Unlocking Competitive Advantage: The Business Imperative of EPS FB and VoNR in the 5G Landscape

​​Wilson Rodriguez​

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As of 2024, the global adoption of Voice over LTE (VoLTE) has reached an impressive milestone, with approximately 6.3 billion subscriptions worldwide. This surge reflects the growing demand for high-quality voice services as operators migrate to IP-based networks. By 2029, nearly 90% of all 4G and 5G connections are projected to make use of IMS-based voice solutions like Evolved Packet System Fallback (EPS FB) and Voice over New Radio (VoNR) for 5G networks. This blog will dig into the strategic business value that VoNR solution brings to operators and how EPS FB can serve as an interim solution until VoNR is fully deployed. We will show how, together, these technologies can optimize existing investments, enhance service offerings, and support future growth. While the technical aspects will be briefly covered, the primary focus will be on how carriers can leverage these solutions to achieve a competitive edge in a rapidly evolving market.

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Strategic Insights: The Business Case

In a rapidly evolving telecommunications landscape, the implementation of EPS FB and VoNR has become a crucial priority for mobile network operators transitioning to 5G. These voice solutions are not just optional features, but essential components that ensure the continuity and quality of voice services as operators migrate from existing 4G networks to the advanced 5G Standalone (5G SA) architecture. For these solutions to be successfully deployed, it is vital that operators build upon a strong and mature Voice over LTE (VoLTE) infrastructure. A solid VoLTE architecture serves as the foundational element to integrate and support EPS FB and VoNR, making it possible to deliver the expected Quality of Service (QoS) and seamless service continuity during this complex transition phase.  

The implementation of EPS FB and VoNR brings tangible benefits for operators and their networks. On one hand, EPS FB, by enabling a seamless fallback to 4G LTE when 5G coverage or device capabilities are insufficient, ensures that voice services are maintained without interruption or degradation. This capability is particularly critical during the initial stages of 5G network deployments, where coverage gaps are common and VoNR may not yet be fully available. On the other hand, VoNR utilizes the full potential of 5G’s lower latency and higher bandwidth to enhance voice communication quality, ensuring that operators can deliver high-definition voice services and a more responsive call experience for their customers.  

Beyond enhancing voice services, the successful deployment of EPS FB and VoNR is a strategic necessity for operators aiming to maximize the advantages promised by the 5G network. The new 5G architecture is designed to support a vast range of applications and services with exceptional performance metrics that go beyond what 4G can offer. By supporting VoNR, operators can fully unlock the capabilities of the 5G Core Network (5GC), eliminate dependency on legacy 4G infrastructure for voice, and set the stage for the introduction of innovative use cases that demand high-performance voice and data integration, such as real-time multimedia communication and augmented reality services.  

Indeed, implementing these technologies does more than just about ensure voice service continuity—it secures a competitive edge in a market that is increasingly embracing the full potential of 5G’s capabilities. With EPS FB and VoNR, operators can provide superior user experience while building on their existing VoLTE investments, establishing a solid base for a smooth and cost-effective evolution to 5G. In this context, a robust VoLTE foundation is not only beneficial but essential, enabling operators to support both the transitional and long-term voice service strategies needed to meet the demands of a connected world that 5G promises to transform.

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The Rising Demand for the Voice Service

The rising demand for uninterrupted and better-quality voice services emphasizes the business case for EPS FB and VoNR. With global VoLTE subscriptions expected to surpass 6 billion in 2024 [Ericsson, 2024], it’s clear that IMS-based voice solutions have become a cornerstone of modern communication. Projections also show that nearly 5.6 billion 5G subscriptions will be active by 2029 [Ericsson, 2024], highlighting a profound shift in consumer expectations. Seamless voice continuity across 4G and 5G networks is no longer optional – it is an essential requirement, not a luxury.

In this context, VoNR, with its high-definition voice quality, faster call setup times, and lower latency, has emerged as an anchor of user satisfaction in the 5G era, empowering operators to meet these evolving demands with greater efficiency. EPS FB complements this by ensuring reliable voice service even when 5G coverage falls short- bridging network gaps during early 5G rollouts and in areas with limited 5G infrastructure. Together, these solutions allow mobile operators to meet market expectations while strengthening their competitive positioning, ultimately creating expanded revenue opportunities and optimizing their investment in 5G voice capabilities.

