IMS_Tutorial3

8/6/2019 IMS_Tutorial3

1/7

Forschungszentrum Telekommunikation Wien

Igor Miladinovic

IMS Tutorial

Part 2: Service Infrastructure

ftw. 2005 IMS Tutorial

Agenda

! Services in IMS a short motivation

! Home Subscriber Server (HSS)

! Authentication

! Charging


Convergence of Services

Back Office

GSM

PSTN

Intern

et

TV

Application

Service Delivery/Session Control

Transmission

Terminals

From the PSTN

Back Office

Collaboratio

n

Intern

et

VoIP

TVoIP

Application


Transmission

Terminals

to todays Triple Play services

Source: Frederic Potter, Cirpack, International SIP 2005


Services in IMS

Source: Frederic Potter, Cirpack, International SIP 2005

=

Application


Transmission

Terminals

Pre

sence

Collaboration

VoIP

TV

oIP

Back Office

Collaboration

Pre

senc

e

Wireline

Access

Wireless

Access

SIP User Agents

SIPSIP

SIP User Agents

HSSDiameter

P-CSCF P-CSCF

SIP

VoIP

TV

oIP

S-CSCF

SIP

Application


Transmission

Terminals

Back OfficeCCF/CGF


2/7


Home Subscriber Server

! The master database for a given user

! Contains subscriptionrelated information tosupport the network entities handlingcall/sessions

! It is an evolution of the central database oftodays GSM provides, the Home LocationRegister (HLR)

! Can be contacted using Diameter protocol


Communication with HSS

! The IMS entities that need to communicate with a

HSS are:- I-CSCF

- S-CSCF

- SIP-AS

- OSA SCS

- CAMEL AS (IM-SSF)

! HSS offers the following interfaces:

- Si- Sh

- Cx


HSS Structure I

HSS

HLR/AUC Functionality for CS

HLR/AUC Functionality for PS

IP Multimedia Functionality


HSS Structure II

! Home Location Register and Authentication Center(HLR/AUC) functionality for PS

- provides support to PS domain entities, such as SSGN andGGSN (necessary for access to PS domain services)

! HLR/AUC functionality for PS

- provides support to CS domain entities, such as MSC servers(necessary for access to CS domain services and for roaming toGSM/UMTS CS domain networks)

! IP Multimedia functionality

- Stores IMS related data including: private and public useridentities, IMS access parameters, service triggering

information, user-specific requirements for S-CSCF capabilities! Communication between HSS functionalities is not

standardized


3/7


The Sh Interface

! Reference point between the HSS and the AS

(SIP-AS or OSA SCS)! The protocol used on this interface is Diameter

! An AS may need some user data from the HSS orto store some service-related data into the HSS

! The HSS maintains a list of ASs that are allowedto obtain or to store data

! Procedures are divided into two categories:- Data handling

- Subscription/notification


Data Handling

! Procedures that make possible to retrieve user

data from the HSS and to store some user datainto the HSS

! User data can be:- Service-related data (transparent or non-transparent)

- Registration information

- Initial filter criteria

- S-CSCF name serving the user

- Addresses of the charging functions- Location information


Subscription/Notification

! Procedures that allow the AS to get a notificationwhen particular data for a specific user is updated

in the HSS! Commands over the Sh Interface :

- User-Data-Request (UDR) the AS uses UDR to requestdata from the HSS

- Profile-Update-Request (PUR) the AS can updatetransparent data in the HSS using this command

- Subscribe-Notification-Request (SNR) the AS sendsSNR to get notified when certain user data changes

- Push-Notification-Request (PNR) the HSS sends PNR tothe AS when particular user data changes


S-CSCF

IMS Authentication Architecture

HSS

P-CSCF

I-CSCF

SIP-AS OSA-SCS

ShSh

Cx

Cx


4/7


Authentication

! The protocol used for authentication between any

two nodes is Diameter with an IMS-specifictailored application that defines new Diametercommand codes and new Attribute Value Pairs(AVPs)

! Authentication is directly coupled to SIPregistration procedures

! It is based on a shared secret and a sequence

number (SQN), which is only available in the HSSand the IP Multimedia Service Identity Module(ISIM) application located in the UE


Authentication during the IMS registration (I)

P-CSCF I-CSCF S-CSCFUE HSS

REGISTERREGISTER

REGISTER

401 Unauthorized401 Unauthorized

401 Unauthorized

UAR

UAA

MAR

MAA


Authentication during the IMS registration (II)

! User sends the private user identity (stored withinthe ISIM application) in the initial REGISTERrequest

!I-CSCF contacts the HSS using User AuthorizationRequest (UAR) and forwards the request to the S-CSCF

! S-SCSF downloads the authentication vector (AV)from the HSS using Multimedia-Auth-Request(MAR)

! S-SCSF rejects the REGISTER request with the401 response that includes a random challenge

(RAND), the network auth. token (AUTN), theintegrity key (IK), and the ciphering key (CK)


Authentication during the IMS registration (I)

