Objective Systems, Inc. December 2004
Abstract: This document summarizes the H.323 (H.225, H.245)
Recommendation into an understandable tutorial. Much of the
material is derived from the Recommendations directly. The salient
features of the Recommendations are presented here; for detailed
information, refer to the Recommendations directly. This document
is provided for instructional purposes only.
H.323 Introduction
Table of Contents
1 ABBREVIATIONS .........................................................................................1
2 H.323 OVERVIEW.........................................................................................1
3 H.323 COMPONENTS ..................................................................................1
3.1 Terminal................................................................................................................. 3
3.2 Gateway ................................................................................................................. 3
3.3 Gatekeeper............................................................................................................. 4
3.4 MCU ....................................................................................................................... 4
4 H.225 .............................................................................................................4
4.1 RAS signaling ........................................................................................................ 4
4.1.1 Gatekeeper discovery...................................................................................... 6
4.1.2 Endpoint registration....................................................................................... 6
4.1.3 Endpoint location............................................................................................ 7
4.1.4 Admissions, bandwidth change, status and disengage.................................... 7
4.1.5 RAS Signaling Channel .................................................................................. 8
4.2 H.225 call signaling ............................................................................................... 8
4.2.1 Setup ............................................................................................................... 9
4.2.2 Facility ............................................................................................................ 9
4.2.3 Alerting ........................................................................................................... 9
4.2.4 Call Proceeding............................................................................................... 9
4.2.5 Connect ........................................................................................................... 9
4.2.6 Status............................................................................................................... 9
4.2.7 Status Inquiry .................................................................................................. 9
4.2.8 Release Complete.......................................................................................... 10
4.2.9 Call Signaling Channel ................................................................................. 10
4.3 H.225 layer........................................................................................................... 10
4.3.1 Use of RTP/RTCP......................................................................................... 10
5 H.245 ...........................................................................................................10
5.1 Master/slave determination................................................................................ 11
5.2 Capabilities Exchange ........................................................................................ 11
5.3 Logical Channel Signaling ................................................................................. 11
5.4 Mode Request ...................................................................................................... 11
5.5 H.245 Control Channel....................................................................................... 12
6 H.323 CALL WALKTHROUGH...................................................................12
6.1 Phase 0: RAS ....................................................................................................... 12
6.2 Phase A: Call setup ............................................................................................. 12
6.2.1 Basic call setup - Neither endpoint registered with Gatekeeper ................... 12
6.2.2 Both endpoints registered, both Gatekeepers routing................................... 13
6.3 Phase B: Initial communication and capability exchange............................... 14
6.4 Phase C: Establishment of audiovisual communication.................................. 15
6.5 Phase D: Call services......................................................................................... 15
6.6 Phase E: Call termination .................................................................................. 15
6.6.1 Call clearing without a Gatekeeper............................................................... 15
6.6.2 Call clearing with a Gatekeeper.................................................................... 16
6.6.3 Call clearing by Gatekeeper.......................................................................... 16
6.7 Summary.............................................................................................................. 17
6.7.1 Channels........................................................................................................ 19
7 AUDIO CODECS.........................................................................................19
8 VIDEO CODECS .........................................................................................19
9 DATA CAPABILITY ....................................................................................20
1 Abbreviations
GK Gatekeeper
GW Gateway
ITU International Telecommunications Union
MC Multipoint controller
MCU Multipoint control unit
QoS Quality of Service
RAS Registration, Admissions, Status
RTP Real-time protocol
RTCP Real-time control protocol
SCN Switched circuit network
Tx Terminal
Figure 1 - Table of abbreviations
2 H.323 Overview
H.323 is an ITU Recommendation describing the protocols involved in making a call
with multimedia capabilities over a packet-based network without guaranteed Quality of
Service (QoS). H.323 consists of an umbrella set of recommendations, including:
• H.225
• H.245
• Audio codecs
• Video codecs
• Data capability
The following diagram shows the components of an H.323 protocol stack:
Audio App Video App A/V Control System Control Data App
G.7xx H.26x
RTP
RTCP
H.225 RAS
Control
H.225 Call
Control
H.245
Unreliable transport Reliable transport
Network layer
Link layer
T.12x
Physical layer
Figure 2 - H.323 layers (gray areas indicate required components)
Note that the network layers are not specified by the Recommendation. Any packet-based
network will suffice.
3 H.323 Components
H.323 defines several different entity types, Terminals, Gateways, Gatekeepers, and
Multipoint Control Units. The most basic configuration is comprised of two Terminals
1
