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Videotelephony

Videotelephony (also known as videoconferencing or video call) is the use of audio and video for simultaneous two-way communication.

There are many terms to refer to videotelephony. Videophones are standalone devices for video calling (compare Telephone). In the present day, devices like smartphones and computers are capable of video calling, reducing the demand for separate videophones. Videoconferencing implies group communication. Videoconferencing is used in telepresence, whose goal is to create the illusion that remote participants are in the same room.

The concept of videotelephony was conceived in the late 19th century, and versions were available to the public starting in the 1930s. Early demonstrations were installed at booths in post offices and shown at various world expositions. In 1970, AT&T launched the first commercial personal videotelephone system. In addition to videophones, there existed image phones which exchanged still images between units every few seconds over conventional telephone lines. The development of advanced video codecs, more powerful CPUs, and high-bandwidth Internet service in the late 1990s allowed digital videophones to provide high-quality low-cost color service between users almost any place in the world. Today, videotelephony is widespread.

Applications of videotelephony include sign language transmission for deaf and speech-impaired people, distance education, telemedicine, and overcoming mobility issues. News media organizations have used videotelephony for broadcasting.

Origin
The concept of videotelephony was first conceived in the late 1870s, both in the United States and in Europe, although the basic sciences to permit its very earliest trials would take nearly a half century to be discovered. The prerequisite knowledge arose from intensive research and experimentation in several telecommunication fields, notably electrical telegraphy, telephony, radio, and television.

Early systems
Simple analog videophone communication could be established as early as the invention of the television. Such an antecedent usually consisted of two closed-circuit television systems connected via coax cable or radio. An example of that was the German Reich Postzentralamt (post office) videotelephone network serving Berlin and several German cities via coaxial cables between 1936 and 1940.

The development of videotelephony as a subscription service started in the latter half of the 1920s in the United Kingdom and the United States, spurred notably by John Logie Baird and AT&T's Bell Labs. This occurred in part, at least with AT&T, to serve as an adjunct supplementing the use of the telephone. A number of organizations believed that videotelephony would be superior to plain voice communications. Attempts at using normal telephony networks to transmit slow-scan video, such as the first systems developed by AT&T Corporation, first researched in the 1950s, failed mostly due to the poor picture quality and the lack of efficient video compression techniques.

During the first crewed space flights, NASA used two radio-frequency (UHF or VHF) video links, one in each direction. TV channels routinely use this type of videotelephony when reporting from distant locations. The news media were to become regular users of mobile links to satellites using specially equipped trucks, and much later via special satellite videophones in a briefcase. This technique was very expensive, though, and was not adopted for applications such as telemedicine, distance education, and business meetings.

Decades of research and development culminated in the 1970 commercial launch of AT&T's Picturephone service, available in select cities. However, the system was a commercial failure, chiefly due to consumer apathy, high subscription costs, and lack of network effect—with only a few hundred Picturephones in the world, users had extremely few contacts they could actually call, and interoperability with other videophone systems would not exist for decades.

Digital
In the 1980s, digital telephony transmission networks became possible, such as with ISDN networks. During this time, there was also research into other forms of digital video and audio communication. Many of these technologies, such as the Media space, are not as widely used today as videoconferencing but were still an important area of research. The first dedicated systems started to appear as ISDN networks were expanding throughout the world. One of the first commercial videoconferencing systems sold to companies came from PictureTel Corp., which had an initial public offering in November, 1984.

In 1984, Concept Communication in the United States created a circuit board for standard personal computers that doubled the video frame rate of typical digital videotelephone systems from 15 to 30 frames per second, and reduced the cost from $100,000 to $12,000. The company also secured a patent for a codec for full-motion videoconferencing, first demonstrated at AT&T Bell Labs in 1986.

Very expensive videoconferencing systems continued to rapidly evolve throughout the 1980s and 1990s. Proprietary equipment, software, and network requirements gave way to standards-based technologies that were available for anyone to purchase at a reasonable cost.

While videoconferencing technology was initially used primarily within internal corporate communication networks, one of the first community service uses of the technology started in 1992 through a unique partnership with PictureTel and IBM, which at the time were promoting a jointly developed desktop based videoconferencing product known as the PCS/1. Over the next 15 years, Project DIANE (Diversified Information and Assistance Network) grew to use a variety of videoconferencing platforms to create a multi-state cooperative public service and distance education network consisting of several hundred schools, libraries, science museums, zoos and parks, and many other community-oriented organizations.

Transition to internet and mobile devices
Advances in video compression allowed digital video streams to be transmitted over the Internet, which was previously difficult due to the impractically high bandwidth requirements of uncompressed video. The DCT algorithm was the basis for the first practical video coding standard that was useful for online videoconferencing, H.261, standardised by the ITU-T in 1988, and subsequent H.26x video coding standards.

In 1992 CU-SeeMe was developed at Cornell by Tim Dorcey et al. In 1995 the first public videoconference between North America and Africa took place, linking a technofair in San Francisco with a techno-rave and cyberdeli in Cape Town. At the 1998 Winter Olympics opening ceremony in Nagano, Japan, Seiji Ozawa conducted the Ode to Joy from Beethoven's Ninth Symphony simultaneously across five continents in near-real-time.

Kyocera conducted a two-year development campaign from 1997 to 1999 that resulted in the release of the VP-210 Visual Phone, the first mobile colour videophone that also doubled as a camera phone for still photos. The camera phone was the same size as similar contemporary mobile phones, but sported a large camera lens and a 5 cm (2 inch) colour TFT display capable of displaying 65,000 colors, and was able to process two video frames per second.

Videotelephony was popularized in the 2000s via free Internet services such as Skype and iChat, web plugins supporting H.26x video standards, and online telecommunication programs that promoted low cost, albeit lower quality, videoconferencing to virtually every location with an Internet connection.

Videotelephony became even more widespread through the deployment of video-enabled mobile phones such as 2010s iPhone 4, plus videoconferencing and computer webcams which use Internet telephony. In the upper echelons of government, business, and commerce, telepresence technology, an advanced form of videoconferencing, has helped reduce the need to travel.

Additional history
In May 2005, the first high definition videoconferencing systems, produced by LifeSize Communications, were displayed at the Interop trade show in Las Vegas, Nevada, able to provide video at 30 frames per second with a 1280 by 720 display resolution. Polycom introduced its first high definition videoconferencing system to the market in 2006. As of the 2010s, high-definition resolution for videoconferencing became a popular feature, with most major suppliers in the videoconferencing market offering it.

Technological developments by videoconferencing developers in the 2010s have extended the capabilities of videoconferencing systems beyond the boardroom for use with hand-held mobile devices that combine the use of video, audio and on-screen drawing capabilities broadcasting in real time over secure networks, independent of location. Mobile collaboration systems now allow people in previously unreachable locations, such as workers on an offshore oil rig, the ability to view and discuss issues with colleagues thousands of miles away. Traditional videoconferencing system manufacturers have begun providing mobile applications as well, such as those that allow for live and still image streaming.

The highest ever video call (other than those from aircraft and spacecraft) took place on May 19, 2013, when British adventurer Daniel Hughes used a smartphone with a BGAN satellite modem to make a videocall to the BBC from the summit of Mount Everest, at 8,848 m above sea level.

The COVID-19 pandemic resulted in a significant increase in the use of videoconferencing. Berstein Research found that Zoom added more subscribers during the first two months of 2020 alone than in the entire year 2019. GoToMeeting had a 20 percent increase in usage, according to LogMeIn. UK based StarLeaf reported a 600 percent increase in national call volumes. Videoconferencing became so widespread during the pandemic that the term Zoom fatigue came to prominence, referring to the taxing nature of spending long periods of time on videocalls. This fatigue refers to the psychological and physiological effects participants involved in videoconferencing. One experimental study from 2021 revealed a link between camera use in videoconferencing and a prediction of fatigue occurrence an individual. Furthermore, a 2022 article in the journal "Computers in Human Behaviour" highlighted a study linking negative attitudes with the use of "self-view" when videoconferencing.

On 21 September 2021, Facebook launched two new versions of its Portal video-calling devices, the Portal Go and Portal Plus. The new video calling devices include the first portable variety of the hardware and number of updates.

Major categories

Videotelephony can be categorized by its functionality and intended purpose, and also by its method of transmission.

Videophones were the earliest form of videotelephony, dating back to initial tests in 1927 by AT&T. During the late 1930s, the post offices of several European governments established public videophone services for person-to-person communications using dual cable circuit telephone transmission technology. In the present day, standalone videophones and UMTS video-enabled mobile phones are usually used on a person-to-person basis.

Videoconferencing saw its earliest use with AT&T's Picturephone service in the early 1970s. Transmissions were analog over short distances, but converted to digital forms for longer calls, again using telephone transmission technology. Popular corporate video-conferencing systems in the present day have migrated almost exclusively to digital ISDN and IP transmission modes due to the need to convey the very large amounts of data generated by their cameras and microphones. These systems are often intended for use in conference mode, that is by many people in several different locations, all of whom can be viewed by every participant at each location.

Telepresence systems are a newer, more advanced subset of videoconferencing systems, meant to allow higher degrees of video and audio fidelity. Such high-end systems are typically deployed in corporate settings.

Mobile collaboration systems are another recent development, combining the use of video, audio, and on-screen drawing capabilities using newest generation hand-held electronic devices broadcasting over secure networks, enabling multi-party conferencing in real time, independent of location. Proximity chat is another alternative mode, focused on the flexibility of small group conversations.

A more recent technology encompassing these functions is TV cams. TV cams enable people to make video calls using video calling services, like Skype on their TV, without using a PC connection. TV cams are specially designed video cameras that feed images in real time to another TV camera or other compatible computing devices like smartphones, tablets and computers.

Webcams are popular, relatively low-cost devices that can provide live video and audio streams via personal computers, and can be used with many software clients for both video calls and videoconferencing.

Each of the systems has its own advantages and disadvantages, including video quality, capital cost, degrees of sophistication, transmission capacity requirements, and cost of use.

