 of the mobile location was provided by the PGW entity through the PCRF entity. On receiving the INVITE request, the E-CSCF entity contacts the location retrieval function (LRF) to obtain the URI identity of the PSAP emergency center. On the basis of information provided by the LRF entity, the E-CSCF entity transfers the request to the entity allocated to the interconnection to the CS network or the IMS network. VoLTE and ViLTE 1.2.1.5. Application servers The application server provides added-value services to the IMS network. For instance, it hosts and executes the supplementary telephone services. The AS entity may affect the SIP session depending on the service required. The S-CSCF entity has to decide whether an application server is necessary for a specific treatment of an SIP request to ensure handling of the appropriate service. The decision is based on the information received from the HSS during mobile registration. The application server may play various roles in the processing of an SIP message: - that of a PROXY SERVER. In this mode, the SIP request from the S-CSCF entity is sent to the application server. The application server can add, remove or modify headers in the SIP message; - that of an User Agent Server (UAS) or a REDIRECT SERVER. In this mode, the response of the application server to the SIP request from the S-CSCF entity is 2xx, 4xx, 5xx or 6xx (UAS) or 3xx (REDIRECT SERVER); - that of an user agent client (UAC). In this mode, the application server generates the SIP request and transmits it to the S-CSCF entity. - that of a Back to Back User Agent (B2BUA). In this mode, the application server receiving an SIP request from the S-CSCF entity terminates the dialog with the UAC entity at the originating side, and generates a new request with an UAS entity at the terminating side. 1.2.1.6. Media processing Media processing is done by the MRF function, divided into two entities, the MRFC and MRFP. The MRFC entity interprets information from the S-CSCF and controls the MRFP on the basis of this