Nokia Siemens Networks CS Fallback (CSFB) Enabling seamless voice services for LTE smartphones
Figure 1: CSFB enables voice and high-speed data on LTE-capable smartphones.
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 packetswitched 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
Data
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 standardsbased 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.
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.
Nokia Siemens Networks has provided CSFB solutions to operators since 2011.
The Nokia Siemens Networks CSFB solution is available now and has been implemented in many live operator networks.
2. LTE-capable smartphones
3. CSFB operation
Operators 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,
CSFB provides LTE voice capability without requiring significant changes in the existing mobile CS core network, for example in routing or charging configurations.
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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.
LTE CSFB Data Voice
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Figure 2: CSFB connects the CS network and the Evolved Packet Core of the LTE network.
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Nokia Siemens Networks CS Fallback (CSFB)
CSFB has no impact on SMS or packet switched data, which is handled in parallel with voice on the 2G/3G network.
2G/3G
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MSS MSS CSFB
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.
MSS MSS
CSFB MSS
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CSFB in every MSS CSFB overlay – dedicated MSSs Figure 3: Options to implement the CSFB – in every MSS or in dedicated MSSs
4. Implementing CSFB
5. Benefits
Options 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)
As 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.
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)
CSFB in every MSS •
Ensures best performance for voice and data
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Software upgrade required in the network
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Minimizes signaling between Visitor- and Home Location Registers (VLR and HLR) No need to update VLR Zero location updates during each mobile terminated call
Zero location updates during each mobile originated call
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.
Overlay CSFB •
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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.
Average call setup time with CSFB is only 1-2 seconds longer than in current 3G networks.
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Enables quick and easy introduction of CSFB in core networks from all vendors No changes required in the network VLR updates result in extra signaling between VLR and HLR
Two location updates during each mobile terminated call
Two location updates during each mobile originated call
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
Figure 4: CSFB in every MSS ensures the best voice and data performance for LTE smartphone users
Nokia Siemens Networks CS Fallback (CSFB)
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Nokia Siemens Networks P.O. Box 1 FI-02022 NOKIA SIEMENS NETWORKS Finland Visiting address: Karaportti 3, ESPOO, Finland Switchboard +358 71 400 4000 (Finland) Switchboard +49 89 5159 01 (Germany) Order-No. C401-00770-B -201208-1-EN Copyright © 2012 Nokia Siemens Networks. All rights reserved. Nokia is a registered trademark of Nokia Corporation, Siemens is a registered trademark of Siemens AG. The wave logo is a trademark of Nokia Siemens Networks Oy. Other company and product names mentioned in this document may be trademarks of their respective owners, and they are mentioned for identification purposes only. This publication is issued to provide information only and is not to form part of any order or contract. The products and services described herein are subject to availability and change without notice.
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