IMS: An Executive Guide

No mobile technology since 3G has created so much interest and discussion as IMS - the IP Multimedia Subsystem. Over the last twelve months there have been thousands of words written about IMS, mostly praising this new technology as the new Holy Grail of mobile communications. Yet despite all this excitement only a small minority really understand what IMS is all about. Addressing this, TelecomsEurope has asked Mac Taylor, ceo of the Moriana Group, to produce this executive guide to IMS.

Operators are currently evaluating the migration from circuit switched technology to next generation packet switched networks based on IP. There are four sets of drivers for this evolution: OpEx cost-reduction, revenue enhancement, disruptive competition and market saturation.

Since 1999, the 3rd Generation Partnership Project (3GPP) has been defining the evolution of the GSM mobile network, from circuit switched to an IP-based network.

In the early stages (3GPP Release 99) the work focused on the 3G radio access network. In the next stage, (3GPP Release 4) the focus included enhanced services and an Open Service Architecture (OSA) component (also known as Parlay/OSA). In the later stages (3GPP Release 5 and Release 6) the work focused on the new IP-based service and control planes of the network, including the specification of the IP Multimedia Subsystem (IMS).

IMS influence is growing

Other standards bodies have subsequently adopted the majority of the 3GPP IMS specifications to underpin their own architectural standards. These fora include the 3rd Generation Partnership Project 2 (3GPP2) under the Multi-Media Domain (MMD) specifications, the Open Mobile Alliance (OMA), and the European Telecommunications Standard Institute (ETSI).

IMS supports convergence of wireline, wireless and IP/broadband networks and it has been adopted by ETSI Telecoms & Internet converged Services & Protocols for Advanced Networks (TISPAN), a fixed network ETSI committee.

New Generation Networks and Services

In addition, ETSI/TISPAN, 3GPP, 3GPP2, OMA, Parlay Group, Java Community Process (JCP) and Internet Engineering Task Force (IETF) are working more generally to deliver open, industry-standard architectural specifications, which detail a set of protocols and APIs to facilitate the deployment of next-generation networks. Some of the key technologies which have been standardised include Session Initiation Protocol (SIP), Parlay/OSA, Parlay X Web Services, JAIN SLEE and SIP Servlets, among others.

IMS specifies a core set of network functional entities, which support access to the SIP-based communication services. IMS does not specify new protocols, but builds upon existing Internet protocols, as specified by IETF, such as SIP, Session Description Protocol (SDP), and Diameter. These are intended to enable the creation of a real-time, peer-to-peer, multimedia network architecture.

Enabling convergence

IMS is intended to be the system that will merge the Internet with the telecom world. IMS enables the convergence of fixed and wireless networks and seamless user roaming irrespective of access technologies, and facilitates services transparency and enables common service and application development.

SIP signalling is the primary method for user registration and session control in the IMS architecture. The Call Session Control Function (CSCF) is the core signalling server in the IMS network architecture. It acts as both a SIP Registrar and a stateful SIP proxy server.


The Home Subscriber Server (HSS) is the 3G IMS equivalent to the HLR, providing AAA (Access, Authorisation, and Accounting) functions and maintaining subscriber profile data in a central location. The CSCF uses Diameter protocols to perform user authorisation and to retrieve subscriber service profiles from the HSS.

Open interfaces

IMS standards define open interfaces for session management, access control, mobility management, service control, and billing. These are intended to provide a SIP network with the carrier-grade attributes of the switched circuit network, but at a lower cost and with increased flexibility.

IMS specifies a SIP-based common interface, IMS Service Control (ISC) by which applications hosted on SIP, Parlay/OSA and CAMEL application servers interact with the IMS core network.

The main integration point between IMS application servers and the IMS core network is through the Serving Call Session Control Function (S-CSCF) network element.

Migration of legacy services to IMS

The investment made in GSM and 3G networks, combined with the radio spectrum issues surrounding voice over IP (VoIP) to the device, mean that it is unlikely that pure IMS to the terminal will be deployed within the foreseeable future. It is thus essential for operators to integrate both new and legacy technologies and span the familiar fixed and mobile divides.

