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Oct/Nov 2008 | Volume 3/Number 5
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Hybrid IMS/TDM Networks

By Richard "Zippy" Grigonis
The initialism “IMS” means “IP-based Multimedia Subsystem”, but the world’s long, piecemeal migration to this grand, common service architecture for wireless and wireline communications means that traditional TDM (Time Division Multiplexed) networks will be very much a part of the picture for many years to come. This shouldn’t be surprising, since IP communications itself was an “outsider” for nearly a decade, a small fish in a large sea of legacy circuit-switched equipment, both at the network operator level as well as in the enterprise. Operators deploying IMS technology gradually over time must support signaling and session control in both NGN and IMS domains while providing connections to Signaling System 7 (SS7) signaling in the PSTN — but the whole idea of a “hybrid” network is actually more complicated than just a “patchwork quilt” of TDM, NGN and IMS, as we shall see.

It was, after all, the appearance of VoIP technology that resulted in the reengineering of Mobile Switching Centers (MSCs) so that they could handle both packet-switched voice traffic along with existing circuit switched traffic, a process that led to the disaggregation of legacy network switches into a control component (the softswitch) that uses a protocol such as the Media Gateway Control Protocol (MGCP) to manage the other major component, the media gateway. The media gateway can handle both packet and circuit switched traffic and it can convert one form to the other under the control of the softswitch. this is why 2.5G networks are able to carry both circuit-switched and packet-switched traffic.

One might wonder how applications and services can function in such a complex environment. Fortunately, vendors have risen to the occasion. Take the Cisco Mobile Office solution, for example, designed so that service providers can offer seamless mobility to enterprise and Small and Medium-sized Business (SMB) customers by integrating mobile and enterprise voice and messaging networks into a single communications environment. For enterprise and SMB customers, operator services based on Cisco Mobile Office integrate existing TDM PBXs and IP PBXs with wireless networks including WiFi, WiMAX, and mobile networks. Providers can now deliver to their customers fixed-mobile convergence (FMC) communications services, accessible from any device over any kind of network, and self-managed by subscribers through a friendly web-based interface. A business customer’s collection of communication devices are now be treated by the service provider as a single entity having access to the converged communication services and call routing capabilities, independent of the underlying infrastructures. Indeed Cisco Mobile Office services can be offered simultaneously on a broad range of network architectures including Signaling System 7 (SS7), VoIP and of course IMS.

In the world of IMS itself, provision has been made for PSTN ingress and egress functions that allow interworking with the PSTN. The physical conversion between TDM and IP for media is done via media gateways, and signaling gateways perform translation at the transport layer between SS7 on MTP (TDM networks) and SS7 on SCTP (IP networks). The Breakout Gateway Control Function (BGCF) determines whether PSTN “breakout” will happen in the current network — in which case the signaling is passed to a Media Gateway Controller (MGC) which allocates a media gateway port — or else the SIP-I (ISUP over SIP) is sent to another IP network where breakout actually occurs (the signaling passes across the IBCF, or Interconnect Border Control Function, to another BGCF in the breakout network).

Service intelligence interworking is generally done from the signaling control plane, by leveraging Signaling System 7 control with Session Initiation Protocol (SIP) control and then extending the signaling service framework from the intelligent network (IN) to both the NGN and IMS domains. In terms of transitioning networks to IMS while maximizing existing architecture investments, the incremental migration of the control layer of SS7 to IP includes deploying SIGTRAN (SS7 over IP) and/or eventually deploying an independent SIP signaling control plane in the NGN. Once a SIP Signaling Router (SSR) for session routing (an idea first championed by Tekelec a few years ago) and a whole unified signaling layer is put in place to unite various signaling and control procedures across multiple network types, hybrid media processing platforms are then brought online to enable uniform media delivery across the many existing access technologies. Another step involves linking the application layer to the control layer by implementing SCIM (Service Capability Interaction Manager) functions to assist in bridging the transition from TDM to NGN, and eventually IMS.

