From Wikipedia, the free encyclopedia H.323
H.323
H.323 is a recommendation from the ITU Telecommu- H.323 was changed when published in 1998.[5] The title,
nication Standardization Sector (ITU-T) that defines the which has since remained unchanged, is "Packet-Based
protocols to provide audio-visual communication ses- Multimedia Communications Systems." The current ver-
sions on any packet network. The H.323 standard ad- sion of H.323 was approved in 2009.[6]
dresses call signaling and control, multimedia transport One strength of H.323 was the relatively early avail-
and control, and bandwidth control for point-to-point ability of a set of standards, not only defining the basic
and multi-point conferences.[1] call model, but also the supplementary services needed
It is widely implemented[2] by voice and videocon- to address business communication expectations.[citation
ferencing equipment manufacturers, is used within var- needed]
ious Internet real-time applications such as GnuGK and H.323 was the first VoIP standard to adopt the In-
NetMeeting and is widely deployed worldwide by service ternet Engineering Task Force (IETF) standard Real-time
providers and enterprises for both voice and video ser- Transport Protocol (RTP) to transport audio and video
vices over IP networks. over IP networks.[citation needed]
It is a part of the ITU-T H.32x series of protocols,
which also address multimedia communications over
ISDN, the PSTN or SS7, and 3G mobile networks.
Protocols
H.323 call signaling is based on the ITU-T Recommen- H.323 is a system specification that describes the use of
dation Q.931 protocol and is suited for transmitting calls several ITU-T and IETF protocols. The protocols that
across networks using a mixture of IP, PSTN, ISDN, and comprise the core of almost any H.323 system are:[7]
QSIG over ISDN. A call model, similar to the ISDN call • H.225.0 Registration, Admission and Status (RAS),
model, eases the introduction of IP telephony into exist- which is used between an H.323 endpoint and a
ing networks of ISDN-based PBX systems, including tran- Gatekeeper to provide address resolution and
sitions to IP-based PBXs. admission control services.
Within the context of H.323, an IP-based PBX might • H.225.0 Call Signaling, which is used between any
be a gatekeeper or other call control element which pro- two H.323 entities in order to establish
vides service to telephones or videophones. Such a device communication.
may provide or facilitate both basic services and supple- • H.245 control protocol for multimedia
mentary services, such as call transfer, park, pick-up, and communication, which describes the messages and
hold. procedures used for capability exchange, opening
While H.323 excels[citation needed] at providing basic and closing logical channels for audio, video and
telephony functionality and interoperability, H.323’s data, control and indications.
strength lies in multimedia communication functionality • Real-time Transport Protocol (RTP), which is used
designed specifically for IP networks[citation needed]. for sending or receiving multimedia information
(voice, video, or text) between any two entities.
Many H.323 systems also implement other protocols that
History are defined in various ITU-T Recommendations to pro-
The first version of H.323 was published by the ITU in vide supplementary services support or deliver other
November 1996[3] with an emphasis of enabling video- functionality to the user. Some of those Recommenda-
conferencing capabilities over a local area network tions are:[citation needed]
(LAN), but was quickly adopted by the industry as a • H.235 series describes security within H.323,
means of transmitting voice communication over a vari- including security for both signaling and media.
ety of IP networks, including WANs and the Internet (see • H.239 describes dual stream use in
VOIP). videoconferencing, usually one for live video, the
Over the years, H.323 has been revised and re-pub- other for still images.
lished with enhancements necessary to better-enable • H.450 series describes various supplementary
both voice and video functionality over packet-switched services.
networks, with each version being backward-compatible • H.460 series defines optional extensions that might
with the previous version.[4] Recognizing that H.323 was be implemented by an endpoint or a Gatekeeper,
being used for communication, not only on LANs, but including ITU-T Recommendations H.460.17,
over WANs and within large carrier networks, the title of
1
From Wikipedia, the free encyclopedia H.323
H.460.18, and H.460.19 for Network address Terminals in an H.323 network are the most funda-
translation (NAT) / Firewall (FW) traversal. mental elements in any H.323 system, as those are the de-
In addition to those ITU-T Recommendations, H.323 im- vices that users would normally encounter. They might
plements various IETF Request for Comments (RFCs) for exist in the form of a simple IP phone or a powerful high-
media transport and media packetization, including the definition videoconferencing system.
