IMS, IP Multimedia Subsystem tutorial
- a summary of IMS, IP Multimedia Subsystem, providing the highlights of its
operation and structure.
IMS, or IP Multimedia Subsystem will have a colossal impact
on the telecommunications industry, both wired and wireless in the coming years.
Although IMS was originally created for mobile applications by 3GPP and 3GPP2,
its use will be far more widespread as fixed line providers are also being
forced to find ways of integrating mobile or mobile associated technologies into
their portfolios. As a result the use of IMS will be cross the frontiers of
mobile, wireless and fixed line technologies. Indeed there is very little within
IMS that is wireless or mobile specific, and as a result there are no barriers
to its use in any telecommunications environment.
IMS itself is not a technology, but rather it is an
architecture. It is based on Internet standards which are currently the major
way to deliver services on new networks. However one of the key enablers for the
architecture is the Session Initiation Protocol (SIP), a protocol that has been
devised for establishing, managing and terminating sessions on IP networks. The
overall IMS architecture uses a number of components to enable multimedia based
sessions between two or more end devices.
One of the elements is a presence server that handles the
user status, and this is a key element for applications such as Push to talk
over Cellular (PoC) where the presence, or user status is key to enabling one
user to be able to talk to another.
With users now needing to activate many sessions using
different applications and often concurrently, IMS provides a common IP
interface so that signalling, traffic, and application development are greatly
simplified. In addition to this an IMS architecture means that subscribers can
connect to a network using multiple mobile and fixed devices and technologies.
With a variety of new applications from Push to talk over Cellular (PoC),
gaming, video and more becoming available, it will be necessary to be able to
integrate them seamlessly for users to be able to gain the most from these new
applications.
It also has advantages for operators as well. Apart from
enabling them to maximise their revenues, functions including billing, and
"access approval" can be unified across the applications on the network, thereby
considerably simplifying this area.
Architecture
IMS provides a unified architecture which can be divided into three layers:
- Transport and Endpoint Layer
- Session Control Layer
- Application Server Layer
Transport and Endpoint Layer
This layer initiates and terminates the SIP signalling,
setting up sessions and providing bearer services including the conversion from
analogue or digital formats to packets. It also provides the media gateways for
converting the VoIP data to the PSTN TDM format.
Session Control Layer
This layer contains what is termed the Call Session Control
Function (CSCF) which provides the endpoints for the registration and routing
for the SIP signalling messages, enabling them to be routed to the correct
application servers. The CSCF also enables QoS to be guaranteed. It achieves
this by communicating with the transport and endpoint layer.
The layer also includes other elements including the Home
Subscriber Server (HSS) that maintains the user profiles including their
registration details as well as preferences and the like. It includes the
presence server essential to many interactive applications such as PoC. A
further element of the session Control Layer is the Media Gateway Control.
Application Server Layer
The control of the end services required by the user is
undertaken by the Application Server Layer. The IMS architecture and SIP
signalling has been designed to be flexible and in this way it is possible to
support a variety of telephony and non-telephony servers concurrently. Within
this layer there is a wide variety of different servers that are supported. This
includes a Telephony Application Server (TAS), IP Multimedia - Service Switching
Function (IM-SSF), Supplemental Telephony Application Server, Non-Telephony
Application Server, Open Service Access - Gateway (OSA-GW), etc.
Summary
The telecommunications industry as a whole is turning to IP
based transport of data along with the introduction of new multimedia services.
To enable this to be achieved telecommunications networks, whether fixed,
cellular or wireless will need to be far more flexible and to achieve this it
will be necessary to implement IMS.
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