message. 13) to 16) Alice's UA entity sends a re-INVITE to Bob's UA entity to resume the parked communication. Recovery is achieved by modifying the attributes of the media flow in the SDP message (a=sendrecv). 17) to 20) Bob's UA entity responds positively with the message 200 20) to 24) Alice's UA entity acknowledges the response with the ACK message. 5.2.1.5. Conference The CONF service is provided by an application server comprising the multimedia resource function controller (MRFC) and MRF processor (MFRP). The MRFP entity enables the media flows to be added together. The MRFC entity plays the part of a UA entity and controls the MRFP entity. The different exchanges relating to establishing the conference service are shown in Figure 5.8. Two UA entities (Alice and Bob) are communicating. Alice decides to invite Carol and to activate the conference service. Alice puts Bob on hold, initializes a session with Carol, establishes a session with the conference bridge (MFRP) and transfers the two sessions established with Bob and Carol to the conference bridge. When Alice's UA entity has established the sessions with Bob, Carol and the conference bridge, she begins the transfer of the session established with Bob to the conference bridge by sending the request REFER to the MRFC entity. Service Profiles The REFER request includes the header Refer - to containing the references of the dialog initialized with Bob's UA entity (Bob's URI, header Call - Id, parameters tag in the headers From and To). Alice P-CSCF S-CSCF P-CSCF Carol Session established between Alice and Bob Audio flow Alice places Bob on HOLD Session established between Alice and Carol Audio flow Session established between Alice and conference bridge Audio flow SIP REFER SIP REFER SIP REFER SIP 202 SIP 202 SIP 202 SIP ACK SIP ACK SIP ACK Transfer of the session established between SIP INVITE Alice and Bob to the conference bridge SIP INVITE SIP INVITE SIP 200 SIP 200 SIP 200 SIP ACK SIP ACK SIP ACK SIP NOTIFY SIP NOTIFY SIP NOTIFY Audio flow SIP 200 SIP