Re: Speed on an ISDN link....

John Carey (
Mon, 10 Jul 1995 08:23:47


Forget about your serial port. ISDN allows a sychronous digital connection.
You can either get an ISDN I/O card which allows your computer to communicate
with your ISDN connection at buss speeds, or you can use either a router or
a bridge to make your ISDN connection using normal network protocols like
ethernet. Those connections will be at network protocol speeds. In this
case, ISDN speed (based upon the number of ISDN channels you have) will
be the limiting factor.

Your serial port has limited bandwidth capabilities in terms of speed
(typically 57600 bps as you noted) and overhead due to the nature of
asychronous communication.

John (

>I've noticed over the last few days a discussion on frame rates over
>ISDN... etc. When connecting to an ISDN line (say PacBell) do you have a
>particular card that goes into your machine or is it a "direct" connection
>to your serial (modem) port via an external adapter?

>We are still out in the woods for ISDN down here so have to use modems. On
>a Mac the serial port speed is only 57600 bp/s so unless a faster port card
>is put in... you have a built in govenor. If you share the serial ports (eg
>have the modem and the printer "running") your port speed drops to about
>19,000 b/ps. We have gotten around the degradation by using dual port
>serial cards that support/allow output to 230,000 bp/s per port. Would be
>interested to hear from the ISDN connected folks as I wonder if a "trick"
>is needed to get the best out of an ISDN connection.

>David Watson School of Health
>Lecturer University of New England
>Email: Armidale, NSW Australia 2351
> GMT +10 Welcome Mat out...
>>>>>>>>>Phone: 61-67-73-3668 Fax: 61-67-73-3666<<<<<<<<<<<<
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