H.323
Encyclopedia
H.323 is a recommendation from the ITU Telecommunication Standardization Sector (ITU-T)
that defines the protocols to provide audio-visual
communication sessions on any packet network. The H.323 standard addresses call signaling and control, multimedia transport and control, and bandwidth control for point-to-point and multi-point conferences.
It is widely implemented by voice and videoconferencing
equipment manufacturers, is used within various Internet
real-time applications such as GnuGK and NetMeeting and is widely deployed worldwide by service providers and enterprises for both voice and video
services over IP
networks.
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.
H.323 call signaling is based on the ITU-T Recommendation Q.931
protocol and is suited for transmitting calls across networks using a mixture of IP, PSTN, ISDN, and QSIG
over ISDN. A call model, similar to the ISDN call model, eases the introduction of IP telephony into existing networks of ISDN-based PBX systems, including transitions to IP-based PBXs.
Within the context of H.323, an IP-based PBX might be a gatekeeper
or other call control element which provides service to telephone
s or videophone
s. Such a device may provide or facilitate both basic services and supplementary services, such as call transfer
, park
, pick-up, and hold
.
While H.323 excels at providing basic telephony functionality and interoperability, H.323’s strength lies in multimedia communication functionality designed specifically for IP networks.
in November 1996 with an emphasis of enabling videoconferencing capabilities over a Local Area Network
(LAN), but was quickly adopted by the industry as a means of transmitting voice communication over a variety of IP networks, including WANs
and the Internet (see VoIP).
Over the years, H.323 has been revised and re-published with enhancements necessary to better-enable both voice and video functionality over Packet-switched networks, with each version being backward-compatible with the previous version. Recognizing that H.323 was being used for communication, not only on LANs, but over WANs and within large carrier networks, the title of H.323 was changed when published in 1998. The title, which has since remained unchanged, is "Packet-Based Multimedia Communications Systems." The current version of H.323 was approved in 2009.
One strength of H.323 was the relatively early availability of a set of standards, not only defining the basic call model, but also the supplementary services needed to address business communication expectations.
H.323 was the first VoIP standard to adopt the Internet Engineering Task Force
(IETF) standard Real-time Transport Protocol
(RTP) to transport audio
and video over IP networks.
Many H.323 systems also implement other protocols that are defined in various ITU-T Recommendations to provide supplementary services support or deliver other functionality to the user. Some of those Recommendations are:
In addition to those ITU-T Recommendations, H.323 implements various IETF Request for Comments
(RFCs) for media transport and media packetization, including the Real-time Transport Protocol
(RTP).
s and codecs defined outside the ITU. Codecs that are widely implemented by H.323 equipment include:
All H.323 terminals providing video communications shall be capable of encoding and decoding video according to H.261 QCIF. All H.323 terminals shall have an audio codec and shall be capable of encoding and decoding speech according to ITU-T Rec. G.711. All terminals shall be capable of transmitting and receiving A-law and μ-law. Support for other audio and video codecs is optional.
s (MCUs), Gateways
, Gatekeepers, and Border Elements. Collectively, terminals, multipoint control units and gateways are often referred to as endpoints.
While not all elements are required, at least two terminals are required in order to enable communication between two people. In most H.323 deployments, a gatekeeper is employed in order to, among other things, facilitate address resolution.
Inside an H.323 terminal is something referred to as a Protocol stack
, which implements the functionality defined by the H.323 system. The protocol stack would include an implementation of the basic protocol defined in ITU-T Recommendation H.225.0 and H.245, as well as RTP or other protocols described above.
The diagram, figure 1, depicts a complete, sophisticated stack that provides support for voice, video, and various forms of data communication. In reality, most H.323 systems do not implement such a wide array of capabilities, but the logical arrangement is useful in understanding the relationships.
(MCU) is responsible for managing multipoint conferences and is composed of two logical entities referred to as the Multipoint Controller (MC) and the Multipoint Processor (MP). In more practical terms, an MCU is a conference bridge not unlike the conference bridges used in the PSTN today. The most significant difference, however, is that H.323 MCUs might be capable of mixing or switching video, in addition to the normal audio mixing done by a traditional conference bridge. Some MCUs also provide multipoint data collaboration capabilities. What this means to the end user is that, by placing a video call into an H.323 MCU, the user might be able to see all of the other participants in the conference, not only hear their voices.
Gateways are widely used today in order to enable the legacy PSTN phones to interconnect with the large, international H.323 networks that are presently deployed by services providers. Gateways are also used within the enterprise in order to enable enterprise IP phones to communicate through the service provider to users on the PSTN.
Gateways are also used in order to enable videoconferencing devices based on H.320
and H.324
to communicate 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 devices.
of the other endpoint.
Gatekeepers may be designed to operate in one of two signaling modes, namely "direct routed" and "gatekeeper routed" mode. Direct routed mode is the most efficient and most widely deployed mode. In this mode, endpoints utilize the RAS protocol in order to learn the IP address of the remote endpoint and a call is established directly with the remote device. In the gatekeeper routed mode, call signaling always passes through the gatekeeper. While the latter requires the gatekeeper to have more processing power, it also gives the gatekeeper complete control over the call and the ability to provide supplementary services on behalf of the endpoints.
H.323 endpoints use the RAS protocol to communicate with a gatekeeper. Likewise, gatekeepers use RAS to communicate with other gatekeepers.
A collection of endpoints that are registered to a single Gatekeeper in H.323 is referred to as a “zone”. This collection of devices does not necessarily have to have an associated physical topology. Rather, a zone may be entirely logical and is arbitrarily defined by the network administrator
.
Gatekeepers have the ability to neighbor together so that call resolution can happen between zones. Neighboring facilitates the use of dial plans such as the Global Dialing Scheme
. Dial plans facilitate “inter-zone” dialing so that two endpoints in separate zones can still communicate with each other.
".
An administrative domain is the collection of all zones that are under the control of a single person or organization, such as a service provider. Within a service provider network there may be hundreds or thousands of gateway devices, telephones, video terminals, or other H.323 network elements. The service provider might arrange devices into "zones" that enable the service provider to best manage all of the devices under its control, such as logical arrangement by city. Taken together, all of the zones within the service provider network would appear to another service provider as an "administrative domain".
