BellSouth is completing the circle of next-generation network architectures between its co-owned wireless carrier Cingular and SBC Communications, announcing today it has selected Lucent Technologies IP Multimedia Subsystem platform as the launch point for voice over IP and possible future converged services.

BellSouth's widely expected decision to go with Lucent will give the RBOC along with SBC and Cingular the same architecture, allowing easy cross-integration between Cingular's cellular network and the two RBOCs' own extensive array of wireline assets. Despite those convergence possibilities, BellSouth is starting with a narrower focus, using the IMS architecture to launch advanced VoIP services over its FastAccess DSL footprint. Lucent will deploy its Lucent Network Controller, an IMS-based softswitch that will host VoIP services, and Lucent's media gateways, bridging the access and next-generation core networks.

In the past, BellSouth has outlined a more cautious path for IMS. Company executives have said they are focusing on offering simplified services that will either give customers a richer user experience or allow BellSouth to deliver current services at lower prices. VoIP is the first logical step to that approach, which BellSouth will likely follow up with video and converging its voice, video and data platforms to launch "blended" services.

When that convergence extends to Cingular's wireless network remains to be seen, but the common control plane the two operators share will certainly make any joint wireless/wireline implementation all the much easier. When SBC and Cingular announced their own choice for IMS, they stressed that they arrived at their decisions to select Lucent independently, but conceded that the coincidence will benefit them both in the future. While IMS is a rapidly standardizing technology, there are still key differences in each vendor's architecture--offering an application that bridges two vendors' IMS platforms, while not impossible, would certainly be more difficult.

The deal also clearly positions Lucent as the definitive leader in IMS in the U.S. and a major player globally. Lucent has one a major piece of each of the four IMS implementations announced so far in the U.S. Unlike the SBC, Cingular and Sprint, however, BellSouth is not starting its IMS implementation in the control plane of the network. All three of the former companies are deploying Lucent's call session control function and home subscriber server elements, largely agreed upon as the two core components of an IMS architecture. Instead, BellSouth's launch appears to be focused on the application layer, specifically to support its new VoIP service. The Lucent Network Controller incorporates the media gateway control functions traditionally found in a softswitch. While the softswitch is integrated into Lucent's overall IMS portfolio and can support other IP-based services in the future, BellSouth won't be able to reap the full benefits of an IMS service-based architecture until it deploys the control plane elements.