IMS: Can it reliably deliver Voice over LTE (VoLTE)?

by Tom Maufer

Recently, a large group of operators and vendors--many of whom are NGN IMS Forum members--announced that their preferred direction on Voice-over-LTE (VoLTE) is a slimmed-down version of IMS. This initiative is being branded as "One Voice." How does the One Voice announcement change the LTE market landscape? Note: The term "VoLTE" also incorporates SMS, a critical service from both an operational and revenue perspective.

IMS has been under development, in both the 3GPP and IETF standards bodies and in the broader market (by major telecommunications network equipment manufacturers) for about 10 years. Operators have deployed it in their networks of varying sizes, and the NGN IMS Forum has been running IMS-related Plugfest interoperability test events for the last 3 years. IMS is as much a 3GPP standard as is LTE, though in terms of standards life cycle, IMS is much more mature than LTE.

With the imminent arrival of LTE behind billions of operator revenue, it is important to point out is that the "I" in IMS stands for "IP." LTE is the first transition point toward the mobile All-IP Network (AIPN). Maybe, for the first time, the network will be aligned with the technology from top-to-bottom?

Regarding IMS, many industry watchers are wondering: "We have been talking about IMS for a long time. Why hasn't IMS happened yet? What's the delay? Is it not ready? Maybe it's just not ready for mobile networks?" Clearly, IMS technology works, as evidenced by the following issues:

  • The 2008 MSF GMI event showed an IMS core supporting IPTV, spanning a global network across five different carriers.
  • Since 2007, IMS Forum Plugfests have demonstrated IMS as the underlying technology that reliably delivers carrier-class VoIP and multimedia services.
  • With the notable exception of GMI, IMS demos have been so VoIP-oriented that it's easy to forgive observers who might miss the bigger potential around a unified signaling infrastructure that supports any IP-based application, not just VoIP.

Delivering the Next-Generation Operations Support System (NGOSS)

IMS is about much more than just a common signaling layer for applications: IMS includes a whole new "brain" for the network, the Next-Generation Operations Support System (NGOSS). The NGOSS is unique, often customized, and constantly changing to deliver new kinds of flexible billing models (e.g., usage-based or per-application) that would be difficult or impossible to roll out today without IMS.

Ultimately, IMS promises leverage for carriers: Deploy a common signaling and OSS infrastructure, leverage them for a long time, drive new revenue streams and monetize the applications running over the IMS signaling core.

All of this sounds fine in theory, but there is considerable inertia around the existing network infrastructure that is evolving in parallel. 3G is being actively rolled out alongside LTE. Can these projects benefit from each other? Is there a natural evolution from (or coexistence with) 3G to LTE?

Realistically, the biggest barrier to the adoption of VoLTE is the huge installed base of 3G (and earlier) customers who are tied into the legacy OSS and mobile switching infrastructure. But if IMS is the pathway for LTE to deliver voice (and SMS), there must be a natural evolution from legacy OSS and legacy signaling to an IMS-based NGOSS and IMS signaling. One Voice articulates exactly how to integrate IMS into an LTE environment to deliver VoLTE, and there are many possible ways to integrate LTE and 3G networks at the signaling level and at the OSS level. Once a firm VoLTE solution emerges, vendors will be able to design bridges to their legacy 3G solutions.

What does IMS offer? Why is it relevant to VoLTE? Ultimately, IMS is invisible to the end-user. IMS is another way for carriers to deliver compelling IP-based services. IMS's NGOSS provides new, flexible billing for revenue streams that don't exist today. IMS is a natural solution for VoLTE because it is relatively mature and is, honestly, more than LTE needs. The One Voice initiative is taking a subset of IMS that is enough for their stated a starting point.

From a long-term investment perspective, using IMS is the strategic approach that scales and creates the maximum leverage of operator and vendor investments in delivering VoLTE. Customers use the mobile web. They make phone calls. Customers won't see IMS, but they vote with valuable revenue dollars in favor of fancy new services on their smart phones and netbooks.

Tom Maufer is Director, Solutions Architecture, at Mu Dynamics, which is the Vice Chair of the Billing/OSS & Security Working Group of the NGN IMS Forum. You can reach Tom at [email protected].