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Date: Wed, 20 Mar 1996 21:58:30 +1000 (EST)
From: Julian Jenkins <kaveman@magna.com.au>
To: "Jordan K. Hubbard" <jkh@time.cdrom.com>
cc: Terry Lambert <terry@lambert.org>, lehey.pad@sni.de, hackers@FreeBSD.org,
        isdn@muc.ditec.de
Subject: Re: ISDN: "modem" or board? (Was: Microsoft "Get ISDN"?) 
In-Reply-To: <4383.827309214@time.cdrom.com>
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On Wed, 20 Mar 1996, Jordan K. Hubbard wrote:

> > > > 1.  Speed of a connection.  Some people say "the bottleneck is the B
> > > >     channel, so you can use async instead".  Well, yes, assuming your
> > > >     machine isn't doing anything else.  To run 2 B channels flat out,
> > > >     you need a 230 kb/s line, which with standard el cheapo 16550As
> > > 
> > > You, uh, would?  64+64 = 128Kb/s using my own calculator! :-)
> > 
> > Hee hee.  Think "allowable baud rates for serial ports".  8-).
> 
> Oh, I know the limitations of that, I was just trying to figure out
> how Greg was managing to take 64+64 and get "230K" from it.  I'm well
> aware that the next step from a 115.2K UART is 230.4Kbaud since they
> generally just double the previous value, but Greg's comments seemed
> to indicate that he was also ADDING the overhead rather than
> subtracting it and coming up with a wholly new bandwidth category for
> ISDN, which would be a neat trick and worth some money were it that
> easy.. :-)

The key point is that the ISDN line is syncronous, so a 64kb/s line can 
carry 64kb/s of data, while the async line from a 16550 or whatever need 
extra start/stop bits. Assuming we can get full capacity from the serial 
line and we hav only one of each start and stop bits, this means that 
each byte transmitted along the async line consists of 10 bits (8 bits 
data, 1 start, 1 stop) so the capacity of the async line that can carry 
the same amount of data as a 64kb/s ISDN line is 64/8*10 = 80kb/s.

Kaveman
kaveman@magna.com.au