The border element is a signaling entity that generally sits at the edge of the administrative domain and communicates with another administrative domain. This communication might include such things as access authorization information; call pricing information; or other important data necessary to enable communication between the two administrative domains.
Peer elements are entities within the administrative domain that, more or less, help to propagate information learned from the border elements throughout the administrative domain. Such architecture is intended to enable large-scale deployments within carrier networks and to enable services such as clearinghouses.
The diagram, figure 2, provides an illustration of an administrative domain with border elements, peer elements, and gatekeepers.
, which allows for efficient message processing in network elements. The 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.
In the simplest form, an H.323 call may be established as follows (figure 3):
In this example, the endpoint (EP) on the left initiated communication with the gateway on the right and the gateway connected the call with the called party. In reality, call flows are often more complex than the one shown, but most calls that utilize the Fast Connect procedures defined within H.323 can be established with as few as 2 or 3 messages. Endpoints must notify their gatekeeper (if gatekeepers are used) that they are in a call.
Once a call has concluded, a device will send a Release Complete message. Endpoints are then required to notify their gatekeeper (if gatekeepers are used) that the call has ended.
When an endpoint is powered on, it will generally send either a gatekeeper request (GRQ) message to "discover" gatekeepers that are willing to provide service or will send a registration request (RRQ) to a gatekeeper that is predefined in the system’s administrative setup. Gatekeepers will then respond with a gatekeeper confirm (GCF). If a GRQ has been sent the endpoint will then select a gatekeeper with which to register by sending a registration request (RRQ), to which the gatekeeper responds with a registration confirm (RCF). At this point, the endpoint is known to the network and can make and place calls.
When an endpoint wishes to place a call, it will send an admission request (ARQ) to the gatekeeper. The gatekeeper will then resolve the address (either locally, by consulting another gatekeeper, or by querying some other network service) and return the address of the remote endpoint in the admission confirm message (ACF). The endpoint can then place the call.
Upon receiving a call, a remote endpoint will also send an ARQ and receive an ACF in order to get permission to accept the incoming call. This is necessary, for example, to authenticate the calling device or to ensure that there is available bandwidth
for the call.
Figure 4 depicts a high-level communication exchange between two endpoints (EP) and two gatekeepers (GK).
voice and video communication.
H.245 provides capabilities such as capability negotiation, master/slave determination, opening and closing of "logical channels" (i.e., audio and video flows), flow control, and conference control. It has support for both unicast
and multicast
communication, allowing the size of a conference to theoretically grow without bound.
Of the functionality provided by H.245, capability negotiation is arguably the most important, as it enables devices to communicate without having prior knowledge of the capabilities of the remote entity. H.245 enables rich multimedia capabilities, including audio, video, text, and data communication. For transmission of audio, video, or text, H.323 devices utilize both ITU-defined codecs and codecs defined outside the ITU. Codecs that are widely implemented by H.323 equipment include:
H.245 also enables real-time data conferencing capability through protocols like T.120
. T.120-based applications generally operate in parallel with the H.323 system, but are integrated to provide the user with a seamless multimedia experience. T.120 provides such capabilities as application sharing T.128, electronic whiteboard T.126, file transfer T.127, and text chat T.134 within the context of the conference.
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 Capability Set (TCS) message is the first message transmitted to the other side.
After sending a TCS message, H.323 entities (through H.245 exchanges) will attempt to determine which device is the "master" and which is the "slave." This process, referred to as Master/Slave Determination (MSD), is important, as the master in a call settles all "disputes" between the two devices. For example, if both endpoints attempt to open incompatible media flows, it is the master who takes the action to reject the incompatible flow.
Once capabilities are exchanged and master/slave determination steps have completed, devices may then open "logical channels" or media flows. This is done by simply sending an Open Logical Channel (OLC) message and receiving an acknowledgement message. Upon receipt of the acknowledgement message, an endpoint may then transmit audio or video to the remote endpoint.
A typical H.245 exchange looks similar to figure 5:
After this exchange of messages, the two endpoints (EP) in this figure would be transmitting audio in each direction. The number of message exchanges is numerous, each has an important purpose, but nonetheless takes time.
For this reason, H.323 version 2 (published in 1998) introduced a concept called Fast Connect, which enables a device to establish bi-directional
media flows as part of the H.225.0 call establishment procedures. With Fast Connect, it is possible to establish a call with bi-directional media flowing with no more than two messages, like in figure 3.
Fast Connect is widely supported in the industry. Even so, most devices still implement the complete H.245 exchange as shown above and performs that message exchange in parallel to other activities, so there is no noticeable delay to the calling or called party.
(ATA), VoIP Phone or "soft phone"). The service provider offers the connection to other VoIP services or to the PSTN. Most service providers charge a monthly fee, then additional costs when calls are made. Using VoIP between two enterprise locations would not necessarily require a VoIP service provider, for example. H.323 has been widely deployed by companies who wish to interconnect remote locations over IP using a number of various wired and wireless technologies.
technologies
allowing two or more locations to interact via two-way video and audio transmissions simultaneously. There are basically two types of videoconferencing; dedicated VTC systems have all required components packaged into a single piece of equipment 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 videoconferencing. Due to the price point and proliferation of the Internet, and broadband in particular, there has been a strong spurt of growth and use of H.323-based IP videoconferencing. H.323 is accessible to anyone with a high speed Internet connection, such as DSL. Videoconferencing is utilized in various situations, for example; distance education, telemedicine
and business.
.
(SIP) that also enables voice and video communication over IP. There are also other ITU-T recommendations used for videoconferencing and videophone services - H.320
(using ISDN) and H.324
(using regular analog phone lines and 3G mobile phones). Some providers (such as Skype) also use their own closed, proprietary formats
. Access Grid
provides broadly similar functionality, with more emphasis on open-source and utilizing multicast. EVO also provides relatively open functionality via Java, and includes H.323 support.
