Which Of The Following Protocols Is Used During The Call Control Process Of Multimedia

Sound-visual advice signaling protocol

H.323
Packet-based multimedia communications systems

Status	In force
Yr started	1996
Latest version	vii.0 December 2009
Organization	ITU-T
Related standards	Q.931
Website	https://www.itu.int/rec/T-REC-H.323

H.323 is a recommendation from the ITU Telecommunication Standardization Sector (ITU-T) that defines the protocols to provide sound-visual advice sessions on whatever packet network.^[1] The H.323 standard addresses phone call signaling and command, multimedia transport and control, and bandwidth control for point-to-point and multi-point conferences.^[2]

It is widely implemented^[3] by voice and videoconferencing equipment manufacturers, is used within various Internet real-fourth dimension 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 office 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, like to the ISDN telephone call model, eases the introduction of IP telephony into existing networks of ISDN-based PBX systems, including transitions to IP-based PBXs.

Inside the context of H.323, an IP-based PBX might be a gatekeeper or other call control element which provides service to telephones or videophones. Such a device may provide or facilitate both basic services and supplementary services, such as call transfer, park, choice-up, and hold.

History [edit]

The offset version of H.323 was published by the ITU in November 1996^[4] 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 speech 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 beingness backward-compatible with the previous version.^[v] Recognizing that H.323 was being used for communication non only on LANs, but over WANs and within large carrier networks, the title of H.323 was changed when published in 1998.^[6] The title, which has since remained unchanged, is "Package-Based Multimedia Communications Systems." The current version of H.323 was approved in 2009.^[7]

One force of H.323 was the relatively early availability of a set up of standards non only defining the bones call model, but likewise the supplementary services needed to accost business advice expectations.^{[ citation needed ]}

H.323 was the first VoIP standard to adopt the Internet Engineering Job Forcefulness (IETF) standard Real-time Send Protocol (RTP) to transport audio and video over IP networks.^{[ commendation needed ]}

Protocols [edit]

H.323 is a system specification that describes the use of several ITU-T and IETF protocols. The protocols that comprise the core of virtually any H.323 system are:^[viii]

H.225.0 Registration, Admission and Status (RAS), which is used between an H.323 endpoint and a Gatekeeper to provide address resolution and admission control services.
H.225.0 Phone call Signaling, which is used between whatsoever two H.323 entities in order to establish advice. (Based on Q.931)
H.245 command protocol for multimedia communication, which describes the messages and procedures used for capability exchange, opening and closing logical channels for audio, video and data, command and indications.
Real-time Transport Protocol (RTP), which is used for sending or receiving multimedia information (voice, video, or text) between whatever ii entities.

Many H.323 systems also implement other protocols that are defined in various ITU-T Recommendations to provide supplementary services back up or deliver other functionality to the user. Some of those Recommendations are:^{[ commendation needed ]}

H.235 series describes security within H.323, including security for both signaling and media.
H.239 describes dual stream use in videoconferencing, normally 1 for live video, the other for still images.
H.450 series describes various supplementary services.
H.460 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 translation (NAT) / Firewall (FW) traversal.

In improver to those ITU-T Recommendations, H.323 implements various IETF Request for Comments (RFCs) for media send and media packetization, including the Real-fourth dimension Ship Protocol (RTP).

Codecs [edit]

H.323 utilizes both ITU-divers codecs and codecs defined outside the ITU. Codecs that are widely implemented past H.323 equipment include:

Audio codecs: Grand.711, M.729 (including Grand.729a), G.723.one, Thou.726, G.722, Grand.728, Speex, AAC-LD
Text codecs: T.140
Video codecs: H.261, H.263, H.264, H.265

All H.323 terminals providing video communications shall exist capable of encoding and decoding video according to H.261 QCIF. All H.323 terminals shall accept an sound codec and shall be capable of encoding and decoding speech according to ITU-T Rec. One thousand.711. All terminals shall exist capable of transmitting and receiving A-police and μ-law. Back up for other audio and video codecs is optional.^[7]

Architecture [edit]

The H.323 system defines several network elements that piece of work together in order to deliver rich multimedia advice capabilities. Those elements are Terminals, Multipoint Control Units (MCUs), Gateways, Gatekeepers, and Border Elements. Collectively, terminals, multipoint command units and gateways are often referred to every bit endpoints. H.323 uses TCP port number 1720.