By cost and quality of service
From the least to the most expensive systems:
 * Web camera videophone and videoconferencing systems, either stand-alone or built-in, that serve as complements to personal computers, connected to other participants by computer and VoIP networks—lowest direct cost, assuming the users already possess computers at their respective locations. Quality of service can range from low to very high, including high definition video available on the latest model webcams. A related and similar device is a TV camera which is usually small, sits on top of a TV, and can connect to it via its HDMI port, similar to how a webcam attaches to a computer via a USB port.
 * Videophones—low to midrange cost. The earliest standalone models operated over either plain old telephone service (POTS) lines on the PSTN telephone networks or more expensive ISDN lines, while newer models have largely migrated to Internet Protocol line service for higher image resolutions and sound quality. Quality of service for standalone videophones can vary from low to high;
 * Huddle room or all-in-one systems —low to midrange cost, newer endpoint category based on standard videoconferencing systems, but defined by the camera, microphone(s), speakers, and codec contained in a single piece of hardware. Typically used in small to medium spaces where beamforming microphone arrays located in the system are sufficient, in lieu of table or ceiling microphones in closer proximity to the in-room participants. Quality of service is comparable to standard videoconferencing systems, varying from moderate to high. Some manufacturers' huddle room systems do not include the codec within the soundbar-shaped unit, rather only camera, microphone, and speakers. These systems are usually still classified as huddle room systems, but, like webcams, rely on a USB connection to an external device, usually a PC, to process the video codec responsibilities. Despite its name, video conferencing systems for Huddle Rooms prevent participants from huddling close together to be seen in the camera. All-in-one systems for these types of rooms range from wide angles such as 110° Horizontal field of view (FOV) to as much as 360° FOV that allow a full view of the room.
 * Videoconferencing systems—midrange cost, usually using multipoint control units or other bridging services to allow multiple parties on videoconference calls. Quality of service can vary from moderate to high.
 * Telepresence systems—highest capabilities and highest cost. Full high-end systems can involve specially built teleconference rooms to allow expansive views with very high levels of audio and video fidelity, to permit an 'immersive' videoconference. When the proper type and capacity transmission lines are provided between facilities, the quality of service reaches state-of-the-art levels.

Security concerns
Computer security experts have shown that poorly configured or inadequately supervised videoconferencing systems can permit an easy virtual entry by computer hackers and criminals into company premises and corporate boardrooms.

Adoption
For over a century, futurists have envisioned a future where telephone conversations will take place as actual face-to-face encounters with video as well as audio. Sometimes it is simply not possible or practical to have face-to-face meetings with two or more people. Sometimes a telephone conversation or conference call is adequate. Other times, e-mail exchanges are adequate. However, videoconferencing adds another option and can be considered when:
 * A live conversation is needed
 * Non-verbal (visual) information is an important component of the conversation
 * The parties of the conversation cannot physically come to the same location
 * The expense or time of travel is a consideration

Bill Gates said in 2001 that he used videoconferencing "three or four times a year", because digital scheduling was difficult and "if the overhead is super high, then you might as well just have a face-to-face meeting". Some observers argue that three outstanding issues have prevented videoconferencing from becoming a widely adopted form of communication, despite the ubiquity of videoconferencing-capable systems. Telcordia Technologies, formerly Bell Communications Research, owns a patent for eye-to-eye videoconferencing using rear projection screens with the video camera behind it, evolved from a 1960s U.S. military system that provided videoconferencing services between the White House and various other government and military facilities. This technique eliminates the need for special cameras or image processing.
 * Eye contact: Eye contact plays a large role in conversational turn-taking, perceived attention and intent, and other aspects of group communication. While traditional telephone conversations give no eye contact cues, many videoconferencing systems are arguably worse in that they provide an incorrect impression that the remote interlocutor is avoiding eye contact. Some telepresence systems have cameras located in the screens that reduce the amount of parallax observed by the users. This issue is also being addressed through research that generates a synthetic image with eye contact using stereo reconstruction.
 * Appearance consciousness: A second psychological problem with videoconferencing is being on camera, with the video stream possibly even being recorded. The burden of presenting an acceptable on-screen appearance is not present in audio-only communication. Early studies by Alphonse Chapanis found that the addition of video actually impaired communication, possibly because of the consciousness of being on camera.
 * Signal latency: The information transport of digital signals in many steps need time. In a telecommunicated conversation, an increased latency (time lag) larger than about 150–300 ms becomes noticeable and is soon observed as unnatural and distracting. Therefore, next to a stable large bandwidth, a small total round-trip time is another major technical requirement for the communication channel for interactive videoconferencing.
 * Bandwidth and quality of service: In some countries, it is difficult or expensive to get a high-quality connection that is fast enough for good-quality videoconferencing. Technologies such as ADSL are usually provided as two separate lines (for uplink/downlink) because each has limited upload speeds and cannot upload and download simultaneously at full speed. As Internet speeds increase, higher quality and high-definition videoconferencing will become more readily available.
 * Complexity of systems: Most users are not technically experienced and want a simple interface. In hardware systems, an unplugged cord or an unresponsive remote control is seen as a failure, contributing to a perceived unreliability. Successful systems are backed by support teams who can provide fast assistance when required.
 * Perceived lack of interoperability: Not all systems can readily interconnect; for example, ISDN and IP systems require a gateway. Popular software solutions cannot easily connect to hardware systems. Some systems use different standards, features, and qualities which can require additional configuration when connecting to dissimilar systems. Free software systems circumvent this limitation by making it relatively easy for a single user to communicate over multiple incompatible platforms.
 * Expense of commercial systems: Well-designed telepresence systems require specially designed rooms which can cost hundreds of thousands of dollars to fit out their rooms with codecs, integration equipment (such as Multipoint Control Units), high fidelity sound systems, and furniture. Monthly charges may also be required for bridging services and high-capacity broadband service.

These are some of the reasons many organizations only use the systems internally, where there is less risk of loss of customers. An alternative for those lacking dedicated facilities is the rental of videoconferencing-equipped meeting rooms in cities around the world. Clients can book rooms and turn up for the meeting, with all technical aspects being prearranged and support being readily available if needed. The issue of eye contact may be solved with advancing technology, including smartphones which have the screen and camera in essentially the same place. In developed countries, the near-ubiquity of smartphones, tablet computers, and computers with built-in audio and webcams removes the need for expensive dedicated hardware.

Components and types

The core technology used in a videotelephony system is digital compression of audio and video streams in real time. The hardware or software that performs compression is called a codec (coder/decoder). Compression rates of up to 1:500 can be achieved. The resulting digital stream of 1s and 0s is subdivided into labeled packets, which are then transmitted through a digital network of some kind (usually ISDN or IP).

The other components required for a videoconferencing system include:
 * Video input: (PTZ / 360° / Fisheye) video camera, or webcam
 * Video output: computer monitor, television, or projector
 * Audio input: microphones, CD/DVD player, cassette player, or any other source of PreAmp audio outlet.
 * Audio output: usually loudspeakers associated with the display device or telephone
 * Data transfer: analog or digital telephone network, LAN, or Internet
 * Computer: a data processing unit that ties together the other components, does the compressing and decompressing, and initiates and maintains the data linkage via the network.

There are basically three kinds of videoconferencing and videophone systems:

 * 1) Dedicated systems have all required components packaged into a single piece of equipment, usually a console with a high quality remote controlled video camera. These cameras can be controlled at a distance to pan left and right, tilt up and down, and zoom. They became known as PTZ cameras. The console contains all electrical interfaces, the control computer, and the software or hardware-based codec. Omnidirectional microphones are connected to the console, as well as a TV monitor with loudspeakers and/or a video projector. There are several types of dedicated videoconferencing devices:
 * 2) Large group videoconferencing are built-in, large, expensive devices used for large rooms such as conference rooms and auditoriums.
 * 3) Small group videoconferencing are either non-portable or portable, smaller, less expensive devices used for small meeting rooms.
 * 4) Individual videoconferencing are usually portable devices, meant for single users, and have fixed cameras, microphones, and loudspeakers integrated into the console.
 * 5) Desktop systems are add-ons (hardware boards or software codec) to normal PCs and laptops, transforming them into videoconferencing devices. A range of different cameras and microphones can be used with the codec, which contains the necessary codec and transmission interfaces. Most of the desktops systems work with the H.323 standard.
 * 6) WebRTC platforms use a web browser instead of dedicated native application software. Solutions such as Adobe Connect and Cisco WebEx can be accessed using a URL sent by the meeting organizer, and various degrees of security can be attached to the virtual room. Often the user must download and install a browser extension to enable access to the local camera and microphone and establish a connection to the meeting. But WebRTC does not require any special software, instead a WebRTC-compliant internet browser itself provides the facilities for 1-to-1 and 1-to-many videoconferencing calls. Several enhancements to WebRTC are provided by independent vendors.

Videoconferencing modes
Videoconferencing systems use several methods to determine which video feed or feeds to display.

Continuous Presence simply displays all participants at the same time, usually with the exception that the viewer either does not see their own feed, or sees their own feed in miniature.

Voice-Activated Switch selectively chooses a feed to display at each endpoint, with the goal of showing the person who is currently speaking. This is done by choosing the feed (other than the viewer) which has the loudest audio input (perhaps with some filtering to avoid switching for very short-lived volume spikes). Often, if no remote parties are currently speaking, the feed with the last speaker remains on the screen.

Echo cancellation
Acoustic echo cancellation (AEC) is a processing algorithm that uses the knowledge of audio output to monitor audio input and filter from it noises that echo back after some time delay. If unattended, these echoes can be re-amplified several times, leading to problems including:
 * The remote party hearing their own voice coming back at them (usually significantly delayed)
 * Strong reverberation, which makes the voice channel useless
 * Howling created by feedback

Echo cancellation is a processor-intensive task that usually works over a narrow range of sound delays.