ITU-T H.235 Series Recommendations
ITU-T H.450 Series Recommendations
ITU-T H.460 Series Recommendations
ITU-T H.500 Series Recommendations
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
that defines the protocols to provide audio-visual
Audio-visual
The term Audio-Visual may refer to works with both a sound and a visual component, the production or use of such works, or to equipment used to create and present such works...
communication sessions on any packet network. The H.323 standard addresses call signaling and control, multimedia transport and control, and bandwidth control for point-to-point and multi-point conferences.
It is widely implemented by voice and videoconferencing
Videoconferencing
Videoconferencing is the conduct of a videoconference by a set of telecommunication technologies which allow two or more locations to interact via two-way video and audio transmissions simultaneously...
equipment manufacturers, is used within various Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
real-time applications such as GnuGK and NetMeeting and is widely deployed worldwide by service providers and enterprises for both voice and video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
services over IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
networks.
It is a part of the ITU-T H.32x series of protocols, which also address multimedia
Multimedia
Multimedia is media and content that uses a combination of different content forms. The term can be used as a noun or as an adjective describing a medium as having multiple content forms. The term is used in contrast to media which use only rudimentary computer display such as text-only, or...
communications over ISDN
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
, the PSTN
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
or SS7, and 3G
3G
3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union...
mobile networks.
H.323 call signaling is based on the ITU-T Recommendation Q.931
Q.931
ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates...
protocol and is suited for transmitting calls across networks using a mixture of IP, PSTN, ISDN, and QSIG
QSIG
QSIG is an ISDN based signaling protocol for signaling between private branch exchanges in a Private Integrated Services Network . It makes use of the connection-level Q.931 protocol and the application-level ROSE protocol...
over ISDN. A call model, similar to the ISDN call model, eases the introduction of IP telephony into existing networks of ISDN-based PBX systems, including transitions to IP-based PBXs.
Within the context of H.323, an IP-based PBX might be a gatekeeper
H.323 Gatekeeper
An H.323 Gatekeeper serves the purpose of Call Admission Control and translation services from E.164 IDs to IP addresses in an H.323 telephony network. Gatekeepers can be combined with a gateway function to proxy H.323 calls and are sometimes referred to as Session Border Controllers...
or other call control element which provides service to telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
s or videophone
Videophone
A videophone is a telephone with a video screen, and is capable of full duplex video and audio transmissions for communication between people in real-time...
s. Such a device may provide or facilitate both basic services and supplementary services, such as call transfer
Call transfer
A call transfer is a telecommunications mechanism that enables a user to relocate an existing call to another phone or attendant console by using the transfer button and dialing the required location. The transferred call is either announced or unannounced....
, park
Call parking
Call park is a feature of some telephone systems that allows a person to put a call on hold at one telephone set and continue the conversation from any other telephone set....
, pick-up, and hold
Hold (telephone)
In telephony, a call may be placed on hold, in which case the connection is not terminated but no verbal communication is possible until the call is removed from hold by the same or another extension on the key telephone system. Music on hold or On Hold Messaging may be played for the caller while...
.
While H.323 excels at providing basic telephony functionality and interoperability, H.323’s strength lies in multimedia communication functionality designed specifically for IP networks.
History
The first version of H.323 was published by the ITUItu
Itu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...
in November 1996 with an emphasis of enabling videoconferencing capabilities over a Local Area Network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
(LAN), but was quickly adopted by the industry as a means of transmitting voice communication over a variety of IP networks, including WANs
Wide area network
A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
and the Internet (see VoIP).
Over the years, H.323 has been revised and re-published with enhancements necessary to better-enable both voice and video functionality over Packet-switched networks, with each version being backward-compatible with the previous version. Recognizing that H.323 was being used for communication, not only on LANs, but over WANs and within large carrier networks, the title of H.323 was changed when published in 1998. The title, which has since remained unchanged, is "Packet-Based Multimedia Communications Systems." The current version of H.323 was approved in 2009.
One strength of H.323 was the relatively early availability of a set of standards, not only defining the basic call model, but also the supplementary services needed to address business communication expectations.
H.323 was the first VoIP standard to adopt the Internet Engineering Task Force
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
(IETF) standard Real-time Transport Protocol
Real-time Transport Protocol
The Real-time Transport Protocol defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and...
(RTP) to transport audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
and video over IP networks.
Protocols
H.323 is a system specification that describes the use of several ITU-T and IETF protocols. The protocols that comprise the core of almost any H.323 system are:- H.225.0H.225.0H.225.0 is part of the H.323 family of telecommunication protocols.The main objective of H.225.0 is the definition of messages and procedures for:* Call Signaling: establish, control and end an H.323 call...
Registration, Admission and StatusRegistration, Admission and StatusH.323 defines packet-based multimedia communications systems that use a RAS channel to carry messages used in the H.323 Gatekeeper discovery and endpoint registration processes...
(RAS), which is used between an H.323 endpoint and a Gatekeeper to provide address resolution and admission control services. - H.225.0H.225.0H.225.0 is part of the H.323 family of telecommunication protocols.The main objective of H.225.0 is the definition of messages and procedures for:* Call Signaling: establish, control and end an H.323 call...
Call Signaling, which is used between any two H.323 entities in order to establish communication. - H.245H.245H.245 is a control channel protocol used with[in] e.g. H.323 and H.324 communication sessions, and involves the line transmission of non-telephone signals. It also offers the possibility to be tunneled within H.225.0 call signaling messages...
control protocol for multimedia communication, which describes the messages and procedures used for capability exchange, opening and closing logical channels for audio, video and data, control and indications. - Real-time Transport ProtocolReal-time Transport ProtocolThe Real-time Transport Protocol defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and...
(RTP), which is used for sending or receiving multimedia information (voice, video, or text) between any two entities.