Real-time Transport Protocol (RTP). Inside an H.323 terminal is something referred to as
a Protocol stack, which implements the functionality de-
Codecs fined by the H.323 system. The protocol stack would in-
clude an implementation of the basic protocol defined in
H.323 utilizes both ITU-defined codecs and codecs de- ITU-T Recommendation H.225.0 and H.245, as well as RTP
fined outside the ITU. Codecs that are widely implement- or other protocols described above.
ed by H.323 equipment include: The diagram, figure 1, depicts a complete, sophisti-
• Audio codecs: G.711, G.729 (including G.729a), cated stack that provides support for voice, video, and
G.723.1, G.726, G.722, G.728, Speex various forms of data communication. In reality, most
• Text codecs: T.140 H.323 systems do not implement such a wide array of ca-
• Video codecs: H.261, H.263, H.264 pabilities, but the logical arrangement is useful in under-
All H.323 terminals providing video communications standing the relationships.
shall be capable of encoding and decoding video accord-
ing to H.261 QCIF. All H.323 terminals shall have an audio Multipoint Control Units
codec and shall be capable of encoding and decoding A Multipoint Control Unit (MCU) is responsible for man-
speech according to ITU-T Rec. G.711. All terminals shall aging multipoint conferences and is composed of two
be capable of transmitting and receiving A-law and μ- logical entities referred to as the Multipoint Controller
law. Support for other audio and video codecs is option- (MC) and the Multipoint Processor (MP). In more practi-
al.[6] cal terms, an MCU is a conference bridge not unlike the
conference bridges used in the PSTN today. The most sig-
H.323 Architecture nificant difference, however, is that H.323 MCUs might be
capable of mixing or switching video, in addition to the
The H.323 system defines several network elements that normal audio mixing done by a traditional conference
work together in order to deliver rich multimedia com- bridge. Some MCUs also provide multipoint data collab-
munication capabilities. Those elements are Terminals, oration capabilities. What this means to the end user is
Multipoint Control Units (MCUs), Gateways, Gatekeepers, that, by placing a video call into an H.323 MCU, the user
and Border Elements. Collectively, terminals, multipoint might be able to see all of the other participants in the
control units and gateways are often referred to as end- conference, not only hear their voices.
points.
points
While not all elements are required, at least two ter- Gateways
minals are required in order to enable communication Gateways are devices that enable communication be-
between two people. In most H.323 deployments, a gate- tween H.323 networks and other networks, such as PSTN
keeper is employed in order to, among other things, fa- or ISDN networks. If one party in a conversation is utiliz-
cilitate address resolution. ing a terminal that is not an H.323 terminal, then the call
must pass through a gateway in order to enable both par-
H.323 Network Elements ties to communicate.
Gateways are widely used today in order to enable the
Terminals legacy PSTN phones to interconnect with the large, inter-
national H.323 networks that are presently deployed by
services providers. Gateways are also used within the en-
terprise in order to enable enterprise IP phones to com-
municate through the service provider to users on the
PSTN.
Gateways are also used in order to enable videocon-
ferencing devices based on H.320 and H.324 to commu-
nicate with H.323 systems. Most of the third generation
(3G) mobile networks deployed today utilize the H.324
protocol and are able to communicate with H.323-based
terminals in corporate networks through such gateway
Figure 1 - A complete, sophisticated protocol stack devices.
