H.323 Protocol Suite
Guoyou He
Helsinki University of Technology
ghe@cc.hut.fi
H.323 Protocol Suite
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Introduction
What is H.323 Protocol
H.323 Version History
H.323 Architecture
Call Signaling
Call Connection Procedures
New Features of H.323 Version 4
Comments on H.323
Conclusions
Introduction
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Numerous multimedia applications and services are
available .
New technology and applications are constantly evolving
(e.g. multimedia computer, compression technique,
multimedia networks, and transport mechnanisms etc.).
Standards and technology for multimedia communications
are evolving at at a prodigious pace.
Many standards have been specified to provide
interoperability for equipment from multiple vendors.
H.323 is one of them.
What is H.323 suite
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H.323 is a standard specified for Packet-based
Multimedia Communications Systems by ITU-T.
H.323 defines systems and functions for audiovisual
services over packet switched networks which may not
provide a guaranteed Quality of Service.
H.323 suite is an ´Umbrella´ standard, references to other
standards and ITU recommendations.
Interoperability with other multimedia networks is the
primary goal for developing H.323.
Version 1 was approved in June, 1996.
What is H.323 Suite (cont.)
H.323 Standards
Network
Non-guaranteed Bandwidth packet-switched
networks (e.g. IP)
Video
H.261, H.263
Audio
G.711, G.722, G.728, G.723, G.729
Call signaling and media H.225
packetisation
Call Control
H.245
Multipoint
H.323
Data
T.120
H.323 Version Suites
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Version 1 – June, 1996
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Named “Visual telephone systems and equipment for local area
networks which provide a non-guaranteed quality of service”
Multimedia conferencing on a LAN
Defined network elements and overall call comdel
Version 2 – January, 1998
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Name changed to “Packet-based multimedia communications
systems”
More efficient procedures for two-party call setup, e.g. fast call
Brought in H.235 Security (Authentication of participant, Integrity of
data, Encryption, and digital signature)
Supplementary Services (H.450), Integration of data conferencing
with T.120
Support media over ATM
H.323 Version Suites (cont.)
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Version 3 – September, 1999
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Geared for better PSTN integration and scability (alternate
resources, multiplexing signaling channels)
Expanded supplementary services (H.450)
Version 4 – November, 2000
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Introduced a lot of new enhancements in many important areas,
including reliability, scalability, and flexibility.
New features help facilitate more scalable Gateway and MCU
Added more new services
New features for usage information reporting, better bandwidth
management, etc.
H.323 Architecture
H.323 Architecture (cont.)
Audio
I/O
Audio
Codec
G.711
G.722
G.723
G.728
G.729
Video
I/O
Data
App.
Video
Codec
Data
Interface
H.261
H.263
RTP/RTCP
UDP
T.120
System Control
User Interface
System Control
CALL
Control
H.225
RAS
H.245
Control Control
H.225
UDP or TCP
IP
L_2 Varies
L_1 Varies
H.323 protocol stack
H.323 Architecture (cont.)
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H.323 standards specifies the elements, protocols,
and procedures providing multimedia
communication over packet-based networks. The
four main enlements are:
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Terminals
Gateways
Multipoint Control Unit (MCU)
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GateKeeper
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H.323 Architecture (cont.)
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Terminals
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Including Video I/O equipment, Audio I/O equipment, User Data Applications,
PC, and System Control User Interface etc.
Used for real-time bidirectional multimedia communications
Compatible with the terminals on the inter-working networks
Client Endpoing on the network
Must support audio
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Video, data support optional
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G.711 (64 Kb/s) is mandatory
G.722 (48, 56, and 64 kbps channels), G.723 (5.3 and 6.3 kbps channels), G.728 (16
kbps channel), and G.729 (8 kbps channel) are specified low bit rate voice
H.261 mandatory if video is supported
H.245, H.225 required for control functions
Real-time Transport Protocol (RTP) used for media packet
H.323 Architecture (cont.)
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Gateways
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Supports interoperability with other terminal types
Provides translation functions between H.323 and circuit-switched
networks
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Translating protocols for call setup and release
Converting media format between different networks
Gateways are optional in H.323 networks
H.323 Architecture (cont.)
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Multipoint Control Unit (MCU)
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Responsible for managing multipoint conferencing between three or
more H.323 terminals.
Two functional parts
Multipoint controller (MC) – mandatory
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Supports the negotiation of capabilities with all terminals
control the resources in the multicast operation
Conference control -> What media streams go where
May also located in terminal, gateway, or gatekeeper
Multipoint processor (MP) – optional
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Is the central processor of some or all the voice, video, and data streams for
a multipoint conference
Mixes, switches, and processes media streams
H.323 Architecture (cont.)
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MCU (Centralized/Decentralized multipoint conference)
H.323 Architecture (cont.)
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MCU (Hybrid multipoint conference)
H.323 Architecture (cont.)
