H.323 - ITU standard protocol for interactive conferencing
MGCP (Media Gateway Control Protocol) - a method of PSTN gateway control or thin device control
Megaco/H.248 - joint IETF/ITU standard protocol that provides full call control intelligence
SIP (Session Initiation Protocol) - a protocol that specifies commands and responses to set up and tear down calls (end-to-end call signaling); text based
RTP (Real-time Transport Protocol) - an IETF media streaming protocol
RTCP (Read-time Transport Control Protocol) - provies out-of-band control information for an RTP flow