What is IMS?
IMS (IP Multimedia Subsystem) is a carrier-grade architecture for delivering multimedia services over IP networks. IMS unifies voice, video, messaging, and additional services on a single platform.
The IP Multimedia Subsystem has become the foundation of modern VoLTE and VoWiFi networks built on the ePDG solution, where voice is no longer carried over traditional circuit switching.
How a call works through IMS
When a user makes a VoLTE call, the smartphone sends a SIP INVITE to the operator’s IMS network. The CSCF validates the session parameters, requests data from the HSS, and determines the call route.
Once the connection is established, voice is transmitted as IP traffic over the LTE or 5G network.
What IMS consists of
The core element of the architecture is the CSCF (Call Session Control Function). It processes SIP requests and manages connection logic.
The HSS (Home Subscriber Server) stores subscriber profiles, authentication parameters, and service data.
Media Gateway nodes connect the IP infrastructure to traditional telephone networks.
Component interaction is built around SIP — the primary signaling protocol in IMS.
Why operators need IMS
Voice, SMS, and data used to exist as separate services with separate infrastructure. IMS moves all communication into an IP environment and enables new services to be launched without rebuilding the entire network.
The platform handles:
- subscriber registration;
- SIP signaling;
- call routing;
- multimedia session management;
- service integration.
This allows operators to centrally manage voice, video, and messaging through a single core.
