8/6/2019 IMS Guerilla School

1/19

IP Multimedia System

Ten Minute Gorilla School

8/6/2019 IMS Guerilla School

2/19

Home Subscriber Server (HSS)

Includes Home Location Register (HLR)function

Includes Authentication Center (AUC) function

Stores subscriber and service related datao User identitieso Registration informationoAccess parameters

o Service triggering informationoAuthentication keys

8/6/2019 IMS Guerilla School

3/19

Call/Session Control Function (CSCF)

Routing of Call Signalingo (SIP proxy)

Authentication & Authorizationo (SIP registrar)

Interface to Home Subscriber Server (HSS)o DIAMETER

Three Typeso Proxyo Servingo Interrogating

8/6/2019 IMS Guerilla School

4/19

Authentication & Authorization UE initiates a SIP

registration to the CSCF CSCF requests the users

profile from the HSS usingDIAMETER

If an acceptable user profileis found an authenticationchallenge is returned

If the response is asexpected a server

assignment is sent to theHSS

On successful assignmentthe registration is confirmed

8/6/2019 IMS Guerilla School

5/19

User Profile

8/6/2019 IMS Guerilla School

6/19

Public Identification

Multiple public useridentities may beassociated with a

service profile May be either sip: or tel:URIs

8/6/2019 IMS Guerilla School

7/19

Core Network Service Authorization

Controls filtering related to subscribed media There may be zero or one instance for each

subscriber profile

Contains an integer which identifies asubscribed media profile in the S-CSCF

Media profile determines allowed SDPparameters

8/6/2019 IMS Guerilla School

8/19

Initial Filter Criteria

Described in XML Determines when an

incoming SIP message

is forwarded to anapplication server Priority value specifies

order triggers areapplied

8/6/2019 IMS Guerilla School

9/19

Service Point Trigger

Matches SIP messageso Addressed resourceo Methodo Headers

Session Caseo Originatingo Terminatingo Terminating unregistered

May match on SDP

content

8/6/2019 IMS Guerilla School

10/19

Call/Session Routing

UE initiates a callsession

INVITE is comparedagainst the filter criteriafor the originating UE

INVITE is forwarded todestination CSCF (itselfin this example)

INVITE is compared

against the filter criteriafor the terminating UE

INVITE is forwarded todestination UE

8/6/2019 IMS Guerilla School

11/19

CSCF Types

Proxy (P-CSCF)o First hop for all UE

originated SIP messages

Interrogating (I-CSCF)o First hop for all inbound

SIP messages fromoutside networks

Serving (S-CSCF)o SIP registraro Controls services

8/6/2019 IMS Guerilla School

12/19

Proxy-CSCF (P-CSCF)

First contact point for users within the IMS All SIP signaling from or to the UE passes

through the P-CSCF

Performs as a proxy as defined in RFC3261 Compress/decompress UE SIP signaling Maintains IPsec security association with UE Perform media policing on SDP payload

8/6/2019 IMS Guerilla School

13/19

Interrogating-CSCF (I-CSCF)

First contact point for users outside the IMS Contacts the HSS to obtain the name of the

S-CSCF serving currently a user

Assign an S-CSCF based on capabilitiesreceived from the HSS when none isassigned

May include optional Topology Hiding Inter-

network Gateway (THIG) function

8/6/2019 IMS Guerilla School

14/19

Serving-CSCF (S-CSCF)

Main point of control in the IMS Located in the Home network Acts as a registrar as defined in RFC3261

Authenticate users using IMS AKA Performs session control by acting as a proxy

and UA as defined in RFC3261 Maintain call state

Decides when to route SIP to applicationservers

8/6/2019 IMS Guerilla School

15/19

Application Server (AS)

Functions layered on top of the IMS core Located in the Home network May originate, terminate or transit SIP

messages Multiple ASs may be involved in a session May bridge to non-IMS functions (e.g.

CAMEL)

8/6/2019 IMS Guerilla School

16/19

Application Server (AS) Arriving SIP message

matches trigger pointfor AS1

Forwarded to AS1 AS1 forwards back

Message matchestrigger point for AS2

Forwarded to AS2

AS2 forwards back

No more trigger pointsmatch

Forwarded todestination

8/6/2019 IMS Guerilla School

17/19

Media Resource Function

Media ResourceFunction Processor(MRFP)o Terminates RTP bearerso Performs media recording

and playback

Media ResourceFunction Controller

(MRFC)o SIP interface to IMS coreo H.248 control of MRFP

8/6/2019 IMS Guerilla School

18/19

Media Gateway Function

Breakout Gateway ControlFunction (BGCF)o Inverse of I-CSCFo Selects a Media Gateway

Media Gateway (MGW)o Interworks RTP with TDM

Media Gateway ControlFunction (MGCF)o SIP interfaces to IMS coreo H.248 control of MGW

o SIGTRAN interface to SGW Signaling Gateway (SGW)

o Interworks ISUP withSIGTRAN

8/6/2019 IMS Guerilla School

19/19

Charging Collection Function

(CCF) Collects billing recordsfrom other networkelements

DIAMETER Correlates Consolidates Filters Processes

Output to Billing System(BS)