A basic requirement will be that the same application should be simultaneously provisioned on different networks using both SS7 and SIP signalling. One way to achieve this is to use converged service gateways that can bridge complex networks like SS7 and emerging IMS signalling based on SIP and Diameter. Note that IMS-ready signalling platforms will have to support a wide variety of application protocols, international variants, and multi-vendor extensions.

Hybrid services

The Parlay/OSA Gateway can also simultaneously connect to the IMS and the 2G environment. It provides a good solution for both the migration of existing services and the creation of new, hybrid services that operate across multiple networks. 

Mass-market telephony services can be migrated to IMS by being re-created on the SIP AS. However the migration of complex, advanced voice and data services requires alternative solutions.

A convergent Service Delivery Platform (cSDP) can use Parlay/OSA APIs to provide a Network Abstraction Layer to provide network-independent voice and data applications. This approach should significantly reduce OpEx and CapEx since service CRM, provisioning, billing, and other IT infrastructure elements as well as business processes will be able to be re-used on both legacy and IMS networks.

Service Logic Execution Environments (SLEEs)

A number of alternate development and execution environments, (for example, JAIN SLEE, J2EE, Parlay/OSA), are currently available in the marketplace. At present, it is not clear which, if any, of these will prevail. Mobile Network Operators (MNOs) must be able to move services between different execution environments without having to recreate the services logic from scratch.

IMS services will vary in scope and requirements. Some services will require low latency characteristics in both simple or complex environments, while other services will depend more on a rich data access or enterprise service coordination. No single execution environment suits all service scenarios and frequently services may span across two or more such execution environments.


Even though multiple execution environments will exist, a single service creation environment is essential. Services should be created once, but deployed in multiple environments. In addition, an IMS infrastructure must be able to support a range of application servers, sourced from different providers and running on an array of industry execution environments.

SOA and Parlay X Web Services

Whilst the basic SIP protocol is relatively straightforward, its extension for IMS (ISC) is far more complex - as an evolving signalling protocol for the next generation networks it has been referred to as 'SS8'. For some value-added applications SIP is inappropriate and a higher-level abstraction is needed.

In the IT industry, Service-Oriented Architecture (SOA) is often implemented using the key web-services technologies (SOAP, WSDL and XML). The Parlay Group has created a similar set of web service interfaces to general telecom capabilities known as Parlay X.

Parlay-X Web Services define a set of telecommunications oriented web-services, which are part of 3GPP Release 6. Parlay-X in effect creates a bridge for the operator from the SOA used by developers and large enterprise customers, to the IMS. The Open Mobile Alliance (OMA) is also developing additional interfaces for specific telecom functions.

A key goal of these web services initiatives is to provide seamless connectivity across wired and wireless networks and among network operators. Aggregated web services will exploit lower-layer network services, such as those specified by IMS, and the higher-layer service network of Web-based applications and services. This should create a set of converged communications services in an emerging service network.

IMS Service Enablers

IMS Service Enablers are either network nodes providing basic service facilities or they are a more complex combination of service facilities and, in the later case, are know as application enablers. Examples include: Third Party Call, Call Notification, Short Messaging, Multimedia Messaging, Payment, Account Management, Terminal Status, Terminal Location, Call Handling, Audio Call, Multimedia Conference, Address List Management, and Presence.

Most of the above enablers will reside on standard application servers which will host Parlay/OSA APIs for service facility access.

Types of services

In value-added relationships with third parties, IMS operators can provide a range of services, including presence, location, QoS, charging, authentication, authorisation, reliable delivery, convenience routing, privacy, security and a host of other functions. 

Examples of innovative service bundles that will be used to serve new market segments in both a pre-IMS and IMS context include:

  • Fixed-mobile convergence (3G, local, long distance, data)
  • Personalised service bundles (multimedia call services, messaging services, data, VPN, content delivery and handset-based applications)
  • Context-aware services that shift services to optimal device based on subscribers presence, location, and profile
  • Seamless billing across all network domains (i.e. circuit switched domain, Internet, WLAN)
  • Personally selectable and configurable rate plans (individual, family, or group-based)
  • Corporate or departmental level rate plans with controllable acceptable usage policies
  • Access independent service transparency (3G, WLAN, WiMAX, UMA, Bluetooth, OFDM)