In this blog article, we are going to discuss what is an IP Multimedia Subsystem (IMS), giving an overview of the IMS architecture and its different layers. We are also going to take a look at ng-voice’s cloud-native IMS and what makes it unique and different from others.

What is an IP Multimedia Subsystem (IMS)? A look into ng-voice’s fully containerized and cloud-native IMS.

The Internet Protocol Multimedia Subsystem, or more commonly known as IP Multimedia Subsystem (IMS), is an architectural framework that was developed by an industry association called 3G.IP in 1999, which later on, was proposed to the 3rd Generation Partnership Project (3GPP). The IMS initially was not about telephony, but more about multimedia applications during the age of 3G. When IMS was introduced, it was not planned as a telephony replacement, but more like a “service extension”. Today, the architectural framework has become standardized across the industry for next-generation mobile networks.

IMS Architectural breakdown

IP Multimedia Subsystem Architectural breakdown

The IMS architectural framework consists of 3 different layers (service/application, control and transport layer). These 3 separated layers provide functions to manage signals and traffic for multimedia applications.


Service Layer/Application layer – The application layer is where it hosts and executes services provided to users (e.g. 4G/5G mobile networks, Wifi or any other network infrastructure)

Control Layer – This layer is responsible for session controlling and management. The control layer regulates the traffic between the transport layer and the service/Application layer by authenticating and distributing traffic (eg. the Proxy-/I-/S-CSCF).

Transport layer – This layer is responsible for supporting the architecture’s core network by acting as a gateway linking access layers and IP networks (ranging from Application servers for supplementary and other services, from SMS, Call-Forwarding or even more sophisticated services such as IP-TV).

The image below shows ng-voice’s IMS architectural framework. 

The IMS or the control layer is responsible for providing session control and governing communications based on subscription information and network policies. Below we have stated the roles of each element in the IMS. 


1- P-CSCF – Proxy Call/Session Control Function

The P-CSCF provides the central entry point into the operator network. All requests need to go through the P-CSCF and that entity should be located close to the connecting endpoint. This is to increase network efficiency by avoiding useless traffic transport and reducing latency.

2- I-CSCF – Interrogating-Call/ Session Control Function

The Interrogating-CSCF (I-CSCF) queries the Home Subscriber Server (HSS) and based on its response routes the message to the correct S-CSCF. If no S-CSCF is indicated by the HSS, the I-CSCF will select S-CSCF.

3- S-CSCF – the Serving Call/Session Control Function

The Serving-CSCF is the worker in the IMS world. It communicates with the HSS using Diameter (Cx-interface) to retrieve information about authentication and in order to retrieve user profiles for triggering different application servers

4 – Emergency Services Implementation (incl. E-CSCF)

The P/I/S-CSCF implementations have been extended to identify emergency registrations. In case of such a registration, the I-CSCF will add an emergency flag in the Diameter User-Auth-Request so that the HSS will ignore roaming restrictions. 

The signaling protocol that is commonly used to communicate is Session Initiation Protocol (SIP). SIP is used for managing, maintaining, routing as well as authorizing and terminating real-time sessions. To put it in simple terms, Session Initiation Protocol (SIP) signals and controls the multimedia communication sessions

What makes ng-voice’s fully containerized and cloud-native IMS unique?

ng-voice’s fully virtualized turnkey IMS/ VoLTE solution enables voice services on any 4G, 5G, WiFi, or any other data network. It is a fully-fledged VoLTE solution, including a lightweight PCRF and a broad range of application servers (e.g. MMTel/TAS, IP-SM-GW, AGW, MRF, Legal Intercept) and is based on 3GPP standard architecture. ng-voice’s solution is cloud-native, easy to deploy, scales for any network size, and integrates with any infrastructure: 


Smallest container sizes: Based on microservices and fully containerized using Kubernetes, our container size of less than 25 MB allows for rapid service delivery, extending the capacity of your platform. 

100% cloud-native and platform-agnostic: Born in the cloud, our IMS runs on any public, private (bare metal) or hybrid cloud environments, and off-the-shelf hardware.

Minimal resource footprint: The most efficient use of hardware resources thanks to cloud-native architecture, very small container size and modular approach.


ng-voice’s fully containerized and cloud-native IMS brings a completely new approach to mobile infrastructure software. Based on Kubernetes architecture, with the smallest container sizes, high level of automation, and efficient use of resources, our IMS is scalable, flexible, and easy to deploy and manage, offering operators and enterprises of any size innovative voice services on LTE/5G networks. To learn more about it, please contact our team at or request a demo.

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Carsten bock | about us
Carsten Bock

Founder and CTO

Publisher Bio

With 20 years+ experience in VoIP and IMS, Carsten led the development and implementation of the first IMS/ VoLTE platform at Telefonica in Germany and was a senior member of the Kamailio management team, developing the IMS extensions in Kamailio. He founded ng-voice in 2011 because he wanted to develop a flexible alternative to existing IMS solutions in the market.

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