Back to mySolvr Home >
Bookmark

Internet Protocol Multimedia Subsystem (IMS) Unvalidated

Definition

The IMS is a component of next generation mobile core networks.  It has been defined by the 3rd Generation Partnership Program (3GPP) as the evolutionary path for all communications.  The specifications are based almost entirely on IETF protocols, chiefly the Session Initiation Protocol (SIP).  While SIP was designed as a peer to peer protocol the IMS is necessarily a centrally controlled system.

The main components in an IMS are:

  • Home Subscriber Server (HSS)
  • Call Session Control Function (CSCF)
  • Application Server (AS)

The HSS contains a database of all the information needed to service the network operator's subscribers' individual selection of services.  The HSS performs all authentication and authorization of subscribers using the DIAMETER protocol.

A CSCF is a SIP proxy used to route SIP messages to other network elements.  There are three types:

  • Serving-CSCF (S-CSCF)
  • Proxy-CSCF (P-CSCF)
  • Interogating-CSCF (I-CSCF)

The S-CSCF acts as the registrar for the subscriber's SIP User Agents (UA) by interworking with the HSS using the DIAMETER protocol.  All SIP traffic involved in a subscriber's sessions will be routed through the S-CSCF in accordance with rules downloaded from the HSS.

A P-CSCF acts as a proxy for SIP traffic from subscribers SIP User Agents (UA) and all SIP traffic sent and received by the UA flows directly between the UA and P-CSCF.

An I-CSCF is a proxy for SIP traffic which determines the S-CSCF to use for a session.  It interworks with the HSS using the DIAMETER protocol.  Initial registration from subscribers and all session establishment SIP messages from outside the operator's network transit the I-CSCF.

An AS provides value added services to subscribers over and above the basic session establishment provided by the network elements above.  The S-CSCF will route SIP traffic to one or more AS based on rules downloaded from the HSS.  The AS interworks with the HSS using the DIAMETER protocol and may interwork with network elements outside the IMS.  For example an AS may provide a prepaid calling service by interworking with the SS7 CAMEL Application Part (CAP) of a 2G cellular network.

Other IMS network elements include:

  • Media Gateway (MGW)
  • Media Gateway Control Function (MGCF)
  • Breakout Gateway Control Function (BGCF)
  • Signalling Gateway (SGW)
  • Media Resource Function Processor (MRFP)
  • Media Resource Function Controller (MRFC)

Type: Networkpedia - Archive ID: 3.3544 - 116 Views - Last Updated: December 04, 2010 21:35:26
Tags: IMS HSS CSCF AS SIP DIAMETER Other Mobile Wireless 3G