Bandwidth requirements

Videophones have historically employed a variety of transmission and reception bandwidths, which can be understood as data transmission speeds. The lower the transmission/reception bandwidth, the lower the data transfer rate, resulting in a progressively limited and poorer image quality (i.e. lower resolution and/or frame rate). Data transfer rates and live video image quality are related but are also subject to other factors such as data compression techniques. Some early videophones employed very low data transmission rates with a resulting poor video quality.

Broadband bandwidth is often called high-speed, because it usually has a high rate of data transmission. In general, any connection of 256 kbit/s (0.256 Mbit/s) or greater is more concisely considered broadband Internet. The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) recommendation I.113 has defined broadband as a transmission capacity at 1.5 to 2Mbit/s. The Federal Communications Commission (United States) definition of broadband is 25 Mbit/s.

Currently, adequate video for some purposes becomes possible at data rates lower than the ITU-T broadband definition, with rates of 768 kbit/s and 384 kbit/s used for some videoconferencing applications, and rates as low as 100 kbit/s used for videophones using H.264/MPEG-4 AVC compression protocols. The newer MPEG-4 video and audio compression format can deliver high-quality video at 2Mbit/s, which is at the low end of cable modem and ADSL broadband performance.

Standards

The International Telecommunication Union (ITU) has three umbrellas of standards for videoconferencing:
 * ITU H.320 is known as the standard for public switched telephone networks (PSTN) or videoconferencing over integrated services digital networks. While still prevalent in Europe, ISDN was never widely adopted in the United States and Canada.
 * ITU H.264 Scalable Video Coding (SVC) is a compression standard that enables videoconferencing systems to achieve highly error resilient Internet Protocol (IP) video transmissions over the public Internet without quality-of-service enhanced lines. This standard has enabled wide scale deployment of high definition desktop videoconferencing and made possible new architectures, which reduces latency between the transmitting sources and receivers, resulting in more fluid communication without pauses. In addition, an attractive factor for IP videoconferencing is that it is easier to set up for use along with web conferencing and data collaboration. These combined technologies enable users to have a richer multimedia environment for live meetings, collaboration and presentations.
 * ITU-T V.80: videoconferencing is generally compatibilized with H.324 standard point-to-point videotelephony over regular (POTS) phone lines.

The Unified Communications Interoperability Forum (UCIF), a non-profit alliance between communications vendors, launched in May 2010. The organization's vision is to maximize the interoperability of UC based on existing standards. Founding members of UCIF include HP, Microsoft, Polycom, Logitech/LifeSize Communications, and Juniper Networks.

Call setup
Videoconferencing in the late 20th century was limited to the H.323 protocol (notably Cisco's SCCP implementation was an exception), but newer videophones often use SIP, which is often easier to set up in home networking environments. It is a text-based protocol, incorporating many elements of the Hypertext Transfer Protocol (HTTP) and the Simple Mail Transfer Protocol (SMTP). H.323 is still used, but more commonly for business videoconferencing, while SIP is more commonly used in personal consumer videophones. A number of call-setup methods based on instant messaging protocols such as Skype also now provide video.

Another protocol used by videophones is H.324, which mixes call setup and video compression. Videophones that work on regular phone lines typically use H.324, but the bandwidth is limited by the modem to around 33 kbit/s, limiting the video quality and frame rate. A slightly modified version of H.324 called 3G-324M defined by 3GPP is also used by some cellphones that allow video calls, typically for use only in UMTS networks.

There is also H.320 standard, which specified technical requirements for narrow-band visual telephone systems and terminal equipment, typically for videoconferencing and videophone services. It applied mostly to dedicated circuit-based switched network (point-to-point) connections of moderate or high bandwidth, such as through the medium-bandwidth ISDN digital phone protocol or a fractionated high bandwidth T1 lines. Modern products based on H.320 standard usually support also H.323 standard.

The IAX2 protocol also supports videophone calls natively, using the protocol's own capabilities to transport alternate media streams. A few hobbyists obtained the Nortel 1535 Color SIP Videophone cheaply in 2010 as surplus after Nortel's bankruptcy and deployed the sets on the Asterisk (PBX) platform. While additional software is required to patch together multiple video feeds for conference calls or convert between dissimilar video standards, SIP calls between two identical handsets within the same PBX were relatively straightforward.

Conferencing layers
The components within a videoconferencing system can be divided up into several different layers: User Interface, Conference Control, Control or Signaling Plane, and Media Plane.

Videoconferencing User Interfaces (VUI) can be either graphical or voice-responsive. Many in the industry have encountered both types of interface, and normally a graphical interface is encountered on a computer. User interfaces for conferencing have a number of different uses; they can be used for scheduling, setup, and making a video call. Through the user interface, the administrator is able to control the other three layers of the system.

Conference Control performs resource allocation, management, and routing. This layer along with the User Interface creates meetings (scheduled or unscheduled) or adds and removes participants from a conference.

Control (Signaling) Plane contains the stacks that signal different endpoints to create a call and/or a conference. Signals can be, but are not limited to, H.323 and Session Initiation Protocol (SIP) Protocols. These signals control incoming and outgoing connections as well as session parameters.

The Media Plane controls the audio and video mixing and streaming. This layer manages Real-Time Transport Protocols, User Datagram Packets (UDP) and Real-Time Transport Control Protocol (RTCP). The RTP and UDP normally carry information such the payload type which is the type of codec, frame rate, video size, and many others. RTCP on the other hand acts as a quality control Protocol for detecting errors during streaming.

Multipoint control
Simultaneous videoconferencing among three or more remote points is possible in a hardware-based system by means of a Multipoint Control Unit (MCU). This is a bridge that interconnects calls from several sources (in a similar way to the audio conference call). All parties call the MCU, or the MCU can also call the parties which are going to participate, in sequence. There are MCU bridges for IP and ISDN-based videoconferencing. There are MCUs which are pure software and others that are a combination of hardware and software. An MCU is characterized according to the number of simultaneous calls it can handle, its ability to conduct transposing of data rates and protocols, and features such as Continuous Presence, in which multiple parties can be seen on-screen at once. MCUs can be stand-alone hardware devices, or they can be embedded into dedicated videoconferencing units.

The MCU consists of two logical components:
 * 1) A single multipoint controller (MC), and
 * 2) Multipoint Processors (MP), sometimes referred to as the mixer.

The MC controls the conferencing while it is active on the signaling plane, which is simply where the system manages conferencing creation, endpoint signaling and in-conferencing controls. This component negotiates parameters with every endpoint in the network and controls conferencing resources. While the MC controls resources and signaling negotiations, the MP operates on the media plane and receives media from each endpoint. The MP generates output streams from each endpoint and redirects the information to other endpoints in the conference.

Some systems are capable of multipoint conferencing with no MCU, stand-alone, embedded or otherwise. These use a standards-based H.323 technique known as decentralized multipoint, where each station in a multipoint call exchanges video and audio directly with the other stations with no central manager or other bottleneck. The advantages of this technique are that the video and audio will generally be of higher quality because they do not have to be relayed through a central point. Also, users can make ad hoc multipoint calls without any concern for the availability or control of an MCU. This added convenience and quality comes at the expense of some increased network bandwidth, because every station must transmit to every other station directly.

Cloud storage
Cloud-based videoconferencing can be used without the hardware generally required by other videoconferencing systems, and can be designed for use by SMEs, or larger international or multinational corporations like Facebook. Cloud-based systems can handle either 2D or 3D video broadcasting. Cloud-based systems can also implement mobile calls, VOIP, and other forms of video calling. They can also come with a video recording function to archive past meetings.

Impact

High speed Internet connectivity has become more widely available and affordable, as has good-quality video capture and display hardware. Consequently, personal videoconferencing systems based on webcams, personal computer systems, software compression, and the Internet have become progressively more affordable by the general public. The availability of freeware (often as part of chat programs) has made software based videoconferencing accessible to many.

The widest deployment of videotelephony now occurs in mobile phones. Nearly all mobile phones supporting UMTS networks can work as videophones using their internal cameras and are able to make video calls wirelessly to other UMTS users anywhere. As of the second quarter of 2007, there are over 131 million UMTS users (and hence potential videophone users), on 134 networks in 59 countries. Mobile phones can also use broadband wireless Internet, whether through the cell phone network or over a local Wi-Fi connection, along with software-based videophone apps to make calls to any video-capable Internet user, whether mobile or fixed.

Deaf, hard-of-hearing, and mute individuals have a particular role in the development of affordable high-quality videotelephony as a means of communicating with each other in sign language. Unlike Video Relay Service, which is intended to support communication between a caller using sign language and another party using spoken language, videoconferencing can be used directly between two deaf signers.

Videophones are increasingly used in the provision of telemedicine to the elderly, disabled, and to those in remote locations, where the ease and convenience of quickly obtaining diagnostic and consultative medical services are readily apparent. In one single instance quoted in 2006: "A nurse-led clinic at Letham has received positive feedback on a trial of a video-link which allowed 60 pensioners to be assessed by medics without traveling to a doctor's office or medical clinic." A further improvement in telemedical services has been the development of new technology incorporated into special videophones to permit remote diagnostic services, such as blood sugar level, blood pressure, and vital signs monitoring. Such units are capable of relaying both regular audio-video plus medical data over either standard (POTS) telephone or newer broadband lines.

Videotelephony has also been deployed in corporate teleconferencing, also available through the use of public access videoconferencing rooms. A higher level of videoconferencing that employs advanced telecommunication technologies and high-resolution displays is called telepresence.

Today the principles, if not the precise mechanisms, of a videophone are employed by many users worldwide in the form of webcam videocalls using personal computers, with inexpensive webcams, microphones, and free video calling Web client programs. Thus an activity that was disappointing as a separate service has found a niche as a minor feature in software products intended for other purposes.

According to Juniper Research, smartphone videophone users will reach 29 million by 2015 globally.

A study conducted by Pew Research in 2010, revealed that 7% of Americans have made a mobile video call.

Government and law
In the United States, videoconferencing has allowed testimony to be used for an individual who is unable or prefers not to attend the physical legal settings or would be subjected to severe psychological stress in doing so, however, there is a controversy on the use of testimony by foreign or unavailable witnesses via video transmission, regarding the violation of the Confrontation Clause of the Sixth Amendment of the U.S. Constitution.