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Gatekeeper (Required functions)
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Admission control for the network
Bandwidth control and management
Address resolution
978-555-4567  204.124.46.19)
E.164 Number
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Network address
Communicates using the RAS protocol
Manages all terminals, gateways, MCUs in an H.323
zone
Optional element in H.323 network, but Must be
consulted if present
H.323 Architecture (cont.)
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Gatekeeper (Optional functions)
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Call control signaling
- May process messages or pass them on
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Call management
-May be based on address translation function
- Call screening
- Call forwarding/redirection
- Call routing
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Time of day
Network congestion
Least cost
Call Signaling
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Addresses
Registration, Admission, and Status (RAS)
H.225 Call Signaling and H.245 Control
Signaling
Addresses
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Each entity has at least one network address
- To uniquely identifies the H.323 entity on the network.
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Some entities may share a network address
- e.g. a terminal and a co-located MC
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For each network address, each H.323 entity may have
several Transport layer Service Access Point (TSAP)
identifiers
- These TSAP identifiers allow multiplexing of several channels
sharing the same network address
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An endpoint may also have one or more alias addresses.
- May represent the endpoint
- May represent the conferences that the endpoint is hosting
Registration, Admission, and
Status (RAS)
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Used between H.323 endpoints and gatekeepers
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Gatekeeper discovery
- determine which Gatekeeper to register with
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Endpoint registration
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Static (well-known Gatekeepr)
Dynamic (multicast)
join a zone, and informs the Gatekeeper of its transport address and alias address
done in the configuration process
occurs before any calls and may occur periodically
Endpoint location
- transport address is determined
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Admission control
- restrict the entry of an endpoint into a zone
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Bandwidth change
- modify the call bandwidth during a call
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Disengagement control
- disassociate an endpoint from a gatekeeper and it zone
Registration, Admission, and
Status (RAS) (cont.)
Gatekeeper discovery
Endpoint
Gatekeeper
GRQ
GCF/GRJ
Registration, Admission, and
Status (RAS) (cont.)
Endpoint registration
Endpoint
Gatekeeper
RRQ
RCF/RRJ
URQ
UCF/URJ
Endpoint initiated
Unregister Request
URQ
UCF
Gatekeeper initiated
Unregister Request
H.225 Call signaling and H.245
Control signaling
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H.225 Call signaling
H.245 Control signaling
H.225 Call Signaling
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Setup connections between H.323
endpoints
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Over which real-time data is transported
Is reliable channel used to carry H.225 call
control messages
Two kind of call signaling method
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Gatekeeper-routed Call Signaling
Direct Call Signaling
Gatekeeper-routed Call Signaling
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Admission messages are on RAS channels
Call-signaling messages are on the call-signaling channel
Gatekeeper cloud
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3
4
5
6
7
8
ARQ
ACF/ARJ
Setup
Setup
ARQ
ACF/ARJ
Connect
Connect
1
2
3
Endpoint 1
Call Signalling Channel Messages
RAS Channel Messages
8
4
5
6
7
Endpoint 2
T1521280-96
Direct Call Signaling
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During the admission confirmation, Gatekeeper indicates that the endpoints can
exchange call-signaling messages directly
Gatekeeper cloud
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1
2
3
4
5
6
ARQ
ACF/ARJ
Setup
ARQ
ACF/ARJ
Connect
2
4
5
3
Endpoint 1
6
Endpoint 2
T1521290-96
Call Signalling Channel Messages
RAS Channel Messages
H.245 Control Signaling
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When Gatekeeper routed call signaling is
used
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H.245 control channel is established directly
between the endpoints
H.245 control channel is routed between the
endpoints through the Gatekeeper
When Direct endpoint call signaling is used
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H.245 control channel can only be connected
directly between the endpoints
H.245 Control Signaling (cont.)
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H.245 control channel is established directly between the endpoints
Gatekeeper cloud
1
2
3
4
5
6
7
8
9
ARQ
ACF/ARJ
Setup
Setup
ARQ
ACF/ARJ
Connect
Connect
H.245 Channel
1
2
3
8
4
5
6
7
9
Endpoint 1
H.245 Control Channel Messages
Call Signalling Channel Messages
RAS Channel Messages
Endpoint 2
T1521300-96
H.245 Control Signaling
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H.245 control channel is routed between the endpoints through the Gatekeeper
Gatekeeper cloud
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2
3
4
5
6
7
8
9
10
ARQ
ACF/ARJ
Setup
Setup
ARQ
ACF/ARJ
Connect
Connect
H.245 Channel
H.245 Channel
1
2
3
Endpoint 1
H.245 Control Channel Messages
Call Signalling Channel Messages
RAS Channel Messages
8
9
4
5
6
7
10
Endpoint 2
T1521310-96
Call Connection Procedures
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Step A: Call setup
Step B: Initial communication and
capability exchange
Step C: Establishment of audiovisual
communication
Step D: Call services
Step E: Call termination
Step A: Call setup
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Different call setup cases
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all combinations of Direct Routed Call
signaling (DRC)/Gatekeeper Routed Call
signaling (GRC) with same or different
Gatekeepers;
Fast connect procedures;
Call forwarding using facility (restarts the
procedure);
setting up conferences
Step A: Call setup (cont.)
