IP Multimedia Subsystem is an architecture framework which is popular to use for delivering IP multimedia services. This network was introduced by a wireless standard body popularly known as 3rd Generation Partnership Project (3GPP). Purpose of introduction of this network was to aid the access of multimedia and voice applications from wireless and wire-line terminals. It was designed as a part of all evolving mobile networks to represent the better approach towards internet service delivery. With passage of time many advancement have been monitored with this network and wireless standard bodies. To make access from one terminal to another easy, different protocols are used and these protocols create fixed-mobile convergence (FMC) to aid the process.
In 1999, for the first time concept of IMS was introduced by 3G.IP industry. Later on same concept moved to other projects and became an integral part of today’s modern 3G mobile phone systems. Some different organizations of 3GPP introduced 3GPP2 as well. Besides this CDMA2000 Multimedia Domain (MMD) on 3GPP IMS was also added with other 8-10 support systems for multimedia session continuity, enhanced emergency sessions and IMS centralized services, inter device transfer, enhancements to the single radio voice call continuity (SRVCC), enhancements to IMS emergency sessions, network-provided location information for IMS, interworking with non-IMS networks, policy and charging control (PCC) etc.
Architecture of IP Multimedia Subsystem:
IMS has divided the networking structure into different independent planes with some standardized interfaces between them. Each of these interfaces is used as reference point for protocols. This point also defines protocol over the interface and the functions between which it operates. Standards are not involved in allocation of functions, this allocation merely depends on the scale of the application, and a single device may contain several functions.
The architecture of IMS is based on following planes
- Application Plane:
This layer in 3GPP IMS architecture is responsible to provide provision and management of services. This is also part of standard interfaces of some common functions including configuration storage, identity management, user status, billing services, and control of voice and video calls and messaging. These functions are held by Home Subscriber Server (HSS), Charging Gateway Function (CGF) and control plane of this architecture.
- Control Plane:
The second and one of the important layers of this architecture is control plane which sits between application and transport planes. This plane is responsible for forwarding the call singles on their route, it also help the transport plane in deciding what traffic should be allowed and it help in generating billing information for each user of a network.
Call Session Control Function is the most important feature performed by this plane as due to it some other important functions are comprises including
- Proxy-CSCF (P-CSCF) which is responsible for security of the messages between the network and the user and allocating resources for the media flows.
- Interrogating-CSCF (I-CSCF) which is responsible for querying the HSS to determine the S-CSCF for a user and may also hide the operator’s topology from peer networks.
- Serving-CSCF (S-CSCF) that performs the duty of processing registrations to record the location of each user, user authentication, and call processing.
User plane provides a core QoS-enabled IPv6 network with access from User Equipment (UE) over mobile, WiFi and broadband networks. Basis purpose of designing this layer was to provide a wide range of IP multimedia server-based and P2P services.
If user plane is not part of IMS architecture then 3GPP can face severe security issues at transport layer and allocation of resources would seem difficult as well.