This is a brief introduction of the general features and attributes of the video telephony services regardless of the network environment where the service might be provided. The video telephony services are classified in the following two main categories:
- Video telephony service for narrow-band networks;
- Video telephony service for broadband networks.
Recommendation F.721 covers the basic video telephony service in the Integrated services digital network (ISDN) due to this classification. Dedicated service Recommendations for higher quality video telephony services are for further study. The higher quality video telephony service will not necessarily employ any fixed information transfer rate as variable bit rate coding may be used. The classification of video telephony services is depicted in Figure 1/F.720.
Video telephony service is an audiovisual conversational teleservice providing bidirectional symmetric real-time transfer of voice and moving colour pictures between two locations (person-to-person) via the networks involved.
The minimum requirement is that under normal conditions the picture information transmitted is sufficient for the adequate representation of fluid movements of a person displayed in head and shoulders view (see Note).
Note – The smoothness of the movements in the reproduced picture is essentially dependent on the amount of motion with respect to the transfer rate of transmitted picture information. The above requirements are supposed to be met under such conditions where the amount of motion is limited or the throughput is high enough not to impair the received picture. Degradation is likely to appear as increased blurring and jerkiness in the reproduced picture. Besides this, other artifacts may occur.
The plain video telephony service includes only the basic user requirements, namely speech and motion picture with essential controls and indications; the service can be enhanced with options providing auxiliary facilities such as transmission of high resolution still images of documents, photographs, drawings, charts, objects, etc. (see Note).
Note – Speech supported only by still picture transmission and/or telewriting is not considered as a part of the video telephony service. This type of communication may either be considered as a separate audiovisual service or as a form of audiographic conference service in a point-to-point configuration.
The video telephony service is likely to be used in much the same way as the ordinary telephone service for individual communication, the enhancement being in the visibility of the communicating parties which implies a number of possible new applications. An essential feature of the service is that it is always provided in conjunction with ordinary telephony allowing the user to intercommunicate with all kinds of audiovisual services by using merely the speech communication facility of a video telephone terminal. In other words, video telephone terminals must be capable of supporting telephony.
A video telephony service may also be used in applications such as communication of speech – and hearing –impaired persons using sign language and remote surveillance where the speech communication facility is of minor importance.
In case the service is provided in a network and terminal environment which offers a number of different quality levels depending, among others, on the transmission medium used and respective charging, it must be possible for the user to select the level/mode of operation and/or type of video telephony service he/she wishes and also change it during the call if provided by the network or supported by the terminals. The latter option may be provided as a supplementary service. Two different types of call shall be possible:
- Point-to-point calls;
- Multipoint calls (see Note).
Note – For multipoint calls a unit for mixing speech signals and/or combining video signals is required. This will be defined in another context later.
Description of various video telephony services
Two primary video telephony service categories have been identified, for narrow-band networks and for broadband networks. The principal features of either type of service are described below.
Video telephony service for narrow-band networks
The video telephony service for narrow-band networks provides end-to-end communication of moving colour pictures with spatial resolution, temporal resolution and quality equivalent to that obtainable by coding the video signal according to Recommendation H.261 (QCIF and CIF format).
The video telephone service may be optionally enhanced by facilities such as the transfer of still pictures, graphics, text and end-to-end control messages.
A basic video telephone teleservice for the narrow-band ISDN has been fully standardized according to Recommendations I.210 and I.240. The stage 1 description for the video telephony teleservice for ISDN is contained in Recommendation F.721.
In the future, basic video telephony services for other narrow-band networks (e.g. radio mobile networks, private networks) could be envisaged.
Video telephony service for broadband networks
The video telephony service for broadband networks provides end-to-end communication of moving color pictures with high spatial and temporal resolution and video quality equivalent to conventional TV standards (PAL, SECAM, NTSC) or better and enhanced voice/sound quality, possibly stereo transmission and optionally facilities for the transfer of still pictures, graphics, text and end-to-end control messages.
Applications related to video telephony
The video telephony service can be utilized in a broad range of applications depending on the Quality of Service for audio and/or video that can be achieved in different types of services.
A video telephony service employing the bearer capabilities of broadband networks is expected to meet the needs of the subsequently listed applications. Because of the high video quality, this service provides, besides the means for face-to-face dialogue, the possibility to transfer any kind of moving scenes. Also, pictures of three-dimensional objects, graphic material, e.g., sketches, drawings, photographs and documents containing text and graphics can be transferred without any restrictions. Commercial and domestic scenes, instruction procedures and films can be transferred to the communication partner.
The constraints imposed by limited transfer capability of narrow-band networks on spatial and temporal image resolutions make certain types of communication less applicable to the video telephony service for narrow-band networks.
Taking into account the aspects mentioned above, the following principle applications of video telephony are possible among others:
- Face-to-face dialogues involving at least head-and-shoulder images;
- Dialogue including interactive viewing of documents such as sketches, diagrams or charts and objects that can be shown on the screen;
- Access of the user to videoconferences;
- Remote video surveillance;
- Communication between hearing and speech impaired persons using the sign language.
Based on these examples, other enhanced videotelephone applications may also emerge. The user should be given the possibility to select the essential parameters best suiting his specific application.
For instance, in applications b) high spatial resolution is required and on the other hand in e), good motion tolerance is important. Moreover, the transfer rate used for audio should be selectable, in particular when it has charging or video quality implications.
Supplementary services and enhancements
The same spectrum of supplementary services supporting telephony is in principle applicable for Videotelephony. Other supplementary services or enhancements, dedicated to video telephony, e.g. video telephony conference services, are for further study. Relevant enhancements are those supporting the transfer of high resolution still pictures and graphics information in a variety of formats like digitized video or other standard document formats. A paper/film scanner or video frame capture may be used as an image input source. Besides this, the possibility to access electronic mail or videotex services with a video telephone terminal may be supported.
Fall back: Procedures performed either by the network or by the calling terminal that allows the calling user to be connected in any case with the called user at any terminal where the call is offered (3.1 kHz telephone terminal or video telephone terminal).
General systems requirements
In order to perform the basic functions necessary for the video telephony service the terminal equipment must include devices capable of:
- capturing participant’s picture(s);
- displaying remote user’s picture(s);
- capturing audio;
- reproducing audio;
- audio coding;
- video coding;
- management of network interfaces.
The terminal equipment also includes devices capable of performing the following functions:
- user control;
- user indication;
Terminals intended to be used in multipoint connections may need additional basic functions related to the multipoint operation. These functions are for further study.
Video telephone terminal equipment
The basic video telephone terminal equipment may include only the basic elements listed in 6.1.
Possible enhancements to the equipment are:
- orientable camera and zoom;
- still picture camera;
- interfaces for an additional camera, an additional screen or a video recorder;
- remote control of a distant camera for some specific applications;
- keyboard for the videotex service;
This list is not exhaustive and is only given as a set of examples. As a general rule, the number of controls that a user has to operate should be kept to a minimum. Training should not be necessary for using the terminal. Clear and concise instructions should be given, for instance on the screen, especially for the supplementary services with which the user may not be very familiar.