share videos from one device to another. The application uses both the UMTS voice and data channels, sending the video feed as an IP stream and the audio as a normal circuit-switched phone call. The SIP stack coordinates the two payloads on the phone, while the IMS core manages the session, determining if both devices are capable of sending and receiving the video stream.

For AT&T, video share in its current form is just the beginning, said Siroos Afshar, the network architect responsible for the carrier's next-generation network design. AT&T's IMS deployment is merely a stage in the carrier's overall converged network plans, which began years before when AT&T began combining all of its transport networks into a common IP MPLS core. As AT&T added more networks through its spate of acquisitions, and the IMS standard matured in the 3rd Generation Partnership Project, AT&T decided to use IMS as the control layer overlaying that core.

“Video share, to the extent possible, was built on top of that target architecture,” Afshar said. “The architecture works by decoupling the access from the application.”

Using that approach, AT&T is prepping itself to live up to its “three screens” advertising plan, launching simultaneously across all of its networks, Afshar said, which means video share could soon be coming to a home phone terminal, a PC or even a TV screen near you.

“Now that we have the application launched, as soon as we decide to support an additional access technology, we can launch it,” Afshar said.

He added, however, that AT&T is approaching the process slowly. The company still hasn't completed the final stage of its target architecture integration; some fine-tuning is necessary to link the AT&T network with its traditional wireline core. But once that integration is complete, the way AT&T launches applications will fundamentally change, Afshar said.

Meanwhile, Sprint has launched an IMS-based IP PBX service for enterprises, which can turn a mobile phone into a fully functioning office extension. The company is saving its big IMS launch for a consumer offering in the first quarter.

Sprint's new Direct Connect push to talk (P2T) service will be its first end-to-end IMS-based application. The service uses the new EV-DO Revision A network and its built-in high-capacity uplink, latency and quality of service capabilities, which Sprint says will bring Nextel-quality P2T to the CDMA network. The carrier also will be able to bridge the Nextel iDEN and Sprint CDMA networks for the first time using the new session control capabilities inherent in IMS.

Like AT&T, Sprint has developed a full SIP stack for the phone, which can be used as the launching point for other IMS applications as well as the glue to tie them all together. Using IMS, Sprint also will be able to link applications to a phone's address book and settings, said Emerino Marchetti, director of network development for Sprint.

Sprint is discovering that IMS has its limitations, however, leading the operator to optimize portions of the network architecture to meet its needs, Marchetti said. The carrier found, for instance, that SIP signaling doesn't always coexist well with the real-time nature of P2T. Marchetti said one of the primary goals of Direct Connect was to reproduce the instantaneous “chirp” of the Nextel network, but initially that wasn't possible because the SIP protocol's long text-based stack wasn't up to the task. Instead, Sprint wrote its own proprietary code to hasten the connection time between sessions; from there, IMS protocols take over, Marchetti said.

“I'd say Direct Connect is 80% IMS,” Marchetti said. “IMS, like any technology, is not perfect.”