Voice over 5G (Vo5G): Why is IMS used in 5G networks?

IMS or IP Multimedia Subsystem is a network architecture used in modern wireless networks, including 4G and 5G, to enable rich communication services. IMS is used in both 4G LTE (Long Term Evolution) and 5G NR (New Radio) networks for IP-based voice calls and SMS services.

IMS (IP Multimedia Subsystem) is needed in 5G NR networks to enable packet-switched voice calls and SMS services through the Voice over NR (VoNR) technology, also known as Voice over 5G (Vo5G). Voice calls and SMS require VoNR (Vo5G) in standalone 5G (5G SA) and VoLTE in non-standalone 5G (5G NSA).

Why is IMS used in 4G LTE and 5G NR mobile networks?

IMS stands for IP Multimedia Subsystem, and it is a network architecture that integrates with the 4G and 5G mobile core networks to enable IP-based real-time services, including traditional voice calls, text messages (SMS) and multimedia messages (MMS).

IMS was initially defined by 3GPP (Third Generation Partnership Project) in Release 5 and is based on the protocols defined by the standards organisation IETF (Internet Engineering Task Force). SIP or Session Initiation Protocol has been standardised by IETF and is the protocol used by IMS for enabling voice calls and other real-time services.

The second and third generations of mobile networks use two separate network components for providing voice, SMS/MMS and data services.

The circuit-switched part delivers voice calling and text messaging in 2G and 3G networks, while the packet-switched function enables mobile data services. However, the IP-based packet-switched part in 2G and 3G networks cannot guarantee Quality of Service (QoS) for voice calls and other real-time communication services.

The vision with 4G LTE networks is to offer all services over the IP network (data), and therefore they do not have a circuit-switched function. IMS or IP Multimedia Subsystem is how 4G LTE networks deliver real-time services such as voice calls, text and picture messages and video calls.

5G mobile networks use a cellular technology New Radio (NR) which consists of a 5G radio network (Next Generation Radio Access Network – NG RAN) and a mobile core network. However, 5G networks can be successfully deployed using a 5G core network or an existing 4G LTE core network.

An end-to-end 5G deployment requires a 5G core network (5G Cloud-Native – 5G CN), but 5G can also be deployed using an existing 4G LTE core network, Evolved Packet Core – EPC. The former deployment model is called standalone 5G or 5G SA, and the latter is called non-standalone 5G or 5G NSA.

Both 5G SA and 5G NSA require IMS (IP Multimedia Subsystem) to deliver voice calls and text and multimedia messages. Voice calls and messages in 5G NR networks are IP-based and are provided through packet-switching technology. IMS works alongside the 4G core network (EPC) to enable Voice over LTE (VoLTE) in 4G LTE networks.

If a 5G deployment employs a 4G core network (EPC), IMS can facilitate voice calls and messages in 5G through the VoLTE technology. However, if a 5G deployment uses a 5G core network (5G CN), IMS works with the 5G core network to deliver voice calls and messages through Voice over New Radio (VoNR) or Voice over 5G (Vo5G).

IMS allows 5G standalone networks to offer Vo5G

Voice over 5G or Voice over New Radio (VoNR) is the 5G version of Voice over LTE (VoLTE) to deliver voice calls, text and multimedia messages in 5G networks. It requires the IMS network architecture to work alongside the cloud-native 5G mobile core network (5G CN).

The fully end-to-end 5G network deployments are called standalone 5G (5G SA), requiring a dedicated 5G core network. 5G core network, called the cloud-native 5G core or 5G CN, uses a service-based architecture (SBA) which can facilitate advanced use cases through technologies like network slicing.

The delivery of voice calls and text messages is not the primary use case for 5G networks. Instead, the primary use case classes for 5G are Enhanced Mobile Broadband (eMBB), Massive Machine Type Communication (mMTC) and Ultra-Reliable Low Latency Communication (uRLLC).

Since 5G networks are designed to be standalone, i.e. work without relying on legacy networks (e.g. 4G LTE), they need to continue offering real-time services like voice calls and messaging that legacy networks already deliver. VoNR or Vo5G is the technology in 5G that allows them to continue offering voice, messaging and potentially other real-time services without any dependency on legacy networks.

5G networks, like 4G LTE networks, are data-only (packet-switched), which means that they do not have a circuit-switched function for traditional voice calls or messages. As a result, 5G networks require an IP-based solution similar to VoLTE for voice calls, SMS and other rich communication services.

The equivalent of VoLTE in the 5G networks is Voice over New Radio or VoNR, also known as Voice over 5G or Vo5G. The fundamental requirement for VoLTE and VoNR is the presence of a mobile core network that can integrate with IMS. In 5G standalone networks, that required is fulfilled by the 5G cloud-native core network (5G CN).

IMS allows non-standalone 5G networks to offer VoLTE

While Voice over LTE (VoLTE) is a 4G technology that delivers voice calls and text and multimedia messages in LTE networks, it can also be used in non-standalone 5G network deployments (5G NSA). In 5G NSA, VoLTE works with the 4G core network (Evolved Packet Core – EPC) to deliver voice and messages.

IMS or IP Multimedia Subsystem is the architecture that works with 4G or 5G mobile core networks to deliver voice calls, messages and other potential real-time services. It has the capability to combine IP telephony with data communications.

IMS in 4G connects the 4G LTE packet-switched core network, Evolved Packet Core, to external telephone networks such as landline (PSTN – Public Switched Telephone Network) and ISDN (Integrated Services Digital Network). Since circuit-switched calling in 2G and 3G networks can offer high-quality carrier-grade voice service, the VoLTE capability is based on requirements to meet the same standards.

Since the non-standalone 5G deployment (5G NSA) works with an existing 4G LTE core network (EPC), IMS can perform the same duties in 5G NSA as it does in 4G LTE through the VoLTE technology. VoLTE has Quality of Service (QoS) requirements such as mouth-to-ear latency, bandwidth, error correction and superior audio codecs to ensure a high-quality calling experience for a user.

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.

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