IMS is an architectural framework that uses SIP to deliver IP multimedia services to mobile users. It consists of common core elements, enablers, and support systems arranged in three layers. The control layer contains various nodes that handle signaling and session management, including the P-CSCF for access, I-CSCF for routing, and S-CSCF for authentication and services.

1. IMS – IP MULTIMEDIA
SUBSYSTEM
Structure, Architectural nodes & Session Establishment
flow
By

2. WHAT IS IMS?
• IP Multimedia Subsystem
• An architectural framework for delivering IP
Multimedia to multiple mobile users.
• Uses IETF (internet) protocol i.e. SIP (Session
Initiation Protocol)

3. STRUCTURE OF IMS

4. IMS STRUCTURE Continued…..
• Consists of common core, enablers, support
systems, internetworking functions enabling
operators & service providers.
• Layered architecture: Application layer,
control layer, connectivity layer.
• Layered architecture allows each layer to
evolve independently as market & technology
demands changes.

5. IMS ARCHITECTURE

6. IMS ARCHITECTURE Continued….
• Definition: IMS is an architecture designed to
support the control layer for packet based
services.
• Types of IMS nodes:
– DATABASE ELEMENTS (HSS, SLF)
– IMS CONTROL ELEMENTS (P-CSCF, I-CSCF, S-CSCF)
– CONTROL PLANE INTERWORKING ELEMENTS
(MGCF, BGCF, S-GW)

7. IMS ARCHITECTURE Continued….
• DATABASE ELEMENTS:
HSS:
 subscriber data & authentication data
 location register
 IMS service profile processing
 accessed by S-CSCF & I-CSCF
 Uses diameter protocol
 SLF:
 accessed by S-CSCF & I-CSCF
 Which HSS stores the user data when 1 or more HSS is present
in the network
 Query contains identification of user
 Response contains the required specific HSS

8. IMS ARCHITECTURE Continued….
• IMS CONTROL ELEMENTS: call session control
functions, acts on control (SIP) signalling
flows.
S-CSCF:
 performs session management
 authenticates the user
 main control point for services
 decides on handling of service requests
 always located in home network

9. IMS CONTROL LAYER Continued….
 P-CSCF:
 main network point towards IMS network
 It is a stateful SIP proxy
 Signalling b/w user & IMS routed through I-CSCF
 Responsible for SIP registration with S-CSCF through I-CSCF
 Responsible for maintaining security with user throughout the
connection.
 Includes PDFs which authorizes use of bearer & QoS resources
 I-CSCF:
 1st point of contact from home to an external network
 main job is to query HSS & find location of S-CSCF.

10. IMS ARCHITECTURE Continued….
• CONTROL PLANE INTERWORKING ELEMENTS:
 MGCF:
Enables communication b/w IMS & CS users
 controls media channels
BGCF: responsible for choosing where a breakout to
the CS domain occurs.
 S-GW:
used to connect signaling networks such as SCTP/IP based &
SS7 networks
 performs signaling conversion

11. IMS SESSION ESTABLISHMENT FLOW