H.323 is an umbrella-like specification that encompasses a large number of state machines that interact in different ways depending upon the presence, absence, and topological relationship of participating entities and the type of session (for example, audio or video). There are many subprotocols within the H.323 specification. In order to understand the overall message flows within an H.323 VoIP transaction, we will concern ourselves with the most common ones that relate to VoIP. Figure 3.10 shows the relevant protocols and their relationships.

Figure 3.10: VoIP-Related H.323 Protocol Stack

H.323 defines a general set of call setup and negotiating procedures the most important in VoIP applications being H.225, H.235, H.245, and members of the Q.900 signaling series. Basic data-transport methods are defined by the real-time protocols RTP and RTCP. H.323 also specifies a group of audio codecs for VoIP communications, the G.700 series:

• H.225/Q.931 Defines signaling for call setup and teardown, including source and destination IP addresses, ports, country code, and H.245 port information.

• H.225.0/RAS Specifies messages that describe signaling, Registration Admission and Status (RAS), and media stream information.

• H.245 Specifies messages that negotiate the terminal capabilities set, the master/slave relationship, and logical channel information for the media streams.

• Real Time Protocol (RTP) Describes the end-to-end transport of real-time data.

• Real Time Control Protocol (RTCP) Describes the end-to-end monitoring of data delivery and QoS by providing information such as jitter and...