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Date:      Mon, 24 May 1999 19:01:06 +0200 (CEST)
From:      Bart van Leeuwen <bart@ixori.demon.nl>
To:        Tom <tom@uniserve.com>
Cc:        Dag-Erling Smorgrav <des@flood.ping.uio.no>, isdn@freebsd.org, blitz@pdq.net, stable@freebsd.org
Subject:   Re: ISDN Terminal adapter no work
Message-ID:  <Pine.BSF.4.05.9905241851250.1785-100000@ixori.demon.nl>
In-Reply-To: <Pine.BSF.4.02A.9905240935480.29433-100000@shell.uniserve.ca>

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On Mon, 24 May 1999, Tom wrote:

> On 24 May 1999, Dag-Erling Smorgrav wrote:
> 
> > "Craig Harding" <crh@outpost.co.nz> writes:
> > > Most TA's act just like very fast modems.
> > 
> > "very" is perhaps not the right word, since an single ISDN channel is
> > less than twice as fast as a 33k6 modem.
> 
>   Nope.
> 
>   First of all, most everyone gets 2B+D ISDN (2 x 64K) anyhow, in fact
> 1B+D (64k) isn't available in most areas of the world.
> 
>   Second, ISDN is sync, but modems are async.  Now some people use various
> async emulation protocols over ISDN (like V.120), but that is a complete
> waste, especially if all you are doing is PPP.
> 
>   Here is what is looks like:
> 
> 64k ISDN channel:  64000 / 8 = 8000 bytes/sec
> 2 x 33.6 modem:    67200 / 10 = 6720 bytes/sec
>

Uusually async modems also support things like mnp4 and 5, v42,
v42bis etc. one of the results of this is that the actual transport of
data across a pstn or leased line with 2 async modems looks pretty much
the same, and also achieves pretty much the same throughput as using 2
sync modems on that same line. Your comparison is only true for a plain
async connection, but thats not what 2 async modems do. (and, when testing
this on a network that has way more throughput then your async connection,
you will plainly see this by achieving approx 110% of the theoretical
async throughput, given you use things like a as high as possible dte-dce
connection and hardware handshaking)
 
>   So a 64k ISDN channel is fair amount faster (about 20%) than two 33.6k
> modems.
>

So... this is only true for a async connection employing 2 plain async
modems without any error correction and such, which are almost
extinct nowadays.
 
Still there are a lot of advantages when using isdn, and esp. when
compared to 2 33k6 connections the fact that its a single connection that
can be setup quickly and requires very little overhead to be managed is
where your potential performance gains are.

Bart




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