IMS 101
A Brief Introduction to The IP Multimedia Subsystem (IMS)
By Matt Byrd
Sales Engineer, MetaSwitch
The IP Multimedia Subsystem (IMS) is an architecture that merges the applications and
capabilities of the Internet with both wireless and wireline telephony, and promotes fixed/mobile
convergence. When realized, IMS will pave the way for true multimedia applications, both voice and
video, to be used through multiple forms of access, including third generation (3G) wireless devices such
as mobile phones and PDAs, as well as through traditional Internet access methods including corporate LANs
and broadband connections.
IMS is also, perhaps more importantly, a blueprint for how carriers and service
providers can architect their networks to deliver these multimedia applications, defined in such a way as
to make it much easier and faster for vendors to produce the underlying, interoperable infrastructure that
enable service delivery. The interfaces and protocols required to deliver these IP multimedia applications
and services are standardized, making it much easier for providers of these new services to develop and
integrate new services that will be delivered over IMS infrastructures. It also provides a much cleaner
mechanism for leveraging the vast wealth of services developed for the Internet using technologies such as
HTTP, Java, etc., allowing these services to be utilized over mobile communications in addition to
"non-mobile" connections. Last, but by no means least, IMS even allows services to be transferred
seamlessly between different access networks when users switch between them (e.g. when a user moves from a
fixed network connection such as a home broadband connection to a mobile network connection such
as a mobile phone).
It is very important to note that the IMS details multiple functional entities (or
service functions) that are the building blocks of the infrastructure. Each of the entities has well defined,
open interfaces associated with them in order to make it easy for vendors to create products that provide the
necessary function and/or interface into the various parts of the system. Vendors are free to create
products that combine multiple functions of the IMS, or may simply choose to produce hardware or software
components that address a single function within the IMS.
A Layered Approach
Perhaps the most powerful concept within IMS is the premise of removing any dependence
of service delivery from the physical network itself (aside from the obvious physical constraints of
bandwidth). This layered approach allows IMS to provide a truly access independent service delivery
mechanism that opens up the entire network for third party service delivery. However, there has also
been a lot of work put into service control within the network, to ensure that access providers aren't
simply "bit-pipe" providers, with all the service revenue going to the service providers.
There are three main layers within IMS: the Transport layer, the Control layer,
and the Service layer. Here's a closer look at all three.
- The Transport layer comprises many different types of access networks. Some examples of packet-based
networks are GPRS, UMTS, CDMA2000, WLAN, PacketCable, and ADSL. The traditional PSTN is an example of
a circuit switched network. Users are connected to the IMS infrastructure through the Transport layer,
either directly through an IMS terminal (such as a 3G wireless handset), or most likely (at least for the
near future) through a non-IMS device that interfaces the IMS infrastructure through a gateway. There are
several gateway functions that are found within the Transport layer which are primarily in place to provide
interworking between legacy networking functions and the IMS.
- The Control layer is the functional area within the IMS that provides all of the session and call
control. The CSCF (Call Session Control Function) is the central routing engine and policy enforcement
point for the network, and uses the SIP protocol for call control. The Home Subscriber Server (HSS) is
also found in the Control layer. The HSS is a centralized database that contains all the pertinent user
information, such as home network location, security information, and user profile information (including
the services for which the user has subscribed and may therefore participate in).
- The Service layer is where the Application Servers (AS) reside. This is where all of the services
are delivered through the IMS interface into the Control layer through standardized protocols, primarily
SIP.
This is a very simplified overview of some of the main functions within the IMS.
Because the IMS is broken out into these layers with specific functions and interfaces defined for each
layer, it makes it much easier for vendors to choose which functions they wish to incorporate into their
equipment, combining these elements in any way they choose. Ultimately, the market will decide what
elements they want in an integrated solution vs. those that they wish to obtain a la carte.
META: A practical IMS Migration strategy
Carriers are faced with a significant challenge in migrating their current networks,
which are typically a mix of IP and TDM technologies, to a fully IMS-compliant network. To address this
challenge, MetaSwitch has introduced META, our Multimedia Enhanced Telephony Architecture. META is a
comprehensive migration strategy that details how MetaSwitch softswitch components and enhanced telephony
applications will evolve to fill the necessary functions of the IMS, ensuring a stepwise transition for
carrier networks. A key feature of META is the ability to simultaneously support legacy devices, including
POTS endpoints and devices (such as GR-303 digital loop carriers), as well as newer IP-based devices such
as IP phones and Broadband Loop Carriers, with feature equivalence to the greatest extent possible.
Summary
The IP Multimedia Subsystem specification presents the most comprehensive blueprint
to date for carriers in architecting their next-generation networks. While there is certainly a lot more
work to be done in certain areas, many vendors and even carriers are aligning their products and solutions
to support the IMS specification. Certainly more vendors will emerge to provide specific applications
that will be delivered over IMS networks. While it will still take several years for the full IMS vision
to become reality, we are already starting to see early implementations of IMS elements within next generation
carrier networks. MetaSwitch provides the necessary migration path to ensure carriers manage this
transition at a pace with which they are comfortable.
To read more about IMS Migration Challenges and Solutions,
download the MetaSwitch White Paper.
Back to the October 2006 newsletter.
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