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VoNR Value Proposition

The deployment of EPS FB and VoNR offers significant operational and financial advantages for Mobile Network Operators. EPS FB optimizes network resources by enabling seamless data and voice service integration across 4G and 5G. This is especially valuable for operators in the preliminary stages of 5G adoption, as they can leverage existing LTE assets to support 5G voice services during the transition. Conversely, VoNR capitalizes on 5G infrastructure investments solely, as 5G SA environment eliminates reliance on 4G. This transition to a 5G SA environment fosters a more cost-effective and scalable network migration while preserving high-quality voice services throughout the deployment phases of both technologies at any stage. Additionally, it supports spectrum efficiency, freeing up valuable spectrum resources, thus helping to reduce costs for carrier companies. Together, these technologies equip operators' networks for the future, laying the groundwork for advanced services such as HD video calls, Augmented Reality (AR), Virtual Reality (VR), and mission-critical communications (Emergency services), which leverage the full potential of 5G's data transmission capabilities.

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Integrating Voice Services in 5G: The Role and Synergy of Existing and new Architectures

The transition to 5G often begins with a Non-Standalone Architecture (NSA), where the existing 4G LTE network manages voice services while new 5G Network Functions handle data. This approach allows operators to utilize their existing Evolved Packet Core (EPC) architecture as a base for establishing voice bearers gradually. In an NSA environment, user equipment (UE) connects to both 4G and 5G networks as both types of Radio Access Networks (RAN) coexist, leveraging 4G for voice communication and 5G for high-capacity data traffic. Voice services across 4G and 5G networks are supported by the IP Multimedia Subsystem (IMS), which facilitates multimedia services and processes signaling messages for VoLTE. This architecture is essential for the seamless migration from VoLTE to EPS FB and then to VoNR, enabling the integration of voice, video, and messaging over an IP network. When a voice call is initiated in an NSA network, the EPC establishes a bearer connecting the UE with IMS, ensuring that calls are connected using VoLTE solution at first stages of 5G deployment.

As operators evolve to 5G Standalone (SA) architecture, EPS FB allows for voice calls to seamlessly transition from 5G to 4G, maintaining consistent Quality of Service (QoS) parameters even when VoNR is not fully supported in the E2E solution. It is crucial for carriers to own robust IMS and EPC architectures, capable of serving the various multimedia services for all subscribers and maintain call sessions connected while handover from 5G to 4G is performed. In the EPC architecture, carriers should trust in their existing Network Elements like Mobility Management Entity (MME), Home Subscriber Server (HSS), Policy and Charging Rules Function (PCRF), Serving Gateway (SGW) and Packet Data Network Gateway (PGW), to allow the bearer to be established for voice services, while the calls are processed by the IMS elements like Proxy Call Session Control Function (P-CSCF), Interrogating Call Session Control Function (I-SCSCF), Serving Call Session Control Function (S-CSCF) IMS Home Subscriber Server (IMS-HSS) and Multimedia Telephony Application Server (MMTel AS) as basic components to connect, maintain and terminate voice calls.

When VoNR service is in use, both voice and data bearers are established and managed by the newly introduced 5G Network Functions like Access and Mobility Management Function (AMF), Session Management Function (SMF), User Plane Function (UPF), Policy Control Function (PCF), Unified Data Management (UDM) and Unified Data Repository (UDR) among others, while the call is handled by the IMS architecture supporting 5G features. Importantly, no mandatory changes are required on the IMS; the Rx Diameter interface continues to be supported by the PCF. Additionally, the 3GPP has also introduced a new N5 interface, operating via the HTTPv2 protocol, as a potential alternative Rx; it remains however as an optional change to the IMS architecture.

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Technical Deep Dive: Why VoNR Performs Better than VoLTE

Evolving from VoLTE, VoNR is designed to operate independently within 5G networks, optimizing the voice service without relying on the 4G infrastructure. VoNR provides enhanced audio quality and reduced latency, thanks to the capabilities of the 5GC and the potential for advanced services such as high-definition voice and video calls.

