IntroductionDigital data

IMS widely known as IP Multimedia Subsystem is an IP multimedia and telephony core network that is defined by 3GPP and 3GPP2 standards and organizations based on IETF Internet protocols. IMS is a set of specifications that describes the Next Generation Networking (NGN) architecture for implementing IP based telephony and multimedia services. It defines a complete architecture and framework that enables the integration of voice, video, data and mobile network technology over an IP-based infrastructure. IPv6

IMS 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. Before going through the details lets have a look into the history of IMS.

History of IMS

IMS was originally defined by an industry forum called 3G.IP, formed in 1999. 3G.IP developed the initial IMS architecture, which was brought to the 3rd Generation Partnership Project. It first appeared in release 5, when SIP-based multimedia was added. 3GPP2 based their CDMA2000 Multimedia Domain on 3GPP IMS, adding support for CDMA2000. 3GPP release 6 added interworking support with WLAN. 3GPP release 7 further added support for fixed networks, by working together with TISPAN release R1.1.

IMS Architecture

IMS subdivides the networking infrastructure into individual functions with standardized interfaces between them. Each interface is called as a “reference point”, which defines both the protocol over the interface and the functions between which it operates. The following fig shows the IMS architecture overview:

IMS architecture diagram
Fig1. IMS Architecture (Ref:

IMS architecture diagram
Fig2. IMS core (Ref:

As shown in fig.1 the architecture is split into three main layers, each of which is described by a number of equivalent names: Transport and Endpoint Layer

Session Control Layer

Application Server Layer

Transport and Endpoint Layer

This layer is involved in initiation and termination of SIP signaling, setting up sessions and providing bearer services. This layer also provides the media gateways for converting the VoIP data to the PSTN TDM format.

Session Control Layer

This layer contains Call Session Control Function (CSCF), whose function involves providing the endpoints for the registration and routing for the SIP signaling messages, enabling them to be routed to the correct application servers. The CSCF guarantees QoS by communicating with the transport and endpoint layer.

Types of CSCF

A Proxy-CSCF (P-CSCF) is a SIP proxy that is the first point of contact for the IMS terminal.

Assigned to an IMS terminal during the registration, and does not change during the duration of the registration.

Authenticates the user and establishes IPsec security integration with the IMS terminal. Other nodes trust the P-CSCF, and do not have to authenticate the user again.

Includes a Policy Decision Function (PDF), which authorizes media plane resources like QoS over the media plane. It is used for policy control, bandwidth management, etc.

Generates charging records

A Serving-CSCF (S-CSCF) is the central node of the signaling plane. It is a SIP server, which also performs session control. It uses Diameter Cx and Dx interfaces to the HSS to download and upload user profiles – it has no local storage of the user. All necessary information is loaded from the HSS.

Handles SIP registrations, which allows it to bind the user location and the SIP address

Decides the destination application servers to which the SIP message will be forwarded, in order to provide the services

Provides routing services, by using Electronic Numbering lookups

it enforces the policy of the network operator

Multiple S-CSCFs coexist in the same network for providing load distribution and high availability. But Its the HSS that assigns the S-CSCF to a user, when it’s queried by the I-CSCF.

An I-CSCF (Interrogating-CSCF) is a SIP proxy, which provides a service locator function.

Its major functions include:

Registration: Assigning a S-CSCF to a user performing SIP registration

Session Flows: Routing a SIP request received from another network to the S-CSCF, or routing intra-domain SIP requests between users on different S-CSCFs.

Charging and Resource utilization

Acts as a Topology Hiding Internetwork Gateway (THIG): A case of I-CSCF, which hides the configuration, capacity, and topology of the network from the outside. The P-CSCF forwards the SIP messages received from the User Equipment to the Interrogating Call Session Control Function (I-CSCF) and/or the Serving Call Session Control Function (S-CSCF), depending on the type of message and procedure. The I-CSCF provides a contact point within an operator’s network allowing subscribers of that network operator, and roaming subscribers, to register. Once registered, the S-CSCF maintains session state for all IMS services.

The layer also includes other elements including the Home Subscriber Server (HSS) or User Profile Server Function (UPSF), which contains the subscription-related information, performs authentication and authorization of the user, and provides user’s physical location information. In short acts as a Maser database.

