This document provides an introduction to the core IMS/VoLTE network elements and protocols. It describes the evolution from circuit-switched to packet-switched voice services and highlights key differences. The core elements discussed include the P-CSCF, I-CSCF, S-CSCF, AS, MRF, MGW, BGCF, and CCF. Basic call flows for registration and VoLTE-to-VoLTE and VoLTE-to-CS calls are presented to illustrate signaling and interactions between elements.

1. Core IMS/VoLTE
An Introduction for CS Core Specialist
Ing. Adrian Lopez Mendoza

2. IMS Core Introduction
• Changes from CS
• Basic Elements

3. CS to PS Voice Evolution

4. Differences in Access Between CS and VoLTE

5. Basic Diagram

6. Voice System Evolution
MSC
CCF
Billing
TAS
Telephony Services
CSCF
Call Session
SBC
MGW
BGCF
P-CSCF
EATF
Open MGW
MRF
Media Resource

7. AS (The Brain)
Aplication Server
• A Server Executes an applicaction (Telephone, OSA, APP)
• The most important applicaction is the Telephone (TAS)
• Receives SIP signalling to handle a service (Control) from S-CSCF
• Connects and ask details about UE to the HSS
• Application Server Layer
Protocols
SIP
Diameter

8. P-CSCF (The Proxy)
Proxy Call Session Control Function
• The first element in the IMS Core from Access
• Receives Control Signalling from GGSN and routes to I-CSCF
• Check identity of the users and confirm this for processing for the S-CSCF
• Session Control Layer
Protocols
SIP

9. I-CSCF (Interrogator)
Interrogating Call Session Control Function
• Responsible for applying network domain security for topology hiding
• Locate and route to signaling from a home or visited P-CSCF to the S-CSCF
• Originates a request to the HSS to obtain the routing information
• Session Control Layer
Protocols
SIP
Diameter

10. S-CSCF (The Service Deliver)
Serving Call Session Control Function
• Enables, obtains, and stores the user profiles from the HSS
• Communicates the registration information
• Selects an AS to handle a call
• Session Control Layer
Protocols
SIP
Diameter

11. MGCF (The Old Man)
Media Gateway Control Function
Protocols
SIP
H.248
Mobile Softswitch Center

12. MRF (The receptionist)
Media Resource Function
• Provides announcement playback
• Receives a request from SCSCF to get a tone an delivered the sound
• Session Control Layer
Protocols
SIP
RTP

13. SCC -AS (The Gateway)
Service Centralization and
Continuity Application Server
• IMS control of services where the media is transported via the CS network
• Supports Single Radio Voice Call Continuity
• Session establishment when using CS access for an IMS service
• Connection with HSS and SCSCF
• Application Server Layer
Protocols
SIP
Diameter

14. CCF (the accountant)
Charging Collection Function
• Collects charging records from the IMS elements for off-line charging
• Obtains the Records from TAS
• Session Control Layer
Protocols
Diameter

15. Basic Call Flows
• Registration
• VoLTE to VoLTE
• VoLTE to CS

16. Registration

17. Call VoLTE to
VoLTE

18. VoLTEtoCS