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| SUMMARY | |
| Protocol |
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H.323 |
| Protocol suite |
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VoIP |
| Layer |
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Transport Layer |
| Related protocols |
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RTSP,
SIP,
RTP,
RTCP,
SDP,
Megaco/H.248,
Q.931,
H.225 |
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| DESCRIPTION |
H.323 is an International Telecommunications Union (ITU) standard that provides specification for computers, equipment, and services for multimedia communication over networks that do not provide a guaranteed quality of service. H.323 computers and equipment can carry real-time video, audio, and data, or any combination of these elements. This standard is based on the Internet Engineering Task Force (IETF) Real-Time Protocol (RTP) and Real-Time Control Protocol (RTCP), with additional protocols for call signaling, and data and audiovisual communications.
Users can connect with other people over the Internet and use varying products that support H.323, just as people using different makes and models of telephones can communicate over Public Switched Telephone Network (PSTN) lines. H.323 defines how audio and video information is formatted and packaged for transmission over the network. Standard audio and video codecs encode and decode input/output from audio and video sources for communication between nodes. A codec (coder/decoder) converts audio or video signals between analog and digital forms.
Also, H.323 specifies T.120 services for data communications and conferencing within and next to an H.323 session. Most importantly, this T.120 support means that data handling can occur either in conjunction with H.323 audio and video, or separately.
Microsoft and more than 120 other leading companies have announced their intent to support and implement H.323 in their products and services. This broad support establishes H.323 as the standard for audio and video conferencing over the Internet.
Benefits
H.323 products and services offer the following benefits to users:
- Products and services developed by multiple manufacturers under the H.323 standard can interoperate without platform limitations. H.323 conferencing clients, bridges, servers, and gateways support this interoperability.
- H.323 provides multiple audio and video codecs that format data according to the requirements of various networks, using different bit rates, delays, and quality options. Users can choose the codecs that best support their computer and network selections.
- The addition of T.120 data conferencing support to the H.323 specification means that products developed under H.323 can offer a full range of multimedia functions, with both data and audiovisual conferencing support.
Versions
The H.323 standard is specified by the ITU Study Group 16. Version 1 of the H.323 recommendation visual telephone systems and equipment for LANs that provide a nonguaranteed quality of service (QoS) as accepted in October 1996. It was, as the name suggests, heavily weighted towards multimedia communications in a LAN environment. Version 1 of the H.323 standard does not provide guaranteed QoS.
The emergence of VoIP applications and IP telephony has paved the way for a revision of the H.323 specification. The absence of a standard for voice over IP resulted in products that were incompatible. With the development of VoIP, new requirements emerged, such as providing communication between a PC based phone and a phone on a traditional switched circuit network (SCN). Such requirements forced the need for a standard for IP telephony. Version 2 of H.323 packet-based multimedia communications systems as defined to accommodate these additional requirements and was accepted in January 1998.
H.323 version 5 was officially approved at the end of July 2003. Unlike previous revisions of the Recommendation, H.323 version 5 aimed to maintain stability in the protocol by introducing only modest additions to the base protocol, rather than introducing sweeping changes as was the case in prior revisions. Look to version 5 as "maintenance" release of H.323 version 4, with just a few new fields added and only one new message type.
Some of the new additions from version 5 to version 4 include:
- Annex M.3 - Tunneling of DSS1 signaling within H.323 systems
- Annex O - Defines how to use the H.323 URL and other DNS services within the context of H.323 systems
- Annex P - Describes how to do modem relay within H.323 systems
- Annex Q - Far-end camera control for video conferences
- Annex R - Provides for fault tolerance-- calls do not drop when a single intermediate signaling entity, such as a softswitch, fails while the calls are in progress. (GEF)
- H.460.1 - Overview of the Generic Extensibility Framework and "author's guide"
- H.460.2 - Number portability (GEF)
- H.460.3 - Circuit Status Map (GEF)
- H.460.4 - Call priority designation (GEF)
- H.460.5 - Transport of duplicate Q.931 IEs (GEF)
- H.460.6 - Extended Fast Connect (GEF)
- H.460.7 - Digit maps (GEF)
- H.460.8 - Querying for alternate routes (GEF)
- H.460.9 - QoS monitoring and reporting (GEF)
- A few additions to H.323v5 and H.225.0v5
Typical H.323 procedures
H.323 protocols suite
The H.323 suite is illustrated here in relation to the OSI model
- H.225.0 protocol is used to describe call signaling, the media (audio and video), the stream packetization, media stream synchronization and control message formats.
