he S-CSCF entity contacts the HSS entity to recover the UE authentication data. To fulfill this function, an exchange of DIAMETER messages with the HSS entity is required. The S-CSCF entity responds with a message 401 unauthorized containing the parameters used for authentication. On receiving the second REGISTER request, The S-CSCF entity authenticates the UE and recovers its profile from the HSS entity. It responds Network Architecture with a message 200 OK containing a header Service Route with its IP address which the UA keeps in its memory. For an outgoing call, on receipt of the first INVITE request from the P-CSCF entity, the S-CSCF entity performs a check on the service required on the basis of the profile recovered during registration. The S-CSCF entity transfers the request either to the application server or to the entity allocated to the interconnection. The IP address of the application server is contained in the profile of the UE entity recovered during registration. For an incoming call, on receipt of the first INVITE request from the I-CSCF entity, the S-CSCF entity performs a check on the service required. The S-CSCF entity transfers the request either to the application server or to the P-CSCF entity.I S-CSCF, as suggested in the latter case, the entity replaces the URI identity of the request with the IP address of the UE entity. The IP address of the P-CSCF entity is recovered on the basis of the header Path, during the registration of the UE entity. The S-CSCF entity forwards the data necessary to generate charging tickets. 1.2.1.4. E-CSCF entity The E-CSCF entity handles emergency calls transmitted by the P-CSCF and routes the request to the public-safety answering point (PSAP) nearest to the UE. The PSAP emergency center can be linked to a fixed or mobile network or to another IMS network. Upon receiving the INVITE request, the E-CSCF entity retrieves the location of the mobile in the header P-Access-Network-Info. The header P-Access-Network-Info was inserted by the P-CSCF entity. The value