While not all elements are required, at to the lowest degree two terminals are required in gild to enable communication betwixt ii people. In nigh H.323 deployments, a gatekeeper is employed in club to, amid other things, facilitate accost resolution.

H.323 network elements [edit]

Terminals [edit]

Figure 1 - A consummate, sophisticated protocol stack

Terminals in an H.323 network are the near fundamental elements in whatsoever H.323 organisation, every bit those are the devices that users would normally encounter. They might exist in the form of a simple IP telephone or a powerful high-definition videoconferencing organisation.

Inside an H.323 last is something referred to every bit a Protocol stack, which implements the functionality divers by the H.323 system. The protocol stack would include an implementation of the basic protocol divers in ITU-T Recommendation H.225.0 and H.245, likewise 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, nigh H.323 systems do non implement such a wide array of capabilities, simply the logical arrangement is useful in understanding the relationships.

Multipoint control units [edit]

A multipoint command unit (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 applied terms, an MCU is a briefing bridge not unlike the conference bridges used in the PSTN today. The most meaning deviation, notwithstanding, is that H.323 MCUs might be capable of mixing or switching video, in addition to the normal audio mixing done past a traditional conference bridge. Some MCUs likewise provide multipoint data collaboration capabilities. What this ways to the end user is that, by placing a video phone 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 [edit]

Gateways are devices that enable communication between H.323 networks and other networks, such as PSTN or ISDN networks. If ane political party in a chat is utilizing a terminal that is not an H.323 last, so the phone 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 big, international H.323 networks that are presently deployed by services providers. Gateways are too used inside 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 use the H.324 protocol and are able to communicate with H.323-based terminals in corporate networks through such gateway devices.

Gatekeepers [edit]

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 command, user authentication, and then forth. Of the various functions performed by the gatekeeper, address resolution is the about of import as it enables two endpoints to contact each other without either endpoint having to know the IP address of the other endpoint.

Gatekeepers may exist designed to operate in one of two signaling modes, namely "direct routed" and "gatekeeper routed" manner. Direct routed way is the most efficient and most widely deployed mode. In this mode, endpoints utilize the RAS protocol in order to acquire the IP address of the remote endpoint and a telephone call is established straight 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 utilize the RAS protocol to communicate with a gatekeeper. Besides, gatekeepers utilize RAS to communicate with other gatekeepers.

A drove 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 take the ability to neighbor together so that call resolution can happen between zones. Neighboring facilitates the use of dial plans such every bit the Global Dialing Scheme. Punch plans facilitate "inter-zone" dialing then that two endpoints in separate zones tin withal communicate with each other.

Border elements and peer elements [edit]

Effigy 2 - An analogy of an administrative domain with edge elements, peer elements, and gatekeepers

Edge Elements and Peer Elements are optional entities similar to a Gatekeeper, simply that do non manage endpoints directly and provide some services that are not described in the RAS protocol. The function of a edge or peer element is understood via the definition of an "administrative domain".

An administrative domain is the collection of all zones that are under the command of a unmarried person or organisation, such every bit a service provider. Within a service provider network in that location may be hundreds or thousands of gateway devices, telephones, video terminals, or other H.323 network elements. The service provider might suit devices into "zones" that enable the service provider to best manage all of the devices under its command, such equally logical organisation past city. Taken together, all of the zones inside the service provider network would announced to another service provider as an "administrative domain".

The border chemical element is a signaling entity that more often than not sits at the edge of the authoritative 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 ii administrative domains.

Peer elements are entities within the administrative domain that, more or less, assist 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 equally clearinghouses.

The diagram, figure ii, provides an illustration of an administrative domain with border elements, peer elements, and gatekeepers.

H.323 network signaling [edit]

H.323 is defined as a binary protocol, 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 diverse communication flows in H.323 systems.

H.225.0 phone call signaling [edit]

Figure 3 - Establishment of an H.323 phone call

Once the address of the remote endpoint is resolved, the endpoint will use H.225.0 Telephone call Signaling in lodge to constitute communication with the remote entity. H.225.0 messages are:^[9]

Setup and Setup acknowledge
Call Proceeding
Connect
Alerting
Information
Release Consummate
Facility
Progress
Status and Status Enquiry
Notify

In the simplest course, an H.323 telephone call may exist established equally 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 chosen political party. In reality, telephone call flows are ofttimes more complex than the ane shown, but most calls that use the Fast Connect procedures defined within H.323 can be established with as few every bit 2 or 3 messages. Endpoints must notify their gatekeeper (if gatekeepers are used) that they are in a call.

