Fallback to the future: Circuit-switched networks as a voice/data solution

The Circuit Switched Fallback (CSFB) feeds data's insatiable bandwidth appetite while maintaining voice QoS, as explained in an interview with industry experts from three companies that collaborated on this new technology.

2Editor’s note: The LTE build out seems at a crossroads, as the technology and availability to transfer voice over an all-IP network while retaining QoS is still not in place. However, the dilemma of offering the exceptional data speeds of LTE with the quality voice services of legacy circuit-switched networks until a Voice over LTE (VoLTE) solution arrives may have found an answer. The Circuit Switched Fallback (CSFB) allows wireless devices to “fall back” to legacy domains to send/receive voice calls. A virtual panel – Drew Sproul, Adax; Venkataraman Prasannan, Radisys; and Niv Kagan and Avi Fisher, SURF – presents details on the CSFB and the partnership resulting in this recent solution. Edited excerpts follow.

CPCI: How close are we to a truly “all-IP” network? What is the status of LTE?

PRASANNAN: If you say, “There is nothing but all-IP network,” no one will agree because there are all kinds of networks in place and a huge amount of copper in the ground that is not all IP. But if you ask, “Are there people who are still deploying the old network? Who’s deploying LTE versus 2.5 G?” you would probably find a small tract of investment in 2.5 G from entities that have some foreseen function, but for the most part everyone else is deploying LTE.

KAGAN: We see our customers starting to deploy LTE networks, and requiring that equipment manufacturers support those different technologies and services. This is driven by the consumer using applications such as real-time video communications, all the way to the operator, who is subsequently required to increase investment in IP, driving an IP network.

ADAX, Radisys, and SURF collaborated on the CSFB for an APAC Tier 1 customer.

CPCI: What is going prevent LTE from arriving at a fate similar to IMS (IP Multimedia System)?

SPROUL: The IMS network standard came out before LTE standards were solidified and when there was not yet the driving demand for multimedia services there is today; Voice was fine and the data speeds weren’t in place to support new multimedia. But tablets and smartphones drove data, video, video streaming – the mantra of “mobile broadband access to the Internet” – and in a way pushed the industry to jump over IMS to LTE, so there are very few IMS subsystems in play today.

PRASANNAN: IMS is being implemented much more slowly on the wireless side, and that’s why it feels like it’s at such a standstill; IMS is getting traction in some of the broadband side infrastructure and getting used. It provides more of a framework and means of dealing with the control and management plane elements, and it has taken much longer to implement that and a service creation aspect in the wireless world.

The LTE promise is the need for speed, and that comes with upgrading the infrastructure to accommodate new network topology and architecture. LTE collapses certain network elements together and makes them more comprehensive or intelligent, if you will. The mobility management that had been very much a part of the Radio Network Controllers (RNCs) and some other base station controllers has been simplified, which translates not only to higher speed but also a cheaper network from an OPEX/CAPEX point of view, ensuring LTE’s viability for the future.

KAGAN: While IMS was driven to provide a next-generation solution for voice infrastructure, LTE is the next generation for both voice and data architecture. As we see subscribers pushing the limits on wireless data usage and willing to pay for wireless data access we believe LTE will continue to deploy for data usage, but constraints on voice are forcing the operators to find other solutions for their currently deployed voice networks, which is where the CSFB comes into play.

CPCI: Generally, what are the past, present, and future of the CSFB?

FISHER: Voice networks have been and are still built on circuit-switched networks, and the technology and network availability is still not in a place to transport voice over an all-IP network to the subscriber. At the same time, adoption of data services is increasing at an exponential rate, forcing operators to adopt technologies for IP networks. The challenge is to maintain all relevant voice services and QoS while providing great data service. This led to the adoption of CSFB, which maintains a circuit-switched network for voice while providing a high-speed data network.

SPROUL: The migration to packet processing requires a middle ground of interworking between the legacy service, switched services signaling, the user plane, and the emerging packet processing-based service. So where we’ve come from is a background of legacy PDM circuit-switched services to LTE, which today is principally focused on building out mobile broadband access to the Internet from smartphones, tablets, and so on, for non-voice services.

KAGAN: We believe that as LTE is adopted, the requirements on CSFB will grow. Initially it will serve as the primary infrastructure for voice communications while it provides the flexibility to migrate to a true all-IP network in the future.

CPCI: How does the CSFB help facilitate the transition from legacy to LTE, and how did the players implement this?

FISHER: The CSFB maintains a circuit-switched voice channel to the subscriber while the transport of media in the backhaul is achieved over IP. This guarantees QoS for voice in wireless networks by interworking with Public Switched Telephone Networks (PSTNs) like wireline, and with circuit-switched networks such as 2G wireless networks that utilize Adaptive Multi-Rate (AMR) over circuit switch (Transcoder and Rate Adaptation Unit (TRAU) frames).