2
From Wikipedia, the free encyclopedia H.323
Gatekeepers Border Elements and Peer Elements are optional enti-
A Gatekeeper is an optional component in the H.323 net- ties similar to a Gatekeeper, but that do not manage end-
work that provides a number of services to terminals, points directly and provide some services that are not de-
gateways, and MCU devices. Those services include end- scribed in the RAS protocol. The role of a border or peer
point registration, address resolution, admission control, element is understood via the definition of an "adminis-
user authentication, and so forth. Of the various func- trative domain".
tions performed by the gatekeeper, address resolution is An administrative domain is the collection of all
the most important as it enables two endpoints to con- zones that are under the control of a single person or or-
tact each other without either endpoint having to know ganization, such as a service provider. Within a service
the IP address of the other endpoint. provider network there may be hundreds or thousands
Gatekeepers may be designed to operate in one of two of gateway devices, telephones, video terminals, or other
signaling modes, namely "direct routed" and "gatekeep- H.323 network elements. The service provider might
er routed" mode. Direct routed mode is the most effi- arrange devices into "zones" that enable the service
cient and most widely deployed mode. In this mode, end- provider to best manage all of the devices under its con-
points utilize the RAS protocol in order to learn the IP trol, such as logical arrangement by city. Taken together,
address of the remote endpoint and a call is established all of the zones within the service provider network
directly with the remote device. In the gatekeeper rout- would appear to another service provider as an "admin-
ed mode, call signaling always passes through the gate- istrative domain".
keeper. While the latter requires the gatekeeper to have The border element is a signaling entity that gener-
more processing power, it also gives the gatekeeper com- ally sits at the edge of the administrative domain and
plete control over the call and the ability to provide sup- communicates with another administrative domain. This
plementary services on behalf of the endpoints. communication might include such things as access au-
H.323 endpoints use the RAS protocol to communi- thorization information; call pricing information; or oth-
cate with a gatekeeper. Likewise, gatekeepers use RAS to er important data necessary to enable communication
communicate with other gatekeepers. between the two administrative domains.
A collection of endpoints that are registered to a sin- Peer elements are entities within the administrative
gle Gatekeeper in H.323 is referred to as a “zone”. This domain that, more or less, help to propagate information
collection of devices does not necessarily have to have an learned from the border elements throughout the ad-
associated physical topology. Rather, a zone may be en- ministrative domain. Such architecture is intended to en-
tirely logical and is arbitrarily defined by the network ad- able large-scale deployments within carrier networks
ministrator. and to enable services such as clearinghouses.
Gatekeepers have the ability to neighbor together so The diagram, figure 2, provides an illustration of an
that call resolution can happen between zones. Neigh- administrative domain with border elements, peer ele-
boring facilitates the use of dial plans such as the Global ments, and gatekeepers.
Dialing Scheme. Dial plans facilitate “inter-zone” dialing
so that two endpoints in separate zones can still commu- H.323 Network Signaling
nicate with each other. H.323 is defined as a binary protocol, which allows for
efficient message processing in network elements. The
Border Elements and Peer Elements syntax of the protocol is defined in ASN.1 and uses the
Packed Encoding Rules (PER) form of message encoding
for efficient message encoding on the wire. Below is an
overview of the various communication flows in H.323
systems.
H.225.0 Call Signaling
Once the address of the remote endpoint is resolved, the
endpoint will use H.225.0 Call Signaling in order to estab-
lish communication with the remote entity. H.225.0 mes-
sages are:
• Setup and Setup acknowledge
• Call Proceeding
Figure 2 - An illustration of an administrative domain with • Connect
border elements, peer elements, and gatekeepers • Alerting
• Information
• Release Complete
3
From Wikipedia, the free encyclopedia H.323
• Facility
• Progress
• Status and Status Inquiry
• Notify
Figure 4 - A high-level communication exchange between two
endpoints (EP) and two gatekeepers (GK)
Figure 3 - Establishment of an H.323 call
that is predefined in the system’s administrative setup.
