IP MULTIMEDIA SUBSYSTEM (IMS)
Why do we need the IMS?
The Main idea of the IMS is to offer Internet services everywhere and at any time using cellular technologies. we need to further clarify what we mean by merging the Internet and the cellular worlds and what the real advantages of doing so are. To do that, we need to introduce the different domains in 3G networks, namely the circuit-switched domain and the packet -switched domain.
The circuit-switched domain is an evolution of the technology used in 2G networks.The circuits in this domain are optimized to transport voice and video, although they can also be used to transport instant messages. The current trend is to substitute it with more efficient packet-switch technology.
The packet-switched domain provides I access to the Internet. While 2G terminals can act as a modem to transmit IP packets over a circuit, 3G terminals use native packet-switched technology to perform data communications. This way, data transmissions are much faster and the available bandwidth for Internet access increases dramatically.
Why do we need the IMS, if all the power of the Internet is already available for 3G users through the packet-switched domain? The answer is threefold: QoS (Quality of Service), charging, and integration of different services. One of the reasons for creating the IMS was to provide the QoS required for enjoying, rather than suffering, real-time multimedia sessions.
Another reason for creating the IMS was being able to charge multimedia sessions appropriately. A user involved in videoconference over the packet-switched domain usually transfers a large amount of information (which consists mainly of encoded audio and video). Depending on the 3G operator, the transfer of such an amount of data may generate large expenses for the user, since operators typically charge by the number of bytes transferred. The IMS provides information about the service being invoked by the user, and with this information the operator decides whether to use a flat rate for the service, apply traditional time-based charging, apply QoS-based charging, or perform any new type of charging.
In addition, users have to be able to execute all their services when roaming as well as from their home networks. To achieve these goals the IMS uses Internet technologies and Internet protocols.
General Principles of the IMS Architecture
GSM has two different modes of operation: circuit-switched and packet-switched.
GSM circuit-switched : The GSM circuit-switched network uses circuit-switched technologies, which are also used in the PSTN (Public Switched Telephone Network).
Circuit-switched networks have two different planes:
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