Mavenir has developed solutions to help mobile operators transition to VoLTE networks from 2G/3G networks. Their VoLTE Interworking Function allows operators to implement Circuit Switched Fallback for VoLTE without requiring upgrades to legacy networks. This improves customer experience and reduces costs. Mavenir's Converged Telephony Application Server further supports the transition to VoLTE while maintaining existing services like SMS and preserving investments in legacy networks. Mavenir's solutions provide an evolutionary path to VoLTE that improves service quality while minimizing additional costs.

1. Voice in VoLTE Networks
An evolutionary path with Circuit Switch Fallback
(CSFB)
For more information on Mavenir Systems, visit our Web site
http://www.mavenir.com/
Every reasonable effort has been made to ensure the information and procedures detailed in this
document are complete and accurate at the time of printing. However, information contained in this
document is subject to change without notice.
© 2012 Mavenir Systems Inc.

2. Mavenir SystemsError! R ef erence source not found.
Error! R ef erence source not found.
1
Voice in LTE networks
An evolutionary path with Circuit Switch Fallback (CSFB)
LTE (Long Term Evolution) is the next-generation mobile technology that delivers true all-IP
communications where all services, including voice, messaging, video and data, use a common IP
infrastructure; there are no separate dedicated speech channels.
The deployment of LTE is accelerating and LTE is now the fastest ever growing mobile technology.
Operators are focusing on LTE as part of their business strategy due to its lower cost of operation (about
5X cheaper to carry data over LTE than 3G and 20x cheaper for voice than compared to 2G) but also as
a way to provide the high throughput, low latency data service demanded by their subscribers with their
media hungry laptops, tablets and smart phones.
Although initially LTE is seen as a way to provide an improved data service, the availability of LTE smart-
phones means that operators needs to address how to continue to provide a high quality voice service on
a technology with no dedicated speech channel. Providing a high quality voice service is not optional as it
is predicted that in 2013 voice revenue will still account for 61% of all mobile service revenue (Source:
GSMA). Similarly, SMS needs to continue not only for its role in subscriber ARPU but as it is just a basic
service expected to be available always.
However, the all-IP nature of LTE presents a challenge to operators as Voice over LTE (VoLTE) requires
the introduction of a new core network architecture based on IMS since VoLTE, being based on VoIP,
doesn’t use the existing circuit switch MSC/VLR infrastructure. The challenge is -- can an MNO invest in
new radio technology, EUTRAN, a new packet core, EPC, and at the same time provide voice and SMS
service continuity via a new IMS core network for their high ARPU subscribers who adopt LTE smart
phones?
The standards bodies recognize that not all operators will be ready or willing to immediately switch to the
new IMS-based VoLTE architecture and as a result, defined an alternative standard for providing voice
and SMS services to LTE devices. This alternative is Circuit Switch Fall Back (CSFB). CSFB continues to
uses the circuit switched MSC network for voice calls. A device registered on the LTE network must
detach from the E-UTRAN and attach (i.e. fallback) to the 2G/3G network prior to originating or receiving
a voice call.
The benefit of CSFB is that it offers a fast time-to-market for LTE voice by not requiring an immediate
transformation to a new IMS core network for voice and messaging services. The continued use of
existing circuit switch infrastructure for voice means that there is no impact on other key elements to
provide a full voice service such as charging, legal intercept and emergency services. CSFB also enables
SMS delivery without changes to HLRs or SMSCs and does this without requiring the device to fall back
to the CS network. Furthermore, CSFB allows the re-use of interconnect, roaming agreements, charging
and settlement processes as well as being mandatory (per GSMA) for incoming roamers from other
3GPP operators.

3. Mavenir SystemsError! R ef erence source not found.
Error! R ef erence source not found.
2
The CSFB Problems
An immediate problem with CSFB is that operators are faced with making additional capital investment in
their legacy core network. CSFB, as defined in 3GPP TS 23.272, introduces the new SGs interface
between CSFB enabled MSCs and LTE MMEs and hence an MSC upgrade is a prerequisite to
introducing CSFB; unless MSC pooling is used this upgrade potentially must be to all MSCs in the
network.
CSFB also introduces ongoing operational complexity as to minimize the likelihood of roaming retry the
TA (Tracking Area) to LA (Location Area) mapping needs to be maintained in the MME. This mapping is
required as it is used by the MME to attempt to predict which MSC a UE would attach if CSFB were
invoked in the event of a mobile terminating call. This mapping needs to be as accurate as possible,
however, some operators state that it is impossible to maintain an accurate TA-LA mapping and as such
impossible to avoid roaming retry with its detrimental impact on customer experience. Furthermore,
roaming retry is not widely supported in networks and introducing support would be yet another
investment in the legacy core network.
Mavenir recognizes the short comings of CSFB and has developed a unique, innovative and standards-
based approach to CSFB, addressing the CAPEX and OPEX issues mentioned above.
The VoLTE Interworking Function
Mavenir’s unique and innovation solution to CSFB
Mavenir addresses the above issues by enabling CSFB to be introduced into an LTE network without
requiring any investment in the legacy network to upgrade MSCs to support the SGs interface. Instead
the investment is made in LTE by introducing a new interworking function (VoLTE IWF) which provides
interworking for CSFB between the LTE MME and existing legacy HLRs and SMSCs without requiring
any MSC upgrade.
The VoLTE IWF, as depicted in Figure 1 below, acts as a MSC/VLR for subscribers registered on the LTE
network.

