It is important to understand that IMS specifications are set up so that the internal functionality of the network entities is not specified in detail. Instead, specifications describe reference points between entities and functionalities supported at the reference points. For instance, how does CSCF obtain user data from databases? Different reference points will be described in Section 2.3. Additionally, GPRS functions are described at the end of this section.
2.2.1. Call Session Control Functions (CSCF)
There are three different kind of Call Session Control Functions (CSCF): Proxy-CSCF (P-CSCF), Serving-CSCF (S-CSCF) and Interrogating-CSCF (I-CSCF). Each CSCF has its own special tasks and these tasks are described in the following subsections. Common to all CSCFs is that they all play a role during registration and session establishment and form the SIP routing machinery. Moreover, all functions are able to send charging data to an offline charging function. ...
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