Continuing out look at the state of the IP Multimedia Subsystem nation, today we hear about IMS advances at AT&T from Siroos Afshar, chief VoIP architect at AT&T Labs. He said: "[AT&T] did not start out by asking how we can use [IMS], rather we decided we needed to combine all our transport network into a single, global IP/MPLS network [that would support] every existing and future service." He continued: "Once a single transport architectural standard was set, and once the 3GPP added requirements [for IMS] to support wireline services with session management, we adopted a 'Common Architecture for Real-Time Services' that we call CARTS, building from IMS."

From a planning and execution perspective, Afshar explained that AT&T recently completed a “CARTS Two” phase the company will share with vendors (under NDA) next year, and that a critical piece of the deployment now underway is to sequence the steps needed to arrive at its target architecture. CARTS Two was important to meet the requirements of both BellSouth and Cingular, as AT&T acquired both companies late last year.

Afshar said that the biggest challenges aren’t with VoIP or making sure that AT&T’s “Three Screen” products like Video Share will work; rather, “the real problem is how to migrate the big, legacy, Class 5 circuit-switched PSTN to CARTS with a solid sequence of events. That sequence includes the IMS Core, managing access technologies - such as PSTN interconnection, Internet access and wireless access - along with executing on plans for transit traffic, IMS and non-IMS peering, CDR collection aggregated endpoints (like a PBX), transcoding and CODECs, and security.

Under the CARTS architecture, the TDM-based PSTN and GSM (wireless) networks aren’t seen as peer networks but as access technologies into AT&T’s network. As Afshar explained: “The service logic comes from an application in CARTS regardless of the using GSM or the PSTN to access it.”

Commenting on what real-life services are using the CARTS IMS architecture today, Afshar said that AT&T is using it to support consumer VoIP, a number of business VoIP features (with about two-dozen IMS-based features) and the AT&T Video Share service, “but the architecture is designed to support thousands of services,” he added.

Steve Taylor is president of Distributed Networking Associates and publisher/editor-in-chief of Webtorials. Larry Hettick is a principal analyst at Current Analysis.