© IMS Essentials 1

What-Is: IMS3GPP IMS – IP Multimedia Subsystem

� IMS is a user focused application platform, enabling IP-Multimedia Services.

� IMS is based on the SIP protocol and Standardized Service Enablers.

Generic IMS Functionality

� User Location Management.

� Service Control.

� Access Network Functionality.

� Border Functionality.

Access Netw

ork

Policy Control

Service Control

Traffic Bearer

Border

Control

SIP: IN

VITE

IMS OperatorA

Repository

SIP Proxy

Enabler

Enabler

Application Server

SGSN

Access Network (IP-CAN)

Access GW(GGSN)

IMS OperatorB

Policy

© IMS Essentials 2

IMS Registration flow

Home Network of Terminal A

Visited Network of Terminal A

Cx: Notify and Get User Profile

5

S-CSCF (A)

P-CSCF (A)

I-CSCF

HSS (A)

1SIP: REGISTER

(IMPI,IMPU)

3

Cx: Discover S-CSCF

Cx Diameter

SIP Protocol

RE

GIS

TE

R

2

4

IMPI

- IMS P

rivate U

ser-I

D

NAI f

ormat

[RFC

-246

8]

e.g. 2

3415

0999

9999

99@im

s.mnc

015.m

cc23

4.o2.com

IMPU

- IMS

Publi

c Use

r-ID

SIP-url fo

rmat

[RFC

-239

6]

e.g. d

avid@

o2.com

ISIM

Home R

ealm

= H

ome O

perat

or

© IMS Essentials 3

IMS Registration Scenario

Visited IMS Home IMS

2. SIP: REGISTER 1 (Private-ID, Public-ID)

P-CSCF S-CSCFHSS

DNS Server

1. Attach to IP-CANActivate Signalling Bearer get IP-Address

4. SIP: REGISTER 1

7. SIP: REGISTER 2 (Challenge Response)

Authenticate

6. 401 UNAUTHORIZED (Random Challenge)

AKA - Authentication & Key Agreement

9. 200 OK

I-CSCF

IPSec Encrypted Tunnel

Gateway

Answer: S-CSCF Name

3. Cx: Status Query (Private-ID, Public-ID)

Answer:(Challenge, Response, Ciphering-Key)

5. Cx: Vector Request

Answer: (User’s Profile )

8. Cx: Registration Notify