Once a call has ended, a device will send a Release Complete bulletin. Endpoints are and so required to notify their gatekeeper (if gatekeepers are used) that the call has concluded.

RAS signaling [edit]

Figure 4 - A high-level communication substitution betwixt two endpoints (EP) and two gatekeepers (GK)

Endpoints employ 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 equanimous of just a few messages. Namely:

Gatekeeper request, pass up and ostend messages (GRx)
Registration request, reject and ostend messages (RRx)
Unregister asking, reject and confirm messages (URx)
Admission asking, refuse and confirm messages (ARx)
Bandwidth request, refuse and ostend message (BRx)
Disengage asking, decline and confirm (DRx)
Location request, reject and ostend 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 command indication and response (SCx)

When an endpoint is powered on, information technology will generally send a gatekeeper asking (GRQ) message to "discover" gatekeepers that are willing to provide service. Gatekeepers will then respond with a gatekeeper confirm (GCF) and the endpoint volition so select a gatekeeper to work with. Alternatively, it is possible that a gatekeeper has been predefined in the system's administrative setup so there is no demand for the endpoint to discover one.

Once the endpoint determines the gatekeeper to work with, it volition try to register with the gatekeeper past sending a registration asking (RRQ), to which the gatekeeper responds with a registration confirm (RCF). At this betoken, the endpoint is known to the network and can make and identify calls.

When an endpoint wishes to place a telephone call, it volition send an access request (ARQ) to the gatekeeper. The gatekeeper will and so resolve the accost (either locally, by consulting another gatekeeper, or past querying another network service) and return the address of the remote endpoint in the access confirm bulletin (ACF). The endpoint tin then place the call.

Upon receiving a call, a remote endpoint will also send an ARQ and receive an ACF in order to go permission to accept the incoming call. This is necessary, for instance, to cosign the calling device or to ensure that at that place is available bandwidth for the call.

Effigy 4 depicts a loftier-level communication commutation between ii endpoints (EP) and two gatekeepers (GK).

H.245 call control [edit]

One time a telephone call has initiated (but not necessarily fully connected) endpoints may initiate H.245 call command signaling in order to provide more than 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 indicate-to-point voice and video advice.

H.245 provides capabilities such as adequacy negotiation, master/slave determination, opening and endmost of "logical channels" (i.eastward., audio and video flows), catamenia command, and conference control. It has support for both unicast and multicast communication, allowing the size of a briefing to theoretically grow without jump.

Capability negotiation [edit]

Of the functionality provided by H.245, adequacy negotiation is arguably the nearly important, as information technology enables devices to communicate without having prior knowledge of the capabilities of the remote entity. H.245 enables rich multimedia capabilities, including sound, video, text, and data communication. For transmission of audio, video, or text, H.323 devices employ both ITU-divers codecs and codecs divers outside the ITU. Codecs that are widely implemented by H.323 equipment include:

Video codecs: H.261, H.263, H.264
Audio codecs: G.711, G.729, Chiliad.729a, Chiliad.723.1, G.726
Text codecs: T.140

H.245 also enables real-fourth dimension data conferencing capability through protocols similar T.120. T.120-based applications generally operate in parallel with the H.323 system, merely are integrated to provide the user with a seamless multimedia experience. T.120 provides such capabilities equally application sharing T.128, electronic whiteboard T.126, file transfer T.127, and text chat T.134 within the context of the briefing.

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) bulletin is the first message transmitted to the other side.

Master/slave conclusion [edit]

After sending a TCS bulletin, H.323 entities (through H.245 exchanges) volition attempt to decide which device is the "chief" and which is the "slave." This procedure, referred to as Master/Slave Determination (MSD), is of import, as the master in a telephone call settles all "disputes" between the 2 devices. For example, if both endpoints endeavor to open incompatible media flows, it is the master who takes the action to reject the incompatible flow.