Many H.323 systems also implement other protocols that are defined in various ITU-T Recommendations to provide supplementary services support or deliver other functionality to the user. Some of those Recommendations are:
- H.235H.235H.235 covers security and encryption for H.323 and other H.245 based terminals The standard addresses authentication by means of several algorithms, including Diffie-Hellman methods, and privacy...
series describes security within H.323, including security for both signaling and media. - H.239H.239H.239 is an ITU-T recommendation from the H.32x Multimedia Communications macrofamily of standards for multimedia communications over various networks....
describes dual stream use in videoconferencing, usually one for live video, the other for still images. - H.450H.450H.450 refers to a set of standards created by the International Telecommunications Union to define several Supplementary Services of the packet based telecommunication protocol known as H.323...
series describes various supplementary services. - H.460H.460H.460 is a series of extensions to the H.323 videoconferencing standard from the ITU. Introduced in 2005, several of the extensions, including H.460.17, H.460.18 and H.460.19, deal with firewall and NAT traversal. For example, they enable end points to communicate with each other without the need...
series defines optional extensions that might be implemented by an endpoint or a Gatekeeper, including ITU-T Recommendations H.460.17, H.460.18, and H.460.19 for Network address translationNetwork address translationIn computer networking, network address translation is the process of modifying IP address information in IP packet headers while in transit across a traffic routing device....
(NAT) / FirewallFirewall (computing)A firewall is a device or set of devices designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass....
(FW) traversal.
In addition to those ITU-T Recommendations, H.323 implements various IETF Request for Comments
Request for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...
(RFCs) for media transport and media packetization, including the Real-time Transport Protocol
Real-time Transport Protocol
The Real-time Transport Protocol defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and...
(RTP).
Codecs
H.323 utilizes both ITU-defined codecCodec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
s and codecs defined outside the ITU. Codecs that are widely implemented by H.323 equipment include:
- Audio codecs: G.711G.711G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation of voice frequencies. It is required standard in many technologies, for example in H.320 and H.323 specifications. It can also...
, G.729G.729G.729 is an audio data compression algorithm for voice that compresses digital voice in packets of 10 milliseconds duration. It is officially described as Coding of speech at 8 kbit/s using conjugate-structure algebraic code-excited linear prediction .Because of its low bandwidth requirements,...
(including G.729a), G.723.1G.723.1G.723.1 is an audio codec for voice that compresses voice audio in 30 ms frames. An algorithmic look-ahead of 7.5 ms duration means that total algorithmic delay is 37.5 ms...
, G.726G.726G.726 is an ITU-T ADPCM speech codec standard covering the transmission of voice at rates of 16, 24, 32, and 40 kbit/s. It was introduced to supersede both G.721, which covered ADPCM at 32 kbit/s, and G.723, which described ADPCM for 24 and 40 kbit/s. G.726 also introduced a new...
, G.722G.722G.722 is a ITU-T standard 7 kHz wideband speech codec operating at 48, 56 and 64 kbit/s. It was approved by ITU-T in November 1988. Technology of the codec is based on sub-band ADPCM ....
, G.728G.728G.728 is an ITU-T standard for speech coding operating at 16 kbit/s. It is officially described as Coding of speech at 16 kbit/s using low-delay code excited linear prediction....
, SpeexSpeexSpeex is a patent-free audio compression format designed for speech and also a free software speech codec that may be used on VoIP applications and podcasts. It is based on the CELP speech coding algorithm. Speex claims to be free of any patent restrictions and is licensed under the revised BSD... - Text codecs: T.140
- Video codecs: H.261H.261H.261 is a ITU-T video coding standard, ratified in November 1988. It is the first member of the H.26x family of video coding standards in the domain of the ITU-T Video Coding Experts Group , and was the first video codec that was useful in practical terms.H.261 was originally designed for...
, H.263H.263H.263 is a video compression standard originally designed as a low-bitrate compressed format for videoconferencing. It was developed by the ITU-T Video Coding Experts Group in a project ending in 1995/1996 as one member of the H.26x family of video coding standards in the domain of the ITU-T.H.263...
, H.264
All H.323 terminals providing video communications shall be capable of encoding and decoding video according to H.261 QCIF. All H.323 terminals shall have an audio codec and shall be capable of encoding and decoding speech according to ITU-T Rec. G.711. All terminals shall be capable of transmitting and receiving A-law and μ-law. Support for other audio and video codecs is optional.
H.323 Architecture
The H.323 system defines several network elements that work together in order to deliver rich multimedia communication capabilities. Those elements are Terminals, Multipoint Control UnitMultipoint Control Unit
A Multipoint Control Unit is a device commonly used to bridge videoconferencing connections.The Multipoint Control Unit is an endpoint on the LAN that provides the capability for 3 or more terminals and gateways to participate in a multipoint conference...
s (MCUs), Gateways
Gateway (telecommunications)
In telecommunications, the term gateway has the following meaning:*In a communications network, a network node equipped for interfacing with another network that uses different protocols....
, Gatekeepers, and Border Elements. Collectively, terminals, multipoint control units and gateways are often referred to as endpoints.
While not all elements are required, at least two terminals are required in order to enable communication between two people. In most H.323 deployments, a gatekeeper is employed in order to, among other things, facilitate address resolution.
Terminals
Terminals in an H.323 network are the most fundamental elements in any H.323 system, as those are the devices that users would normally encounter. They might exist in the form of a simple IP phone or a powerful high-definition videoconferencing system.Inside an H.323 terminal is something referred to as a Protocol stack
Protocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
, which implements the functionality defined by the H.323 system. The protocol stack would include an implementation of the basic protocol defined in ITU-T Recommendation H.225.0 and H.245, as well as RTP or other protocols described above.
The diagram, figure 1, depicts a complete, sophisticated stack that provides support for voice, video, and various forms of data communication. In reality, most H.323 systems do not implement such a wide array of capabilities, but the logical arrangement is useful in understanding the relationships.
Multipoint Control Units
A Multipoint Control UnitMultipoint Control Unit
A Multipoint Control Unit is a device commonly used to bridge videoconferencing connections.The Multipoint Control Unit is an endpoint on the LAN that provides the capability for 3 or more terminals and gateways to participate in a multipoint conference...