On audio quality, both VoNR and VoLTE utilize the Enhanced Voice Services (EVS) codec, which provides high-definition audio by supporting a range of audio bandwidths, from narrowband to full band. The EVS codec adapts to different bit rates and provides greater resilience under fluctuating network conditions, offering superior voice clarity. While EVS capabilities are available in both technologies, VoNR is more likely to harness the codec’s full potential due to the higher available bandwidth in 5G. Furthermore, 5G's advanced interference mitigation technologies, such as beamforming, massive MIMO, and smarter spectrum utilization, reduce signal degradation even in challenging environments. These enhancements ensure that VoNR can deliver ultra-HD voice with minimal audio distortion.  On the contrary, VoLTE, constrained by LTE’s narrower bandwidth and potentially higher susceptibility to interference, may experience limitations in leveraging the EVS codec’s full capabilities under certain conditions.

On call setup time, VoNR provides an improvement over VoLTE, though the difference is subtle and often dependent on network conditions. VoLTE typically achieves call setup in around 2-3 seconds, while VoNR can reduce this to roughly 1.5-2 seconds [3] [4]. This improvement is a result of the optimized signaling process in 5GC, where the direct access to the IMS requires less hops and the communication between Network Functions (NFs) is faster on the SBI. Additionally, the 5G SA architecture is theoretically built to support end-to-end latency as low as 1 millisecond under ideal conditions; while in real-world 5GC deployments it is around 5-20 milliseconds, it still presents a substantial improvement over the 30-50 milliseconds or higher latency delivered by LTE networks [5]. Although this latency difference may seem minor in some applications, it is particularly advantageous in high-density urban areas where users may switch between calls frequently and expect barely instant connection times.

In large LTE networks, operators often deploy Diameter Routing Agents (DRAs) to manage the complexity of signaling and reduce the mesh that arises when multiple nodes (e.g., MMEs, HSSs, SGWs, PGWs, PCRFs and IMS entities) must communicate. The DRA centralizes the Diameter protocol routing, streamlining the signaling path and improving efficiency. However, this adds one or more additional steps in the call flow, which can marginally increase call setup time.

Meanwhile, 5G SA networks leverage the Network Repository Function (NRF) as part of the 5GC architecture. The NRF acts as a central registry for network functions, enabling the discovery of service endpoints. This discovery information is cached within several NFs, allowing faster identification of the next hop in the communication flow. Powered by the HTTPv2 Service-Based Interface (SBI), this architecture eliminates the need for intermediaries like the DRA. The SBI allows direct communication between 5GC functions (e.g., AMF, SMF, UPF, and IMS), reducing the signaling complexity and hops in the call flow.

Hence, while VoLTE often involves additional signaling layers and intermediaries to optimize large-scale network performance, which typically requires 6-7 hops from the user equipment to the IMS, VoNR benefits from a more efficient and modernized architecture that inherently avoids such complexities. The number of hops to establish a call in the 5G SA network is reduced to around 3-4 hops resulting in a faster call setup time.

If you would like to explore the technical flows behind voice call establishment in 5G, please scroll down to the appendix.

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reailize Commitment to Excellence: Modernizing the Voice Service Infrastructure of Tier-1 Operators  

We have leveraged our track record of expertise in the telco network to help Tier-1 Operators around the world advance their Voice Service roadmaps.  

In one engagement, we have partnered with a leading mobile operator in North America to upgrade their IMS network that is reaching the out-of-service state. The scope included designing and implementing multiple IMS cores running on the latest SIP release, as well as validating new SMS over IP, Online Charging, and other customized flows. Our Subject Matter Experts have run a thorough assessment of the existing IMS capabilities and capacity and reviewed the network trends to propose a new IMS architecture suitable for the projected services use. The implementation engagement covered deployment, routing validation, integration with the existing network elements, and extensive testing, part of which was automated. We have successfully achieved an error-free implementation with minimal impact on service continuity, while reducing the delivery time by up to 70% compared to the vendor’s proposal, significantly accelerating time-to-market.  

We have also engaged with the core engineering team to leverage the IMS modernization from the PGWs, enabling approximately eight migrations through traffic re-routing to new SBCs. 3 million voice users were migrated with imperceptible service interruptions through meticulous planning and execution.  

