Mavenir csfb

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Mavenir csfb

  1. 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. 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. 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. 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. 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. 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.

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