Section: 4.3.2.2.2 S-CSCF online charging / IMS Gateway Function
Content: As stated above, the S-CSCF does not trigger any online charging events and thus does not include the CTF online charging enhancements described in clause 4.3.2.1 (in contrast, it does have a CTF for offline charging, as described in clause 4.3.1.1). Instead, the ISC interface is employed by the S-CSCF online charging, implying that online charging is transparent to the S-CSCF and appears like any other service logic controlled by a SIP application server. Therefore, if support for Ro based online charging is required instead of / or in addition to application server or MRFC, a special CTF is needed in order to mediate between the Ro based SBCF and the SIP based service control. This role is taken by the IMS Gateway Function (IMS GWF), which translates between SIP service control towards the S-CSCF and Ro Credit-Control on the OCS side. From the perspective of the online charging architecture, the IMS GWF is an online charging capable CTF; from the perspective of the S-CSCF, the IMS GWF is a SIP application server and is triggered the same way. It is out of scope of the 3GPP standards whether the IMS GWF is embedded in the S-CSCF, embedded in the OCS/SBCF, or exists as a stand-alone component.