LTE Migration and Circuit Switched Fallback

(Jun 8 2011) 4G , LTE

1. By Drew Sproul, Marketing Director, Adax.

   Three themes define today’s 4G/LTE landscape. Service providers are building mobile broadband access to the Internet; we’re not in the telephony business any more. Second, like it or not, a web of networked economies, nationally and globally, is being spun before our very eyes. And lastly, that’s not a smartphone on your hip, in your pocket, briefcase or purse—it’s a personal computer. We are truly at an inflection point in network history.

   These themes are based on high-speed, broadband mobile data networks. 4G/LTE network build-out must address this dramatic and continuing rise in mobile data traffic. The incentives are clearly there. Studies presented at Mobile World Congress 2011 show that subscribers are indeed willing to pay for a better, richer, faster experience. Higher bandwidth and more content are worth the money.  If we build it, they will use it and pay for it.

   But lest we forget, voice is still the major contributor to network service provider (NSP) revenue and LTE requires not-so-ubiquitous VoIP. How then will NSPs meet the exploding demand for high-quality, video/data-oriented bandwidth and still provide basic voice, roaming and text services? With Voice Over LTE/GAN (VOLGA) dead and Voice over LTE (VoLTE) not yet ready, what is to be done? Hold off fielding LTE and forego new revenue streams? Not likely.

   This is where Circuit Switched Fallback (CSFB) comes in. It is an idea whose time has come. That’s a good description of this good idea from earlier 3GPP specifications.  While the market focus (and hype) was on the “All-IP Network,” other minds saw the difficulties inherent in achieving this vision. Not that they are insurmountable, but there will be hurdles in deploying the major network overhaul.  One such hurdle is voice.

   CSFB addresses this need by resolving LTE’s lack of backward compatibility with circuit-switched services. All-IP LTE rollouts have to include VoIP or not roll out at all. At first this omission was addressed by the VOLGA initiative, which was deemed too narrow and proprietary. This led to VoLTE. Unfortunately, VoLTE will take time to roll out with the same roaming and connectivity as 3G. Not to mention the resource conflict with the data side of things.

   CSFB allows network operators to carry voice traffic over existing GERAN/UTRAN networks from multi-mode LTE UE devices. This very practical goal is realized by a very clever innovation: network awareness in the MME. Where overlapping networks exist, the MME may carry maps of TAs to LAs that allow the UE to utilize CS services all managed from the MME. If no VoIP services are available, the UE is instructed to access the alternate network for voice calls.

   The CSFB standard explicitly states there need be no change to User Plane transport services. While true, agile minds continue to seek ways to combine CSFB with the IP/EP Core. Their solution is User Plane interworking at the Network Edge. This approach brings CS-based voice calls into existing VoIP networks quickly and efficiently. In the UTRAN interworking between ATM-IP is performed on already AMR-encoded voice. For truly traditional voice over DS0s, the ATCA I-TDM specification allows T1/E1 and DSP cards to pass traffic seamlessly between them and their associated networks. Most recently Adax, Surf Solutions and RadiSys delivered such solutions for both GERAN and UTRAN networks.

   Broadband mobile access to the Internet is well under way. The future for wireless networking never looked brighter, even in these dark times. It’s great to be part of an industry that forges ahead with applications, devices, services and networks that change the world while making use of what exists today.

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