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This is not a very exciting subject, but it's one which needs considering when videoconferencing links are being established between different systems and/or across national boundaries.

ISDN standards

Global Connections uses basic rate ISDN (Integrated Services Digital Network) for its videoconferencing. ISDN is a sophisticated transmission protocol, which can be implemented in a number of different ways by different telecommunications companies.

In the UK (and Europe in general) basic ISDN lines work at 64K bits/sec, and the 2 lines in a basic set-up can be used together to give a total throughput of 128K bits/sec.

In the US, basic ISDN lines work at 56K bits/sec, with two channels giving a throughput of 112K bits/sec.

(The reason for this, loosely speaking, is that in the US the call signalling information uses up 8K (64K-56K) of the line's capacity, whereas in the UK the signalling information travels separately.)

What standards to use?

Whilst most equipment can cope with these differences, it can be a source of problems, and it's certainly something that needs testing.

Videoconferencing standards

There is, unsurprisingly, a range of videoconferencing standards, so ensuring compatibility is not as straightforward as it might at first appear.

The main standard of relevance to ISDN-based systems is called H.320. H.320 - compatible systems should be able to link to each other, allowing you (at least) to 'talk and wave' - that is, users can speak to, hear from, and see each other.

The other important standard is called T.120. T.120 compatible systems should be able to share applications (and therefore documents and other files) with each other.

In theory, any two systems should be able to communicate fully and successfully, provided they comply with these standards.

In practice, it isn't quite that simple. There are various levels of H.320 compliance, which are seldom made clear in manufacturers' literature. (H.320 is actually a collection of standards, one of which is T.120.) So, whilst links may work, two different systems will operate at their 'lowest common denominator'. The best way to assess the quality of a link is to try it out!

Videoconferencing over the Internet (that is, videoconferencing over TCP/IP networks) is governed by the H.323 standard. (Like H.320, this is also a collection of video, audio and other standards.)

The (free) Microsoft Netmeeting program supports T.120 over Internet and point-to-point connections, and is a de facto standard for document sharing. However, it is not always possible to use Netmeeting on point to point videoconference links between systems of different manufacture. (H.323 and H.320 systems do not necessarily work together: it depends on how they have been implemented.) Again, the moral is 'try it and see'!

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