Introduction to IMS
IP Multimedia System or IMS describes the NGN (New Generation Networking) architecture for implementing IP-based multimedia and telephony services, defined by the 3GPP (3rd Generation Partnership Project). IMS supports multiple access types, including WCDMA, GSM, WLAN, CDMA2000 and any other packet data applications.
It uses Internet Engineering Task Force (IETF) protocols (e.g. SIP or Session Initiation Protocol) to ease the integration with the Internet whenever possible. It will make Internet technologies like instant messaging, web browsing, video conferencing and e-mail available to everyone from any location. IMS is not intended to standardize application but to ease the access of multimedia application from wire line or wireless terminals.
Development of IMS
In 1999, 3G.IP defined and developed the first initial architecture of the protocol IP Multimedia System (IMS). 3GPP (3rd Generation Partnership Project) bought this architecture to work for 3rd Generation (3G) mobile phone in UMTS (Universal Mobile Telecommunications System) networks. Later on, 3rd Generation Partnership Project 2 (3GPP2), a different organization, integrated 3GPP IMS on their CDMA2000 Multimedia Domain (MMD).
In Release 6, 3GPP added Inter-operability between IMS using different IP-connectivity networks, interworking with WLAN, routing group identities, Push to Talk service, multiple registration and forking.
In Release 7, working together with TISPAN release R1.1, 3GPP added support for fixed networks, fixed broadband connection to the IMS, Policy and Charging Control (PCC), interworking with non-IMS networks, emergency sessions, the function of PSTN Emulation Service (PES) and Access Gateway Control Function (AGCF) and the voice call continuity between packet switching and circuit switching networks (VCC).
In Release 8, 3GPP added support for Multimedia Session Continuity, System Architecture Evolution (SAE), Long Term Evolution (LTE), IMS centralized services and enhanced emergency services.
Architecture of IMS
IMS architecture provides the wide range of IP-based services over both circuit switching and packet switching networks. The major IMS network elements include –
• Access Network – The user can connect to IMS network in many ways. They can directly register on IMS networks through IMS terminals (like computers, mobile phones, PDA etc.), even if they are in another country or network while roaming.
• Core Network – It can be divided into two parts –
a) Home Subscriber Server (HSS) – It is also known as User Profile Server Function (USPF). HSS is a master user database which contains the authorization and authentication of user, subscribers profile and location, and IP information of the user.
b) User Identities – Various user identities are associated with IMS networks like IP Multimedia Public Identity (IMPU), Wild carded Public User Identity, IP Multimedia Private Identity (IMPI) and Globally Routable User Agent URI (GRUU).
• Call Session Control Function (CSCF) – It is basically a SIP (Session Initiation Protocol) Server which controls the session routing, call and file protocols.
• Application Server (AS) – It uses SIP to host and execute services with the Serving Call Session Control function. It can be located in third-party network or within the home network.
• Media Servers or Media Resource Function (MRF) – MRF or Media Servers provide media related functions i.e. medina manipulation (audio or video mixing).
• Breakout Gateway Control Function (BGCF) – It controls and allocates the resources to IP sessions.
Session handling
It is one of the most important features of IMS. It can be divided into two parts–
• Trigger Point – Trigger Point provide five services :
• A match on the SIP method.
• A match on the request URI.
• Originating and terminating the state of the registered or unregistered user.
• A regular expression match on SIP header.
• A regular expression match on Session Description Protocol (SDP) payload.
• Initial Filter Criteria (iFC) – It is used to represent the subscription of a user to an application. iFC contains:
• A Trigger Point.
• An Application Server URI.