- H.245 control protocol for multimedia communication, describes the messages and procedures used for opening and closing logical channels for audio, video and data, capability exchange, control and indications.
- H.450 describes the Supplementary Services
- H.235 describes security in H.323.
- H.239 describes dual stream use in videoconferencing, usually one for live video, the other for presentation.
Terminal Characteristics
- Gateway Characteristics
A gateway provides translation of protocols for call setup and release, conversion of media formats between different networks, and the transfer of information between H.323 and non-H.323 networks. An application of the H.323 gateway is in IP telephony, where the H.323 gateway connects an IP network and SCN network.
- Gatekeeper Characteristics
Gatekeepers provide call-control services for H.323 endpoints, such as address translation and bandwidth management as defined within RAS. Gatekeepers in H.323 networks are optional. If they are present in a network, however, terminals and gateways must use their services. The H.323 standards both define mandatory services that the gatekeeper must provide and specify other optional functionality that it can provide.
The H.225 RAS is used between H.323 endpoints (terminals and gateways) and gatekeepers for the following:
- gatekeeper discovery (GRQ)
- endpoint registration
- endpoint location
- admission control
- access tokens
The RAS messages are carried on a RAS channel that is unreliable. Hence, RAS message exchange may be associated with timeouts and retry counts.
- Gatekeeper Discovery
The gatekeeper discovery process is used by the H.323 endpoints to determine the gatekeeper with which the endpoint must register. The gatekeeper discovery can be done statically or dynamically. In static discovery, the endpoint knows the transport address of its gatekeeper a priori.
- Endpoint Registration
Registration is a process used by the endpoints to join a zone and inform the gatekeeper of the zone's transport and alias addresses. All endpoints register with a gatekeeper as part of their configuration.
- Endpoint Location
Endpoint location is a process by which the transport address of an endpoint is determined and given its alias name or E.164 address.
- Other Control
The RAS channel is used for other kinds of control mechanisms, such as admission control, to restrict the entry of an endpoint into a zone, bandwidth control, and disengagement control, where an endpoint is disassociated from a gatekeeper and its zone.
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| EXAMPLES |
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| PROTOCOL RELATIONS |
■ Parent layer
■ Child layer
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| GLOSSARY |
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DSS1
DSS1 (Digital Subscriber System No. 1), also known as Euro-ISDN or E-DSS1 (European DSS1), is a digital signalling protocol (D channel protocol) used for the ISDN. The interface is also called NET3 for BRI and NET5 for PRI lines. It can also be called CTR4.
Audio codec
An audio codec is a computer program that compresses/decompresses digital audio data according to a given audio file format or streaming audio format. Most codecs are implemented as libraries which interface to one or more multimedia players, such as XMMS, Winamp or Windows Media Player.
Codec
Codec is a portmanteau of either "Compressor-Decompressor" or "Coder-Decoder," which describes a device or program capable of performing transformations on a data stream or signal. Codecs can both put the stream or signal into an encoded form (often for transmission, storage or encryption) and retrieve, or decode that form for viewing or manipulation in a format more appropriate for these operations.
DNS
DNS(Domain Name System or Service or Server), an Internet service that translates domain names into IP addresses. Because domain names are alphabetic, they're easier to remember. The Internet however, is really based on IP addresses. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name www.example.com might translate to 198.105.232.4.
The DNS system is, in fact, its own network. If one DNS server doesn't know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned.
Gatekeeper
An H.323 Gatekeeper is an entitiy that manages an H.323 zone, providing address information/translation and other services to the terminals within the zone, and for Gatekeepers managing other zones.
Gateway
A network device used to translate between two different protocols. Used to interconnect two networks that use incompatible protocols. It is a node on a network that serves as an entrance to another network. In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages. In homes, the gateway is the ISP that connects the user to the internet.