In the simplest form, an H.323 call may be established Gatekeepers will then respond with a gatekeeper confirm
as follows (figure 3): (GCF). If a GRQ has been sent the endpoint will then select
In this example, the endpoint (EP) on the left initiated a gatekeeper with which to register by sending a regis-
communication with the gateway on the right and the tration request (RRQ), to which the gatekeeper responds
gateway connected the call with the called party. In re- with a registration confirm (RCF). At this point, the end-
ality, call flows are often more complex than the one point is known to the network and can make and place
shown, but most calls that utilize the Fast Connect pro- calls.
cedures defined within H.323 can be established with as When an endpoint wishes to place a call, it will send
few as 2 or 3 messages. Endpoints must notify their gate- an admission request (ARQ) to the gatekeeper. The gate-
keeper (if gatekeepers are used) that they are in a call. keeper will then resolve the address (either locally, by
Once a call has concluded, a device will send a Release consulting another gatekeeper, or by querying some oth-
Complete message. Endpoints are then required to notify er network service) and return the address of the remote
their gatekeeper (if gatekeepers are used) that the call endpoint in the admission confirm message (ACF). The
has ended. endpoint can then place the call.
Upon receiving a call, a remote endpoint will also
RAS Signaling send an ARQ and receive an ACF in order to get permis-
Endpoints use the RAS protocol in order to communicate sion to accept the incoming call. This is necessary, for
with a gatekeeper. Likewise, gatekeepers use RAS to com- example, to authenticate the calling device or to ensure
municate with peer gatekeepers. RAS is a fairly simple that there is available bandwidth for the call.
protocol composed of just a few messages. Namely: Figure 4 depicts a high-level communication ex-
• Gatekeeper request, reject, and confirm messages change between two endpoints (EP) and two gatekeepers
(GRx) (GK).
• Registration request, reject, and confirm messages
(RRx) H.245 Call Control
• Unregister request, reject, and confirm messages Once a call has initiated (but not necessarily fully con-
(URx) nected) endpoints may initiate H.245 call control signal-
• Admission request, reject, and confirm messages ing in order to provide more extensive control over the
(ARx) conference. H.245 is a rather voluminous specification
• Bandwidth request, reject, and confirm message with many procedures that fully enable multipoint com-
(BRx) munication, though in practice most implementations
• Disengage request, reject, and confirm (DRx) only implement the minimum necessary in order to en-
• Location request, reject, and confirm messages (LRx) able point-to-point voice and video communication.
• Info request, ack, nack, and response (IRx) H.245 provides capabilities such as capability negotia-
• Nonstandard message tion, master/slave determination, opening and closing of
• Unknown message response "logical channels" (i.e., audio and video flows), flow con-
• Request in progress (RIP) trol, and conference control. It has support for both uni-
• Resource availability indication and confirm (RAx) cast and multicast communication, allowing the size of a
• Service control indication and response (SCx) conference to theoretically grow without bound.
• Admission confirm sequence (ACS)
When an endpoint is powered on, it will generally Capability Negotiation
send either a gatekeeper request (GRQ) message to "dis- Of the functionality provided by H.245, capability nego-
cover" gatekeepers that are willing to provide service or tiation is arguably the most important, as it enables de-
will send a registration request (RRQ) to a gatekeeper vices to communicate without having prior knowledge of
4
From Wikipedia, the free encyclopedia H.323
the capabilities of the remote entity. H.245 enables rich After this exchange of messages, the two endpoints
multimedia capabilities, including audio, video, text, and (EP) in this figure would be transmitting audio in each di-
data communication. For transmission of audio, video, or rection. The number of message exchanges is numerous,
text, H.323 devices utilize both ITU-defined codecs and each has an important purpose, but nonetheless takes
codecs defined outside the ITU. Codecs that are widely time.
