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Forging the future of IMS

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What is IMS    Transmitter

The IP Multimedia Subsystem is a collection of 3GPP and ITU standards which propose a technology infrastructure to enable an all IP based Telecommunications Network based around the use of protocols such as SIP and DIAMETER. IMS evolved from an early specification from an industry consortium which sought to create an IP based core networking standard for telecommunications network. It was envisaged that by defining an all IP control layer and adopting IETF track standards such as SIP, network operators could reduce network management overhead, medium to long-term Capital Expenditure and increase Average Revenue Per User (ARPU) through new IP-based services. These services would potentially use capabilities from the telecommunications network control plane and the Internet, hence they’ve been called Converged Network Services.

The main nodes of the IMS core network are:

 

  • The Home Subscriber Server (HSS), or User Profile Server Function (UPSF), is a master user database that supports the IMS network entities that actually handle calls. It contains the subscription-related information (user profiles), performs authentication and authorization of the user, and can provide information about the user's physical location. It is similar to the GSM Home Location Register (HLR) and Authentication Centre (AUC).

 

  • A Proxy-CSCF (P-CSCF) is a SIP proxy that is the first point of contact for the IMS terminal. It can be located either in the visited network (in full IMS networks) or in the home network (when the visited network isn't IMS compliant yet).

 

  • A Serving-CSCF (S-CSCF) is the central node of the signalling plane. It is a SIP server, but performs session control too. It is always located in the home network. It uses Diameter Cx and Dx interfaces to the HSS to download and upload user profiles — it has no local storage of the user. All necessary information is loaded from the HSS.

 

  • An Interrogating-CSCF (I-CSCF) is another SIP function located at the edge of an administrative domain. Its IP address is published in the Domain Name System (DNS) of the domain (using NAPTR and SRV type of DNS records), so that remote servers can find it, and use it as a forwarding point (e.g. registering) for SIP packets to this domain. The I-CSCF queries the HSS using the Diameter Cx interface to retrieve the user location (Dx interface is used from I-CSCF to SLF to locate the needed HSS only), and then routes the SIP request to its assigned S-CSCF.