BGCF (Border Gateway Control Function): It is used to select the network in which the connection to the Public Switched Telephone Network will be made. It either forwards to another BGCF or to a MGCF controlling the access for the PSTN. MGCF (Media Gateway Control Function): It controls the media gateway call management that is to send or receives calls from or to PSTN or circuit switched network. It uses the SIP messages to or from the CSCF / BGCF and uses Media Gateway Control messages to or from the Media Gateway.

MGW (Media Gateway): Responsible for media processing for calls to or from PSTN / Circuit Switched Network.

The MRF (Media Resource Function) provides media related functions such as media manipulation. Media Resource Function Controller (MRFC): Signaling plane node that acts as a SIP User Agent to the S-CSCF, and which controls the MRFP. Media Resource Function Processor (MRFP): media plane node responsible for implementation of media-related functions.

Application Server Layer

The control of the end services required by the user is undertaken by the Application Server Layer.

Servers Supported in this layer are:

Telephony Application Server (TAS): The Telephony Application Server (TAS) is a back-to-back SIP user agent that maintains the call state. TAS contains the service logic that provides the basic call processing services including digit analysis, routing, call setup, call waiting, call forwarding, conferencing, etc.

IP Multimedia – Service Switching Function (IM-SSF): Provides the interworking of the SIP message to the corresponding Customized Applications for Mobile Networks Enhanced Logic (CAMEL), ANSI-41, Intelligent Network Application Protocol (INAP) or Transaction Capabilities Application Part (TCAP) messages.

Supplemental Telephony Application Server: Standalone independent servers that provide supplemental telephony services at the beginning of a call, at the end, or in the middle, by triggers.

Non-Telephony Application Server: These application servers interwork with endpoint clients to provide services such as IM, PTT, or presence-enabled services.

Open Service Access – Gateway (OSA-GW): The interworking between SIP and the Parlay API is provided in the Open Services Access – Gateway (OSAGW) that is part of the application server layer of the 3GPP IMS architecture.

Benefits and Implementation of IMS

The benefits of IMS over the existing cellular network infrastructure can be demonstrated in the following aspects.

Reduced time-to-market new multimedia services: The IMS infrastructure provides a standardized platform and reusable components. The standardized interface and common features provided by IMS infrastructure helps the service provider to market new multimedia services in short period of time.

Quality of Service (QoS): IMS specifies Quality of Service within the IP network and thus takes advantage of the QoS mechanism to improve and guarantee the transmission quality.

IMS allows all services to be available irrespective of the users’ location: IMS uses Internet technologies and protocols to allow users to move across the countries and still be able to execute all the services. So, all services are available to the user irrespective of the location.

Application of IMS Some of the IMS implementation are as discussed below:

Push to Talk over Cellular (PoC)

Multiple and simultaneous Ringing / find-me, follow-me : Call routed to a predefined list of destinations (sequentially or in parallel)

Multimedia Push: This service allows users to push some multimedia content (e.g.: a greeting card)

Push RingTone:Calling party selects which ring tone shall ring on the destination number/address

Real Time Video Sharing: Real time peer-to-peer, multimedia streaming service

Interactive Gaming

Shared folders: Content sharing enables users to share files between terminals.

Voice Messaging: form of instant messaging where the content of the message is an audio file.

Instant Messaging services: communication service that allows end-users to send and receive messages instantly.

Video-conferencing: IP Multimedia Subsystem (IMS) Video-conferencing service extends the point-to-point video call to a multi-point service.

IMS enabled Voice and Video Telephony

Presence Server

IP Centrex

Media Streaming and Download parental control

Incoming Call Screening and programmable Control

Outgoing VoIP call Barring/Control

Convergent Instant Messaging System (mixing SIP, USSD, SMS, text-to-speech)

Enhanced Policy controller (check and controls SIP message sequences and format)


IMS is part of a huge 3G gamble fuelled by the mobile telephony operators to obtain control of the vast new Internet medium and monetize it.

The IMS offers a robust infrastructure for developing and deploying multiple network services and data from a common network across different access networks. The development and research going on in this field is bound to make IMS the norm for all broadband access.