The present document specifies the@ stage three@ Protocol Description of the Conference (CONF) service based on stage one and two of the ISDN CONF supplementary service. Within the Next Generation Network (NGN) the stage 3 is specified using the IP-Multimedia Call Control Protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP). The present document specifies centralized conferencing@ using a conference focus@ distributed conferencing is out of scope. The present document does not cover the cases of : a) cascading conference services; and b) the support of the PSTN/ISDN conference service hosted in the PSTN.