Videoconferencing
Video conference is called for video and audio transmission of synchronous information exchange between people in more than one place in extension to localized meeting (conference) with the help of technical equipment. The different images and sounds at the locations are processed by the participants influenced for the respective devices. The terminals have at least one camera and a microphone as input devices and screen and speakers as output devices. With only two participants, the term video telephony is more accurate.
- 2.3.1 The H.323 protocol
- 2.3.2 The protocol H.320
- 2.3.3 The protocol T.120
- 2.3.4 The H.239 standard
- 2.3.5 The SIP protocol
- 2.3.6 proprietary peer -to-peer systems
History
First development of technologies for videoconferencing began already in the thirties, in parallel with the development of television. Despite new technologies in the 1970s and carried thereby resumption of development videoconferences were not widespread until the beginning of the third millennium. A breakthrough obtained improved compression method of the now fully digitized voice and video data, standardization of previously proprietary process and the general availability of PCs and digitized loops for initial success. With the increasing migration to IP -based switching technology videoconferencing is now benefiting more and more from the availability of broadband connections. Consequently, experts predict a growing dissemination of this communication technology.
Technical aspects
Loggerhead video conference: Web Conference with telephone circuit
A similar form is the web conference with simultaneous telephone conference. To this end, the participants need only a PC with an Internet connection and a telephone. On the PC itself has no camera and no microphone is needed. The participants will be informed by e- mail from a host (a difference for video conference in which all participants are equal ) invited and received in an Internet link to the web conference. In parallel they dial into a conference call. The number of participants can go into the thousands.
During the conference, participants can be muted. This is common especially for large conferences. It is also possible to give each participant the word. The web conference allows both pure slide presentations as well as the display screen contents of a participant on all participant screens. There are chats and areas for questions and answers. The messages can be sent publicly to all participants depending on the settings confidential to certain users or.
In some cases, the terms webcast and webinar be used for this purpose.
The device- technical basis of the transmission
The standards body ITU- T defines four major groups of devices:
Video conference terminal
The video conferencing market offers a variety of plant or implementation variants, their equipment is largely dependent on the intended use.
Desktop systems
For desktop systems, the necessary components are installed in a PC. It is this addition of an external camera ( now mostly USB webcam ) also needs a microphone or headset. A distinction is hardware-based ( encoding and decoding on a riser card) and software-only desktop systems. In addition to the relatively low cost desktop systems have the advantage that the user has during the videoconference full access to its data and applications installed on the PC programs. Desktop systems are therefore particularly suitable where at conferences, a common data processing is to take place, perhaps with the help of the software NetMeeting or VNC.
Set-top boxes / roll -abouts
These compact systems are special devices are needed to operate them usually only have one monitor and the appropriate network connections (ISDN and / or LAN). Due to the low weight and ease of installation, these devices are also suitable for mobile use.
Solutions
Solutions are modular, high-performance systems. With various features system configurations are possible for almost any application. Powerful cameras, room microphones and large monitors also allow in large conference rooms the installation of these systems, which allow the integration of other peripheral devices such as document cameras.
Other devices
This includes being developed mobile video conferencing via UMTS or video telephony.
Multipoint Control Unit ( MCU)
MCUs are star distributor - also called Reflector - for group conferences. You are always required when more than two participants want to participate in a conference. There are hardware and / or software solutions that manage and control one or more multi-point conferences. The MCU is connected to all the participants. It manages and controls the incoming and outgoing video and audio streams. In Germany MCUs are used, for example, within the DFN-Verein for the operation of its service Video Conference. However, companies and large national administrations such as in NRW put an MCU for video communication. MCUs support styles for the H.323 and SIP protocols.
Gatekeeper
The gatekeeper is a key logical component of the video conference which can organize among other things the connection between the terminals and the MCU. It can also forward the data streams as a proxy. All devices which are associated with a gatekeeper, are located in the same zone (similar to the prefix numbers when phone). A gatekeeper address translations are performed.
Gateway
A gateway connects different networks together and is implemented using the OSI layers 4 to 7. It convert gateway protocols to each other, but can also couple two networks take over. For joint use of ISDN and IP terminals, the use of a gateway is imperative.
Protocols as the basis of transmission
An important part of the technical basis form the Protocols H.320, H.323 and T.120. These protocols are so-called umbrella standards, which include other protocols on individual aspects.
The H.323 protocol
The main protocol for the operation of the Internet is the H.323 protocol. The standard regulates the cooperation for video telephony terminals, which are connected via a LAN / WAN. Within the framework of H.323 to control the connection and the implementation of IP addresses is regulated in the H.225 protocol, using H.245, the devices agree on which services they support. This applies particularly to the video compression by H.261, H.263 or H.264, and audio encoding to G.711 G.729.
The protocol H.320
The protocol H.320 regulates the operation of narrow-band video telephony terminals (eg ISDN, satellite, radio relay ) and contains, like H.323, further frame specifications.
The protocol T.120
With the help of the protocol T.120 data applications to be realized within a video conference. It includes nine guidelines that specify the connection establishment and termination, flow control, cooperation with MCUs, the use of whiteboard, file transfer and application sharing in detail.
The H.239 standard
A classic video conference has an audio, a video, and optionally a data channel. H.239 defines the method to use a second video channel when using the protocols H.320 and H.323, for example, a presentation to show a video or a picture of a second camera. H.239 replaces proprietary processes (eg, People Content, DuoVideo ). However, all these methods can not be an application sharing, because no mutual access to the data is possible.
The SIP protocol
The Session Initiation Protocol (SIP ) has been developed for the transmission of multimedia applications. SIP is similar to HTTP removed and is not compatible with H.323 or H.320. It is used to negotiate the communication modalities, the communication is agreed within the SIP packets over the Session Description Protocol ( SDP). The data transfer itself is then usually directly between the endpoints with other Internet protocols such as the Real -time Transport Protocol.
Proprietary peer -to-peer systems
Peer-to- Peer ( P2P ) video conference systems dispense with central Group and the communication server, as given in H.323 systems by gatekeepers and MCU. Instead, it is transferred to the terminal group and QoS management. P2P video conferencing systems are usually desktop systems. There are proprietary systems and are not subject to standardization. Example of P2P video conferencing systems are the system Bravis BTU, the system daViKo ( developed at the HTW Berlin) and Skype.
Social Dynamics
Video conferences are an independent communicative situation that is not just in the middle between traditional telephone and face -to -face communication. The most important problem is that of the separate locations of the participants give different perceptual conditions.
Answering the question of whether the participants in a videoconference increasing the use of the video or audio stream, it depends on the topic of the conversation. If nonverbal or deictic aspects play a major role, the video moves to the fore, for all other topics, participants, concentrate mostly on the audio stream.
A common communication problem in videoconferencing is up to one second for a long time lag between recording and broadcasting. This lag often means that each of the other caller is perceived as " slow" or a transmission pause is collected and interpreted to mean that the expected, but seemingly lack of response is dropped into word. Other problems include the difficulty of making eye contact, especially because often the self- image is preferred and technical problems such as picture or no sound or Grobpixelierung.