In a military investigation in North Carolina, Afghan witnesses have testified via videoconferencing.

In Hall County, Georgia, videoconferencing systems are used for initial court appearances. The systems link jails with courtrooms, reducing the expenses and security risks of transporting prisoners to the courtroom.

The U.S. Social Security Administration (SSA), which oversees the world's largest administrative judicial system under its Office of Disability Adjudication and Review (ODAR), has made extensive use of videoconferencing to conduct hearings at remote locations. In Fiscal Year (FY) 2009, the U.S. Social Security Administration (SSA) conducted 86,320 videoconferenced hearings, a 55% increase over FY 2008. In August 2010, the SSA opened its fifth and largest videoconferencing-only National Hearing Center (NHC), in St. Louis, Missouri. This continues the SSA's effort to use video hearings as a means to clear its substantial hearing backlog. Since 2007, the SSA has also established NHCs in Albuquerque, New Mexico, Baltimore, Maryland, Falls Church, Virginia, and Chicago.

Education

Videoconferencing provides students with the chance to learn by participating in two-way communication forums. Because it is live, videotelephony allows teachers to access remote or otherwise isolated learners. Students from diverse communities and backgrounds can come together to learn about one another through practices known as telecollaboration (in foreign language education) and virtual exchange, although language barriers will continue to be present. Such students are able to explore, communicate, analyze, and share information and ideas with one another.

Through videoconferencing, students can visit other parts of the world, including museums and other cultural and educational sites. Such virtual field trips can provide enriched learning opportunities to students, especially those who are geographically isolated or economically disadvantaged. Small schools can use these technologies to pool resources and provide courses, such as in foreign languages, which could not otherwise be offered.

Some benefits that videoconferencing can provide to education include:
 * faculty members keeping in touch with classes while attending conferences;
 * faculty members attending conferences 'virtually'
 * guest lecturers brought in classes from other institutions;
 * researchers collaborating with colleagues at other institutions on a regular basis without loss of time due to travel;
 * schools with multiple campuses collaborating and sharing professors;
 * schools from two separate nations engaging in cross-cultural exchanges;
 * faculty members participating in thesis defenses at other institutions;
 * administrators on tight schedules collaborating on budget preparation from different parts of campus;
 * faculty committee auditioning scholarship candidates;
 * researchers answering questions about grant proposals from agencies or review committees;
 * alternative enrollment structures to purely in-person attendance;
 * student interviews with employers in other cities, and
 * teleseminars.

Medicine and health
Videoconferencing is a highly useful technology for real time telemedicine and telenursing applications, such as diagnosis, consulting, prevention, treatment, and transmission of medical images. With videoconferencing, patients may contact nurses and physicians in emergency or routine situations; physicians and other paramedical professionals can discuss cases across large distances. Rural areas can use this technology for diagnostic purposes, thus saving lives and making more efficient use of health care money. For example, a rural medical center in Ohio used videoconferencing to successfully cut the number of transfers of sick infants to a hospital 70 mi away. This had previously cost nearly $10,000 per transfer.

Special peripherals such as microscopes fitted with digital cameras, videoendoscopes, medical ultrasound imaging devices, otoscopes, etc., can be used in conjunction with videoconferencing equipment to transmit data about a patient. Recent developments in mobile collaboration on hand-held mobile devices have also extended video-conferencing capabilities to locations previously unreachable, such as a remote community, long-term care facility, or a patient's home.

Business
Videoconferencing can enable individuals in distant locations to participate in meetings on short notice, with time and money savings. Technology such as VoIP can be used in conjunction with desktop videoconferencing to enable low-cost face-to-face business meetings without leaving the desk, especially for businesses with widespread offices. The technology is also used for remote work. One research report based on a sampling of 1,800 corporate employees showed that, as of June 2010, 54% of the respondents with access to videoconferencing used it "all of the time" or "frequently".

Intel Corporation have used videoconferencing to reduce both costs and environmental impacts of its business operations.

Videoconferencing is also currently being introduced on online networking websites, in order to help businesses form profitable relationships quickly and efficiently without leaving their place of work. This has been leveraged by banks to connect busy banking professionals with customers in various locations using video banking technology.

Videoconferencing on hand-held mobile devices (mobile collaboration technology) is being used in industries such as manufacturing, energy, healthcare, insurance, government, and public safety. Live, visual interaction removes traditional restrictions of distance and time, often in locations previously unreachable, such as a manufacturing plant floor thousands of miles away.

In the increasingly globalized film industry, videoconferencing has become useful as a method by which creative talent in many different locations can collaborate closely on the complex details of film production. For example, for the 2013 award-winning animated film Frozen, Burbank-based Walt Disney Animation Studios hired the New York City-based husband-and-wife songwriting team of Robert Lopez and Kristen Anderson-Lopez to write the songs, which required two-hour-long transcontinental videoconferences nearly every weekday for about 14 months.

With the development of lower-cost endpoints, the integration of video cameras into personal computers and mobile devices, and software applications such as FaceTime, Skype, Teams, BlueJeans and Zoom, videoconferencing has changed from just a business-to-business offering to include business-to-consumer (and consumer-to-consumer) use.

Although videoconferencing has frequently proven its value, research has shown that some non-managerial employees prefer not to use it due to several factors, including anxiety. Some such anxieties can often be avoided if managers use the technology as part of the normal course of business. Remote workers can also adopt certain behaviors and best practices to stay connected with their co-workers and company.

Researchers also find that attendees of business and medical videoconferences must work harder to interpret information delivered during a conference than they would if they attended face-to-face. They recommend that those coordinating videoconferences make adjustments to their conferencing procedures and equipment.

Press
The concept of press videoconferencing was developed in October 2007 by the PanAfrican Press Association (APPA), a Paris France-based non-governmental organization, to allow African journalists to participate in international press conferences on developmental and good governance issues.

Press videoconferencing permits international press conferences via videoconferencing over the Internet. Journalists can participate on an international press conference from any location, without leaving their offices or countries. They need only be seated by a computer connected to the Internet in order to ask their questions.

In 2004, the International Monetary Fund introduced the Online Media Briefing Center, a password-protected site available only to professional journalists. The site enables the IMF to present press briefings globally and facilitates direct questions to briefers from the press. The site has been copied by other international organizations since its inception. More than 4,000 journalists worldwide are currently registered with the IMF.

Sign language

One of the first demonstrations of the ability for telecommunications to help sign language users communicate with each other occurred when AT&T's videophone (trademarked as the Picturephone) was introduced to the public at the 1964 New York World's Fair—two deaf users were able to communicate freely with each other between the fair and another city. Various universities and other organizations, including British Telecom's Martlesham facility, have also conducted extensive research on signing via video telephony.

The use of sign language via videotelephony was hampered for many years due to the difficulty of its use over slow analog copper phone lines, coupled with the high cost of better quality ISDN (data) phone lines. Those factors largely disappeared with the introduction of more efficient and powerful video codecs and the advent of lower-cost high-speed ISDN data and IP (Internet) services in the 1990s.

21st-century improvements
Significant improvements in video call quality of service for the deaf occurred in the United States in 2003 when Sorenson Media Inc. (formerly Sorenson Vision Inc.), a video compression software coding company, developed its VP-100 model stand-alone videophone specifically for the deaf community. It was designed to output its video to the user's television in order to lower the cost of acquisition and to offer remote control and a powerful video compression codec for unequaled video quality and ease of use with video relay services. Favorable reviews quickly led to its popular usage at educational facilities for the deaf, and from there to the greater deaf community.

Coupled with similar high-quality videophones introduced by other electronics manufacturers, the availability of high-speed Internet, and sponsored video relay services authorized by the U.S. Federal Communications Commission in 2002, VRS services for the deaf underwent rapid growth in that country.

Using such video equipment in the present day, the deaf, hard-of-hearing, and speech-impaired can communicate between themselves and with hearing individuals using sign language. The United States and several other countries compensate companies to provide video relay services (VRS). Telecommunication equipment can be used to talk to others via a sign language interpreter, who uses a conventional telephone at the same time to communicate with the deaf person's party. Video equipment is also used to do on-site sign language translation via Video Remote Interpreting (VRI). The relatively low cost and widespread availability of 3G mobile phone technology with video calling capabilities have given deaf and speech-impaired users a greater ability to communicate with the same ease as others. Some wireless operators have even started free sign language gateways.

Sign language interpretation services via VRS or by VRI are useful in the present day where one of the parties is deaf, hard-of-hearing, or speech-impaired (mute). In such cases the interpretation flow is normally within the same principal language, such as French Sign Language (LSF) to spoken French, Spanish Sign Language (LSE) to spoken Spanish, British Sign Language (BSL) to spoken English, and American Sign Language (ASL) also to spoken English (since BSL and ASL are completely distinct from each other), German Sign Language (DGS) to spoken German, and so on.

Multilingual sign language interpreters, who can also translate as well across principal languages (such as a multilingual interpreter interpreting a call from a deaf person using ASL to reserve a hotel room at a hotel in the Dominican Republic whose staff speaks Spanish only, therefore the interpreter has to use ASL, spoken Spanish, and spoken English to facilitate the call for the deaf person), are also available, albeit less frequently. Such activities involve considerable mental processing efforts on the part of the translator, since sign languages are distinct natural languages with their own construction, semantics and syntax, different from the aural version of the same principal language.

With video interpreting, sign language interpreters work remotely with live video and audio feeds, so that the interpreter can see the deaf or mute party, and converse with the hearing party, and vice versa. Much like telephone interpreting, video interpreting can be used for situations in which no on-site interpreters are available. However, video interpreting cannot be used for situations in which all parties are speaking via telephone alone. VRS and VRI interpretation requires all parties to have the necessary equipment. Some advanced equipment enables interpreters to control the video camera remotely, in order to zoom in and out or to point the camera toward the party that is signing.