implemented by H.323 equipment include: For this reason, H.323 version 2 (published in 1998)
• Video codecs: H.261, H.263, H.264 introduced a concept called Fast Connect, which enables
• Audio codecs: G.711, G.729, G.729a, G.723.1, G.726 a device to establish bi-directional media flows as part of
• Text codecs: T.140 the H.225.0 call establishment procedures. With Fast Con-
H.245 also enables real-time data conferencing capability nect, it is possible to establish a call with bi-directional
through protocols like T.120. T.120-based applications media flowing with no more than two messages, like in
generally operate in parallel with the H.323 system, but figure 3.
are integrated to provide the user with a seamless multi- Fast Connect is widely supported in the industry.
media experience. T.120 provides such capabilities as ap- Even so, most devices still implement the complete H.245
plication sharing T.128, electronic whiteboard T.126, file exchange as shown above and performs that message ex-
transfer T.127, and text chat T.134 within the context of change in parallel to other activities, so there is no no-
the conference. ticeable delay to the calling or called party.
When an H.323 device initiates communication with
a remote H.323 device and when H.245 communication is
established between the two entities, the Terminal Capa-
Use cases
bility Set (TCS) message is the first message transmitted
to the other side. H.323 and Voice over IP services
Voice over Internet Protocol (VoIP) describes the trans-
Master/Slave Determination
mission of voice using the Internet or other packet
After sending a TCS message, H.323 entities (through switched networks. ITU-T Recommendation H.323 is one
H.245 exchanges) will attempt to determine which device of the standards used in VoIP. VoIP requires a connection
is the "master" and which is the "slave." This process, re- to the Internet or another packet switched network, a
ferred to as Master/Slave Determination (MSD), is impor- subscription to a VoIP service provider and a client (an
tant, as the master in a call settles all "disputes" between analogue telephone adapter (ATA), VoIP Phone or "soft
the two devices. For example, if both endpoints attempt phone"). The service provider offers the connection to
to open incompatible media flows, it is the master who other VoIP services or to the PSTN. Most service
takes the action to reject the incompatible flow. providers charge a monthly fee, then additional costs
Logical Channel Signaling when calls are made.[citation needed] Using VoIP between
Once capabilities are exchanged and master/slave deter- two enterprise locations would not necessarily require a
mination steps have completed, devices may then open VoIP service provider, for example. H.323 has been wide-
"logical channels" or media flows. This is done by simply ly deployed by companies who wish to interconnect re-
sending an Open Logical Channel (OLC) message and re- mote locations over IP using a number of various wired
ceiving an acknowledgement message. Upon receipt of and wireless technologies.
the acknowledgement message, an endpoint may then
transmit audio or video to the remote endpoint. H.323 and Videoconference services
A videoconference, or videoteleconference (VTC), is a set
Fast Connect
of telecommunication technologies allowing two or more
locations to interact via two-way video and audio trans-
missions simultaneously. There are basically two types
of videoconferencing; dedicated VTC systems have all re-
quired components packaged into a single piece of equip-
ment while desktop VTC systems are add-ons to normal
PC’s, transforming them into VTC devices. Simultaneous
videoconferencing among three or more remote points
is possible by means of a Multipoint Control Unit (MCU).
There are MCU bridges for IP and ISDN-based videocon-
ferencing. Due to the price point and proliferation of the
Internet, and broadband in particular, there has been a
Figure 5 - A typical H.245 exchange strong spurt of growth and use of H.323-based IP video-
conferencing. H.323 is accessible to anyone with a high
A typical H.245 exchange looks similar to figure 5:
5
From Wikipedia, the free encyclopedia H.323
speed Internet connection, such as DSL. Videoconferenc- [4] ITU-T Recommendation H.323, in force, superseded
ing is utilized in various situations, for example; distance and withdrawn component.
education, telemedicine, Video Relay Service, and busi- [5] ITU-T Recommendation H.323 (02/1998), Packet-
ness.[citation needed] based multimedia communications systems.
[6] ^ ITU-T Recommendation H.323 (12/2009), Packet-
International Conferences based multimedia communications systems.