P-CSCF I-CSCF S-CSCFUE HSS

REGISTERREGISTER

REGISTER

401 Unauthorized401 Unauthorized

401 Unauthorized

REGISTERREGISTER

REGISTER

200 OK200 OK

200 OK

UAR

UAA

MAR

MAA

UAR

UAA

SAR

SAA


5/7


Authentication during the IMS registration (III)

! UE processes the received parameters to the ISIMapplication which verifies AUTN (based on the

shared secret and the SQN), calculates the result(RES) (based on the shared secret and RAND),and calculates IK

! Afterwards, UE sends the second REGISTERrequest with the authentication challengeresponse (RES) to the S-CSCF

! S-CSCF compares it with XRES and if theverification is successful, the S-CSCF replies with

the 200 OK response! Additionally, the S-CSCF sends the Server-

Assignment-Request (SAR) to the HSS


Charging in IMS

! Specified in TR 23.815 (Charging implications of

IMS architecture)! Two models of charging in IMS

- Offline charging

- Online charging

! Offline charging- Charging information is mainly collected after the session

- A user typically receives a bill on a monthly basis

!

Online charging- IMS entities interact with the online charging system

- Charging system interacts in real time with the usersaccount


Offline Charging Architecture (I)

S-CSCFS-CSCF

I-CSCFI-CSCF

ASAS

MGCFMGCF

BGCFBGCF

P-CSCFP-CSCF

GGSNGGSN

SGSNSGSN

BSBS

BSBSMRFCMRFC

Home(B)

Visited(B)

Rf

Ga

Rf

Rf

Rf

Rf

Rf

Charging informationflow

CGFCGF

CCF

CGFCGF

CCF

Ga

S-CSCFS-CSCF

I-CSCFI-CSCF

ASAS

MGCFMGCF

BGCFBGCF

P-CSCFP-CSCF

GGSNGGSN

SGSNSGSN

BSBS

BSBS MRFCMRFC

Home(A)

Visited(A)

Rf

Ga

Rf

Rf

Rf

Rf

Rf

CGFCGF

CCF

CGFCGF

CCF

Ga

Source: 3GPP TR 23.815 V5.0.0 (2002-03) ftw. 2005 IMS Tutorial

Offline Charging Architecture (II)

! The central point is the Charging CollectionFunction (CCF)

- CCF receives accounting information from IMS entities via

the Rf interface, creates Charging Data Records (CDRs),and transfers CDRs to the Billing System (BS)

- CCF can be implemented as a separate network element oras an integrated functionality resident in the IMS entities

! Charging Gateway Function (CGF) collect charginginformation from SGSN and GGSN

! The Billing System (BS) creates the actual bill (for

example, number of sessions, duration, destination,and type of sessions)


6/7


Offline Charging Interfaces

! Rf:

- Based on Diameter and the Diameter Application for theRf/Ro interface

- Diameter messages over this interface are AccountingRequests (ACRs) and Accounting Answers (ACAs)

- IMS session-related ACRs are called start, interim, andstop

- Non-session-related ACRs are called event ACRs

! Bi:- Interface between CCF and BS

- Not specified, recommendation is the FTP over TCP/IP


Online Charging Architecture (I)

Source: 3GPP TR 23.815 V5.0.0 (2002-03)

Charginginformation

flow

ISC

SGSNSGSNCAP

Online Charging System

Home(A) + Visited(A)

Re

Rb

Ro

Ro

Re

Rc

MRFCMRFC

AS(s)AS(s)

ISC

SGSNSGSNCAP

Online Charging System

Home(B) + Visited(B)

Re

Rb

Ro

Ro

Re

Rc

Account

CorrelationFunctionBearer

ChargingFunction

Account

CorrelationFunction Bearer

ChargingFunction

RatingFunction

RatingFunction

S-CSCFS-CSCF S-CSCFS-CSCF

AS(s)AS(s)

MRFCMRFC SCCF

CPCF

SCCF

CPCF

Session

ChargingFunction

Session

ChargingFunction

EventChargingFunction

EventChargingFunction


Online Charging Architecture (II)

! IMS entities able to perform online charging are:S-CSCF, AS, and Multimedia Resource Function

Controller (MRFC)! Event Charging Function performs charging

authorization for event charging

! Session Charging Function performs chargingaccording to session resource usage (includingauthorization) based on received SIP messagesover the ISC interface (Ro instead of ISC?)

!Bearer Charging Function over CAMEL App. Part(CAP) interface allows permission for bearer usage


! Rating function performs unit, price, and tariffdetermination

!

Correlation function is corresponding forproducing charging data regarding a single IMSsession based on charging data from differentsources

! Ro interface is similar to the Rf interface withsome additional Diameter credit control AttributeValue Pairs (AVPs)

!Rb, Rc, and Re interfaces are still notstandardized

Online Charging Architecture (III)


7/7


Lessons Learnt

! HSS is the master database for a given user in

IMS! HSS extends HLR functionality with the IMS

functionality

! ASs communicate with a HSS over the Shinterface

! IMS implements SIP authentication based on ashared secret during the registration process

! IMS charging specifies two different architectureto allow offline as well as online charging

IMS_Tutorial3

Documents

Transcript of IMS_Tutorial3