(MCU) is responsible for managing multipoint conferences and is composed of two logical entities referred to as the Multipoint Controller (MC) and the Multipoint Processor (MP). In more practical terms, an MCU is a conference bridge not unlike the conference bridges used in the PSTN today. The most significant difference, however, is that H.323 MCUs might be capable of mixing or switching video, in addition to the normal audio mixing done by a traditional conference bridge. Some MCUs also provide multipoint data collaboration capabilities. What this means to the end user is that, by placing a video call into an H.323 MCU, the user might be able to see all of the other participants in the conference, not only hear their voices.
Gateways
Gateways are devices that enable communication between H.323 networks and other networks, such as PSTN or ISDN networks. If one party in a conversation is utilizing a terminal that is not an H.323 terminal, then the call must pass through a gateway in order to enable both parties to communicate.Gateways are widely used today in order to enable the legacy PSTN phones to interconnect with the large, international H.323 networks that are presently deployed by services providers. Gateways are also used within the enterprise in order to enable enterprise IP phones to communicate through the service provider to users on the PSTN.
Gateways are also used in order to enable videoconferencing devices based on H.320
H.320
H.320 is an umbrella recommendation by the ITU-T for running Multimedia over ISDN based networks. The main protocols in this suite are H.221, H.230, H.242, audio codecs such as G.711, and video codecs such as H.261 and H.263....
and H.324
H.324
H.324 is an ITU-T recommendation for voice, video and data transmission over regular analog phone lines. It uses a regular 33,600 bit/s modem for transmission, the H.263 codec for video encoding and G.723.1 for audio....
to communicate 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 devices.
Gatekeepers
A Gatekeeper is an optional component in the H.323 network that provides a number of services to terminals, gateways, and MCU devices. Those services include endpoint registration, address resolution, admission control, user authentication, and so forth. Of the various functions performed by the gatekeeper, address resolution is the most important as it enables two endpoints to contact each other without either endpoint having to know the IP addressIP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
of the other endpoint.
Gatekeepers may be designed to operate in one of two signaling modes, namely "direct routed" and "gatekeeper routed" mode. Direct routed mode is the most efficient and most widely deployed mode. In this mode, endpoints utilize the RAS protocol in order to learn the IP address of the remote endpoint and a call is established directly with the remote device. In the gatekeeper routed mode, call signaling always passes through the gatekeeper. While the latter requires the gatekeeper to have more processing power, it also gives the gatekeeper complete control over the call and the ability to provide supplementary services on behalf of the endpoints.
H.323 endpoints use the RAS protocol to communicate with a gatekeeper. Likewise, gatekeepers use RAS to communicate with other gatekeepers.
A collection of endpoints that are registered to a single Gatekeeper in H.323 is referred to as a “zone”. This collection of devices does not necessarily have to have an associated physical topology. Rather, a zone may be entirely logical and is arbitrarily defined by the network administrator
Network administrator
A network administrator, network analyst or network engineer is a person responsible for the maintenance of computer hardware and software that comprises a computer network...
.
Gatekeepers have the ability to neighbor together so that call resolution can happen between zones. Neighboring facilitates the use of dial plans such as the Global Dialing Scheme
Global Dialing Scheme
The Global Dialing Scheme is numbering plan for H.323 audio-visual communication networks . Based on the numerology provided by the United Nations International Telecommunications Union, GDS numerology resembles the international telephone system numbering plan, with some exceptions.The Global...
. Dial plans facilitate “inter-zone” dialing so that two endpoints in separate zones can still communicate with each other.
Border Elements and Peer Elements
Border Elements and Peer Elements are optional entities similar to a Gatekeeper, but that do not manage endpoints directly and provide some services that are not described in the RAS protocol. The role of a border or peer element is understood via the definition of an "administrative domainAdministrative domain
-Definition:An administrative domain is a service provider holding a security repository permitting to easily authenticate and authorize clients with credentials.This particularly applies to computer network security....
".
An administrative domain is the collection of all zones that are under the control of a single person or organization, such as a service provider. Within a service provider network there may be hundreds or thousands of gateway devices, telephones, video terminals, or other H.323 network elements. The service provider might arrange devices into "zones" that enable the service provider to best manage all of the devices under its control, such as logical arrangement by city. Taken together, all of the zones within the service provider network would appear to another service provider as an "administrative domain".
The border element is a signaling entity that generally sits at the edge of the administrative domain and communicates with another administrative domain. This communication might include such things as access authorization information; call pricing information; or other important data necessary to enable communication between the two administrative domains.
Peer elements are entities within the administrative domain that, more or less, help to propagate information learned from the border elements throughout the administrative domain. Such architecture is intended to enable large-scale deployments within carrier networks and to enable services such as clearinghouses.
The diagram, figure 2, provides an illustration of an administrative domain with border elements, peer elements, and gatekeepers.
H.323 Network Signaling
H.323 is defined as a binary protocolBinary protocol
A binary protocol is a protocol which is intended or expected to be read by a machine rather than a human being, as opposed to a plain text protocol such as IRC, SMTP, or HTTP...
, which allows for efficient message processing in network elements. The syntax of the protocol is defined in ASN.1 and uses the Packed Encoding Rules
Packed Encoding Rules
Packed encoding rules are ASN.1 encoding rules for producing a compact transfer syntax for data structures described in ASN.1, defined in 1994....
(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 establish communication with the remote entity. H.225.0 messages are:- Setup and Setup acknowledge
- Call Proceeding
- Connect
- Alerting
- Information
- Release Complete
- Facility
- Progress
- Status and Status Inquiry
- Notify
In the simplest form, an H.323 call may be established as follows (figure 3):
In this example, the endpoint (EP) on the left initiated communication with the gateway on the right and the gateway connected the call with the called party. In reality, call flows are often more complex than the one shown, but most calls that utilize the Fast Connect procedures defined within H.323 can be established with as few as 2 or 3 messages. Endpoints must notify their gatekeeper (if gatekeepers are used) that they are in a call.
Once a call has concluded, a device will send a Release Complete message. Endpoints are then required to notify their gatekeeper (if gatekeepers are used) that the call has ended.