Descriptive names and terminology
The name videophone never became as standardized as its earlier counterpart telephone, resulting in a variety of names and terms being used worldwide, and even within the same region or country. Videophones are also known as video phones, videotelephones (or video telephones) and often by an early trademarked name Picturephone, which was the world's first commercial videophone produced in volume. The compound name videophone slowly entered into general use after 1950, although video telephone likely entered the lexicon earlier after video was coined in 1935.

Videophone calls (also: videocalls, video chat) as well as Skype and Skyping in verb form differ from videoconferencing in that they expect to serve individuals, not groups. However that distinction has become increasingly blurred with technology improvements such as increased bandwidth and sophisticated software clients that can allow for multiple parties on a call. In general everyday usage the term videoconferencing is now frequently used instead of videocall for point-to-point calls between two units. Both videophone calls and videoconferencing are also now commonly referred to as a video link.

Webcams are popular, relatively low-cost devices that can provide live video and audio streams via personal computers, and can be used with many software clients for both video calls and videoconferencing.

A videoconference system is generally higher cost than a videophone and deploys greater capabilities. A videoconference (also known as a videoteleconference) allows two or more locations to communicate via live, simultaneous two-way video and audio transmissions. This is often accomplished by the use of a multipoint control unit (a centralized distribution and call management system) or by a similar non-centralized multipoint capability embedded in each videoconferencing unit. Again, technology improvements have circumvented traditional definitions by allowing multiple-party videoconferencing via web-based applications.

A telepresence system is a high-end videoconferencing system and service usually employed by enterprise-level corporate offices. Telepresence conference rooms use state-of-the-art room designs, video cameras, displays, sound systems and processors, coupled with high-to-very-high capacity bandwidth transmissions.

Typical uses of the various technologies described above include calling one-to-one or conferencing one-to-many or many-to-many for personal, business, educational, deaf Video Relay Service and tele-medical, diagnostic and rehabilitative purposes. personal videocalls to inmates incarcerated in penitentiaries, and videoconferencing to resolve airline engineering issues at maintenance facilities, are being created or evolving on an ongoing basis.

Other names for videophone that have been used in English are: Viewphone (the British Telecom equivalent to AT&T's Picturephone), and visiophone, a common French translation that has also crept into limited English usage, as well as over twenty less common names and expressions. Latin-based translations of videophone in other languages include vidéophone (French), Bildtelefon (German), videotelefono (Italian), both videófono and videoteléfono (Spanish), both beeldtelefoon and videofoon (Dutch), and videofonía (Catalan).

A telepresence robot (also telerobotics) is a robotically controlled and motorized videoconferencing display to help give a better sense of remote physical presence for communication and collaboration in an office, home, school, etc. when one cannot be there in person. The robotic avatar device can move about and look around at the command of the remote person it represents.

Popular culture

In science fiction literature, names commonly associated with videophones include telephonoscope, telephote, viewphone, vidphone, vidfone, and visiphone. The first example was probably the cartoon "Edison's Telephonoscope" by George du Maurier in Punch 1878. In «In the year 2889», published 1889, the French author Jules Verne predicts that «The transmission of speech is an old story; the transmission of images by means of sensitive mirrors connected by wires is a thing but of yesterday.» In many science fiction movies and TV programs that are set in the future, videophones were used as a primary method of communication. One of the first movies where a videophone was used was Fritz Lang's Metropolis (1927).

Other notable examples of videophones in popular culture include an iconic scene from the 1968 film 2001: A Space Odyssey set on Space Station V. The movie was released shortly before AT&T began its efforts to commercialize its Picturephone Mod II service in several cities and depicts a video call to Earth using an advanced AT&T videophone—which it predicts will cost $1.70 for a two-minute call in 2001 (a fraction of the company's real rates on Earth in 1968). Film director Stanley Kubrick strove for scientific accuracy, relying on interviews with scientists and engineers at Bell Labs in the United States. Dr. Larry Rabiner of Bell Labs, discussing videophone research in the documentary 2001: The Making of a Myth, stated that in the mid-to late-1960s videophones "...captured the imagination of the public and... of Mr. Kubrick and the people who reported to him". In one 2001 movie scene a central character, Dr. Heywood Floyd, calls home to contact his family, a social feature noted in the Making of a Myth. Floyd talks with and views his daughter from a space station in orbit above the Earth, discussing what type of present he should bring home for her.

A portable videophone is also featured prominently in the 2009 science fiction movie Moon, where the story's protagonist, Sam Bell, also calls home as well to communicate with loved ones. Bell, the lone occupant of a mining station on the far side of the Earth's moon, finally succeeds in making his video call after an extended work period, but becomes traumatized when viewing his daughter.

Other earlier examples of videophones in popular culture included a videophone that was featured in the Warner Bros. cartoon, Plane Daffy, in which the female spy Hatta Mari used a videophone to communicate with Adolf Hitler (1944), as well as a device with the same functionality has been used by the comic strip character Dick Tracy, who often used his "2-way wrist TV" to communicate with police headquarters. (1964–1977).

By the early 2010s videotelephony and videophones had become commonplace and unremarkable in various forms of media, in part due to their real and ubiquitous presence in common electronic devices and laptop computers. Additionally, TV programming increasingly used videophones to interview subjects of interest and to present live coverage by news correspondents, via the Internet or by satellite links. In the mass market media, the popular U.S. TV talk show hostess Oprah Winfrey incorporated videotelephony into her TV program on a regular basis from May 21, 2009, with an initial episode called Where the Skype Are You?, as part of a marketing agreement with the Internet telecommunication company Skype.

Additionally, videophones have been featured in:
 * "Lisa's Wedding", an episode of The Simpsons which depicted a Picturephone (1995).
 * a Beyoncé Knowles pop single and music video called "Video Phone" from her album I Am... Sasha Fierce (2008).

Unsorted

 * Greenberg, Alan D. Taking the Wraps off Videoconferencing in the US Classroom, Wainhouse Research, April 2009.
 * Kopytoff, Verne G. Hewlett-Packard Sells Its Video Conferencing Business, The New York Times, June 1, 2011.
 * Lawlor, Julia. Videoconferencing: From Stage Fright to Stage Presence, The New York Times, August 27, 1998.
 * Lohr, Steve. As Travel Costs Rise, More Meetings Go Virtual, The New York Times, July 22, 2008.
 * Miller, Claire Cain. Logitech Buying a Maker Of Videoconference Tools, The New York Times, November 11, 2009.
 * Miller, Claire Cain. Logitech Breaks Into Videoconferencing, The New York Times, November 10, 2009 on line, and November 11, 2009, in print, p. B3. Discusses the acquisition of LifeSize Communications.
 * Millman, Howard. The Videoconference as a Bicoastal Pas de Deux, The New York Times, July 12, 2001.
 * O'Brien, Kevin. Stranded Travelers Turn to Videoconferencing, The New York Times, April 19, 2010. Article discusses the increased use of videoconferencing due to the eruption of an Icelandic volcano which severely curtailed air travel for several months.
 * Ramirez, Anthony. More Than Just a Phone Call; Video Conferencing And Photocopies, Too, The New York Times, September 15, 1993. Discusses the deployment of videoconferencing rooms in several hundred Kinkos locations.
 * Shannon, Victoria. Videoconferencing's virtual leap forward, The New York Times, August 29, 2007.
 * Sharkey, Joe. A Meeting in New York? Can't We Videoconference?, The New York Times, May 11, 2009 online, and in print on May 12, 2009, p. B6 of the New York edition.
 * Vance, Ashlee. Cisco Buys Norwegian Firm for $3 Billion, The New York Times, October 1, 2009 online, and October 2, 2009, in print, p. B7. Discusses the acquisition of Tandberg.
 * Wang, Ses Open source tool detects videoconferencing equipment vulnerabilities, Help Net Security, 17 February 2012.
 * Wayner, Peter. Jerky Pictures and Sound Are History. Videoconferencing Is All Grown Up., The New York Times, June 16, 2005.

General

 * Adeshina, Emmanuel. In-Person Visits Fade as Jails Set Up Video Units for Inmates and Families, The New York Times website, August 7, 2012, pg. A15 of the New York Edition.
 * Bajaj, Vikas. Transparent Government, Via Webcams in India, The New York Times, July 18, 2011, pg.B3. Published online: July 17, 2011.
 * Davis, Andrew W.; Weinstein, Ira M. The Business Case for Videoconferencing, Wainhouse Research, March 2005.
 * Fairlie, Rik. Skype Puts Video Calls on Your TV, The New York Times, July 2, 2010.
 * Hoffman, Jan. When Your Therapist Is Only a Click Away, The New York Times, September 25, 2011, pg. ST1. Also published September 23, 2011 online at www.nytimes.com.
 * Miller, Hugh., Department of Social Sciences, Nottingham Trent University, June 16, 2002. Discusses social issues related to videotelephony.
 * Perlroth, Nicole. Cameras May Open Up the Board Room to Hackers, The New York Times online, January 22, 2012. A version of this article appeared in print on January 23, 2012, on page B1 of the New York edition with the headline: "Conferences Via the Net Called Risky".
 * ProAV Magazine. Being There, ProAV Magazine, November 7, 2008.
 * Saint Louis, Catherine. With Enough Bandwidth, Many Join The Band, The New York Times, January 10, 2012 (online), January 11, 2012 (in print, New York Edition, pg. A1). Retrieved online January 11, 2012. Synopsis: a look at the pros and cons of videotelephony used for private, individual, music lessons.
 * Steinberg, Neil. Dude, Where's My Video Phone?, Forbes.com website, October 15, 2007

Environmental benefits

 * Biello, David. Can Videoconferencing Replace Travel?, Scientific American, March 18, 2009.
 * Coroama, V.C., et al. Effects of Internet-Based Multiple-Site Conferences on Greenhouse Gas Emissions, Telematics and Informatics, 2011, doi:10.1016/j.tele.2011.11.006
 * Pachner, Joanna. The Next Best Thing to Being There, Toronto: The Globe and Mail, October 13, 2009.

