VoLTE stands for Voice over LTE, and it is a technology that facilitates real-time communication services, including voice calls and SMS in 4G LTE (Long Term Evolution) networks. While VoLTE was originally developed for 4G LTE networks, it can also be used in 5G New Radio (NR) networks.
VoLTE or Voice over LTE is a 4G technology built for LTE networks but is also used in 5G NR. The non-standalone 5G deployment model, 5G NSA, utilises an existing 4G LTE core network (Evolved Packet Core – EPC). EPC works alongside the IMS to support voice calls and SMS in 5G NSA through VoLTE.
5G New Radio (NR) networks can be deployed in two ways: standalone 5G (5G SA) and non-standalone 5G (5G NSA). These two deployment models determine which technology is used in 5G to enable real-time services such as voice calls and text and multimedia messages. VoLTE is one of the two technologies that allow real-time communication services in 5G networks.
Non-standalone 5G delivers voice calls and SMS through VoLTE
The non-standalone 5G deployment takes advantage of an existing 4G LTE core network instead of investing in a dedicated 5G NR cloud-native core network. Since VoLTE is dependent on the core network and IMS, the 4G core network (EPC) allows it to be used in 5G non-standalone deployments.
The earliest 5G network deployments are non-standalone and use a combination of a 5G radio access network and a 4G core network. VoLTE requires the LTE core network, Evolved Packet Core (EPC), and a new network entity, IP Multimedia Subsystem (IMS).
The 4G mobile core network, EPC, provides the packet-switched function required for data communication whereas the IMS architecture offers the integration of telephony with data communication.
VoLTE is enabled by the interworking of IMS and EPC. This combination integrates with the LTE radio access network in 4G LTE and the 5G radio access network in non-standalone 5G deployment.
VoLTE requires the packet-switched 4G LTE core network
Voice calling has been an essential part of mobile communications ever since mobile networks were introduced in the early 1980s. Since then, the technologies behind voice calling have evolved considerably, but the value of carrier-grade voice calls in high quality has not changed.
The voice calls and SMS in 2G and 3G networks are based on circuit-switched technology, which offers high quality and reliability but limited efficiency. 4G LTE and 5G NR networks do not have a circuit-switched capability, and all services are delivered over the packet-switched data network.
In 2G and 3G networks, the mobile core network has two separate entities MSC (Mobile Switching Centre) for the circuit-switched part and SGSN (Serving GPRS Support Node) for the packet-switched part. The circuit-switched part delivers voice and texts, whereas the packet-switched part provides mobile data.
4G LTE networks change that and use EPC – Evolved Packet Core, which is a packet-switched core network. 5G New Radio networks also follow the data-only vision and use a packet-switched mobile core network called 5G Cloud-Native core network – 5G CN. The voice calls and messages in 4G and 5G networks are IP-based and delivered as data packets by the packet-switched core network.
Since a data network is not natively designed to ensure guaranteed and reliable delivery of real-time services like voice and SMS, the IP Multimedia Subsystem (IMS) architecture is employed to integrate telephony with data networks.
IMS can work with a 4G core network or a 5G core network to provide necessary integration with external networks like PSTN and ISDN. IMS ensures the Quality of Service (QoS) requirements, including bandwidth, latency, audio codecs, etc., to deliver high-definition voice calls in a reliable way.
5G networks are expected to coexist with the 4G LTE networks for a long time to jointly cater to a wide range of use cases such as high-speed broadband, IoT (Internet of things) device support, self-driving cars, smart cities and many more.
Standalone 5G does not need the VoLTE technology
The standalone 5G deployments use the 5G mobile core network and therefore do not support VoLTE technology. Instead, 5G standalone deployments use an equivalent technology called Voice over New Radio (VoNR), also known as Voice over 5G (Vo5G), to enable voice calls and messages.
The support for VoLTE requires the 4G LTE core network, Evolved Packet Core – EPC and IMS. Since standalone 5G deployments (5G SA) use a dedicated cloud-native 5G core network (5G CN) instead of EPC, the VoLTE capability is not available in 5G SA. But that does not mean that 5G networks cannot deliver voice calls and messages over the packet-switched network.
5G NR networks use Voice over New Radio (VoNR) which is an equivalent of VoLTE but requires a dedicated cloud-native 5G core network to work alongside the IP Multimedia Subsystem (IMS). VoNR is also called Voice over 5G or Vo5G. VoNR or Vo5G is not limited to 5G only, and it can also be used in 4G networks.
4G LTE and 5G NR networks are expected to co-exist for a long time which is why they are designed to work with each other nicely through multiple deployment scenarios. Just like the 5G radio access network is able to utilise an existing 4G LTE core network (EPC), the 4G radio access network can also employ a 5G core network (5GCN).
In non-standalone 5G deployments (5G NSA), the 5G radio base station (gNodeB or gNB) can work with the 4G core network, Evolved Packet Core – EPC. On the other hand, it is also possible for an upgraded version of the 4G radio base station (next-generation eNodeB or ng-eNB ) to work with the Cloud Native 5G Core (5G CN).
While 4G networks rely on the packet-switched VoLTE technology for essential voice calling services, they also have a backup for instances where VoLTE is not supported by the phone or the network.
Circuit-switched fallback or CSFB is a technique used in a 4G LTE network that allows a 4G LTE phone to switch to 2G or 3G networks for voice calls and SMS if VoLTE is unavailable. The data technology in 5G networks is more mature and they do not use a circuit-switched backup for voice calls and messages.
I have written dedicated posts on Voice over LTE (VoLTE) and Voice over New Radio (VoNR) to help you find more details on these topics.
Here are some helpful downloads
Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:
Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.
Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.