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Chapter 1

Carriers

An important aspect of running desktop videoconferencing software is that of the medium over which which the basic communication is to take place. Where videoconferencing is to be used on a site such as a university campus (local area), the two main possibilities at present are to use a Local Area Network (LAN) or to use ISDN which is an emerging technology based on telephone connections which is becoming more widespread.

Full screen broadcast quality video in its raw state (at 25 frames a second) requires transmission speeds in the 100-200 Megabit/second range. This is far beyond the capacity of current LANs (up to 10 Mbit/sec) or basic ISDN connections (up to 128 Kbit/sec). The only way to reduce the bandwidth requirements to reasonable proportions is to use a codec (coder/decoder) to compress the video stream in some way before transmission and to decompress at the receiving end. Compression ratios of well over 100:1 can be achieved by a combination of image compression and frame-change coding but this still leaves bandwidth requirements at quite a high level. Typical codecs tend to introduce artifacts (features that are not in the original picture) and perform less well when there is a lot of movement between frames. A description of some relevant compression methods is given in Appendix 2.

Further reduction in bandwidth requirements, beyond that gained by compression, requires reduction in the size of the video picture and/or the frame rate. Two standard formats that are widely used are: FCIF (Full Common Intermediate Format) which uses a 352 x 288 pixel rectangle at 15 frames per second; and QCIF (Quarter CIF) which uses a 176 x 144 pixel rectangle. It can be seen therefore that at current communications speeds the quality of transmitted video is going to be relatively poor. This is illustrated vividly in Appendix 5 where we have included a table of videoconferencing bandwidth requirements for various parameter values.

LANs

LAN technology uses a broadcast network. Its main advantages over ISDN are that it is potentially fast (up to 10 Mbit/sec), and it is usually already available in many organisations and so no cost is required either in terms of manpower or money in order to provide the basic carrier. It also has the advantage of being able to support inexpensive multiway conferencing (see below). However, because it is a broadcast network technology, it cannot guarantee constant speed of communication, and indeed when a LAN becomes heavily loaded with other traffic, videoconferencing can become unusable. As will be seen later, though, current LANs can be used with some effectiveness under favourable conditions.

ISDN

ISDN comes in two main forms called Basic Rate and Primary Rate. Basic Rate provides two 64 Kbit/sec channels for data and a control channel. Primary Rate provides thirty 64 Kbit/sec channels for data and a control channel. Present desktop systems are geared (because of their price range) to Basic Rate ISDN and so we restrict our comments to this in the present report.

The overriding advantage of using ISDN over a LAN is that it provides a constant speed of communication which is dedicated to the conference participants. Typically, one 64 Kbit/sec channel would be used for video and the other for audio communications. The main disadvantage of ISDN, apart from its lower speed, is that, usually, current telephone switches and possibly wiring cannot be used and special connections have to be installed by a telecommunications supplier (e.g. BT, Mercury). These connections are relatively expensive (at the time of writing, from BT, they are 400 installation and 84/quarter rental plus the cost of the calls).

The lower 64 Kbit/sec speed of ISDN relative to a LAN means that video transmission frame rates tend to be around 1-5 frames/second giving very poor motion reproduction. On the positive side is the fact that the dedicated sound channel gives excellent voice communication.

A Basic ISDN connection only supports one-to-one conferencing with another similar connection. Multi-way conferencing requires connection to extra equipment (Multipoint Control Units) which is likely to be expensive (we have not been able to ascertain how expensive at the time of writing).

Ideally it would be possible for an institution to use its existing internal telephone system (PBX) but, in our case, enquiries indicate that our internal system is not capable of supporting ISDN and an expensive upgrade to our exchange would be required costing tens of thousands of pounds. It seems at present that the only option for us (and probably other similar sites), if ISDN is required, is to install single connections via BT.

[Note: The MICE (Multimedia Integrated Conferencing for European Researchers) project has been developing facilities to interface ISDN users to the Internet. Readers are recommended to consult the MICE WWW server for further information. Dan Kegets' ISDN page contains an abundance of ISDN information and includes sections on videoconferencing and ISDN shareware. "The joys of obtaining ISDN in the UK" is worth exploring. This site is connected to the Internet via ISDN.]

For the present study we used our local Ethernet TCP/IP based network which the Showme software had been designed to run on. Part of the study was directed at seeing how the software would run in this environment.


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