Historical and technical

 * Ebel, H. "Subjective Assessment of Picture Interference in the Videophone" in 4th International Symposium on Human Factors in Telephony, Bad Wiessee, Germany, September 23–27, 1968, Information Gatekeepers Inc., pp. 289–298.
 * Fischer, K.; Ebel, H.; "Problems Of How To Position Users In Front Of The Videophone" in: 4th International Symposium on Human Factors in Telephony, Bad Wiessee, Germany, September 23–27, 1968, Information Gatekeepers Inc., pp. 269–282.
 * Hall, A.D. "Experiments with Picturephone Service" in: Bell Laboratories RECORD, 1964, Vol. 42, pp. 114–120.
 * Ives, Herbert E. Two-Way Television and a Pictorial Account of its Background in: Bell Laboratories Record, 1930, Vol. 8, pp. 399–404, and viewable online here.
 * Ives, Herbert E. Picture Transmission and Television in: Bell Labs Quarterly, April 1932, Vol. 11, pp. 118–142.
 * Jaur, Francis. "Usage Domestiques du Visiophone", Technologies de l'Information et Société, 2, 1990, pp. 89–102. (French)
 * Schunack, Von Johannes; Maly, Rolf. Die Fernseh-Sprechtechnik in: Fernseh A.G.: 10 Jahre, Berlin: Hausmitteilungen Aus Forshunsg Und Betrieb Der Fernseh Aktiengellschaft, July 1939, pp. 138–143. (PDF, German)
 * Schulte, Olaf A. "The Next Best Thing To Being There"—Ein Überblick Zu 25 Jahren Videokonferenzforschung (An Overview of 25 years of videoconferencing research) in: Medien & Kommunikations-wissenschaft, Baden-Baden: Hans-Bredow-Institut, April 2002, pp. 551–570 (PDF, German).

title: Videotelephony


text: User talk:64.115.210.23

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: User talk:64.115.210.23


text:
John Pringle Nichol

John Pringle Nichol FRSE FRAS (13 January 1804 – 19 September 1859) was a Scottish educator, phrenologist, astronomer and economist who did much to popularise astronomy in a manner that appealed to nineteenth century tastes.

Early life
Born at Huntly Hill, near Brechin, Angus, Nichol was the son of a gentleman farmer and was educated at the local grammar school and then studied mathematics and natural philosophy (physics) at King's College, University of Aberdeen. He then changed to study divinity. He was licensed as a preacher and became a highly effective communicator, but the impact of phrenological thinking led him to abandon the Church for education.

Nichol held a number of posts in education and journalism and corresponded with many leading thinkers of the times, including John Stuart Mill. He clearly made some impression in economics as James Mill and Nassau Senior nominated him as Jean-Baptiste Say's successor as professor of political economy at the Collège de France though he was at the time too ill to take the post.

Astronomy
In 1836 and in competition with Thomas Carlyle, Nichol was appointed Regius Professor of Astronomy at the University of Glasgow. He became an enthusiastic and effective lecturer and made a profound impression on William Thomson, 1st Baron Kelvin with his introduction of the "Continental" approach to mathematical physics of Jean Baptiste Joseph Fourier. He lived at the Glasgow Observatory.

Nichol turned to popular lecturing and authored a number of popular and successful books about astronomy, especially championing the nebular hypothesis. In 1841 George Eliot wrote:

William John Macquorn Rankine declared Nichol's Dictionary of the Physical Sciences to be:

Private life
In 1831 Nichol married Jane Tullis of Cupar in Fife (1813-1851).

Their eldest son, John Nichol became a literary critic and writer. Jane died in 1850. Nichol married secondly Elizabeth Pease in 1853, a prominent reformer and member of the Darlington Pease family, much against her family's wishes. His daughter, Agnes Jane Nichol, married the mathematician William Jack FRSE (1834–1924).

Nichol was a member of the Edinburgh Phrenological Society.

During the late 1840s, his health declined and, stemming from his physician's prescription, Nichol became addicted to opiates. He recorded an account of his drug-addiction illness and its cure by hydrotherapy at the Ben Rhydding Hydro in his book Memorials from Ben Rhydding (1852).

He died at Glenburn House in Rothesay but is buried in Grange Cemetery in south Edinburgh.

Obituaries

 * Monthly Notices of the Royal Astronomical Society, 19 (1858–9), 141; 20 (1859–60), 131;
 * The Times, 23 September 1859, 10b

title: John Pringle Nichol


text:
Duke of Mercœur

The Seigneurs and Dukes of Mercœur were a line of powerful lords deriving their name from the estate of Mercœur in Auvergne, France. The line became extinct in the 14th century, and passed by inheritance to the dauphins of Auvergne, counts of Clermont. In 1426 it passed to the Bourbons by the marriage, of Jeanne de Clermont, dauphine of Auvergne, to Louis I, Count of Montpensier. It formed part of the confiscated estates of the Constable de Bourbon, and was given by Francis I and Louise of Savoy to Antoine, Duke of Lorraine, and his wife, Renée of Bourbon, sister of the Constable. Nicholas of Lorraine, son of Duke Antoine, was created Duke of Mercœur and a peer of France in 1569. His son Philippe Emmanuel left a daughter, who married the duc de Vendôme in 1609.

House of Lorraine

 * Nicholas 1569–1577
 * Philippe Emmanuel 1577–1602

House of Bourbon-Vendôme
The title became extinct in 1712.
 * Françoise 1602–1649, with her husband César de Bourbon, duc de Vendôme 1609–1649
 * Louis 1649–1669
 * Louis Joseph 1669–1712

Other creations
The title was re-created in 1723 for Louis François I de Bourbon, prince de Conti, who sold it back to the crown in 1770. In 1773, it was re-created for Charles, Count of Artois, but was exchanged in 1778 for Poitou.

title: Duke of Mercœur


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Talk:Waldegrave School

This article has been kept following this VFD debate. Sjakkalle (Check!)  09:13, 25 July 2005 (UTC)

1977 or 1980 or both?
There seems to be some conflict of opinion about the date of formation of the school, or at least the sequence of schools moving to the Fifth Cross Road site vacated by Thames Valley Sixth Form College at the end of the '76-'77 academic year. See http://www.twickenham-museum.org.uk/schools/timeline.asp?offset=100. What really happened? Did one of Twickenham County or Kneller Girls move there first in 1977 or did they both move there together in 1977 or did the 5th X Road site stand empty until 1980? --KenBailey (talk) 23:46, 12 May 2011 (UTC)
 * A: Kneller first. KenBailey (talk) 21:43, 13 May 2011 (UTC)

Article Assessment June 2011
I have moved this article up from Stub to Start class. The article is much more substantive than a stub, but does not qualify as C Class, as there are a number of unreferenced areas, noted by a number of citation needed tags. For some suggestions to expand this article have a look at the WikiProject Schools article guidelines found here - WP:WPSCH/AG.

title: Talk:Waldegrave School


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User:Pyrotec

Pyrotec, from the Greek word for fire or heat, was the username chosen to represent my interests: the history of railways and canals; industrial chemical based products, such as matches; Gunpowder, etc. However, for the past four years, or so, I've mostly been involved in reviewing WP:GANs.

Barnstars
On behalf of Wikiproject Good Articles, I would like to express our gratitude to you for your contributions to the Sweeps process, for which you completed 46 reviews. Completion of this monstrous task has proven to be a significant accomplishment not only for our project, but for Wikipedia. As a token of our sincere appreciation, please accept this ribbon.

User:Pyrotec/GA reviews

title: User:Pyrotec


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Richard Solomon (psychologist)

Richard Lester Solomon (October 2, 1918 – October 12, 1995) was a psychologist well known for his work with in comparative psychology, as well as his opponent-process theory of emotion.

Solomon attended Brown University, where he earned a bachelor's degree (A.B.) in 1940, a master's degree (A.M.) in 1942, and a doctorate (Ph.D.) in 1947.

Solomon won several awards for his scientific achievements, including the Distinguished Scientific Contribution award of the American Psychological Association and the Howard Crosby WARREN Medal of the Society of Experimental Psychologists. Additionally, he was elected to both the American Academy of Arts and Sciences and the National Academy of Sciences. He also held several honorary posts, and edited the Psychological Review.

During his time at Harvard University, Solomon conducted research into avoidance learning. In his experiments, he placed dogs into shuttle boxes with two chambers. The lights would then come on in the side where the dog was. A few seconds later, one half of the chamber would become electrified. To avoid shock, the dog would run to the other chamber. Eventually, the dogs learned to avoid shock entirely by running to the other side in the interval between lighting and electrification.

title: Richard Solomon (psychologist)


text:
Anton Gindely

Anton Gindely (Antonín Gindely, 3 September 1829, Prague – 24 October 1892) was a Bohemian historian, a son of a Hungarian German father and a Czech mother, born in Prague. As a distinguished historian, Gindely has left number of valuable historical works, written in German and Bohemian.

The development of Gindely's perspective on Bohemian history reflected his different personal trajectories through the tumultuous events that happened during the first half of 19th century. He studied in Prague and in Olomouc, and, after travelling extensively in search of historical material, became professor of history at the German Charles-Ferdinand University of Prague and archivist for Bohemia in 1862. During the same time, Gindely was entrusted with the control of the library of Prince George Lobkowitz, which brought him in close connection with the literary interests of the leading families of the Bohemian nobility. He principally occupied himself during the final years of Bohemian independence. He died in Prague.

Gindely's chief work is his Geschichte des dreissigjährigen Krieges (Prague, 1869–1880), which has been translated into English (New York, 1884); and his historical work is mainly concerned with the period of the Thirty Years' War. 'Unity of the brethren' was one of his focused areas, which was evolved out of a more radical Hussite Movement. This was included a two volume "History of the Bohemian Brethren" (1857–58).

