IMS Layers & Stack

- details of the OSI layers as used within the IMS stack, including the Transport & Endpoint, Session Control, & Application Layers.

When referring to communication between systems a layered model is often used. The OSI, Open System Interconnection reference model is widely used.

This seven layer OSI stack is used as the basis for many systems, and the same is true for IMS.

While the OSI layer system is a generalised system, it is easily adapted for use with IMS, or any other system.


OSI layers and IMS

The OSI layer reference model was developed by the International Organisation for Standardisation, ISO. It is a model that is used in communications systems to divide the communication channel into various levels of tasks.

OSI layers and IMS
OSI layer reference model and IMS

Each layer provides some distinct and well defined services to the adjacent layer further up the stack.

However, above layer 5 (and sometimes layer 4) the distinction can become a little less defined, and some services overlap the layers. Often layer 5 and above may become more specific to the environment, e.g. IMS in which they are used. As a result, many systems will have their own terminology and particular definitions for these layers.


IMS layers & stack basics

There are three main IMS layers that are tailored for this application. They correspond to layers 4 and above. These are:

  • Transport and Endpoint Layer [equates to OSI layer 4]
  • Session Control Layer [equates to OSI layer 5]
  • Application Server Layer [equates to OSI layers 6 & 7]

IMS Transport and Endpoint Layer

This IMS layer initiates and terminates the SIP signalling, setting up sessions and providing bearer services including the conversion from analogue or digital formats to packets.

This IMS layer also contains all of the media processing facilities including media gateways. These can be used to convert VoIP bearer streams to the PSTN TDM format. They can also be used to provide many media-related services such as conferencing, playing announcements, collecting in-band signalling tones, speech recognition, and speech synthesis.


IMS 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.

By Ian Poole


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