visual telephone system

Visual telephone systems and terminal equipment

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Visual telephone systems and terminal equipment 

This is a brief introduction of H.320 and this Recommendation H.320 covers the technical requirements for narrow-band visual telephone systems services defined in H.200/F.720-series Recommendations, where channel rates do not exceed 1920 kbit/s.

NOTE – It is anticipated that this Recommendation H.320 will be extended to a number of Recommendations each of which would cover a single video conferencing or videophone service (narrow-band, broadband, etc.). However, large parts of these Recommendations would have identical wording, while in the points of divergence the actual choices between alternatives have not yet been made; for the time being, therefore, it is convenient to treat all the text in a single Recommendation.

The service requirements for visual telephone services are presented in ITU-T Recs F.720 for video telephony and F.702 for a video conference; video and audio coding systems and other technical aspects common to audiovisual services are covered in other Recommendations in the H.200/F.700-series.

System description

A generic visual telephone system is shown in Figure 1. It consists of terminal equipment, network, Multipoint Control Unit (MCU) and other system operation entities.

visual telephone system


A configuration of the terminal equipment consisting of several functional units is also shown in Figure 1. A video I/O equipment includes cameras, monitors and video processing units to provide functions such as split-screen scheme. Audio I/O equipment includes microphones, loudspeakers and audio processing units to provide such functions as acoustic echo cancellation (see ITU-T Rec. G.167). Telematic equipment includes visual aids such as electronic blackboard, text conversation facility and still picture transceiver to enhance basic visual telephone communication.

The system control unit carries out such functions as network access through end-to-network signalling and end-to-end control to establish a common mode of operation and signalling for proper operation of the terminal through end-to-end signalling. The video codec carries out redundancy reduction coding and decoding for video signals, while audio codec does the same thing for audio signals. The delay in the audio path compensates video codec delay to maintain lip synchronization.

The mux/demux unit multiplexes transmitting video, audio, data and control signals into a single bit stream and demultiplex a received bit stream into constituent multimedia signals. Network interface makes necessary adaptation between the network and the terminal according to the user-network interface requirements defined in the I.400-series Recommendations (see Note).

NOTE – For leased line networks, the network interface is defined in ITU-T Rec. G.703 for bit rates in the range of 64 kbit/s to 2048 kbit/s. An alternative interface is defined in ITU-T Rec. X.21. For n × H0 channels, timeslot allocation is given in clause 5/G.704 for the G.703 interface. It is stressed that interworking towards ISDN requires synchronous operation of the leased line network.


Visual telephone signals are classified into video, audio, data and control as follows:

  • Audio signals are continuous traffic and require real-time transmission. NOTE – In order to reduce the average bit rate of audio signals, voice activation can be introduced (in which case the audio signals are no longer continuous).
  • Video signals are also continuous traffic; the bit rate allocated to video signals should be as high as possible, in order to maximize the quality within the available channel capacity.
  • Data signals include still pictures, facsimile and documents, or other facilities such as text conversation; this signal may occur only occasionally as required and may temporarily displace all or part of the audiovisual signal content. It should be noted that data signals are associated only with optional enhancements to the basic visual telephone system; therefore, the opening of a path to carry such signals is preceded by negotiation between the terminals.
  • Control signals are some system control signals by definition. The path for the terminal-to-network control signals is provided in the D-channel, while the path for the terminal-to-terminal control signals are provided in BAS or service channel only when necessary by the mechanism defined in ITU-T Rec. H.221.


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