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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.
The IP Multimedia Subsystem (IMS) was originally conceived as a ground-breaking platform for a wide range of multimedia services - think gaming, IPTV and streaming. Its name, "Multimedia" system, hints at this ambitious scope. The aim was to create a unified, converged platform capable of handling all operator services seamlessly. Over time, however, IMS has found its niche primarily in telephony. A particularly interesting chapter in the history of IMS is VoHSPA (Voice over High-Speed Access), which attempted to bring telephony over 3G using IMS. Remarkably, almost all chipsets and handsets at the time supported the technology, and it passed early tests with flying colours. But despite the promise, operators clung to their tried-and-tested legacy systems, leaving this innovative approach largely unexplored. Today, IMS systems are an integral part of modern telecommunications and are the standard for the new generation of networks, supporting advanced voice services such as VoLTE, VoNR and VoWiFi.
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, therefore, operators now need an IMS as the voice solution for launching VoLTE (Voice over LTE) and VoNR (Voice over New Radio).
The IMS architecture 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
The IMS core is a foundational architecture used to deliver multimedia services over IP-based networks. Here are some of the key design aspects that define the nature with which the IMS core was conceived:
The IMS uses different types of identities to manage user access and services:
The IMS uses several interfaces to communicate with external systems:
Session management in IMS follows a detailed process:
Unlike traditional VoIP or OTT services, IMS and mobile networks use a SIM or eSIM for authentication. A SIM (Subscriber Identity Module) is not just a simple key or password storage but functions more like a small computer. The phone itself does not validate the subscriber based on credentials; instead, it forwards the authentication data to the SIM, which handles the verification process.
The primary role of the SIM card is to verify the legitimacy of the subscriber. When you make a call, send an SMS, or share media, the SIM guides your phone on which network to connect to and enables the network to bill you for the services you use. This process relies on the IMSI (International Mobile Subscriber Identity) number and an authentication key that verifies the IMSI—essentially functioning as the login credentials for your mobile account. When you power on your phone, it retrieves the IMSI from the SIM and sends it to the network. The network then checks its database to find the matching authentication key for that IMSI. To complete the authentication, the network generates a random number, referred to as “X,” and signs it with your authentication key to create a new number, “Y.” This number “X” is sent to your phone, which passes it to the SIM. The SIM then signs the random number “X” with its own authentication key, producing a new number, “Z.” This process ensures that both the device and the network can confirm the identity of the subscriber securely.
Although complex, this process takes only a few milliseconds and provides a high level of security, as sensitive information is never transmitted over the network.
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 sales@ng-voice.com or request a demo.