In enterprises, the gateway node often acts as a proxy server and a firewall. The gateway is also associated with both a router, which use headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway.
It is also a computer system located on earth that switches data signals and voice signals between satellites and terrestrial networks and an earlier term for router, though now obsolete in this sense as router is commonly used.
H.225
H.225.0 is a key protocol in the H.323 VoIP architecture defined by ITU-T. H.225.0 describes how audio, video, data and control information on a packet based network can be managed to provide conversational services in H.323 equipment. H.225.0 has two major parts: Call signaling and RAS (Registration, Admission and Status).
H.235
H.235 provides enhancements within the framework of the H.3xx-Series Recommendations to incorporate security services such as Authentication and Privacy (data encryption). H.235 should work with other H series protocols that utilize H.245 as their control protocol.
H.239
H.239 describes dual stream use in videoconferencing, usually one for live video, the other for presentation.
H.245
H.245 is an H.323 protocol for capability negotiation, messages for opening and closing channels for media streams; for example - media signaling.
H.261
H.261 describes a video stream for transport using the real-time transport protocol, RTP, with any of the underlying protocols that carry RTP.
H.263
H.263 specifies the payload format for encapsulating an H.263 bitstream in the Real-Time Transport Protocol (RTP). Three modes are defined for the H.263 payload header. An RTP packet can use one of the three modes for H.263 video streams depending on the desired network packet size and H.263 encoding options employed.
H.323
H.323 is an umbrella recommendation from the ITU-T, that defines the protocols to provide audio-visual communication sessions on any packet network. It is currently implemented by various Internet real-time applications such as NetMeeting and GnomeMeeting. It is a part of the H.32x series of protocols which also address communications over ISDN, PSTN or SS7. H.323 is commonly used in Voice over IP (VoIP) and IP-based videoconferencing.
H.450
H.450 refers to a set of standards created by the International Telecommunications Union (ITU) to define several Supplementary Services of the packet based telecommunication protocol known as H.323. It parallels another set of standards known as QSIG which define similar services for ISDN based networks.
H.450.1
The H.450.1 protocol deals with the procedures and signalling protocol between H.323 entities for the control of supplementary services. This signalling protocol is common to all H.323 supplementary services. The protocol is derived from the generic functional protocol specified in ISO/IEC 11582 for Private Integrated Services Networks (PISN).
H.450.2
H.450.2 is a Call Transfer supplementary service for H.323. The H.450.2 protocol describes the procedures and signalling protocol for the call transfer supplementary service in H.323 networks.
H.450.3
The H.450.3 is a call diversion supplementary service for H.323. It describes the procedures and signalling protocol for the call diversion supplementary service in H.323 networks.
IETF
IETF (Internet Engineering Task Force) is the main standards organization for the Internet. The IETF is a large open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. It is open to any interested individual.
ISDN
ISDN (Integrated Services Digital Network), an international communications standard for sending voice, video, and data over digital telephone lines or normal telephone wires. ISDN supports data transfer rates of 64 Kbps (64,000 bits per second).
ITU
The International Telecommunication Union (ITU) is an international organization established to standardize and regulate international radio and telecommunications. It was founded as the International Telegraph Union in Paris in May 17, 1865, and is today the world's oldest international organization. Its main tasks include standardization, allocation of the radio spectrum, and organizing interconnection arrangements between different countries to allow international phone calls. It is one of the specialized agencies of the United Nations, and has its headquarters in Geneva, Switzerland, next to the main United Nations campus.
Internet
A global network connecting millions of computers. More than 100 countries are linked into exchanges of data, news and opinions.
Unlike online services, which are centrally controlled, the Internet is decentralized by design. Each Internet computer, called a host, is independent. Its operators can choose which Internet services to use and which local services to make available to the global Internet community. Remarkably, this anarchy by design works exceedingly well.
There are a variety of ways to access the Internet. Most online services, such as America Online, offer access to some Internet services. It is also possible to gain access through a commercial Internet Service Provider (ISP).