RAS Signaling
Endpoints use the RAS protocol in order to communicate with a gatekeeper. Likewise, gatekeepers use RAS to communicate with peer gatekeepers. RAS is a fairly simple protocol composed of just a few messages. Namely:- Gatekeeper request, reject, and confirm messages (GRx)
- Registration request, reject, and confirm messages (RRx)
- Unregister request, reject, and confirm messages (URx)
- Admission request, reject, and confirm messages (ARx)
- Bandwidth request, reject, and confirm message (BRx)
- Disengage request, reject, and confirm (DRx)
- Location request, reject, and confirm messages (LRx)
- Info request, ack, nack, and response (IRx)
- Nonstandard message
- Unknown message response
- Request in progress (RIP)
- Resource availability indication and confirm (RAx)
- Service control indication and response (SCx)
- Admission confirm sequence (ACS)
When an endpoint is powered on, it will generally send either a gatekeeper request (GRQ) message to "discover" gatekeepers that are willing to provide service or will send a registration request (RRQ) to a gatekeeper that is predefined in the system’s administrative setup. Gatekeepers will then respond with a gatekeeper confirm (GCF). If a GRQ has been sent the endpoint will then select a gatekeeper with which to register by sending a registration request (RRQ), to which the gatekeeper responds with a registration confirm (RCF). At this point, the endpoint is known to the network and can make and place calls.
When an endpoint wishes to place a call, it will send an admission request (ARQ) to the gatekeeper. The gatekeeper will then resolve the address (either locally, by consulting another gatekeeper, or by querying some other network service) and return the address of the remote endpoint in the admission confirm message (ACF). The endpoint can then place the call.
Upon receiving a call, a remote endpoint will also send an ARQ and receive an ACF in order to get permission to accept the incoming call. This is necessary, for example, to authenticate the calling device or to ensure that there is available bandwidth
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
for the call.
Figure 4 depicts a high-level communication exchange between two endpoints (EP) and two gatekeepers (GK).
H.245 Call Control
Once a call has initiated (but not necessarily fully connected) endpoints may initiate H.245 call control signaling in order to provide more extensive control over the conference. H.245 is a rather voluminous specification with many procedures that fully enable multipoint communication, though in practice most implementations only implement the minimum necessary in order to enable point-to-pointPoint-to-point telecommunications
In telecommunications, a point-to-point connection refers to a communications connection between two nodes or endpoints. An example is a telephone call, in which one telephone is connected with one other, and what is said by one caller can only be heard by the other...
voice and video communication.
H.245 provides capabilities such as capability negotiation, master/slave determination, opening and closing of "logical channels" (i.e., audio and video flows), flow control, and conference control. It has support for both unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
and multicast
Multicast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
communication, allowing the size of a conference to theoretically grow without bound.
Capability Negotiation
Of the functionality provided by H.245, capability negotiation is arguably the most important, as it enables devices to communicate without having prior knowledge of the capabilities of the remote entity. H.245 enables rich multimedia capabilities, including audio, video, text, and data communication. For transmission of audio, video, or text, H.323 devices utilize both ITU-defined codecs and codecs defined outside the ITU. Codecs that are widely implemented by H.323 equipment include:
- Video codecs: H.261H.261H.261 is a ITU-T video coding standard, ratified in November 1988. It is the first member of the H.26x family of video coding standards in the domain of the ITU-T Video Coding Experts Group , and was the first video codec that was useful in practical terms.H.261 was originally designed for...
, H.263H.263H.263 is a video compression standard originally designed as a low-bitrate compressed format for videoconferencing. It was developed by the ITU-T Video Coding Experts Group in a project ending in 1995/1996 as one member of the H.26x family of video coding standards in the domain of the ITU-T.H.263...
, H.264 - Audio codecs: G.711G.711G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation of voice frequencies. It is required standard in many technologies, for example in H.320 and H.323 specifications. It can also...
, G.729G.729G.729 is an audio data compression algorithm for voice that compresses digital voice in packets of 10 milliseconds duration. It is officially described as Coding of speech at 8 kbit/s using conjugate-structure algebraic code-excited linear prediction .Because of its low bandwidth requirements,...
, G.729a, G.723.1G.723.1G.723.1 is an audio codec for voice that compresses voice audio in 30 ms frames. An algorithmic look-ahead of 7.5 ms duration means that total algorithmic delay is 37.5 ms...
, G.726G.726G.726 is an ITU-T ADPCM speech codec standard covering the transmission of voice at rates of 16, 24, 32, and 40 kbit/s. It was introduced to supersede both G.721, which covered ADPCM at 32 kbit/s, and G.723, which described ADPCM for 24 and 40 kbit/s. G.726 also introduced a new... - Text codecs: T.140
H.245 also enables real-time data conferencing capability through protocols like T.120
T.120
T.120 is an ITU-T recommendation that describes a series of communication and application protocols and services that provide support for real-time, multipoint data communications...
. T.120-based applications generally operate in parallel with the H.323 system, but are integrated to provide the user with a seamless multimedia experience. T.120 provides such capabilities as application sharing T.128, electronic whiteboard T.126, file transfer T.127, and text chat T.134 within the context of the conference.
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 Capability Set (TCS) message is the first message transmitted to the other side.
Master/Slave Determination
After sending a TCS message, H.323 entities (through H.245 exchanges) will attempt to determine which device is the "master" and which is the "slave." This process, referred to as Master/Slave Determination (MSD), is important, as the master in a call settles all "disputes" between the two devices. For example, if both endpoints attempt to open incompatible media flows, it is the master who takes the action to reject the incompatible flow.
Logical Channel Signaling
Once capabilities are exchanged and master/slave determination steps have completed, devices may then open "logical channels" or media flows. This is done by simply sending an Open Logical Channel (OLC) message and receiving an acknowledgement message. Upon receipt of the acknowledgement message, an endpoint may then transmit audio or video to the remote endpoint.
Fast Connect
A typical H.245 exchange looks similar to figure 5:
After this exchange of messages, the two endpoints (EP) in this figure would be transmitting audio in each direction. The number of message exchanges is numerous, each has an important purpose, but nonetheless takes time.
