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1.2.2. H.323 Protocol Stack
The H.323 specification is an umbrella specification for the many different protocols that comprise the overall H.323 protocol stack. Figure 2 shows the H.323 protocol stack.
The purpose of each protocol is summarized briefly as follows:
- H.245 - Specifies messages for opening and closing channels for media streams, and other commands, requests and indications.
- Q.931 - Defines signaling for call setup and call teardown.
- H.225.0 - Specifies messages for call control including signaling, Registration Admission and Status (RAS), and the packetization and synchronization of media streams.
- Real Time Protocol (RTP) - The RTP specification (RFC 18890) is an IETF draft standard that defines the end-to-end transport of real-time data. RTP does not guarantee quality of service (QoS) on the transmission. However, it does provides some techniques to aid the transmission of isochronous data including:
- Real Time Control Protocol (RTCP) - RTCP is part of the RTP specification (RFC 18890) and defines the end-to-end monitoring of data delivery and QoS by providing information such as:
The H.245, Q.931 and H.225.0 combination provide the signaling for the establishment of a connection, the negotiation of the media format that will be transmitted over the connection, and call teardown at termination. As indicated in Figure 2, the call signaling part of the H.323 protocol is carried over TCP, since TCP guarantees the in-order delivery of packets to the application.
The RTP and RTCP combination is for media handling only. As indicated in Figure 2, the media part of the H.323 protocol is carried over UDP and therefore there is no guarantee that all packets will arrive at the destination and be placed in the correct order.
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