There are so many different components involved in IMS. The scope of the components varies depending on the parties participating in a specific communication (e.g., IMS phone to IMS phone, IMS phone to non-IMS phone, etc.)
These components work together within the IMS architecture to deliver a wide range of IP-based multimedia services, ensuring seamless communication across various types of networks.
CSCF (Call Session Control Function):
- P-CSCF (Proxy CSCF): Acts as the first point of contact for the IMS user, handling signaling between the user's device and the IMS network. It also performs security functions and session management.
- I-CSCF (Interrogating CSCF): Routes incoming requests to the appropriate S-CSCF and acts as a contact point within an operator's network for all connections destined for a subscriber of that network.
- S-CSCF (Serving CSCF): Performs session control for IMS users, handling registration, session setup, and routing. It also enforces service policies and interacts with the HSS for subscriber data.
HSS (Home Subscriber Server): A central database that contains user-related and subscription-related information. It supports user authentication, service profiles, and location services.
SLF (Subscription Locator Function): If multiple HSSs are used, the SLF determines which HSS contains the specific subscriber's data.
MGCF (Media Gateway Control Function): Controls media gateways that interface with the circuit-switched networks, enabling voice and multimedia calls across network boundaries.
MGW (Media Gateway): Transfers media streams between the packet-switched IMS networks and the circuit-switched networks, performing necessary conversions.
BGCF (Breakout Gateway Control Function): Determines the route for outgoing calls to external networks, especially for routing calls to the PSTN (Public Switched Telephone Network) or other networks not using IMS.
MRF (Media Resource Function):
- MRFC (Media Resource Function Controller): Controls the media resources in the MRF, deciding on the use of resources based on the application logic.
- MRFP (Media Resource Function Processor): Executes media processing tasks such as tone generation, media mixing, and media streaming under the control of the MRFC.
AS (Application Servers): Host and execute IMS services, interacting with the S-CSCF to provide services like VoIP, conferencing, messaging, and presence.
PCRF (Policy and Charging Rules Function): Determines policy rules and charging control decisions for data flows within the user's session, ensuring that resources are used according to the operator’s policies.