IMS, or IP Multimedia Subsystem, is a framework for delivering multimedia services over IP networks. In an IMS call flow, here’s a simplified explanation of how it works:
Registration: When a user’s device, like a phone, connects to the network, it goes through a registration process. The device sends a REGISTER message to the IMS network, indicating its presence and capabilities.
Session Establishment: When you want to make a call, your device sends an INVITE message to the IMS network, indicating the destination. The IMS core then locates the recipient and sends an INVITE to their device.
Session Description: Both devices exchange session descriptions that outline the type of media (audio, video) they can handle, codecs, and other settings. This is done using SDP (Session Description Protocol).
Media Negotiation: The devices and the network agree on the best possible media format to use for the call, considering the capabilities of both ends.
Routing and Transcoding: If necessary, the IMS core might perform media transcoding to ensure both devices can communicate seamlessly even if they support different codecs.
Call Setup: Once media settings are agreed upon, the IMS core coordinates the call setup, connecting the two devices.Call Handling: During the call, the IMS core manages things like Quality of Service (QoS), call control (hold, transfer), and other features.
Call Termination: When the call ends, one of the devices sends a BYE message to the other, indicating the desire to disconnect. The IMS core then tears down the session.
Deregistration: When a device disconnects from the network or goes out of coverage, it sends a deregistration request to the IMS core.
Keep in mind that this is a simplified overview, and in a real IMS environment, there are numerous network elements and protocols involved to ensure the call quality, security, and various services work smoothly.