Dialogic 05-2239-009 User Manual

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Dialogic Corporation

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, 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