TISPAN (Telecoms & Internet converged Services & Protocols for Advanced Networks), situated in the ETSI standards body, has long worked at standardizing converged networks around the IMS core architecture. This means re-jiggering fixed network access so that it interfaces to IMS and, although the original main thrust of the work was to figure out a way to move existing PSTN functionality onto an IP core, it was soon joined by the matters of interworking, PSTN emulation and producing PSTN equivalence services (malicious call trace, lawful intercept, etc.) service.

All this eventually leads us to the matter of service mediation, which enables new IMS subscribers to immediately leverage existing services, since operators no longer need to replicate their services in the IMS domain, thus allowing them to minimize costs while providing both pre-IMS and IMS services. Here again, a well-designed SCIM can not only orchestrate applications, but also mediates multiple services across various technologies, and allows for seamless service blends that can be offered to any subscriber at any location.

Realizing that an instant cutover to an IMS-based network is usually impractical and that service providers must continue to engage their NGN and TDM networks during the migration process, Tekelec (a developer of high-performance network applications for next-gen fixed, mobile and packet networks) and HP in 2007 combined their expertise in SS7 Signal Transfer Points (STPs) and Home Location Register (HLR) platforms to form an “Open IMS” vision, to help network operators migrate to IMS. Their joint Open IMS solution supports the Call Session Control Function (CSCF), Media Resource Function (MRF) and HSS functionality, plus multiple application capability, on a single, standards-based platform. It delivers service enablers such as MRF, presence server, electronic numbering (ENUM), group list management and voice call continuity (VCC).

Around the same time, Tekelec unveiled SIP Signaling Router functionality on their TekCore Session Manager product. TekCore SSR enables operators to expand their NGNs, at the same time maintaining a seamless migration path to the IMS. The Tekelec SSR provides centralized session routing for softswitches and enables core signaling capabilities that inter-work between TDM, NGN and future IMS domains. Subscribers can take advantage of all these capabilities, enjoying existing, next-gen and future IMS-based services regardless of access type; thus, consistent service delivery over hybrid networks is assured as they migrate to all-IP environments.

The non-trivial process of incrementally converging disparate networks and providing multi-protocol support to maintain seamless connectivity between TDM, VoIP and IMS has also been taken up by Veraz Networks, whose ControlSwitch User Services Core (CS-USC) is an IP softswitch built to smooth the migration path to IMS. With the CS-USC, providers can deliver multimedia services via any mix of traditional and IP-based voice networks. The platform is based on a sophisticated, modular, programmable, distributed, scalable, high-availability architecture with open interfaces to media devices, application servers and backoffice systems. CS-USC is consistent with 3GPP IMS, TISPAN and MSF standards. The CS-USC supports web – IMS integrated services via Veraz Portal Connect, and linkages to nomadic/mobile users and devices are maintained in conjunction with the multiservice and multi-access IMS core. The Veraz CS-USC also has programmatic interfaces including TL1 and SOAP/XML for integration into the service provider’s existing OSS/BSS, thus streamlining business operations.

Veraz Networks has had some of its new equipment in trials for about a year and is rapidly becoming a major global provider (50+ countries) of IP softswitches, media gateways and digital compression products that enable voice, video and other multimedia services. As the IMS service architecture slowly percolates throughout the world, service providers have found that products such as those from Veraz allow them to quickly and efficiently migrate from traditional voice networks to all- IP, fixed-mobile and multimedia networks consistent with the emerging IMS standards.

Operators Saddled with Saddles

The hybrid network will be with us for at least 15 more years, during which time network operators must deliver seamless services and support service mediation and interaction between legacy, mobile, VoIP and IMS networks, delivering older “siloed” or “stovepipe” services alongside newer IMS-based applications. As we’ve seen, technologies exist to help them. At some point, however, the dawn will come, and the final buildout to IMS (or whatever we call it then) will occur.

Richard Grigonis is Executive Editor of TMC’s IP Communications Group.

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Veraz Networks

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