H.323 has been used in the industry to enable large-scale [7] See ITU-T Recommendations of the H.323 System
international video conferences that are significantly for a detailed list.
larger than the typical video conference. One of the most [8] "EVO The Collaboration Network".
widely attended is an annual event called Megaconfer- http://evo.caltech.edu/evoGate. Retrieved
ence. 2010-03-08.
Alternatives ITU-T Recommendations of the
The IETF produced a standard called the Session Initia- H.323 System
tion Protocol (SIP) that also enables voice and video com-
• ITU-T Recommendation H.323, Packet-based
munication over IP. There are also other ITU-T recom-
multimedia communications systems.
mendations used for videoconferencing and videophone
• ITU-T Recommendation H.225.0, Call signalling
services - H.320 (using ISDN) and H.324 (using regular
protocols and media stream packetization for packet-based
analog phone lines and 3G mobile phones). Some
multimedia communication systems.
providers (such as Skype) also use their own closed, pro-
• ITU-T Recommendation H.245, Control protocol for
prietary formats. Access Grid provides broadly similar
multimedia communication.
functionality, with more emphasis on open-source and
• ITU-T Recommendation H.246, Interworking of H-series
utilizing multicast. EVO also provides relatively open
multimedia terminals with H-series multimedia terminals
functionality via Java, and includes H.323 support.[8]
and voice/voiceband terminals on GSTN and ISDN.
• ITU-T Recommendation H.283, Remote device control
See also logical channel transport.
• ITU-T Recommendation H.341, Multimedia
• Global Dialing Scheme (GDS)
management information base.
• H.323 Gatekeeper
ITU-T H.235 Series Recommendations
• H.323 Gateway
• ITU-T Recommendation H.235.1, H.323 security
• Next Generation Networks
framework: Baseline security profile.
• Internet Engineering Task Force (IETF)
• ITU-T Recommendation H.235.2, H.323 security
• International Telecommunication Union (ITU)
framework: Signature security profile.
Telecommunications Standardization Sector (ITU-T)
• ITU-T Recommendation H.235.3, H.323 security: Hybrid
• Multipoint Control Units (MCU)
security profile.
• Videoconferencing
• ITU-T Recommendation H.235.4, H.323 security: Direct
• Voice over IP (VoIP)
and selective routed call security.
• Session Initiation Protocol (SIP)
• ITU-T Recommendation H.235.5, H.323 security:
• LifeSize Communications
Framework for secure authentication in RAS using weak
• Polycom
shared secrets.
• Tandberg
• ITU-T Recommendation H.235.6, H.323 security
• Radvision
framework: Voice encryption profile with native H.235/
• RTP audio video profile
H.245 key management.
• ITU-T Recommendation H.235.7, H.323 security
References framework: Usage of the MIKEY key management protocol
for the Secure Real Time Transport Protocol (SRTP) within
[1] Davidson, Jonathan; James Peters, Jim Peters, Brian
H.235.
Gracely. "H.323". Voice over IP fundamentals. Cisco
• ITU-T Recommendation H.235.8, H.323 security: Key
Press. pp. 229–230. ISBN 9781578701681.
exchange for SRTP using secure signalling channels.
http://books.google.com/
• ITU-T Recommendation H.235.9, H.323 security:
books?id=S5P7-Xtq7W8C&pg=PA229.
Security gateway support for H.323.
[2] H.323 Forum List of Products and Services
[3] ITU-T Recommendation H.323 (11/1996), first
version of H.323.
6
From Wikipedia, the free encyclopedia H.323
ITU-T H.450 Series Recommendations • ITU-T Recommendation H.460.14, Support for Multi-
• ITU-T Recommendation H.450.1, Generic functional Level Precedence and Preemption (MLPP) within H.323
protocol for the support of supplementary services in systems.
H.323. • ITU-T Recommendation H.460.15, Call signalling
• ITU-T Recommendation H.450.2, Call transfer transport channel suspension and redirection within H.323
supplementary service for H.323. systems.