Logical channel signaling [edit]

One time capabilities are exchanged and main/slave determination steps accept completed, devices may then open up "logical channels" or media flows. This is washed by simply sending an Open up Logical Channel (OLC) message and receiving an acknowledgement message. Upon receipt of the acknowledgement bulletin, an endpoint may and so transmit sound or video to the remote endpoint.

Fast connect [edit]

Figure 5 - A typical H.245 exchange

A typical H.245 exchange looks similar to figure five:

Later on this exchange of messages, the 2 endpoints (EP) in this figure would be transmitting audio in each direction. The number of message exchanges is numerous, each has an important purpose, simply nonetheless takes fourth dimension.

For this reason, H.323 version 2 (published in 1998) introduced a concept called Fast Connect, which enables a device to plant bi-directional media flows as part of the H.225.0 telephone call institution procedures. With Fast Connect, information technology is possible to establish a call with bi-directional media flowing with no more than than 2 messages, similar in figure three.

Fast Connect is widely supported in the manufacture. Nevertheless, most devices all the same implement the complete H.245 substitution as shown above and perform that bulletin exchange in parallel to other activities, so there is no noticeable delay to the calling or called political party.

Utilise cases [edit]

H.323 and vocalisation over IP services [edit]

Vocalization over Net 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 some other parcel switched network, a subscription to a VoIP service provider and a customer (an counterpart telephone adapter (ATA), VoIP Phone or "soft phone"). The service provider offers the connection to other VoIP services or to the PSTN. Well-nigh service providers charge a monthly fee, and so additional costs when calls are made.^{[ citation needed ]} Using VoIP betwixt two enterprise locations would non 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 [edit]

A videoconference, or videoteleconference (VTC), is a set of telecommunication technologies assuasive two or more locations to collaborate via two-way video and audio transmissions simultaneously. There are basically 2 types of videoconferencing; defended VTC systems accept all required components packaged into a single piece of equipment while desktop VTC systems are add-ons to normal PC'southward, transforming them into VTC devices. Simultaneous videoconferencing among 3 or more remote points is possible past ways of a Multipoint Command Unit (MCU). In that location are MCU bridges for IP and ISDN-based videoconferencing. Due to the price indicate 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 loftier speed Internet connection, such every bit DSL. Videoconferencing is utilized in diverse situations, for example; distance education, telemedicine, Video Relay Service, and business.^{[ citation needed ]} ^{[ needs update ]}

Alternatives [edit]

IAX2 - Inter-Asterisk commutation, a binary protocol, designed to reduce overhead especially in regard to voice streams. Defined in RFC 5456.
The IETF produced a standard chosen the Session Initiation Protocol (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 telephone lines and 3G mobile phones).
Jingle (Jabber/XMPP extension) also enables video and vox over IP.
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 back up.^[x]

See likewise [edit]

Global Dialing Scheme (GDS)
H.323 Gatekeeper
Next-generation network
Internet Engineering Job Force (IETF)
International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T)
Multipoint Control Units (MCU)
Videoconferencing
Vox over IP (VoIP)
Session Initiation Protocol (SIP)
LifeSize Communications
Polycom
RTP audio video contour

References [edit]

^ "H.323: Bundle-based multimedia communications systems". www.itu.int. Archived from the original on 2021-04-28. Retrieved 2021-04-29 .
^ Davidson, Jonathan; James Peters; Jim Peters; Brian Gracely (2000). "H.323". Voice over IP fundamentals . Cisco Printing. pp. 229–230. ISBN978-1-57870-168-1.
^ H.323 Forum Listing of Products and Services
^ ITU-T Recommendation H.323 (xi/1996), starting time version of H.323.
^ ITU-T Recommendation H.323, in force, superseded and withdrawn component.
^ ITU-T Recommendation H.323 (02/1998), Packet-based multimedia communications systems.
^ ^a ^b ITU-T Recommendation H.323 (12/2009), Packet-based multimedia communications systems.
^ Meet ITU-T Recommendations of the H.323 Organisation for a detailed list.
^ ITU-T H.225.0 Recommendation
^ "EVO The Collaboration Network". Archived from the original on 2010-02-18. Retrieved 2010-03-08 . ^{[ not specific enough to verify ]}

External links [edit]

H.323 Information site
H.323 Plus open source H.323 projection
GNU (OpenSource) Gatekeeper