On Sun, 1 Nov 1998 17:11:03 +0000, David Meldrum wrote:
On the subject of 'always on' ISDN:
>Can you let us know *exactly* how that works please.
This is what is called 'Always On Dynamic ISDN' (AO/DI).
A typical ISDN line has 3 channels, 2 Bs and 1 D. The B channels are
64 kbps each and the D channel is much smaller in bandwidth (9600 bps).
The B channels have dedicated physical bandwidth along the
'circuit-switched' network. That means data is transmitted in one long
stream along that channel of bandwidth that is yours and yours alone.
When you finish the call, that bandwidth is released to other users and
so on.
This is called 'bandwidth on demand'
The D channel is normally used to intiate the call. However, it can be
used to handle small amount of data like email. It can do this because
it handles data differently. It sends data in 'packets' that can find
their own way along the 'packet-switched' network. In other words, the
packets make their own way along any available stream and then get
re-assembled the other end.
Any time you make a 'circuit-switched' call, say, to the Web, you use
one or both B channels. If the TA (and the ISP) support 'AO/DI', once
that call is finished, the D channel kicks in and maintains the
connection to download email and 'push' updates etc. Small amounts of
data that do not require the 64 kbps bandwidth.
>Thanks for your consistently sensible advice + one-liners :-)
My... (one line)
Pleasure... (two lines)
Risk
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Ahmad Risk MB BCh
The Good Doctor Ltd
http://mednetics.org
Tel: +44 1273 724866
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