SPROUL: Specifically, in a Radisys ATCA server platform, we’re bringing in Transition-Minimized Differential Signaling (TMDS) zeros from the GERAN network through our T1E1 ports – voice over 64-kilobit DS zero slots; it doesn’t get any more legacy than that. Then our smartcard interworks that voice into Internal Time-Division Multiplexing (I-TDM) packets, which are Ethernet packets based off of an MPLS (Multi-Protocol Label Switching) framework. Then it is sent to the SURF DSP card.

When we set up the connection we exchange Media Access Control (MAC) addresses – no IP involved here. We’re setting up an IPDM channel between us and start sending these packets over the backplane. Then the SURF card gets the IPDM, does whatever magic it needs to do for voice encryption/decryption, and sends it out onto the IP network.

PRASANNAN: In this particular instance, the customer is building a network element which requires a particular functionality and incorporates a Line Interface Card, a DSP resource, and because a modular AMC form factor is being used as a host to incorporate the two, the customer is left with the task of putting these three pieces together, integrating them, and building an application on top of it.

Radisys, ADAX, and SURF brought our contributions together to make this integration as seamless as possible, accelerating time to market. At Radisys we have a framework and a mechanism to make this happen more naturally with our Alliance Partner Program, in which all of the players were already involved.

CPCI: How long will it take to complete the transition to LTE, and what is the CSFB’s longevity?

SPROUL: There is still a lot of legacy gear and service in the field that needs to be interconnected, and it’s not just a holding pattern while those are decommissioned and replaced. What’s actually happening is that LTE is an add-on to existing service networks to provide data services, and therefore you get upgraded legacy systems with the CSFB.

The transition depends on how TEMs (Telecom Equipment Manufacturers) address the VoLTE and circuit-switched connection. What the VoLTE initiative requires is an IMS services plane for LTE networks to send voice traffic for true VoIP. In an LTE network, this is gobbledygook, but in an LTE/IMS system, the network just picks it up like it always has. For now, all the arrangements that support voice and text messaging will remain in place.

FISHER: The complete transition will only occur once the LTE network provides the same level of quality for the voice channel over IP as over circuit-switched network. Then there will be a truly all-IP network from the subscriber to the network and beyond, but we haven’t seen a deployed technology that is able to achieve this yet, and therefore it will take at least a few years before the CSFB will not be required anymore.

CPCI: If there were no such thing as AdvancedTCA, would it have to be invented or is CSFB more agnostic?

SPROUL: ATCA is the perfect vehicle for CSFB because you can re-architect your existing concentrator networks by leaving the RAN in place and architecting access gateways and concentrators to legacy networks like GERAN and UTRAN.

So now you’ve got TDM and ATM (Asynchronous Transfer Mode)-based services that you want to get onto your IP core; ATCA-based products facilitate that by on the one hand speaking legacy via zero or ATM, and on the other speaking IP, and the beauty of ATCA is that the Ethernet IP cores are built into the architecture.

That is a crucial component of the ATCA architecture, and the second main point as the CSFB gets to deployment is the need for High Availability (HA) and preventing loss of service. Because ATCA already has a redundancy built into it, it makes it easier for things like HA for the user plane to be implemented. That’s all an integral, fundamental part of the ATCA architecture; so yes, if ATCA didn’t exist it would have to be invented.

CPCI: How significant do you anticipate ATCA to be in your next win? What are the promising ATCA markets moving forward?

SPROUL: Our next two opportunities in this domain are in China, and ATCA is crucial because of, again, scalability, flexibility, and the challenge that we’re addressing now: interworking. Adax has always been an advocate of open system standards, both in terms of software and hardware development and platforms. We sell to the broad telecom market, and an open standards-based interoperability is one of our core business and technical principles.

KAGAN: LTE is built in a flat IMS architecture and therefore promotes independent nodes of different scale and functionality, as opposed to the legacy approach of a centralized large switching system. This approach promotes different TCA architectures based on the network requirements.

On a global scale, we feel that the promising ATCA markets are those with a high concentration of users that adopt technologies requiring high-performance, real-time computing and communication technologies, especially in video with its vast requirements for real-time processing and ultra-high bandwidth in the all-IP domain.

PRASANNAN: We are seeing it pretty well balanced, and our ATCA split is one-third, one-third, one-third (Europe, Asia, and North America). It tends to be driven by where the TEMs are, and if you look across those geographies, there are definitely equipment manufacturers present in all three.

The telecom market will continue to be present and grow at a fairly attractive rate, but ATCA is also spreading its wings to other market segments. I’ve seen some network monitoring, which is a segment that has a lot of ATCA presence, and I’m sure it will continue to grow in that space. Another opportunity I can cite is Aerospace and Defense, which is looking for more of the ruggedized alignment and performances dictating higher processing power and higher network performance; it’s just a combination of time and the artifact of the segment.

Telecom itself was a fairly long cycle compared to enterprise, and Aerospace and Defense is an even longer cycle: 15 to 20 years. You start seeing growth after the fourth or fifth year. That’s the nature of the segment and it also takes time for those things to come up for re-evaluation on that long of a cycle, so you have to wait it out.