y by the sum of the different APN-AMBR parameters, provided it is less than the value indicated by the HSS entity. 1.2. IMS network 1.2.1. Functional architecture The functional architecture of the IP multimedia subsystem (IMS) network is described in Figure 1.7. The IMS network includes the following entities: - call session control function (CSCF), involving P-CSCF (Proxy-CSCF), S-CSCF (Serving-CSCF), I-CSCF (Interrogating-CSCF) and E-CSCF (Emergency-CSCF); - application servers (AS), involving telephony application server (TAS); - multimedia resource function (MRF), involving MRFC (MRF Controller) and MFRP (MRF Processor). Traffic stream Control stream P-CSCF E-CSCF Interconnection I-CSCF S-CSCF with CS network with IMS network Figure 1.7. Functional architecture of IMS network Network Architecture 1.2.1.1. P-CSCF entity The P-CSCF entity is the first point of contact for the mobile in the IMS network. It performs the function of a PROXY SERVER. It receives the requests from the UE or from the S-CSCF entity and transfers them respectively to the S/I-CSCF entity or to the UE entity. The P-CSCF entity may also act as an user agent (UA) in abnormal operating conditions, when it has to terminate or generate SIP transactions. During registration, the P-CSCF entity forwards the SIP REGISTER request to the I-CSCF entity determined on the basis of the domain name provided by the UE entity. Into this message, it adds a header path containing its identity. This identity is preserved by the S-CSCF entity. During session establishment, the P-CSCF entity forwards the SIP INVITE request received from the S-CSCF entity (or from the UE entity respectively) to the UE entity (or respectively to the S-CSCF entity). To carry out the transfer, the P-CSCF entity has to retrieve the IP addresses of the UE entity (or respectively of the S-CSCF entity): - the SIP INVITE request received from the S-CSCF entity contains the IP address of the UE entity instead of the uniform resource identifier (URI); - the SIP INVITE request received fr