IP Multimedia Subsystems

IP Multimedia Subsystem

Gaurav Gautam

University College Cork CS6316 1

History Analog (Telephone Voice Switches) Digital (Voice IP

• Concept “softswitch” emerged(i.e. Open API to bridge a PSTN and VOIP)[1].

IMS• emerging architectural framework, • based on SIP and IP, for offering multimedia, VoIP

services and fixed mobile convergence.


standardized reference architecture

Introduction• Defined by 3GPP and 3GPP2 standards and

organizations based on IETF Internet protocols.• It is access independent as it supports IP to IP

session over wireline IP, 802.11, 802.15, CDMA, packet data along with GSM/EDGE/UMTS and other packet data applications.

• standardized reference architecture


The IMS will provide a multi-service multi-protocol,

multi-access, IP based network - secure, reliable and

trusted – Multi-services: Any type of service may be delivered

by a common QoS enabled core network, Multi-access: diverse access networks (WiFi, WiMAX,

UMTS, CDMA2000, xDSL,Cable, etc.) can interface with IMS


IMS Layers


Applications (Voice , Video,Web,Messaging)

CSCF (SCTP,SIP,RTSP)

Access (PSTN)

Transport (Wi-Fi,IP,,GSM,CDMA,SS7)

IP

AccessNetwork

Services

Control

Transport

Devices

High Level Functioning

•Transfer Layer

responsible for the abstraction of the actual access networks.

doing initial IP provisioning

facilitating the registration of devices with the higher layers.

•Control Layer authentication, routing distribution of IMS traffic between the transport layer and the service layer. For example, a pay-per-download service Call Session Control Function (CSCF) which facilitates the correct interaction between

the application servers, media servers, and the Home Subscriber Service (HSS)


High Level Functioning(cont)

•Service Layer

Final Level of Abstraction

provides multimedia services to the overall IMS network

contains network elements which connect to the Serving-CSCF (Call

Session Control Function) using the IP multimedia Subsystem Service

Control Interface (ISC).

Elements of IMS Service layer :- SIP Application Server,OSA Service

Capability Server and IM-SSF


IMS Architecture Basics

Main Elements :- User equipment: As the name implies, the user equipment or

UE is part of the IMS architecture resides with the user - it is the endpoint.

Access network: This is the portion of the IMS architecture through which the overall network is accessed.

Core network: This is a major element within the IMS architecture and provides all the core functionality.

Application layer: The application layer contains the web portal and the application servers, which provide the end user with service and enhanced service controls.


IMS architecture functional view


Simplified view of the functional IMS architecture

Main Elements of IMS

•IMS CSCF - Call Session Control Server

provides the registration of the endpoints

provides routing for the SIP signalling messages

links to the interworking and transport layer to

provide QoS


Slide Title•HSS - Home Subscriber Server

subscriber data base for the home network(Implemented using DIAMETER protocol).

•SLF (Subscription Locator Function) –Need when there are more than a single HSS(User address mapped to HSS).


•BGCF - Breakout gateway control function SIP proxy which processes requests for routing from an S-CSCF when the S-CSCF has determined that the session cannot be routed using DNS or ENUM/DNSIincludes routing functionality based on telephone numbers


Slide Title•MGCF - Media Gateway Controller Function

interworks the SIP signallingmanages the distribution of sessions across multiple media gateways

•MSCF - Media server function controlThis manages the use of resources on media servers.

•SIP-AS - SIP applications serverservice execution platform on which one or more services are deployed.


Common IMS core overview


CSCF – Call Session Control Function Basic Features :-

• Used to process SIP processing.• intercepts call signaling and passes it to the

application services for them to handle.• Comprises all functional modules required to

manage signaling from end-user equipment, allocating application servers, establishing emergency connections and controlling the communication with other networks[2].


Proxy- Call Session Control Function (P-CSCF)

•This SIP proxy is first point of contact for IMS terminal.

•Secure SIP signaling transmission.

•Interface towards the Policy control architecture for

authorization of the media.

•Detection of IMS Emergency services.


Interrogating Call Session Control Function (I-CSCF)

located at the edge of an administrative domain Queries HSS to retrieve the address of the S-CSCF and assign it

to a user performing SIP registration. Forwards SIP request or reponse to the S-CSCF.


Serving Call Session Control Function (S-CSCF)

Central node for the provision of the SIP signaling.

Responsible for routing and translation of the sessions. Assists in authenticating the user upon registration and

retrieve the users service profile[3].


Role of SIP in IMS Main signaling protocol used in IMS. To establish, modify and terminate multimedia

sessions. There is single protocol.Works end to end. Designed to enable additional multimedia

sessions. SIP signaling packets within IMS are normally

processed by Call Session Control Function(CSCF).


References

•[1]http://searchnetworking.techtarget.com/definition/

•[1] Lucent IP Multimedia Subsystem Overview

•[2]

http://www.ericsson.com/fj/ourportfolio/products/call-session

-control-function-cscf

•[3]White Paper : Introduction to IMS,Ericsson (March 2007)


IP Multimedia Subsystems

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