LAN
Local-area network (LAN) is a computer network that spans a relatively small area. Most LANs are confined to a single building or group of buildings. However, one LAN can be connected to other LANs over any distance via telephone lines and radio waves. A system of LANs connected in this way is called a wide-area network (WAN).
Most LANs connect workstations and personal computers. Each node (individual computer ) in a LAN has its own CPU with which it executes programs, but it also is able to access data and devices anywhere on the LAN. This means that many users can share expensive devices, such as laser printers, as well as data. Users can also use the LAN to communicate with each other, by sending e-mail or engaging in chat sessions.
Microsoft
Microsoft founded in 1975 by Paul Allen and Bill Gates, Microsoft Corporation is one of the largest and most influential companies in the personal computer industry. In addition to developing the de facto standard operating systems -- DOS and Windows -- Microsoft has a strong presence in almost every area of computer software, from programming tools to end-user applications.
Node
In networks, node is a processing location. A node can be a computer or some other device, such as a printer. Every node has a unique network address, sometimes called a Data Link Control (DLC) address or Media Access Control (MAC) address.
In tree structures, node is a point where two or more lines meet.
PSTN
PSTN (Public Switched Telephone Network), which refers to the international telephone system based on copper wires carrying analog voice data. This is in contrast to newer telephone networks base on digital technologies, such as ISDN and FDDI.
QoS
QoS (Quality of Service) refers to the capability of a network to provide better service to selected network traffic over various technologies, including Frame Relay, Asynchronous Transfer Mode (ATM), Ethernet and 802.1 networks, SONET, and IP-routed networks that may use any or all of these underlying technologies.
RAS
RAS (Registration, Admission and Status) is a management protocol between endpoints (terminals and gateways) and gatekeepers. The RAS is used to perform registration, admission control, bandwidth changes, status, and disengage procedures between endpoints and gatekeepers.
RTCP
The RTP control protocol (RTCP) is based on the periodic transmission of control packets to all participants in the session, using the same distribution mechanism as the data packets. The underlying protocol must provide multiplexing of the data and control packets.
RTP
RTP (Real-Time Transport Protocol) is an Internet protocol for transmitting real-time data such as audio and video. RTP itself does not guarantee real-time delivery of data, but it does provide mechanisms for the sending and receiving applications to support streaming data. Typically, RTP runs on top of the UDP protocol, although the specification is general enough to support other transport protocols.
T.120
T.120 is a comprehensive specification that solves several troublesome problems that have historically slowed market growth for applications of this nature. Perhaps most importantly, T.120 resolves complex technological issues in a manner that is acceptable to both the computing and telecommunications industries.
T.125
The T.120 family of protocols describe protocols and services for multipoint Data Conferencing including multilayer protocols which considerably enhance multimedia, MCU and codec control capabilities, permitting greater MCU operational sophistication beyond that described in H.231 and H.243.
Terminal
Terminal is a device that enables you to communicate with a computer. Generally, a terminal is a combination of keyboard and display screen. In networking, a terminal is a personal computer or workstation connected to a mainframe. The personal computer usually runs terminal emulation software that makes the mainframe think it is like any other mainframe terminal.
URL
URL (Uniform Resource Locator) is the global address of documents and other resources on the World Wide Web. The first part of the address indicates what protocol to use, and the second part specifies the IP address or the domain name where the resource is located.
For example, the two URLs below point to two different files at the domain pcwebopedia.com. The first specifies an executable file that should be fetched using the FTP protocol; the second specifies a Web page that should be fetched using the HTTP protocol:
ftp://www.webpage.com/example.exe
http://www.webpage.com/index.html
Video codec
A video codec is a device or software module that enables the use of compression for digital video. The compression usually employs lossy data compression.
VoIP
Voice over Internet Protocol, a category of hardware and software that enables people to use the Internet as the transmission medium for telephone calls by sending voice data in packets using IP rather than by traditional circuit transmissions of the PSTN. One advantage of VoIP is that the telephone calls over the Internet do not incur a surcharge beyond what the user is paying for Internet access, much in the same way that the user doesn't pay for sending individual e-mails over the Internet.
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| REFERENCES |
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| OTHER PROTOCOLS OF TCP/IP SUITE |
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