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Interrogating-CSCF (I-CSCF) 
The I-CSCF node is a SIP proxy located at the edge of an administrative 
domain, and is therefore the first contact within the home network from 
a visited network. The I-CSCF node has an interface to both the HSS and 
the SLF database nodes, and retrieves user location information from the 
HSS and can then route the SIP messages to the appropriate destination. 
The I-CSCF node has the opportunity to encrypt the parts of the SIP 
messages have contains sensitive information of the domain. The I-CSCF 
node is usually located in the home network but can be located in the 
visited network in special circumstances.
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Serving-CSCF (S-CSCF) 
The S-CSCF node is the central node of the signaling plane. It is a SIP 
server but performs session control and can act as a SIP registrar as well. 
This node has the responsibility for the session management in the IMS 
network. One of the main functions is to provide SIP routing services, 
e.g. translation services between SIP URI and telephone numbers. The S-
CSCF is always located at the home network. 
4.1.1.3
 
The Application Servers (AS) 
The AS is a SIP entity that hosts and executes services. The AS can operate in different 
modes depending on which service it is suppose to operate. These modes can be SIP UA 
or B2BUA. Different types of AS are SIP AS, OSA-Service Capability Server (SCS) and 
IP Multimedia Service Switching Function (IM-SSF). These types of AS all behave as SIP 
application servers towards the IMS network. The AS can be located in the home 
network or it can be in an external third-party network. 
4.1.1.4
 
The Media Resource Function (MRF) 
This node provides a source of media in the home network which can be used to do any 
sort of media analysis. The MRF node is divided into two parts, a signaling plane node 
and media plane node called MRF Controller (MRFC) and MRF Processor (MRFP) 
respectively. The MRFC acts like a SIP User Agent (UA), and has an interface towards 
the S-CSCF. It interprets information coming from the AS and the S-CSCF and controls 
the MRFP accordingly. The MRFP provides resources to be controlled by the MRFC 
and implements all the media-related functions. It also mixes the incoming media 
streams. The MRF is always located in the home network.