VoIP can replace phone-to-phone signaling that the PBX provides in traditional environments
Legacy protocol support is still required for analog phones and connections to the PSTN
The two most popular standards for phone-to-phone signaling are H.323 and SIP
H.323 was created by the ITU as a video conferencing standard but grew to become an ambitious PBX-replacement recommendation
Microsoft NetMeeting and OhPhone are H.323 softphones
A gatekeeper and gateway form the softPBX nucleus on an H.323 network. These two elements often run on a single server
H.245, H.225, and RAS are the three layers of a phone-to-phone signaling session on an H.323 network
SIP was created by the IETF as a media-session management protocol; it has proven a great match for telephony applications on the Internet
SIP defines less of the network than H.323 does, leaving to the application developer the details of application, session, and presentation layers
SIP doesn't address legacy interfacing at all
SIP is seen by many PBX vendors as a way of signaling trunk connections to other vendors ' equipment
SDP is SIP's capabilities negotiation protocol
RTP is the packetization and framing mechanism used by SIP and H.323
IAX, MEGACO/H.248, and Cisco SCCP are other prevalent signaling protocols
IAX does not use RTP for packetization; it frames signaling and sound data in the same packet construct
Heterogeneous signaling is required when an endpoint of one signaling protocol wishes to communicate with an endpoint of another