Perhaps the most important of his numerous other works are: He wrote a history of Gabriel Bethlen in Hungarian, and edited the Monumenta historiae Bohemica. Gindely's posthumous work, Geschichte der Gegenreformation in Böhmen, was edited by T. Tupetz (1894).
 * Geschichte der böhmischen Brüder (Prague, 1857–1858)
 * Rudolf II. und seine Zeit (1862–1868)
 * A criticism of Wallenstein, Waldstein während seines ersten Generalats (1886)

title: Anton Gindely


text:
Dan Jacobson

Dan Jacobson (7 March 1929 – 12 June 2014) was a South African novelist, short story writer, critic and essayist of Lithuanian Jewish descent.

Early life and career
Dan Jacobson was born 7 March 1929, in Johannesburg, South Africa, where his parents' families had come to avoid the persecution of Jews and to escape poverty in their European homelands. His father, Hyman Michael Jacobson, was born in Ilūkste, Latvia, in 1885. His mother, Liebe (Melamed) Jacobson, was born in Kelme, Lithuania, in 1896. Jacobson had two older brothers, Israel Joshua and Hirsh, and a younger sister, Aviva. His mother's family emigrated to South Africa in 1919, after the death of his grandfather. His grandfather, Heshel Melamed, was a rabbi, and refused to leave Lithuania after traveling to the United States and finding that many Jews were not following their religion. Jacobson later wrote in his memoir Heshel's Kingdom about his travels back to Lithuania to find out more information about his grandfather.

When Dan was four, the family moved from Johannesburg to Kimberley, which was then under British control. The city had once been a huge diamond mining center, but the mines had closed and the town was in decline. However, the De Beers Consolidated Mines Company continued to have great influence. He attended a public school and learned English. During his childhood, he became aware of the ways that different people were treated based on their race, religion, economic status, and social status. In his autobiography Time and Time Again, he refers to the many classes of people in his community: "The Africans lived either in rooms in the back yards of their employers' houses or in sprawling, dusty, tatterdemalion 'locations'; the Cape Coloureds (South African name for people of mixed race) lived in their parts of town; the whites in theirs. Interspersed among these groups were smaller communities: Indians and Chinese among the non-whites, Jews and Greeks among the whites. As for the major division among the whites themselves, that between English-speaking and Afrikaans-speaking, or Briton and Boer.… All these peoples met in the streets, they did business with one another, but just about every aspect of their social life was severely segregated. To sit together in the same room with anyone of a darker skin than their own was a moral impossibility for most whites." He later recalled that many of his Jewish friends and acquaintances sympathized with the blacks in South Africa. He began to observe the ways that the government, churches, and the newspapers justified the ill treatment of blacks.

At the age of 11, an event occurred that affected Jacobson for the rest of his life. After helping a boy rescue his book bag from a filthy trash bin, he went to school unaware that he had gotten dirt on the back of his legs. When his teacher mentioned the dirt in front of the class, several of the boys made fun of him and led the class in ignoring him for six to eight weeks. He was stunned at the mob mentality, seeing how a few leaders of the class could control the actions of the entire group. Paul Gready writes in Research in African Literatures: "A childhood experience of bullying and ostracism was something from which Jacobson was 'never to wholly recover."'

The Jewish community in Kimberley was a strong one. More Lithuanian Jews traveled to South Africa in the early 20th century than to any other country except the United States. Many were hoping to follow in the footsteps of Sammy Marks, a Lithuanian who had made his fortune in the diamond mines. The Jewish community grew even closer together in the 1930s as Nazism rose, and they felt connected to Jews around the world. Jacobson's parents were not particularly religious, but his father insisted that the children attend synagogue and Hebrew lessons, because, as Jacobson later wrote, "To have done less, especially as the Nazi madness swept across Europe, would have seemed to him spineless, even treacherous." Jacobson attended, but usually under protest. Jacobson attended Boys' High School in Kimberley and graduated at the age of 16. He went on to the University of Witwatersrand in Johannesburg, where he graduated at the top of his class with a bachelor's degree in English Literature. Following his graduation in 1948, he worked as a laborer in an Israeli kibbutz for about a year. Then he got a job as a teacher at a Jewish school in London. In less than a year, he was asked to leave, according to a published memoir, because he told the pupils about evolution, thereby contradicting the teachings of Orthodox Judaism.

He returned to Johannesburg in 1951 and worked for the South African Jewish Board of Deputies as a public relations assistant and then as a journalist for Press Digest. In 1952 he returned to Kimberley to work as a correspondence secretary on his father's cattle farm. During this period he became determined to be a writer. His first literary success occurred with a short story entitled "The Box", which was published in Commentary. It was followed by other short stories in Harpers Bazaar, The New Yorker, and other magazines.

In February 1954, he married Margaret Pye, whom he had met while working in London.

His first novel, The Trap, was released in 1955, followed by A Dance in the Sun in 1956. Both books draw upon his childhood experiences. Together the two books earned him a yearlong creative writing fellowship at Stanford University in California. During his time at Stanford, he completed his third novel, The Price of Diamonds, which was also set in South Africa but was a lighthearted comedy–mystery with a moral message. All three books dealt with prejudice and racism. He returned to England in 1957, determined to seek greater depth in his writing. In 1959, he received the John Llewelyn Rhys Award for fiction for his collection of short stories, A Long Way from London. In 1960, The Evidence of Love was published. It dealt with the racism involved in a romantic relationship between a black man and a white woman who were put in prison for getting married. In 1964, he received the W. Somerset Maugham Award for his first collection of essays, Time of Arrival. One of his short stories, "The Zulu and the Zeide", was adapted into a musical and produced on Broadway in 1965. In 1966, he published The Beginners, a longer, in-depth novel following the lives of a Jewish family after their emigration to South Africa. It was a great literary success.

Over the next two decades, he continued to write while holding various teaching positions. In 1965–66, he was visiting professor at Syracuse University in New York. A Dance in the Sun, his second novel, was produced as the play Day of the Lion in Cleveland in 1968. He was a visiting fellow at the State University of New York at Buffalo in the summer of 1971. In 1974, he became vice chair of the Literature Panel of the Arts Council of Great Britain. In 1981, he was a Fellow at the Humanities Research Centre at Australian National University. He also took a position at the University of London as a lecturer; from 1979 to 1987 he was a reader in English. In 1988, he became a professor of English, a position he would hold until his retirement in 1994.

Jacobson's first five novels all focused on South Africa. His writing focus then shifted to moral and ethical issues involving all of humanity. Although he had no interest in learning the Bible as a child, he developed a strong interest in the Old Testament as an adult. In The Rape of Tamar, he retells the story of the rape of Tamar by her half-brother Amnon, the brother of Absolom. The book was adapted into a play called Yonadeb, after the narrative character in the book, and was produced in London in 1985.

In The Story of Stories: The Chosen People and its God, he provided a study of the Bible. His goal was to have a textual analysis as a narrative. The book was criticized by many Jews, probably because he refers to God as "an imaginative creation". Themes that continued to reveal themselves in his works included race relations, class consciousness, human nature, universal traits, group mentality, corruption, betrayal, guilt, power, and social morality. In an article that he wrote for Commentary in 2000, entitled "My Jewish Childhood", Jacobson said: "It is always going to be difficult to get socially and racially diverse people to live harmoniously together within a single polity." In 1985, his autobiography, Time and Again: Autobiographical Essays, was published. Each of the 13 chapters tells of an event in his life that shaped his way of thinking about the human race. The book won the J. R. Ackerley Prize for Autobiography. The God-Fearer, published in 1992, is a story of persecution. Jews are in the majority and oppress a group called the "Christers". In a Washington Post article, Anne Roiphe observed: "By making the majority Jewish, Jacobson makes it clear that power is the source of oppression: not that the power is German or Christian, but that it has the weight of numbers. … The horror of the story lies not in gruesome details or heated prose, but in the calm truth of what we call normal behavior when we try to save our skin at any cost."

In the middle 1990s, Jacobson turned to nonfiction. In 1994 he published The Electronic Elephant: A South African Journey about his travels back to South Africa to observe the changes in the land and the culture since his childhood. In 1998, Heshel's Kingdom provided a moving story of his travels to Lithuania to learn more about the life of his grandfather. He started with his grandfather's identity document, spectacles, an address book, an old photograph, and the memories of relatives. Sadly, he found no trace of his grandfather, and, indeed, not even the cemetery he was buried in remained. He did find that in 1941, within six weeks, the Nazis essentially wiped out the Lithuanian Jewish community, killing 210,000 Jewish people.

Works
His early novels, including The Trap, his first published novel, focus on South African themes. His later works have been various in kind: they include works of fantasy and fictional treatments of historical episodes, as well as memoirs, critical essays, and travel books. Among the awards and prizes he has received are the John Llewellyn Rhys Prize 1959 (A Long Way from London and Other Stories); Somerset Maugham Award 1964 (Time of Arrival and Other Essays); The Jewish Chronicle Award 1977 (The Confessions of Josef Baisz); the J. R. Ackerley Prize for Autobiography 1986 (Time and Time Again). In 2000 he edited and translated from the Dutch Een mond vol Glas by Henk van Woerden, an imaginative re-creation of the circumstances leading to the assassination of the South African president Dr Hendrik Verwoerd, in the country's House of Assembly. His 2005 novel All For Love was long listed for the Man Booker Prize.

Honours
Dan Jacobson received an Honorary D. Litt. from the University of the Witwatersrand, Johannesburg, and on retirement from his position at University College London was elected a Fellow of the college. He was elected a Fellow of the Royal Society of Literature in 2007.

Papers
Collections of his papers can be found at the Harry Ransom Humanities Research Center, Austin, Texas; Oxford University, England; and, in South Africa, at Witwatersrand University Library, Johannesburg, the National English Literary Museum, Grahamstown, and the Africana Library, Kimberley. He died in London on 12 June 2014.

