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