July 11, 2006

Cable IMS: Adolescent Growing to Adult

By Justin J. Junkus

Cable's version of IMS at Expo, in many ways, looked a lot like a teen-age football star about to enter college. It's poised for a major role in fixed mobile convergence (FMC), but still needs some definition before it can join DOCSIS and PacketCable as an anchor technology for the industry.

The good news is that vendors appear to understand the basic IMS layered architecture and are implementing standards-based products. Diagrams showing IMS layers for applications, control, transport/access, and client are almost as universal as the OSI seven-layer model, as are the interfaces for legacy interworking.

Similarities

The migration from PacketCable-compliant VoIP to IMS has become a generic story, with the feature side of the CMS morphing into the first IMS applications server and the call control function going to the IMS call session control function (CSCF). Interfacing to the legacy TDM and packet worlds remains the job of a media gateway. New IMS applications in general rely heavily on the databases contained in the control layer home subscriber server (HSS) for provisioning, policy and the interaction needed for multimedia capability. Key interfaces are the "Sh" interface between applications servers and an HSS and the ISC interface between applications servers and a CSCF.

Differences

The separation of architectural blocks is largely a software concept, however, and vendors differ in physical implementations. Both Alcatel and Nortel have chosen the Advanced Telecom Computing Architecture (ATCA) developed by the PCI Industrial Computer Manufacturers Group (PICMG) as the hardware platform for IMS implementations. The key motivator for that decision is the physical standardization of chassis, blades and signal interconnection contained in ATCA, theoretically making it easier to mix and match component vendors at the board level within a standard chassis. However, like many "standards," second generation ATCA implementations often contain proprietary "improvements" that make the hardware implementations unique to the vendor. In contrast, Siemens and Gallery-IP have chosen to offer IMS middleware that runs on off-the-shelf Sun servers.

Interoperability across vendors is therefore still an issue with IMS. Nortel's Elaine Smiles points out that vendors need to participate in interoperability tests such as the Open Mobile Alliance (OMA) Test Fest events, and operators need to consider systems integration as part of IMS implementations.

FMC

Fixed-mobile convergence (FMC), as the driving application for cable IMS, received a lot of attention both at Expo booths and in the technical sessions. Russ Coffin, Nortel strategic architect, presented a paper that discussed the role of the Packet Cable 2.0 voice call continuity (VCC) function as a bridge between the cellular circuit-switched domain and IP connectivity access networks that support wireless LAN VoIP. To accomplish seamless roaming and call handoffs, all IMS-VCC user voice calls are anchored in the IMS domain, irrespective of call origination. The VCC application serves a dual role as an application server in IMS and as a standard service control point with interfaces to mobile switching centers.

Another paper by Jay Bestermann from Arris and Jack Fijolek from UTStarcom included a discussion of initial FMC trials in Europe and Brazil. The European lab trial proved that the technical concept of bidirectional roaming between GSM and IP networks actually worked and provided experience with provisioning challenges and matching terminology across cellular and VoIP fixed line networks. The Brazil Telecom trial was conducted with actual subscribers in a pre-IMS configuration where one operator owned both fixed and mobile networks. It determined the tradeoffs between single number and dual number FMC implementations.

- Justin J. Junkus