• ITU-T Recommendation H.450.3, Call diversion • ITU-T Recommendation H.460.16, Multiple message
supplementary service for H.323. release sequence capability.
• ITU-T Recommendation H.450.4, Call hold • ITU-T Recommendation H.460.17, Using H.225.0 call
supplementary service for H.323. signalling connection as transport for H.323 RAS messages.
• ITU-T Recommendation H.450.5, Call park and call • ITU-T Recommendation H.460.18, Traversal of H.323
pickup supplementary service for H.323. signalling across network address translators and
• ITU-T Recommendation H.450.6, Call waiting firewalls.
supplementary service for H.323. • ITU-T Recommendation H.460.19, Traversal of H.323
• ITU-T Recommendation H.450.7, Message waiting media across network address translators and firewalls.
indication supplementary service for H.323. • ITU-T Recommendation H.460.20, Location number
• ITU-T Recommendation H.450.8, Name identification within H.323 systems.
supplementary service for H.323. • ITU-T Recommendation H.460.21, Message broadcast
• ITU-T Recommendation H.450.9, Call completion for H.323 systems.
supplementary service for H.323. • ITU-T Recommendation H.460.22, Negotiation of
• ITU-T Recommendation H.450.10, Call offering security protocols to protect H.225.0 Call Signalling
supplementary service for H.323. Messages.
• ITU-T Recommendation H.450.11, Call intrusion ITU-T H.500 Series Recommendations
supplementary service for H.323. • ITU-T Recommendation H.501, Protocol for mobility
• ITU-T Recommendation H.450.12, Common management and intra/inter-domain communication in
Information Additional Network Feature for H.323. multimedia systems.
ITU-T H.460 Series Recommendations • ITU-T Recommendation H.510, Mobility for H.323
• ITU-T Recommendation H.460.1, Guidelines for the use multimedia systems and services.
of the generic extensible framework. • ITU-T Recommendation H.530, Symmetric security
• ITU-T Recommendation H.460.2, Number Portability procedures for H.323 mobility in H.510.
interworking between H.323 and SCN networks.
• ITU-T Recommendation H.460.3, Circuit maps within
H.323 systems.
External links
• ITU-T Recommendation H.460.4, Call priority
designation and country/international network of call General
origination identification for H.323 priority calls. • H.323 Definition and overview
• ITU-T Recommendation H.460.5, H.225.0 transport of • H.323 Forum
multiple Q.931 information elements of the same type. • H.323 Information site
• ITU-T Recommendation H.460.6, Extended Fast • H.323 Tutorial and resources
Connect feature. • Implementing H.323 (Zip)
• ITU-T Recommendation H.460.7, Digit maps within
H.323 systems. Papers
• ITU-T Recommendation H.460.8, Querying for alternate • H.323 Protocol Overview (technical)
routes within H.323 systems. • H.323 Overview (less technical)
• ITU-T Recommendation H.460.9, Support for online • H.323 Call flow covering H.225, Q.931, H.245, RTP and
QoS-monitoring reporting within H.323 systems. RTCP protocols (PDF)
• ITU-T Recommendation H.460.10, Call party category • H.323 Call flow (communication example)
within H.323 systems. • H.323 List of papers and presentations
• ITU-T Recommendation H.460.11, Delayed call
establishment within H.323 systems. Projects
• ITU-T Recommendation H.460.12, Glare control
• H.323 Plus open source H.323 project
indicator within H.323 systems.
• Xmeeting for Mac OS X
• ITU-T Recommendation H.460.13, Called user release
• GNU (OpenSource) Gatekeeper
control.
• Ekiga: open source VoIP and video conferencing
application for GNOME
7
From Wikipedia, the free encyclopedia H.323
Retrieved from "http://en.wikipedia.org/w/index.php?title=H.323&oldid=466965969"
Categories:
• ITU-T recommendations
• Teleconferencing
• VoIP protocols
• Videotelephony
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