4. Mavenir SystemsError! R ef erence source not found.
Error! R ef erence source not found.
3
Figure 1
Rather than attempting to predict the MSC to which a UE will fallback, the VoLTE IWF ensures that
mobile terminated call is routed to the actual MSC serving the UE after it fallsback to the circuit switch
network. This means that roaming retry is never invoked due to CSFB and hence there is no compromise
to the user experience.
The VoLTE IWF is a signaling only solution with no impact on the voice bearer and hence is a simple,
cost-effective and scalable solution to introduce.
In addition, regarding the CAPEX challenges of CSFB, the VoLTE IWF also addresses the operational
issues by simplifying the provisioning of CSFB in the MME. The VoLTE IWF uses a static TA-LA
mapping with a single LAI for all IWFs in the network and all the E-UTRAN TAs are mapped to this single
LA in the MME. This removes the requirement to maintain dynamically an accurate TA-LA mapping in all
MMEs.
SMS delivery for LTE attached devices is performed by the VoLTE IWF. As the VoLTE IWF requires no
MSC upgrades it offers the fastest time to market approach to provide SMS service to LTE enabled
device. This is significant as in some markets, there are regulatory requirements to inform users of
charges and operators use SMS as the notification mechanisms. To ensure service parity for SMS, the
VoLTE IWF supports X1/X3 interfaces for Lawful Intercept of SMS messages as well as CAP for prepaid
charging of SMS based on information retrieved from the HLR.
By addressing the capital expenditure, operational complexity and user experience issues with CSFB,
Mavenir’s VoLTE IWF facilitates the rapid deployment of LTE with CSFB for Voice and SMS service
parity. This VOLTE IWF is also an investment in the new LTE network providing the first step on an
evolutionary path to full VoLTE.

5. Mavenir SystemsError! R ef erence source not found.
Error! R ef erence source not found.
4
Evolving from CSFB to VoLTE
As LTE coverage becomes more extensive and the number of LTE subscribers and devices increases,
the operator will want to evolve from using CSFB for voice calls to using VoLTE where possible. VoLTE
enables a better user experience by supporting HD voice but also because it avoids the additional latency
to call setup introduced by CSFB. Other reasons for switching from circuit switching call to VoLTE include
LTE’s spectrum efficiency and the ability of IMS to introduce new converged services.
VoLTE introducing a Telephony Application Server (TAS) as call control is performed in the IMS core
network rather that the MSCs.
This move to VoLTE presents challenges in a couple of areas. Firstly, call continuity; until LTE is
ubiquitous it is necessary to seamlessly hand-off an active call from the LTE packet E-UTRAN network to
circuit-switch and thus ensure that calls are not dropped as users move out of LTE coverage. This call
continuity is addressed in the standards by SR-VCC (Single Radio Voice Call Continuity) but, similar to
CSFB, SR-VCC requires an upgrade to all the MSCs in the legacy network to support the Sv interface
between MSC and MME. The MSC upgrades don’t stop with Sv, if mid-call controls are to remain in IMS
TAS after the hand-off from E-UTRAN then the MSCs require further upgrades to support the i2 interface
required by ICS (IMS Centralized Services).
A second challenge in VoLTE is that to maintain service parity the operator needs to either invest
additional capital to provide feature parity for calls controlled by the IMS TAS. There is also complexity as
IN Services (WIN/CAMEL/INAP) are not addressed by VoLTE and any use of IN services requires that
the operator deploys an additional IM-SSF for IN inter-working. An upgrade of the IN system may also be
required to accommodate the new SIP call model.
Mavenir addresses both of these issues allowing the operator a smooth evolution to VoLTE. Firstly, the
VoLTE IWF extends to not only act as an interworking function for CSFB but also an interworking function
for SR-VCC and thus enable call handoff without requiring capital and operational investment in the
legacy MSC/VLR infrastructure. It removes the need to upgrade MSCs to support the Sv interface.
Mavenir also provide a best in class Converged Telephony Application Server (CTAS). In addition to
IR.92 support, the CTAS includes an integral IM-SSF to preserve IN services and enable new services,
such as video or multi-device. The CTAS is a pure IP-based TAS that enables operators to offer all
services in the IMS domain and is the only solution that implements a CAMEL/INAP compliant Basic Call
State Machine (BCSM) with its built-in IM-SSF supporting IN triggers to preserve critical services,
mapping the SIP call model as required. Additionally, the CTAS has an integrated Mobility Server (SCC
AS) supporting SR-VCC handover.

6. Mavenir SystemsError! R ef erence source not found.
Error! R ef erence source not found.
5
Figure 2
The CTAS and VoLTE IWF are both based on the same carrier grade Mavenir mOne™ Convergence
Platform. Mavenir’s unique approach permits fast time to market for LTE and VoLTE whilst ensuring that
investments are made in the new network, yet preserving existing investments in the existing network.
Summary
Mobile operators are launching LTE networks and need to continue to provide voice with the same
seamless mobility and service parity to that offered on the existing network. At the same time, these
operators wish to minimize further investment in their legacy networks but may not be ready or willing to
move immediately to a full IP-based Voice over LTE service (VoLTE).
Mavenir’s VoLTE interworking function (VoLTE IWF) provides a unique and innovation approach allowing
the rapid launch of LTE services using CSFB for LTE devices without requiring investment or upgrades
in the legacy MSC/VLR infrastructure. This VoLTE IWF extends to include SR-VCC interworking enabling
the introduction of VoLTE without requiring any further investment or upgrades to legacy infrastructure to
support call continuity via SR-VCC. Mavenir’s Converged TAS (CTAS) allows further investment
protection by supporting IN-triggers for new IMS services and optimizes the new network by optionally
incorporating the IN-SSF and SCC functions a single platform.
Mavenir’s unique approach permits fast time to market for LTE and VoLTE whilst ensuring that
investments are made in the new network yet preserving existing investments in the existing network.