For this reason, H.323 version 2 (published in 1998) introduced a concept called Fast Connect, which enables a device to establish bi-directional
Bi-directional
Bi-directional may refer to:* Bi-directional text* Two-way communication* Bi-directional railway signalling* Controls at each end of a tram or railcar...
media flows as part of the H.225.0 call establishment procedures. With Fast Connect, it is possible to establish a call with bi-directional media flowing with no more than two messages, like in figure 3.
Fast Connect is widely supported in the industry. Even so, most devices still implement the complete H.245 exchange as shown above and performs that message exchange in parallel to other activities, so there is no noticeable delay to the calling or called party.
H.323 and Voice over IP services
Voice over Internet Protocol (VoIP) describes the transmission of voice using the Internet or other packet switched networks. ITU-T Recommendation H.323 is one of the standards used in VoIP. VoIP requires a connection to the Internet or another packet switched network, a subscription to a VoIP service provider and a client (an analogue telephone adapterAnalog telephony adapter
An analog telephony adapter, or analog telephone adapter, is a device used to connect one or more standard analog telephones to a digital telephone system or a non-standard telephone system....
(ATA), VoIP Phone or "soft phone"). The service provider offers the connection to other VoIP services or to the PSTN. Most service providers charge a monthly fee, then additional costs when calls are made. Using VoIP between two enterprise locations would not necessarily require a VoIP service provider, for example. H.323 has been widely deployed by companies who wish to interconnect remote locations over IP using a number of various wired and wireless technologies.
H.323 and Videoconference services
A videoconference, or videoteleconference (VTC), is a set of telecommunicationTelecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
technologies
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...
allowing two or more locations to interact via two-way video and audio transmissions simultaneously. There are basically two types of videoconferencing; dedicated VTC systems have all required components packaged into a single piece of equipment while desktop VTC systems are add-ons to normal PC's
Personal computer
A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
, 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 videoconferencing. Due to the price point and proliferation of the Internet, and broadband in particular, there has been a strong spurt of growth and use of H.323-based IP videoconferencing. H.323 is accessible to anyone with a high speed Internet connection, such as DSL. Videoconferencing is utilized in various situations, for example; distance education, telemedicine
Telemedicine
Telemedicine is the use of telecommunication and information technologies in order to provide clinical health care at a distance. It helps eliminate distance barriers and can improve access to medical services that would often not be consistently available in distant rural communities...
and business.
International Conferences
H.323 has been used in the industry to enable large-scale international video conferences that are significantly larger than the typical video conference. One of the most widely attended is an annual event called MegaconferenceMegaconference
The Megaconference are special non-profit world-wide events which use the H.323 protocol to create a virtual conference involving hundreds of locations and thousands of people. Everyone in the world with H.323 equipment is invited to participate. They are the world’s largest video conferences. The...
.
Alternatives
The IETF produced a standard called the Session Initiation ProtocolSession Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
(SIP) that also enables voice and video communication over IP. There are also other ITU-T recommendations used for videoconferencing and videophone services - H.320
H.320
H.320 is an umbrella recommendation by the ITU-T for running Multimedia over ISDN based networks. The main protocols in this suite are H.221, H.230, H.242, audio codecs such as G.711, and video codecs such as H.261 and H.263....
(using ISDN) and H.324
H.324
H.324 is an ITU-T recommendation for voice, video and data transmission over regular analog phone lines. It uses a regular 33,600 bit/s modem for transmission, the H.263 codec for video encoding and G.723.1 for audio....
(using regular analog phone lines and 3G mobile phones). Some providers (such as Skype) also use their own closed, proprietary formats
Comparison of VoIP software
VoIP software is used to conduct telephone-like voice conversations across Internet Protocol based networks. VoIP stands for "Voice over IP". For residential markets, VoIP phone service is often cheaper than traditional public switched telephone network service and can remove geographic...
. Access Grid
Access Grid
Access Grid is a collection of resources and technologies that enables large format audio and video based collaboration between groups of people in different locations. The Access Grid is an ensemble of resources, including multimedia large-format displays, presentation and interactive...
provides broadly similar functionality, with more emphasis on open-source and utilizing multicast. EVO also provides relatively open functionality via Java, and includes H.323 support.
See also
- Global Dialing Scheme (GDS)Global Dialing SchemeThe Global Dialing Scheme is numbering plan for H.323 audio-visual communication networks . Based on the numerology provided by the United Nations International Telecommunications Union, GDS numerology resembles the international telephone system numbering plan, with some exceptions.The Global...
- H.323 GatekeeperH.323 GatekeeperAn H.323 Gatekeeper serves the purpose of Call Admission Control and translation services from E.164 IDs to IP addresses in an H.323 telephony network. Gatekeepers can be combined with a gateway function to proxy H.323 calls and are sometimes referred to as Session Border Controllers...
- H.323 GatewayH.323 GatewayAn H.323 gateway is an optional type of endpoint that provides interoperability between H.323 endpoints and endpoints located on a switched-circuit network , such as the PSTN, the ISDN or an enterprise voice network....
- Next Generation NetworksNext Generation NetworkingNext-generation network is a broad term used to describe key architectural evolutions in telecommunication core and access networks. The general idea behind the NGN is that one network transports all information and services by encapsulating these into packets, similar to those used on the...
- Internet Engineering Task Force (IETF)Internet Engineering Task ForceThe Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
- International Telecommunication Union (ITU) Telecommunications Standardization Sector (ITU-T)ITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
- Multipoint Control Units (MCU)Multipoint Control UnitA Multipoint Control Unit is a device commonly used to bridge videoconferencing connections.The Multipoint Control Unit is an endpoint on the LAN that provides the capability for 3 or more terminals and gateways to participate in a multipoint conference...
- VideoconferencingVideoconferencingVideoconferencing is the conduct of a videoconference by a set of telecommunication technologies which allow two or more locations to interact via two-way video and audio transmissions simultaneously...
- Voice over IP (VoIP)Voice over IPVoice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...
