video call. The authentication parameters are used to control access to the mobile for attachment to the EPS network or registration to the IMS network. The service profile determines the services that mobile has subscribed. The S-CSCF entity accesses the HSS entity to recover the authentication data and the service profile. The I-CSCF entity accesses the HSS entity to retrieve the identity of the S-CSCF entity that has attached the mobile. The TAS entity can access the HSS entity to retrieve service data necessary for the performance of the supplementary telephone service. The subscription locator function (SLF) is a database that records the identity of the HSS entity where the mobile subscription was recorded, in the case where several HSS entities are deployed. 1.3.2. Protocol architecture The S6a interface is the reference point between the MME and HSS entities. This interface supports the DIAMETER signaling. The Cx interface is the reference point between, on the one hand, the I-CSCF or S-CSCF entities, while the HSS entity on the other. This interface supports the DIAMETER signaling. Network Architecture The Sh interface is the reference point between the TAS and HSS entities. This interface supports the DIAMETER signaling. The Dx interface is the reference point between, on the one hand, the I-CSCF or S-CSCF entities and the other hand, the SLF entity. This interface supports the DIAMETER signaling. The Dh interface is the reference point between the SLF and TAS entities. This interface supports the DIAMETER signaling. 1.4. Charging associated with IMS network 1.4.1. Functional architecture In the case of online charging, the user's account is consulted before granting the permission to use the network resource. That account is decreased during the communication. When it reaches zero, the communication is cut off. This mode of charging corresponds to pre-paid service. 1.4.1.1. Offline charging The functional architecture of the offline charging system (OFCS) is described in Figure 1.8. The charging trigge