Circuit-Switched Fallback

Supporting Voice Services in LTE Migration

The Challenge of Supporting Voice while Deploying LTE

The move to LTE is advancing full-speed ahead. This buildout of new equipment and services is complex and will not happen overnight. The ever-increasing demand for highspeed, high-volume data applications requires network service providers (NSPs) to focus on providing data services first and foremost in their initial LTE deployments. With Voice-over-LTE (VoLTE) not yet a ubiquitous solution, how can NSPs ensure support for voice and deploy LTE?

Since voice remains the major source of NSP revenue,it must be protected and maintained. Subscribers expect the high quality of voice services, especially roaming, to continue unaffected on their new smart, video- data-centric phones. The challenge then is how to preserve voice services and build out mobile broadband services in the absence of IMS/ VoIP. One solution is circuit-switched fallback (CSFB).

Today’s 2G/3G legacy networks currently enabling the voice revenue stream have sufficient capacity for continued voice service support. It only makes sense to let this gear continue to generate voice revenue. CSFB allows NSPs to preserve the sunken investment in existing circuit-switched networks. This extended life of legacy equipment and its associated revenues stream is a double bonus. Revenues remain the same without the expense of new CAPEX; and with some extensions, it can even promote the move to the all-IP network.

Circuit-Switched Fallback Provides a Solution

CSFB provides new LTE data-centric deployments with backward compatibility to circuit-switched services. As specified in 3GPP TS 23.272, CSFB is the preferred solution for the early and even later stages of LTE. It allows network operators to carry voice traffic over existing GERAN/UTRAN networks from multimode LTE user equipment (UE) devices. This practical goal is realized by a clever innovation: network awareness in the mobility management entity (MME). Where overlapping networks exist, the MME may carry maps of UTRAN tracking areas (TAs) to LTE location areas (LAs) that allow the UE to utilize circuit-switched services, all managed from the MME in conjunction with the mobile switching center (MSC). Without the IP multimedia subsystem (IMS),VoIP services are not available in the LTE network so the UE is instructed to access the alternate network for voice calls.


CSFB enables mobile operators to quickly and economically support services in conjunction with their LTE network roll-outs. It allows mobile devices to “fall back” to GSM or UMTS domains for incoming or outgoing voice calls, and subscribers maintain access to the wide array of rich circuitswitched capabilities, including international roaming, while enjoying LTE broadband access to the Internet and protected corporate VPNs.

CSFB with Enhanced VoIP Services

Implementing CSFB does not require changes to the GERAN or UTRAN user plane transport services. Existing signaling links and associated transport protocols can be retained if desired, however many media gateways in the field support VoIP. Adding VoIP services to the CSFB gateway is clearly a benefit. Network equipment providers (NEPs) and NSPs realize that interworking legacy voice to IP at the earliest network entry point possible facilitates the transition to the all-IP network. Voice interworking is being added to the CSFB formula by many forward-looking telecom equipment manufacturers based on multi-vendor, commercial-off-the-shelf (COTS) ATCA equipment (see Figure 2).


This enhancement 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 using real-time transport protocol(RTP), which facilitates not only voice but multi-media streaming over IP. For 2G voice over DS0s, the ATCA I-TDM specification allows T1/E1 and DSP cards to pass traffic seamlessly between them and their associated networks. Such solutions for both GERAN and UTRAN networks can be provided on a single ATCA platform with multi-vendor, industry-standard AMC cards as a sub-system for OEM/VAS applications.

The CSFB gateway architecture illustrated in Figure 2 for existing TDM-based network services uses a standard ATCA carrier-grade chassis, equipped with switches, SBCs and carrier blades hosting TDM T1/E1 and VoIP/DSP AMC cards. Legacy voice and SS7 signaling enter the system on TDM links via the T1/E1 ports. Voice channels are interworked to IP using I-TDM and sent to the VoIP/DSP card, which transmits VoIP packets to the network. Data services are handled by the LTE network or the legacy interface when there is no LTE connection. The 3G solution maps ATM voice traffic to IP on an advanced AMC ATM card. RTP is added on the intelligent carrier blade or an SBC, all of which are again ATCA COTS products.

These components for an enhanced CSFB gateway are the foundation for OEMs and TEMS to build advanced CSFB solutions including fully redundant systems of cards and blades that may be added, removed and re-allocated with no loss of service. The flexible ATCA architecture fulfills the promise of cost-effective, multi-vendor solutions and short time-to-market through close cooperation between committed ecosystem partners.

LTE in support of streaming video and data services is rolling out to everyone’s satisfaction. Voice continues to be supported via dual and tri-mode phones without service interruption of any kind. CSFB allows the integration of these two types of services based on different networks types seamlessly and effectively. Enhanced CSFB gateways will bring the legacy voice into IP all that much quicker, paving the way for the all-IP network. All of this is made possible by industry-standard protocols and an ecosystem of ATCA network equipment. The future never looked so bright, at least in this one corner of the world.



Andrew (Drew) Sproul is currently director of marketing at Adax, Inc. During his 20+ year career in telecom, Drew has held management positions in sales and marketing at Adax, Trillium, and ObjectStream. Drew has a BA in human services from Western Washington University in Bellingham, WA.

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