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Both endpoints registered to the same Gatekeeper, direct call signaling
Endpoint 1
Gatekeeper 1
Endpoint 2
ARQ (1)
ACF/ARJ (2)
Setup (3)
Call proceeding (4)
ARQ (5)
ACF/ARJ (6)
Alerting (7)
Connect (8)
T1527160-97
RAS Messages
Call Signalling Messages
Step B: Initial communication
and capability exchange
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Capability exchange
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Endpoints establish the H.245 control channel after call setup
message from step A
H.245 procedures are used over the H.245 control channel
- for the capability exchange
- to open the media channels
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Master/Slave determination
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To resolve conflicts between two endpoints which both:
- can be the MC for a conference
- are attempting to open a bidirectional channel
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H.245 tunneling
Step B: Initial communication
and capability exchange
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H.245 Control signaling flow, control channel is established directly between the endpoints
Endpoint 1
Gatekeeper
Endpoint 2
TerminalCapabilitySet(9)
TerminalCapabilitySetAck(10)
TerminalCapabilitySet(11)
TerminalCapabilitySetAck(12)
OpenLogicalChannel(13)
OpenLogicalChannelAck(14)
OpenLogicalChannel(15)
OpenLogicalChannelAck(16)
H.245 Message
Step C: Establishment of
audiovisual communication
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The audio and video streams
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transmitted in the logical channels setup in H.245,
transported over dynamic Transport layer Service
Access Point (TSAP) Identifiers using an unreliable
protocol.
Data communications
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transmitted in the logical channels setup in H.245,
transported using a reliable protocol.
Step C: Establishment of audiovisual
communication (cont.)
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Media stream and media control flow
Endpoint 1
Gatekeeper
Endpoint 2
RTP Media Stream(17)
RTP Media Stream(18)
RTCP Messages(19)
RTCP Messages(20)
RTP media stream and RTCP Messages
Step D: Call services
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Bandwidth change
Status
Conference expansion
Multicast cascading
H.450 Supplementary services
Step D: Call services
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Bandwidth change may requested by both Gatekeeper or endpoints during a conference
Endpoint 1
Gatekeeper
Endpoint 2
BRQ(21)
BCF/BRJ(22)
CloseLogicalChannel(23)
OpenLogicalChannel(24)
BRQ(25)
BCF/BRJ(26)
OpenLogicalChAck(27)
RAS messages
H.245 messages
Step E: Call termination
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Any endpoint can request terminate a call
All logical channels are closed
Terminating a conference is done by MC
Endpoint 1
Gatekeeper
Endpoint 2
End Session Command(28)
End Sesion Command(29)
Release Complete (30)
DRQ(31)
DCF(33)
H.225 Signaling Messages
RAS Message
H.245 Message
DRQ(32)
DCF(34)
New Feature of H.323 Version 4
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Scalability, Reliability, and Flexibility
Services
“Must Have” Features
Further Features are under developing
on H.323
Scalability, Reliability, and
Flexibility
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Gateway Decomposition with H.248
Alternate Gatekeepers
Endpoint Capacity Reporting
Gateway Decomposition with
H.248
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Decomposition gateway
Alternate Gatekeepers
Endpoint Capacity Reporting
Services
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HTTP-based Service Control
Stimulus-based Control
Call completion
Must Have” Features
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Usage Information Reporting
Tones and Announcements
Alias Mapping
Better Bandwidth Management (multicast)
Fax Enhancements
Tunneling other protocols
H.323 specific URL
Call Credit-related capabilities
Multiplexing audio and video
Further Features are under
developing on H.323
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Generic Extensibility Framework
Inter-working or integrating with other
protocols
Mobility
Robustness
Mobility
Comments on H.323
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H.323 is a very complex system with all of kinds of features for
multimedia communications
H.323 allows the use of multiple codecs
H.323 is scalable
H.323 is a proven technology used in large networks. It has excellent
integration with PSTN.
Multimedia conferencing shows the real potential for H.323 used in
multimedia communication
Many equipment manufacturers, software vendors, and service
providers have built products and services supporting H.323. It
greatly supports the success of H.323.
Under the challenge of new technologies
Conclusions
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H.323 is organized around four major facilities: (a) terminals, (b)
Gateways, (c) Gatekeeper, and (d) MCUs
A principal technology for the transmission of real-time audio, video,
and data communication over packet-based networks
Multipoint and point-to-point sessions.
A rich and complex specification
Scalable, reliable, flexible system,
Supplementary services, and new features are powerful
Faces great compression from some newly developed protocols
Reducing the complexity of H.323, and simplifying its usage are
essential