Subject: Re: AF_ISDN (was: Downloading code to IO processor)
To: Bill Studenmund <>
From: Martin Husemann <>
List: tech-kern
Date: 06/20/1997 09:59:12
Bill wrote:
> Take a look at what Gordon Ross did with the 8530 driver. It's broken
> up into a zsc, the Zilog Serial port Chip, and various slaves, like
> zstty, which is the tty driver, and a kbd and ms (mouse) drivers for
> the sun3.

This is fine as long as clients attach to only one master and know the
concrete attach interface defined by this master. In the ISDN case we have
different controllers (the ISDN cards and their drivers), connected to one or
more outside busses (the S0 bus in european ISDN's) and common clients that
should know nothing about the controllers and their details, just how to
attach to their common bus interface. If you would call that a master isdn
device or a bus doesn't realy matter.

> What exactly is a BRI? Well, more exactly, if you have multiple ISDN
> cards, why would having only one BRI make you want to have only one
> ISDN bus?

A BRI (basic rate interface) is "the plug in the wall", a 196kBit/s 
synchronous interface. And no, it usualy doesn't make sense to have multiple
cards in one computer attached to the same BRI, as long as the cards don't
have special mutualy exclusive features (I haven't seen any falling in this
category). I use it for testing only, and if that's hard to achive, I
wouldn't mind.

I only have one BRI at home, so pretending I have two (by attaching two
isndbusses) would not work, there are resources to be maintained on a per bus
basis (and others on a per card one). But you can always swap plugs... I.e.:
you can't allocate the B2 channel on card X and card Y concurrently, if they
are connected to the same BRI. Actually, most of the time you don't care,
just let the ISDN assign the channel to you - although some snooping for
"busy" situations is at least recommended.