Books

 * The Trap (1955)
 * A Dance in the Sun (1956)
 * The Price of Diamonds (1958)
 * The Zulu and the Zeide (1959)
 * The Evidence of Love (1960)
 * No Further West (1961)
 * Time of Arrival (1963) (essays)
 * Beggar My Neighbor (1964) (short stories)
 * The Beginners (1966)
 * Through the Wilderness and Other Stories (1968)
 * The Rape of Tamar (1970)
 * The Boss (1971)
 * Inklings (1973) (short stories)
 * The Wonder-Worker (1973)
 * The Confessions of Josef Baisz (1979)
 * The Story of the Stories: The Chosen People and Its God (1982) (non-fiction)
 * Time and Time Again: Autobiographies (1985)
 * Her Story (1987)
 * Adult Pleasures: Essays on Writers and Readers (1989)
 * Hidden in the Heart (1991)
 * The God-Fearer (1992)
 * The Electronic Elephant (1994)
 * Heshel's Kingdom (1998)
 * A Mouthful of Glass – translated and edited from Henk van Woerden's Een Mond vol Glas (2000)
 * All for Love (2005)
 * Literary Genius: 25 Classic Writers Who Define English & American Literature (2007) (Illustrated by Barry Moser)

Cited in others' works
At the conclusion of his book Austerlitz, W. G. Sebald has his anonymous narrator take from his rucksack a copy of Dan Jacobson's Heshel's Kingdom (1998), the account of Jacobson's journey in the 1990s to Lithuania in search of traces of his grandfather Heshel's world. The orthodox rabbi Heshel Melamed's sudden death in 1919 had provided an opportunity for his widow and nine children to leave Lithuania for South Africa, which, in light of events two decades later, ironically, had been a gift of life. "On his travels in Lithuania Jacobson finds scarcely any trace of his forebears, only signs everywhere of the annihilation from which Heshel's weak heart had preserved his immediate family when it stopped beating."

title: Dan Jacobson


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Category:Chocolatiers

This category should contain only persons (Chocolatiers), chocolate companies you can find under category:Chocolate companies.

title: Category:Chocolatiers


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Daniel Jacobson

Daniel Jacobson may refer to:

 * Dan Jacobson (1929–2014), South African writer
 * Daniel P. Jacobson (born 1961), American politician in New Jersey

title: Daniel Jacobson


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List of railway companies in Switzerland

The following is a list of railway companies which operate routes on Swiss territory.

Standard gauge
The following is a complete list of all railway companies which operate routes on Swiss territory. It also includes routes of foreign railway companies (e.g. Deutsche Bahn), but not routes of Swiss companies in neighbouring countries.

Not included are railway companies which do not operate their own routes (e.g. Cisalpino, Hupac or the former Lokoop) as well as operators of short connecting goods lines.

If there is no abbreviation shown, it means that this company always appears with its full name.

Tramways

 * BLT Baselland Transport AG (Basel)
 * BVB Basler Verkehrs-Betriebe (Basel)
 * RiT Riffelalptram (Zermatt)
 * SVB BERNMOBIL Städtische Verkehrsbetriebe Bern (Bern)
 * TN Transports publics du littoral neuchatelois (Neuchâtel)
 * TPG Transports Publics Genevois (Geneva)

Closed or merged

 * ABB Altstätten-Berneck-Bahn (later RhStB)
 * AF Strassenbahn Altdorf-Flüelen (closed 26 March 1951)
 * CBV Tramway Chillon-Byron-Villeneuve (later VMCV)
 * ESZ Elektrische Strassenbahnen im Kanton Zug (closed 21 May 1955)
 * MRA Trambahn Meiringen-Reichenbach-Aareschlucht (closed 16 September 1956)
 * RhStB Rheintalische Strassenbahn (later RhV)
 * RhV Rheintalische Verkehrsbetriebe (closed 2 June 1973)
 * SSS Strassenbahn Schwyz-Seewen (later SStB)
 * SStB Schwyzer Strassenbahnen (closed 15 December 1963)
 * StrStM Strassenbahn St. Moritz (closed 18 September 1932)
 * SchSt Schaffhauser Strassenbahn (closed 6 March 1970)
 * StSS Strassenbahn Schaffhausen-Schleitheim (closed 1 October 1964)
 * StSt Strassenbahn Stansstad-Stans (closed)
 * StStW Städtische Strassenbahn Winterthur (closed 2 November 1951)
 * STI Steffisburg-Thun-Interlaken (closed 31 May 1958)
 * STL Tramway Locarno (closed 30 April 1960)
 * SVB Spiezer Verbindungsbahn (closed 25 September 1960)
 * (TB) Tramway Bellavista (closed 31 October 1913)
 * TC Tramway de La Chaux-de-Fonds (closed 15 June 1950)
 * TEL Tram Lugano (closed 17 December 1959)
 * TEM Tram Mendrisio (closed 31 December 1950)
 * TF Tramway de Fribourg (closed 1 April 1965)
 * TL Tramways lausannois (closed 6 January 1964)
 * TP Tramway-funiculaire Trait-Planches (closed 11 November 1912)
 * TrB Tramway de Bienne/Biel (closed 9 December 1948)
 * TrL Tram Luzern (closed 11 November 1961)
 * TrMB Tramway Martigny-Bourg (closed 31 December 1956)
 * TStG Trambahn der Stadt St. Gallen (closed 1 October 1957)
 * UOeB Uster-Oetwil-Bahn (closed 10 January 1949)
 * VMC Tramway Vevey-Montreux-Chillon (later VMCV)
 * VMCV Tramway Vevey-Montreux-Chillon-Villeneuve (closed 19 January 1958)
 * WMB Wetzikon-Meilen-Bahn (closed 13 May 1950)
 * ZBB Zuger Berg- und Strassenbahn (closed 10 May 1959)


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UDP hole punching

UDP hole punching is a commonly used technique employed in network address translation (NAT) applications for maintaining User Datagram Protocol (UDP) packet streams that traverse the NAT. NAT traversal techniques are typically required for client-to-client networking applications on the Internet involving hosts connected in private networks, especially in peer-to-peer, Direct Client-to-Client (DCC) and Voice over Internet Protocol (VoIP) deployments.

UDP hole punching establishes connectivity between two hosts communicating across one or more network address translators. Typically, third-party hosts on the public transit network are used to establish UDP port states that may be used for direct communications between the communicating hosts. Once port state has been successfully established and the hosts are communicating, port state may be maintained either by normal communications traffic, or in the prolonged absence thereof, by keep-alive packets, usually consisting of empty UDP packets or packets with minimal, non-intrusive content.

Overview
UDP hole punching is a method for establishing bidirectional UDP connections between Internet hosts in private networks using network address translators. The technique is not applicable in all scenarios or with all types of NATs, as NAT operating characteristics are not standardized.

Hosts with network connectivity inside a private network connected via a NAT to the Internet typically use the Session Traversal Utilities for NAT (STUN) method or Interactive Connectivity Establishment (ICE) to determine the public address of the NAT that its communications peers require. In this process another host on the public network is used to establish port mapping and other UDP port state that is assumed to be valid for direct communication between the application hosts. Since UDP state usually expires after short periods of time in the range of tens of seconds to a few minutes, and the UDP port is closed in the process, UDP hole punching employs the transmission of periodic keep-alive packets, each renewing the life-time counters in the UDP state machine of the NAT.

UDP hole punching will not work with symmetric NAT devices (also known as bi-directional NAT) which tend to be found in large corporate networks. In symmetric NAT, the NAT's mapping associated with the connection to the known STUN server is restricted to receiving data from the known server, and therefore the NAT mapping the known server sees is not useful information to the endpoint.

In a somewhat more elaborate approach both hosts will start sending to each other, using multiple attempts. On a Restricted Cone NAT, the first packet from the other host will be blocked. After that the NAT device has a record of having sent a packet to the other machine, and will let any packets coming from this IP address and port number through. This technique is widely used in peer-to-peer software and Voice over Internet Protocol telephony. It can also be used to assist the establishment of virtual private networks operating over UDP. The same technique is sometimes extended to Transmission Control Protocol (TCP) connections, though with less success because TCP connection streams are controlled by the host OS, not the application, and sequence numbers are selected randomly; thus any NAT device that performs sequence-number checking will not consider the packets to be associated with an existing connection and drop them.

Flow
Let A and B be the two hosts with internal IP addresses iAddrA and iAddrB respectively, each in its own private network; NA and NB are the two NAT devices with external IP addresses eAddrA and eAddrB respectively; S is a public server with a known IP address.
 * 1) A and B each begin a UDP conversation with S; the NAT devices NA and NB create UDP translation states and assign temporary external port numbers ePortA and ePortB.
 * 2) S examines the UDP packets to get the source port used by NA and NB (the external NAT ports ePortA and ePortB).
 * 3) S informs B about the values of eAddrA:ePortA and informs A about the values of eAddrB:ePortB.
 * 4) A sends a packet to eAddrB:ePortB and B sends a packet to eAddrA:ePortA (not necessarily at the same time nor in any particular order).
 * 5) * When those packets leave their private network, each NAT device adds an entry to its translation table:
 * 6) ** NA examines A's packet and finds an entry already exists (iAddrA, iPortA, eAddrA, ePortA) but the destination is different (S) . NA will create a new entry with the same translation for traffic sourced from A ( iAddrA) to B (eAddrB).
 * 7) ** NB examines B's packet and finds an entry already exists (iAddrB, iPortB, eAddrB, ePortB) but the destination is different (S). NB will create a new entry with the same translation for traffic sourced from B (iAddrB) to A (eAddrA).
 * 8) * Depending on the state of each NAT's translation table upon arrival of the remote's packet:
 * 9) ** If the new entry was already added to NA's translation table before the arrival of B's packet, then NA passes B's packet, but otherwise drops it.
 * 10) ** If the new entry was already added to NB's translation table before the arrival of A's packet, then NB passes A's packet, but otherwise drops it.
 * 11) Once both NAT's have those tuples added, then "holes" have been "punched" in the NATs, allowing both hosts to directly communicate.
 * If both hosts have Restricted cone NATs or Symmetric NATs, the external NAT ports will differ from those used with S. On some routers, the external ports are picked sequentially, making it possible to establish a conversation through guessing nearby ports.

title: UDP hole punching


text: User:Cookiemoose

title: User:Cookiemoose