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Videoconferencing at UVa

Bandwidth

The choice between using ISDN or LAN (Local Area Network, including Internet connectivity) cannot be made simply by comparing bandwidth. One reason is that a comparison of bandwidth between the two types of transmission protocols is actually difficult to make. Other factors involve quality of the perceived image and the reliability of the channel.

Graphic: 1 ISDN Line Desktop conferencing systems that utilize ISDN typically use one ISDN phone line.A standard ISDN line has two data channels that combine to provide the capability to transfer 128,000 bits of data per second.

Graphic: 3 ISDN Lines Group conferencing systems may use three combined ISDN lines to provide the capability to transfer 384,000 bits of data per second. The audio and the video signals must share the bandwidth.

Graphic: One Frame = 202,752 bits Desktop video images are standard sizes. A QCIF video frame is 176 by 144 pixels, or 25,344 pixels. Eight bits are typically required to encode a pixel. This means that a single frame of video is 202,752 bits.

Graphic: 15 frames per second = 4,041,280 bits Motion picture films are shown at 24 frames per second. The human eye perceives a jerky motion at low frame rates. A commonly accepted lower bound on video signals is 15 frames per second. 15 frames of video is 3,041,280 bits. Fortunately, the video frames can be compressed before sending and uncompressed when they are received.

Graphic: Compressed Image = 404,128 bits A compression ratio of 10:1, which will degrade the image slightly, means that one second (15 frames) of compressed video can "fit" into 304,128 bits.

Graphic: ISDN Compression Unfortunately, a single-line ISDN desktop conferencing system can only transmit 128,000 bits per second. The top "speed" that can be transmitted on ISDN works out to more like six frames per second with just over 6,000 bits left over for the audio signal.

That means that the best that single-line ISDN can deliver is still a jerky image. Sacrificing the image quality by compressing even more can yield a higher frame rate.

An ethernet LAN can transmit 10,000,000 bits per second. Two computers linked by this LAN could transmit 5,000,000 bits per second to each other. Even carving out sufficient bits for a good-quality audio signal leaves plenty of bandwidth for 20 frames per second of uncompressed video. However, it is seldom the case that there are only two stations on a LAN. It is very probable that 100 stations could be using the network, which lowers each station's "share" of the bandwidth to just 100,000 bits per second (4 frames per second of compressed video). Of course, LAN bandwidth is not divided equally among the stations. Some stations will be using more bandwidth than others at any given time, which allows some stations to achieve high bandwidths when they need it if other stations are not competing for the network. Thus a station on a LAN could exchange high quality video at some times but not others, depending on its neighbors' activities.

Framerate comparison between two, eight and one hundred stations The hardware component of some video conferencing systems may limit the bandwidth, also. The CODEC (the hardware component that compresses and decompresses the video signal) may have been designed for use with ISDN-based systems, and therefore may be limited to a 128 or 384 Kbps transfer rate, despite the availability of more bandwidth on a LAN.

Some LAN-based videoconference software stops sending a video signal if the frame rate degrades below a pre-determined level. For that reason, those video conference systems refuse to exchange video when the network is busy. During those times, the guaranteed performance of an ISDN connection is attractive, even if the image may not be.

Network technology is constantly evolving. New protocols and equipment for providing a guaranteed portion of network bandwidth are being deployed. ATM (asynchronous transfer mode) has the ability to reserve network bandwidth. However, the network connection from one desktop to another will typically not be implemented completely using ATM. Instead, the backbone of the network may be ATM, while the connections within a building are still the equivalent of a shared LAN. Similarly, RSVP (resource reservation protocol) for IP networks can provide a guaranteed amount of bandwidth, but it must be implemented along the entire network path. Connections to sites outside one's own local network are the ones most in need of a Quality of Service guarantee, but are the ones least likely to have the necessary protocols implemented along the entire connection. These new network developments are promising, but will probably not be available to the average desktop videoconference user for several years.

Page Updated: 2012-02-16