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Figure 4-1: Overview of the 3GPP IMS architecture 
The nodes of the IP Multimedia Core Network subsystems are described below. 
4.1.1.1
 
The Home Subscriber Server (HSS): 
This node can be compared to the Home Location Register (HLR) in a GSM network. It 
is the master database for a given user and contains all the information about a user-
related subscription data required to handle multimedia sessions. This is among others 
information concerning location, security, and user profile. This makes it possible for a 
user to find and communicate with other end users. An IMS network can contain more 
then one HSS. If this is the case, there is a need for another database called Subscriber 
Location Functions (SLF). This is a simple database that maps users’ addresses to HSS.
4.1.1.2
 
The Call/Session Control Function (CSCF): 
This node is a SIP server and an essential node. It is needed for session management and 
support for Quality of Service (QoS) provisioning in the core network. Together with the 
HSS database the CSCF node is capable of routing any SIP requests/messages to devices 
and other networks. There are three different types of CSCF. 
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Proxy-CSCF (P-CSCF) 
The P-CSCF node is the first point of contact between the IMS terminal 
and the IMS network, and includes several functions. Some of these 
functions are the handle of emergency control, charging and resource 

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4.1 TMMS Service based on IMS: : Overview 
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allocations, and security functions. The P-CSCF node has two main 
functions. The primary function is to be in the QoS Policy Enforcement 
Point within the visited network. Secondly it is responsible for providing 
the local control for emergency services.
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