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gsm isgnaling 3G IMS     IP Multimedia Subsystem

1.   Introduction

IP Multimedia Subsystem (IMS) is set of standards, developed by the wireless standards body 3rd Generation Partnership Project (3GPP), for providing IP based multimedia services including IP telephony. 3GPP defines IMS to be a new mobile network infrastructure that enables the convergence of data, speech and mobile network technology over an IP-based infrastructure. The IMS uses open standards to provide cross platform interoperability and fixed–mobile convergence.
The 3GP Project originally released specifications (3GPP R5) to provide Internet services over GPRS. Subsequently, the specifications were updated to support Wireless LAN, CDMA2000 and fixed line networks. The IEFTF ( Internet engineering task force) protocol were adopted by 3GPP in development of IMS to ease integration with the Internet. The use of protocols such as Session Initiation Protocol (SIP) was a step toward this direction.


2.   Why IMS?

The integration of Internet, mobile & wire line services bring rich features & new set of conveniences both for users as well operators. The benefits to both are as given below:

2.1   Benefit to Users

The IMS aims to provide following facilities to its user:
  • The use of Internet technologies will allow a seamless communication irrespective of user’s location and the platform of communication, he is using.The users will be required to use a single public identity for the services used.
  • All services, such as voice, video, SMS, conferencing, IPTV etc. will be available on the device being used. This will provide user with rich media experience.
  • The users will be able to personalize their communication need such as preferred medium of communication depending on who is calling etc.
  • Users will be able to work in converged landline & wireless environment without breaking the communication.
  • Safe and secure communication.

 2.2   Benefits to Service Providers

The IMS has been designed to keep in mind the flexibility of operation, services & expandability. The proper implementation of IMS will lead to following benefits to operator:
  • Use of IP backbone by IMS will provide an optimized transport and coding.

  • IMS provides a common standardized platform with reusable components for development of new services. This will reduce development time and allow roll out of new multimedia services quickly, as per the market requirement.

  • Quality of service (QoS) has been as issue with IP services. The IMS has been designed keeping in mind quality of service (QoS) requirement, so that the guaranteed bandwidth requirement can be met.

  • IMS allow flexibility in charging users as per the services utilized by them.

  • The open standards used by IMS allow inter-networking and interoperability between various service providers. This enables them to provide seamless communication to user irrespective of network he is in and the device he is using.

  • The open architecture, available common services and converged IP backbone will reduce system complexity, thereby bringing down the CapEx and OpEx for the operators.

  • The IMS design has been done keeping in mind operators & allows operators to deploy third party developers for developing application without loosing the control of their network.

3.   IMS Architecture

According to the 3GPP, IMS is not intended to standardize applications itself, but to aid the access of multimedia and voice applications across wireless and wire line terminals. To do this, a horizontal control layer, which provide isolation between access network and service layer, is used.
The IMS architecture is unified and layered. The IMS system architecture manages the movement of media packets through the network, as well as provide the necessary integration required between various types of users. The core network of IMS manages and routes the packets to media and application servers for processing, using reusable functions. The applications servers provide various services required. The IMS specifications set define service control, protocols, routing, and charging processes across the network.

3.1   Simplified IMS architecture

The diagram in Fig 1 given below shows the simplified architecture of IMS.
Figure 1 Simplified IMS architechture
The IMS architecture consists of three layers connectivity/transport layer, the control layer, and the application layer

3.1.1Connectivity/transport layer

The connectivity layer consists of necessary hardware for IP backbone as well as access network. It provides necessary interface for all types of networks & devices requiring access to IMS. All the devices are connected to IMS network through this layer. Thus, it acts as entry and exit point to the network.

Control layer

The control layer consists of necessary hardware for managing the call setup, modification and release. The most important component of control layer is CSCF (Call Session Control Function) server, basically a SIP server, which manages call, session routing and file protocols. The control layer also contains other servers to provide as provisioning, charging and operation & management functions. The interfacing with other network is provided by a set of gateways. The Home Subscriber Server (HSS) database stores the unique service profile for each end user.

Application layer

The application layer consists of SIP application servers to host, process and store data and provide various services to users. The third party providers hoist the value added service on application servers (AS) leaving network control to service providers.