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2. Technological Issues
The technologies used to deliver video conferences currently have a dramatic effect on the quality of the communication achievable. This report concentrates on the use of ISDN with compressed video where as earlier examples, often referred to as Interactive Television use transportation media such as microwave links or satellite links and provide high cost, full motion video links. In order to understand the issues in video conferencing, a basic understanding of transmission technology is required. The issues are those of:
Bandwidth (or baud rate) refers to the amount of information (bits) which can be transmitted along a carrier every second. The bandwidth required depends on the application. Thus textual data can be transmitted slowly, or using a narrow bandwidth because it is not required in real time and the printed information contains little information relative to sound and video. Sounds such as speech, contain more information than the printed word and have to be carried at the same speed as normal speech for conversation to be possible; thus a wider bandwidth is required. If moving pictures have also to be sent and real time transmission is required then we have a lot of information to send very quickly, thus a high bandwidth is required.
- video compression
- delivery method
Consider the amount of information that must be transmitted if a video image is to be sent in real time.
A screen has 625 lines and there are 625 points on each line.
A point is stored in 24 bits.
To transmit in real time, 25 pictures must be sent every second.
This adds up to 234,375,000 bits of information to be transmitted every second.
Telephone lines are required to carry voice information at roughly 2400 bps which they manage. A single ISDN telephone line can carry information at 64 Kbps. So consider how many telephone lines would be required to carry the amount of information for a video image! It would not be possible. Thus alternative to telephone lines, such as satellite or fibre optics which have extremely high bandwidth would have to be used. This would reduce accessibility of the technology and increase costs. An alternative is to compress the video image thus less information is transmitted along a lower bandwidth medium. We will look at both solutions.
An analogue, full motion, signal takes up a great deal of bandwidth. It can be converted to digital signals (and vice versa ) and compressed using codecs. One way to reduce the bandwidth required is to compress the video image; that is digitise the signal and then remove as much extraneous data as possible. A video signal changes 25 times per second, not all the picture changes in each frame and so with a compressed image only the changes are sent. Thus the more changes, the less compression can occur. Compressed video can be squeezed into as few as two telephone lines, or up to 24. The greater the compression, the greater the loss of clarity, continuity of motion and colour information.
There are already several levels of compression being adopted:
- Video on desktop computers
- 64kps. Allows video intergrated into the screen. However this rate is not good enough for full video conferences, it would suffice for one to one video phone situations. Many people in education do not feel it is adequate.
- Group video conferencing
- Between128kps and 2Mbps. 384 kbps is providing a good quality reception for conferencing and is used in many educational environments.
- Digital Broadcasting
- Uses 2- 6 Mbps rates. The quality is greatly increased over the previous compression levels but costs are also higher
- 25-45 Mbps is adopted by High Definition Television. This is a relatively new technology and it is not universally accepted.
Some vendors provide equipment that can operate at a variety of compression levels. Remember the more visuals and movements to be transmitted the greater the transmission requirements and hence the higher the cost of both transmission and site equipment. There are recommended ways of coping with video compression (see Appendix One).
By standard telephone lines
The advantage of this method is one of accessibility. However, this only allows 64 kbps transmission rates. This is relatively untested for education. The picture and sound quality will be poor and the picture jerky.
ISDN can in theory be carried through any telecommunications delivery medium: fibre optics, or telephone. This method is rapidly being taken up by educational establishments. There are several advantages for the education sector:
- Two way voice, data and graphics can be carried simultaneously over the telephone network.
- May become the standard telephone system and therefore reach all homes, offices and educational establishments.
- It is relatively inexpensive.
By satellite broadcast
- A common use of satellites is for one way television transmission with audio only feedback.
- The advantage is that its cost is independent of distance, where as cable transmission costs increase with distance. This method is used where very large distances and many sites are involved or there are natural barriers to the laying of any cable technology, such as mountain ranges or oceans.
- The disadvantages include lack of visuals from receive site, unavailability of transmission time, or particular time may be expensive and the cost of the equipment installation.
By VSAT (very small aperture terminal)
This method uses narrow band transmission (256-384 kbps). They can be used as receive only, or for data transmission to a central point, with a more powerful “hub”.
For education, it is possible to use VSAT in a “mesh” system with each site capable of both transmit and receive, to any other site on the system, thus allowing any site to originate teaching materials. Full two way motion video with sound is possible.
Transmission costs are likely to be low but ground stations are in the range of 50,000 dollars.
By co-axial cable
Cable is a useful medium if delivering from a single point (TV Station) to many different sites. It is a common form of television broadcasting in North America and becoming more common in the UK.
Co-axial cable can allow up to 40 TV channels to be carried without compression. In Canada, at least one of these must be dedicated to educational programming ( The Knowledge Network).
It can also be used for two way video communication provided there is a suitable equipment at both ends.
The cost of laying cable is high so would only be of benefit for educational video conferencing if the cable was already laid. This is the case in the USA, Canada and some parts of the UK.
By fibre-optic cable
This has a far greater capacity than co-axial cable.
It is currently being installed by major telecommunication companies. It will form a backbone for services that will often transfer to copper wire for local delivery.
There is therefore a growing range of ways to deliver video conferencing. The most appropriate choice of system will depend partly on the physical configuration of sites to be connected, the applications which are required, the amount of traffic to be carried, and the distances between sites.
- There are two video conferencing standards: H261 and H320.
- H261 standard refers to the visual and audio part of the conference. This standard has been in existence for a number of years and any system complying with it will at least be able to see and hear people using another compliant system.
- Only H320 affects Desktop Video Conferencing. H320 is supposed to link together different software standards by setting factors such as voice and picture syncing. This is not well implemented. The different interpretation of the standard means that if using a desktop system, to guarantee success, the equipment and software at both the send and receive site must be identical. This rule holds even if both systems say they comply to H320.
- Every country does not have International ISDN, different countries have their own form. This is particularly apparent between the USA (56 kbps) and Europe (64 kbps) although most equipment can now deal with this difference.
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