There are 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.
- Handles SIP signaling between the user's device and the IMS network.
- Performs security functions such as authentication and integrity protection.
- Manages session establishment and maintenance.
- Compresses/decompresses SIP messages for efficient transmission.
- I-CSCF (Interrogating CSCF):
- Routes incoming SIP requests to the appropriate S-CSCF.
- Acts as a contact point within an operator's network for all connections destined for a subscriber of that network.
- Performs topology hiding to protect network structure.
- Interacts with the HSS to obtain subscriber location information.
- S-CSCF (Serving CSCF):
- Performs session control for IMS users.
- Handles user registration and authentication.
- Manages session setup, modification, and termination.
- Enforces service policies and triggers application servers.
- Interacts with the HSS for subscriber data and service profiles.
HSS (Home Subscriber Server):
- Central database containing user-related and subscription-related information.
- Supports user authentication and authorization.
- Stores service profiles and preferences.
- Provides location information for users.
- Interfaces with CSCF entities for subscriber data retrieval.
SLF (Subscription Locator Function):
- Used when multiple HSSs are deployed in the network.
- Determines which HSS contains the specific subscriber's data.
- Provides HSS address information to CSCF entities.
MGCF (Media Gateway Control Function):
- Controls media gateways that interface with circuit-switched networks (e.g., PSTN).
- Handles signaling conversion between SIP (IMS) and ISUP/BICC (circuit-switched).
- Coordinates call setup and teardown across network boundaries.
- Works with MGW to enable voice and multimedia calls between IMS and legacy networks.
MGW (Media Gateway):
- Transfers media streams between packet-switched IMS networks and circuit-switched networks.
- Performs media format conversion (e.g., codec transcoding).
- Handles echo cancellation and tone generation.
- Works under the control of MGCF for media path management.
BGCF (Breakout Gateway Control Function):
- Determines the route for outgoing calls to external networks.
- Handles breakout to PSTN or other non-IMS networks.
- Selects the appropriate MGCF for call routing.
- Supports policy-based routing decisions.
MRF (Media Resource Function):
- MRFC (Media Resource Function Controller):
- Controls the media resources in the MRF.
- Decides on the use of resources based on application logic.
- Interfaces with application servers to provide media services.
- MRFP (Media Resource Function Processor):
- Executes media processing tasks such as tone generation, media mixing, and media streaming.
- Handles conferencing, announcements, and interactive voice response (IVR).
- Operates under the control of the MRFC.
AS (Application Servers):
- Host and execute IMS services and applications.
- Interact with the S-CSCF to provide services like VoIP, conferencing, messaging, and presence.
- Support third-party and operator-provided services.
- Enable service innovation and customization.
PCRF (Policy and Charging Rules Function):
- Determines policy rules and charging control decisions for data flows within the user's session.
- Ensures resources are used according to the operator’s policies.
- Interfaces with gateways to enforce QoS and charging rules.
- Supports dynamic policy management for multimedia sessions.