Nokia Siemens Networks CS Fallback (CSFB)Enabling seamless voice services for LTE smartphones

Nokia Siemens Networks CS Fallback (CSFB) 2

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1. Enabling voice for LTE smartphones

CS Fallback (CSFB) will be the bridging technology to ensure that LTE smartphone users can enjoy ultra-fast data services as well as high-quality voice services on 2G/3G networks. CSFB will enable devices in packet-switched LTE networks to change over to CS networks for incoming and outgoing voice calls. In many cases, CSFB will co-exist with Voice over Long Term Evolution (VoLTE) well into the future.

Going forward, IMS-based Voice over LTE (VoLTE) will be the logical choice to provide voice services over the

same technology that is driving the data revolution. There are compelling reasons, however, to provide CS-based voice services for LTE smartphone users, even in markets were LTE networks have been commercially launched. The Next Generation Mobile Networks Alliance (NGMN) has recommended standards-based CSFB to enable non-IMS roaming subscribers to use both LTE data services and voice services in Circuit Switched (CS) networks. With CSFB, when an LTE smartphone makes or receives a voice call, it connects automatically to the 2G/3G network. The ongoing LTE data session is also transferred to the 2G/3G network. Once the call is terminated, the smartphone reverts to the LTE network. This process is seamless to the end-user.

Nokia Siemens Networks has provided CSFB solutions to operators since 2011.

2. LTE-capable smartphonesOperators worldwide are implementing LTE data networks. During the past six months, a number of LTE-capable smartphones have also been introduced, and as this evolution continues, end-users will expect to be able to make voice calls as well as access data services with their new devices (Figure 1). To support a seamless voice and data experience,

smartphone vendors and operators have implemented standardized CSFB functionality. Without CSFB in the network, the LTE data experience would not be available for LTE smartphone users as devices with CSFB capability have to connect to the 2G/3G network as required by standard. For operators who have not yet implemented VoLTE, CSFB provides a solution to enable both voice and data services on LTE smartphones.

With CSFB, operators can provide not only quality voice for LTE subscribers but also data continuity, Short Messaging Services (SMS), emergency calls, voice roaming and other CS services currently available in 2G/3G networks.

The Nokia Siemens Networks CSFB solution is available now and has been implemented in many live operator networks.

3. CSFB operationCSFB provides LTE voice capability without requiring significant changes in the existing mobile CS core network, for example in routing or charging configurations.

The interface between the CS network and the Evolved Packet Core (EPC), which controls the LTE connection, is the cornerstone of CSFB. The standard SGs interface enables CSFB between these two networks. (Figure 2)

The Nokia Siemens Networks solution provides the main core network functions on the same hardware. The capacity can be dynamically allocated between functions, which is a key advantage for future VoLTE implementation.

CSFB has no impact on SMS or packet switched data, which is handled in parallel with voice on the 2G/3G network.

Figure 2: CSFB connects the CS network and the Evolved Packet Core of the LTE network.

Figure 1: CSFB enables voice and high-speed data on LTE-capable smartphones.

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4. Implementing CSFBOptions for configuring the connection between the CS and Evolved Packet Core networks are versatile and flexible. CS Fallback can be implemented in every Mobile Switching Center Server (MSS) or in a dedicated MSS (known as CSFB overlay). (Figure 3)

With the Nokia Siemens Networks CSFB overlay configuration, there is no need to update all the existing MSSs or software. As the number of users with CSFB capable smartphones will be limited at first, the additional traffic from the CSFB overlay will not cause changes in the network. (Figure 4)

5. BenefitsAs CSFB utilizes current CS networks, all features related to voice services are available (e.g. SMS, emergency calls, roaming agreements, voices services, charging, etc). With the Nokia Siemens Networks approach, CSFB MSS can control the Media Gateway in the network, which enables smooth integration of existing ringing and message tones and announcements to subscribers.

CSFB does not cause interruptions in data sessions when subscribers make a voice call. The data connection returns to the LTE network seamlessly when the voice call is over.

In several tests and field trials, Nokia Siemens Networks has demonstrated that the call setup time with CSFB is only a fraction longer than in current 3G networks. The connection time depends to a large extent on the network structure and technologies.

Average call setup time with CSFB is only 1-2 seconds longer than in current 3G networks.

CSFB is part of the Nokia Siemens Networks VoLTE solution. Together, they comprise a highly cost effective solution that reuses existing network investments to provide a seamless experience when 2G/3G and LTE networks are used in parallel. Designed to provide full IMS capability in the same hardware, the solution helps reduce investment and operational costs.

CSFB can be provided as a software upgrade to Nokia Siemens Networks’ core network elements, which are compatible with our IMS-based VoLTE solution. For this reason and to ensure a smooth evolution to VoLTE, Nokia Siemens Networks recommends implementing CSFB in every MSS before CSFB traffic reaches a significant level.

Figure 4: CSFB in every MSS ensures the best voice and data performance for LTE smartphone users

Figure 3: Options to implement the CSFB – in every MSS or in dedicated MSSs

CSFB in every MSS CSFB overlay – dedicated MSSs

• CSFB in every MSS is the recommended way to implement CSFB • Overlay CSFB is ideal for fast and easy introduction of CSFB • Overlay CSFB is upgradable to CSFB in every MSS when subscriber numbers increase

• Enables quick and easy introduction of CSFB in core networks from all vendors

• No changes required in the network

• Ensures best performance for voice and data • Software upgrade required in the network

CSFB in every MSS Overlay CSFB

• VLR updates result in extra signaling between VLR and HLR

• Minimizes signaling between Visitor- and Home Location Registers (VLR and HLR)

• No need to update VLR

• Two location updates during each mobile terminated call

• Zero location updates during each mobile terminated call

• Two location updates during each mobile originated call

• Zero location updates during each mobile originated call

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