Tutorial: Internet Multimedia Subsystem (IMS)


Internet Multimedia Subsystem (IMS)

What Is IMS?

Communications networks are rapidly evolving into policy-based, packet-oriented networks designed to provide a particular quality-of-services (QoS) for subscribers while reducing the costs associated with capital expansions, network operations, and management.  If you are involved with telecommunications planning, engineering, deployment, strategy, marketing, or services creation, it is critical that you understand the technology and business implications of IMS.

Internet Multimedia Subsystem (“IMS”) is a policy based system that provides greater flexibility to operators for the development and launch of multimedia applications.   While IMS has often been thought of as a data/application enabler, many operators are actually looking to it as the next generation technology for both voice and data, providing services to subscribers in a manner that has never before been seen.

IMS will provide carriers with a new set of operating capabilities, flexibilities, and challenges.  In the near future, network professionals will need to augment their knowledge of circuit switched networks with an understanding of packet oriented technologies.


Third Generation Partnership (3GPP), Telecoms & Internet converged Services & Protocols for Advanced Networks (TISPAN), and 3GPP2 (Third Generation Partnership 2) all have variants of the general IMS technology that address their specific requirements.   Since mobile networks have more demanding policy and signaling requirements, they need a somewhat customized version of the standard.  Similarly, 3GPP and 3GPP2 differ in a number of areas, e.g., authentication, which calls for different IMS strategies for 3GPP from those employed in Multimedia Domain (MMD), a close cousin of IMS in the CDMA 3GPP2 arena.   Also in the standardization mix is TISPAN, an arm of the European Telecommunications Standards Institute (ETSI) that deals with the convergence of Internet with fixed networks.  There are working groups that help 3GPP and TISPAN share ideas and approaches toward the next generation technology, especially in the area of fixed and mobile convergence.


Historical Milestones

  • 1999 – 3G.IP, the industry forum that developed the initial IMS architecture, comes into being.
  • 2000 – Work from 3G.IP is brought to 3GPP working group as part of their standardization efforts.
  • 2001 – 3GPP Release 4 introduces the capability of an all-IP core network as well as a bearer independent core network.
  • 2002 – 3GPP Release 5 introduces IMS and HSDPA.
  • 2004 – 3GPP Release 6 adds enhancements to IMS.
  • Mid 2007 – 3GPP Release 7 will reduce the delay that affects VoIP


Principles and Operation

It would be hard to do justice to the complex architecture of IMS in this short article.  What follows is a very high-level description of IMS.  The signaling protocol for IMS is based on SIP.  As a network technology, its architecture includes many other components.  As requests for resources enter the network, a P-CSCF, which may also include PEF/PDF and SBC functions, will route the signaling to the appropriate I-CSCF, and can be located in either the visited or home network.  The P-CSCF will also provide authentication and charging functions.

The P-CSCF is the first point of contact with the network and an IMS based terminal.  Resident in the P-CSCF may be the PEF/PDF (policy enforcement/decision point), and can be located in either the home or visited network.  Additionally, the P-CSCF may take on functions of a Session Border Controller.

The I/S-CSCF will handle SIP registrations, decide which application servers or SCIM need to be included, will provide ENUM lookups for routing, and enforces policies for load purposes.  Also, the S-CSCF is interfaced to the HSS where the subscriber data is housed for policy information relative to the subscriber’s profile.

Also involved are additional nodes such as media servers, BGCF, PSTN gateways, charging, SLF within SDM/HSS, and a host of other elements.


Business Implications and Applications

The coming fixed/mobile convergence offer the promise of a new communications paradigm where a subscriber could perform tasks such as using a cellular telephone as a TV remote control, programming a DVR while away from home, transferring calls from a mobile phone to a landline, all without interrupting communication.

It allows a network operator to rationalize much of the investment needed for new – and legacy – applications and services across a shared, common policy based enablement layer made possible by IMS.  Since the network itself is a contended asset, the challenge that operators face is how to provide policy management and support across all levels of the network while maintaining the quality of service parity.   IMS provides a way to meet that challenge as well.

Many operators are looking at IMS as being the new core network technology that will allow them to cap the (ANSI-41 or MAP) networks sometime in 2008 or 2009.

In a nutshell, IMS changes the core network from a circuit switched environment to that of a policy based, packet domain.  Policy engines and definitions, from subscriber management and profiles to access policy and admission control to policies governing composite services and third party applications, define how the network is to function and operate.  Moreover, policy decision functions and policy enforcement functions reside within the network to dynamically control which traffic traverses the network, and how the traversal is to occur.

By understanding the basics of IP and networking, IPv4, IPv6, SIP, and VoIP, engineers can build a solid base of knowledge ahead of the roll out and commercialization of IMS networks and applications.

How to Learn More About It


  • Engineers, network designers, technicians, and specialists who work today with legacy circuit switched equipment and need to quickly ramp up their knowledge of IMS, should consider:  IMS: The Technology, Applications, and Challenges (IMS, 2 days).  This course studies IMS from all angles including the technology, status of wireless and wireline standards, key challenges posed by the technology, financial drivers for its adoption, deployment, security considerations, and the future of telecommunications networks, including a flat, all-IP infrastructure.  It also takes a look at the issue of network policy and how the different levels and types of policies for QoS and admission control have important bearing on traffic engineering in the evolving networks.
  • Finally, if you come from a traditional telecommunications background and have not been exposed to the concepts of data networks and the increasingly important Internet Protocol (IP), you may wish to consider our courses on IPv4 or IPv6: Internetworking with TCP/IP Version 6 (IPv6, 2-3 days).


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