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Global Call IP Technology Guide — November 2007 31
Dialogic Corporation
IP Overview
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.
Figure 2. 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, the packetization and synchronization
of media streams, and Registration, Admission, and Status (RAS).
Real Time Protocol (RTP)
The RTP specification is an IETF draft standard (RFC 1889) 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:
• information about the type of data being transmitted
• time stamps
• sequence numbers
Real Time Control Protocol (RTCP)
RTCP is part of the IETF RTP specification (RFC 1889) and defines the end-to-end monitoring
of data delivery and QoS by providing information such as:
• jitter, that is, the variance in the delays introduced in transmitting data over a wire
• average packet loss
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.
RTP
(Media Streaming)
UDP
IP
TCP
H.255.0
(RAS)
H.245
(Logical
Channel
Signaling)
H.225.0
(Q.931
Call
Signaling)
RTCP
(Monitoring
and QoS)
Audio Codecs
G.711, G.723.1,
G.726, G.729, etc.
Application