In another engagement, the IMS network deployment team presented challenges related to the extensive testing needed for integrations with peripheral networks and traffic use cases. The repetitive nature of these tasks leads to lengthy timelines and high resource demands, making it critical to ensure the network meets service standards rapidly. To address these challenges, we proposed an automation-driven approach using the Katalon application. Our solution automates call testing based on predefined instructions mapped to Android's interface, streamlining the Acceptance Test Plan (ATP) and reducing manual efforts. This approach has improved operational efficiency, reducing ATP execution time by up to 50%. This was reflected in OPEX cuts where manual interventions are reduced and resources were reallocated to higher-priority tasks, finally enabling the mobile operator to meet the VoLTE service standards effectively.  

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Future Trends in 5G

As we look ahead, the growth and implementation of EPS FB and VoNR in 5G networks are expected to enable many advanced, high-value applications across multiple industries. These technologies will empower critical applications such as smart homes, smart cities, autonomous vehicles, and countless IoT solutions that demand ultra-reliable and low-latency communication. While these applications are predominantly data-intensive and are enabled by 5G SA’s advanced capabilities and network slicing, voice communication remains a foundational component for support functions such as emergency alerts, real-time command and control, and user interaction. In this context, these applications rely on the low latency of VoNR in a fully operating 5G environment, or EPS FB during initial stages. For instance, intercom systems or voice-command features require stable, continuous, and high-quality voice support that only VoNR and EPS FB solutions can provide in 5G networks. These use cases clearly illustrate how EPS FB and VoNR will facilitate rapid and seamless voice and data transfers, driving innovations in emergency services, traffic management, and even remote surgeries. This underscores the essential role of robust voice solutions in the connected environments of the future.  

Beyond immediate use cases, the expansion of 5G is set to foster increasingly immersive experiences driven by Artificial Intelligence (AI), Augmented Reality (AR), and Virtual Reality (VR) that are becoming mainstream globally. As telecommunication networks adopt predictive maintenance and automated troubleshooting powered by AI, EPS FB and VoNR will be crucial in ensuring smooth connectivity for these applications. High-definition video calls, real-time machine interactions in manufacturing, and complex cloud-based applications will rely on these voice protocols, which will continue to evolve alongside the shift to cloud-native, IP-based frameworks.

In the early stages of 5G adoption, EPS FB plays a critical role in ensuring seamless voice call capabilities for users operating in 5G SA mode until VoNR becomes widely available. By leveraging the mature LTE infrastructure, EPS FB allows operators to roll out 5G SA networks while maintaining uninterrupted voice services, as well as in scenarios where VoNR is unsupported due to device limited capabilities, RAN features, or core network elements limitations. Without EPS FB, users in 5G SA-only areas lacking VoNR coverage would lose voice call functionality, severely impacting the user experience.  

EPS FB introduces signaling procedures between the 4G and 5G cores, which can increase call setup times and signaling load due to inter-system flows such as bearer control handovers. However, it remains essential for service continuity during the transitional phase of 5G deployment. Over time, as VoNR coverage expands and matures, reliance on EPS FB is expected to decrease. Until then, EPS FB serves as a vital bridge, enabling operators to provide a smooth user experience while advancing toward comprehensive 5G capabilities, including immersive services being adopted globally.  

Looking further into the future, the outlook for 6G suggests an even deeper integration of these technologies, positioning EPS FB and VoNR not just as current solutions but as vital foundations for the stable, high-performance connectivity that markets will increasingly and continually demand in both 5G and beyond.

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Conclusion

As carriers navigate the complex transition to 5G, the deployment of EPS FB and VoNR technologies represents both a technical and strategic necessity to meet consumer expectations. At Reailize, we understand Mobile Operators’ commitment to maintain competitive relevance and agility in an evolving digital ecosystem, this is why we have developed agile methodologies to smoothen the adoption of recent technologies while ensuring that high levels of customer experience are kept intact.

