Yong Qiu Liu's Web Page--H.320

Networking service systems and related standards

2.2 H.320 ISDN Conferencing

As an overview document for videoconferencing^[29], H.320 is an ITU recommendation, specifying compatibility amongst terminals produced by different vendors. Known as an "umbrella" standard, H.320 defines a range of protocols for video, audio and control^[30].

There are a variety of protocols that are implemented within H.320. Two kinds of requirements, mandatory requirements and optional requirements, are specified in H.320. Mandatory requirements support all H.320 compatible systems communicating with one another, whereas optional requirements specify some extended functionality for the systems. So different varieties of protocols are supported by different classes of H.320. Here are three classes: ^[30]

Class 1: Minimum level of H.320 support. It supports H.261 for video compression, QCIF (Quarter Common Intermediate Format) resolution, 7.5 frames per second, Motion compensation decoding, and G.711 audio protocol.

Class 2: Supports Class 1 and some optional features. It supports at least everything covered in the Class 1 of H.320, but may support as few or as many of the Class 3 options as the manufacturer would like

Class 3: Supports Class 1 and all optional features. It supports H.261 for video compression, CIF (or FCIF - Full Common Intermediate Format) resolution; 7.5, 10, 15, or 30 frames per second; pre- or post-processing; Motion Compensation Encoding and Decoding; and G.711, G.722, and G.728 audio protocols.

Picture Resolution, Frame Rate, Pre and Post Processing, Motion Compensation, and Audio are the factors that affect the system quality.^[29]

Picture Resolution

H.320 implements two types of resolutions: CIF (Common Intermediate Format) with a picture resolution of 352 * 288, which is also known as FCIF -- Full CIF, and (Quarter Common Intermediate Format) with a picture resolution of 176 x 144. At QCIF resolution pictures are often blocky and may seem distorted.

Class 1 systems will only support QCIF and Class 2 systems may support CIF whereas Class 3 must support CIF.^[30]

Frame Rate

Frame Rate (fps) is the number of times the picture image is refreshed per second. H.320 systems can support frame rates of 7.5, 10, 15, or 30 fps. This factor affects the smoothness of the video. The higher the frame rate is, the more fluid and less jerky the video is. When the frame rate is higher than 24, there is no obvious difference of the animation of the video when watched by a human’s eye.

Class 1 systems support at least 7.5 frames per second. Class 2 systems will support up to 15 fps and Class 3 must support up to 30 fps.^[30]

If a higher-class system connects to a lower-class system, the frame rate of the lower class system will be selected as the common frame rate.^[29]

Video Pre- and Post-processing

Pre-processing eliminates the unnecessary noise from the background before the video is encoded and ensures that only real motion gets encoded. This will save encoding time by preventing the encoder from doing any unnecessary work . This is not required for Class 1 systems.

Post-processing compensates for the degradation of the video especially during fast motion. It reduces the blocking and noisy effects, enhances the frame rate and eliminates the jerky motion effect caused by low frame rates.^[29]

Motion Compensation

There are two concepts of motion compensation: motion estimation, which determines what the motion vector should be, and the actual motion compensation, which makes sure that only sections of each frame that have experienced motion are encoded. Motion estimation is performed at the encoder and motion compensation is performed at both the encoder and the decoder. When using lower bit rates this is especially important.^[29]

Class 1 systems only support the compensation decoding and Class 3 systems usually support more efficient encoding where as the functionalities in Class 2 systems are between Class 1 and Class 3. ^[30]

Audio Considerations

H.320 specifies three types of audio:

G.711 -- 48-64 Kbps Narrowband: It provides telephone quality sound (narrow-band, 3khz) and is the only audio protocol required for a system to be H.320 compliant. A Class 1 system only has to support G.711.

G.722 -- 48-64 Kbps Wideband: It provides stereo quality sound (wideband, 7khz) and is usually supported by Class 2 systems. Class 3 systems must support G.722 offering the user the best audio quality available.

G.728 -- 16 Kbps Narrowband: This is very helpful for videoconferences being held at speeds lower than 256Kbps (especially at single BRI connections of only 128Kbps). This allows more bandwidth for video. A manufacturer that is capable of supporting G.728 will have a major impact to the audio quality and the video quality at lower data rates. Also it can give the user the selection of which standard he prefers to get the best communication effect.

But if a Class 3 system communicating with a Class 1 system, G.711 will be the common standard to be used.^[29]

Data rates also directly affect the perceived quality of the communication although it is not specified in H.320. Higher data rate results in better video quality. The video quality at T-1 (1536 Kbps) will be ideal. But, because of the cost, many users are using 768 Kbps as a trade-off between high quality video and cost. 384 Kbps is probably the most common data rate in use today. With the development of ISDN, 128 Kbps is becoming more popular.^[29]

There are many factors that affect the overall quality of an H.320 system. The more optional features that are implemented the better the overall quality will be. The user should understand their environments and their required communication quality, so as to select the most suitable rate for their application. Furthermore, the quality of the other end, to which the user is communicating will also affect the overall quality of the communication. "A lesser capable system will always force the high quality system to downgrade its quality in order for the conference to be held".^[29]

There are many ITU standards implemented in H.320. For the video codec, H.261 is used. G.711, G.722 and G.728 are included for the audio codec.

Last update April 1, 2002