Is IMS needed in 5G networks?

IMS or IP Multimedia Subsystem is a network architecture used in modern wireless networks for rich communication services. One of the most apparent use cases for IMS in 4G LTE networks is Voice over LTE or VoLTE. However, IMS is not limited to 4G LTE only, but it is also a key component in the 5G New Radio (5G NR) networks. This post aims to explain why IMS is used in 5G NR networks.

What is IMS – IP Multimedia Subsystem?

Mobile networks initially provided voice and data services through different network components. The circuit-switched part delivers the voice calling in 2G and 3G networks, whereas the packet-switched function enables mobile data. The packet-switched part is IP based, and in 2G and 3G networks, it cannot ensure service quality for real-time services, including voice calls. The vision with 4G LTE networks was to offer all services over the IP network, and therefore they do not have a circuit-switched function. IMS or IP Multimedia Subsystem is how 4G LTE networks provide real-time services such as voice calls, instant messaging and even video calls. IMS was originally 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.

What IMS does for 4G networks

IMS is the architecture behind voice calling in 4G LTE networks which combines IP telephony with data communications. In LTE, IMS works with the LTE mobile core network, Evolved Packet Core (EPC), to enable real-time services, including Voice over LTE (VoLTE) for voice calls and text messages (SMS). IMS connects the 4G LTE packet-data-network 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 has requirements that allow it to meet the same standards. 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.

What IMS does for 5G networks

5G networks, just like 4G networks, are also data-only (packet-switched), which means no circuit-switched function for traditional voice calls. As a result, they require an IP-based solution similar to VoLTE for voice calls, SMS and other rich communication services. The equivalent of VoLTE in the 5G network is Voice over New Radio or VoNR, also known as Voice over 5G or Vo5G. 5G networks can be deployed in two ways and can be either stand-alone or non-stand-alone. A stand-alone 5G network (SA) has a 5G radio network as well as a 5G core network. On the other hand, a non-stand-alone 5G network (NSA) has a 5G radio network, but it works with an existing 4G LTE core network (EPC- Evolved Packet Core). The fundamental requirement for VoLTE and VoNR is the presence of an equivalent core network. NSA works with the LTE core network (EPC); therefore, it can only facilitate Voice over LTE (VoLTE), whereas the SA deployment has a 5G NR core network and consequently, it can facilitate Voice over NR (VoNR).


IMS or IP Multimedia Subsystem is needed in 5G New Radio (NR) deployments to facilitate rich communication services, including voice calls and SMS through Voice over New Radio (VoNR). Non-stand-alone 5G deployments (NSA) use the existing VoLTE services for 5G devices, whereas stand-alone 5G deployments (SA) use VoNR for 5G devices.

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 some extra support especially when preparing for a new job, or studying a new topic, or maybe just 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 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 product overview and product roadmap.

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