Like any major technological shift, successful planning and careful staging are essential to fully realize the benefits of the investment. With a proven track record of successful technology adoption among Tier-1 operators in the USA, Mexico, Latin America, and EMEA, our team’s extensive experience in E2E network architectures, signaling protocols, and past technology rollouts enables us to support carriers through every phase of this transition.

Interested to know how we can support your tech upgrades? Contact us today to learn more about our Network Modernization solution!

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Appendix: Exploring the Processes Behind Voice Services in 5G

Before delving into the technical details, it’s important to understand that, prior to initiating the voice service request in 5G network, the user must undergo a structured series of processes to connect, communicate and effectively perform the call. The first two processes, 5G registration and PDU Session Establishment, are essential and common steps that all devices follow when attaching to the 5G network. Additionally, devices must register with the IMS architecture to initiate and receive calls and access other multimedia services. These procedures form the backbone of seamless voice services in the 5G environment and are briefly outlined below:

• 5G Registration: It is the first step that a device should take to connect to the 5G network. In this process, UE establishes a Radio Connection with gNB, gNB receives all the signaling to register the UE to the 5GC. AMF handles the authentication and authorization process by querying AUSF/UDM and UDR, which then performs the assignment of a temporary identity to enable the exchange of encryption and authentication keys to secure communication. At the end of this process, the UE is registered and active in the 5G network securely.

• PDU Session Establishment: the device establishes PDU (Protocol Data Unit) sessions in the 5G network based on the services it will use, identified through Data Network Names (DNNs). In normal conditions, the UE initiates PDU session establishment procedure for both default data and IMS DNNs to establish a connection to the network and IMS respectively. In this process, SMF and UPF manage the configuration and maintenance of these sessions, ensuring that the device can transmit and receive data over 5G towards the endpoints.

• IMS registration: after the IMS bearer is established between the UE and the IMS, the former sends a SIP REGISTER signaling to P-CSCF as a first communication with the IMS architecture. The P-CSCF then routes the SIP message to I-CSCF which initiates authentication and authorization for the device. In this process, I-CSCF, S-CSCF and IMS-HSS interwork to validate user data, server capabilities and permissions to access to IMS network, then the IMS-HSS generates authentication vectors that are used by the UE to calculate a response and finally establish an encrypted IPSec connection between UE and P-CSCF. S-CSCF may trigger a third-party registration to external servers like MMTel AS to provide the multimedia services if allowed for the user profiles.

Once the subscribers are registered in IMS, they can initiate or receive voice calls or other multimedia services. EPS FB and VoNR flows will occur upon device, RAN or 5GC limitations, but they are triggered when the device initiates or receives a call. In both cases, P-CSCF sends an AAR request to PCF to supply session related information, PCF then updates the session policies to SMF which proceeds to request the PDU session modification to the UE to enable call QoS. In the case where the UE, RAN and 5GC support VoNR, the call flow is completed at this step and there is no mechanism triggered.

Contrarily, EPS FB will be the path forward, and the UE and RAN will enable it through any of these 2 mechanisms to seamlessly handover the call to 4G network based on their capabilities: 5G to 4G Release with Redirection or 5G to 4G Inter-RAT Handover.  

EPS FB and VoNR common call flows

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EPS FB - 5G to 4G Release with Redirection

In this method, the 5G radio connection is released during the 5G RRC Release procedure, instructing UE to reselect a 4G cell, where a new radio connection can be established for the VoLTE call. In this scenario, the UE context is transferred from the AMF to the MME, and the Tracking Area Update procedure is utilized to update the serving area.

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EPS FB - 5G to 4G Inter-RAT Handover

In this method, the N26 interface (GTPv2) between AMF and MME invokes the Forward Relocation procedure, where session contexts and user plane tunnels in the core network are sent from the AMF/SMF to the MME/S-GW.

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End notes:

[1] https://www.ericsson.com/en/reports-and-papers/mobility-report/dataforecasts/mobile-subscriptions-outlook

[2] https://www.ericsson.com/en/reports-and-papers/mobility-report/dataforecasts/voice-services-outlook

[3] https://www.3gpp.org/technologies/volte-vonr

[4] https://www.mdpi.com/2079-9292/11/15/2412

[5] https://www.ericsson.com/en/blog/2022/8/who-cares-about-latency-in-5g

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