- Session Initiation Protocol (SIP)Session Initiation ProtocolThe Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
- LifeSize Communications
- PolycomPolycomPolycom is a multinational corporation with approximately 3,200 employees worldwide and an annual revenue of approximately $1.2 billion in 2010. The company manufactures and sells telepresence and voice communications solutions.-Company History 1990:...
- TandbergTandbergThis article is about the video-conferencing vendor, a Cisco company. Other companies with the same name such as Tandberg Television or Tandberg Data for data storage: see here and here...
- Radvision
- RTP audio video profile
ITU-T Recommendations of the H.323 System
ITU-T H.323 Core Recommendations- ITU-T Recommendation H.323, Packet-based multimedia communications systems.
- ITU-T Recommendation H.225.0, Call signalling protocols and media stream packetization for packet-based multimedia communication systems.
- ITU-T Recommendation H.245, Control protocol for multimedia communication.
- ITU-T Recommendation H.246, Interworking of H-series multimedia terminals with H-series multimedia terminals and voice/voiceband terminals on GSTN and ISDN.
- ITU-T Recommendation H.283, Remote device control logical channel transport.
- ITU-T Recommendation H.341, Multimedia management information base.
ITU-T H.235 Series Recommendations
- ITU-T Recommendation H.235.1, H.323 security framework: Baseline security profile.
- ITU-T Recommendation H.235.2, H.323 security framework: Signature security profile.
- ITU-T Recommendation H.235.3, H.323 security: Hybrid security profile.
- ITU-T Recommendation H.235.4, H.323 security: Direct and selective routed call security.
- ITU-T Recommendation H.235.5, H.323 security: Framework for secure authentication in RAS using weak shared secrets.
- ITU-T Recommendation H.235.6, H.323 security framework: Voice encryption profile with native H.235/H.245 key management.
- ITU-T Recommendation H.235.7, H.323 security framework: Usage of the MIKEY key management protocol for the Secure Real Time Transport Protocol (SRTP) within H.235.
- ITU-T Recommendation H.235.8, H.323 security: Key exchange for SRTP using secure signalling channels.
- ITU-T Recommendation H.235.9, H.323 security: Security gateway support for H.323.
ITU-T H.450 Series Recommendations
- ITU-T Recommendation H.450.1, Generic functional protocol for the support of supplementary services in H.323.
- ITU-T Recommendation H.450.2, Call transfer supplementary service for H.323.
- ITU-T Recommendation H.450.3, Call diversion supplementary service for H.323.
- ITU-T Recommendation H.450.4, Call hold supplementary service for H.323.
- ITU-T Recommendation H.450.5, Call park and call pickup supplementary service for H.323.
- ITU-T Recommendation H.450.6, Call waiting supplementary service for H.323.
- ITU-T Recommendation H.450.7, Message waiting indication supplementary service for H.323.
- ITU-T Recommendation H.450.8, Name identification supplementary service for H.323.
- ITU-T Recommendation H.450.9, Call completion supplementary service for H.323.
- ITU-T Recommendation H.450.10, Call offering supplementary service for H.323.
- ITU-T Recommendation H.450.11, Call intrusion supplementary service for H.323.
- ITU-T Recommendation H.450.12, Common Information Additional Network Feature for H.323.
ITU-T H.460 Series Recommendations
- ITU-T Recommendation H.460.1, Guidelines for the use of the generic extensible framework.
- ITU-T Recommendation H.460.2, Number Portability interworking between H.323 and SCN networks.
- ITU-T Recommendation H.460.3, Circuit maps within H.323 systems.
- ITU-T Recommendation H.460.4, Call priority designation and country/international network of call origination identification for H.323 priority calls.
- ITU-T Recommendation H.460.5, H.225.0 transport of multiple Q.931 information elements of the same type.
- ITU-T Recommendation H.460.6, Extended Fast Connect feature.
- ITU-T Recommendation H.460.7, Digit maps within H.323 systems.
- ITU-T Recommendation H.460.8, Querying for alternate routes within H.323 systems.
- ITU-T Recommendation H.460.9, Support for online QoS-monitoring reporting within H.323 systems.
- ITU-T Recommendation H.460.10, Call party category within H.323 systems.
- ITU-T Recommendation H.460.11, Delayed call establishment within H.323 systems.
- ITU-T Recommendation H.460.12, Glare control indicator within H.323 systems.
- ITU-T Recommendation H.460.13, Called user release control.
- ITU-T Recommendation H.460.14, Support for Multi-Level Precedence and Preemption (MLPP) within H.323 systems.
- ITU-T Recommendation H.460.15, Call signalling transport channel suspension and redirection within H.323 systems.
- ITU-T Recommendation H.460.16, Multiple message release sequence capability.
- ITU-T Recommendation H.460.17, Using H.225.0 call signalling connection as transport for H.323 RAS messages.
- ITU-T Recommendation H.460.18, Traversal of H.323 signalling across network address translators and firewalls.
- ITU-T Recommendation H.460.19, Traversal of H.323 media across network address translators and firewalls.
- ITU-T Recommendation H.460.20, Location number within H.323 systems.
- ITU-T Recommendation H.460.21, Message broadcast for H.323 systems.
- ITU-T Recommendation H.460.22, Negotiation of security protocols to protect H.225.0 Call Signalling Messages.
ITU-T H.500 Series Recommendations
- ITU-T Recommendation H.501, Protocol for mobility management and intra/inter-domain communication in multimedia systems.
- ITU-T Recommendation H.510, Mobility for H.323 multimedia systems and services.
- ITU-T Recommendation H.530, Symmetric security procedures for H.323 mobility in H.510.
General
- H.323 Definition and overview
- H.323 Forum
- H.323 Information site
- H.323 Tutorial and resources
- Implementing H.323 (Zip)
Papers
- H.323 Protocol Overview (technical)
- H.323 Overview (less technical)
- H.323 Call flow covering H.225, Q.931, H.245, RTP and RTCP protocols (PDF)
- H.323 Call flow (communication example)
- H.323 List of papers and presentations