From: Niall O Broin (niall at domain linux.ie)
Date: Tue 16 Jul 2002 - 15:52:04 IST
On Tue, Jul 16, 2002 at 03:05:07PM +0100, Matthew French wrote:
> Presumably there are people who want ADSL for Internet access, but still
> need ISDN to dial up to their office or other remote locations that do not
> have virtual access.
>
> There may even be people out there who use ISDN as a substitute for two
> separate phone lines. *gasp*
Yes, and who don't want to pay Eircom to downgrade their ISDN line to POTS.
> > or do you suddenly get far higher bitrate from ADSL over ISDN?
>
> Lower. The problem is that the frequency ISDN uses mix with the lower
> frequencies of standard ADSL. ADSL over ISDN essentially removes these lower
> frequencies, thus removing a corresponding chunk of available bandwidth.
>
> Although I think one can still get 512kbps over such a line?
ISDN uses frequencies between voice and 1MHz so ADSL/ISDN uses the
frequencies between 1MHz and 16MHz (AFAIR that's the upper bound). ADSL/POTS
uses the frequencies between voice and 16MHz. ADSL breaks the frequencies
available to it into a large number of channels and uses those which it
finds useable (there's a certain amount :-) of signal degradation at those
frequenceis on telephone cable) to provide the amount of bandwidth the
customer has purchased. So, in terms of what's offered to the customer,
there's no practical difference ADSL wise between ADSL/ISDN and ADSL/POTS.
Niall
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