Date: Sat, 2 Jan 1999 18:44:14 +0100 (CET) From: hm@kts.org (Hellmuth Michaelis) To: dave@turbocat.de (David Wetzel) Cc: freebsd-isdn@FreeBSD.ORG Subject: Re: Suggestion (Was Re: Rates calculations and the U.K. ) Message-ID: <m0zwV5y-00002iC@bert.kts.org> In-Reply-To: <199901021524.QAA00394@cat.turbocat.de> from David Wetzel at "Jan 2, 1999 4:24:25 pm"
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David Wetzel wrote:
> > This is impossible. You need a second card (or a chipset being able to
> > "listen" on the tx wires) to be able to do this for everything on a given
> > bus.
>
> Someone told me that his ISDN card (which is in a linux box) can do this.
>
> Would you please explain what is possible and what not?
Think about the following scenario: You have an S0 bus, an NT and two devices
on the S0 bus, i.e. one ISDN telephone (TE1) and an machine running i4b (TE2):
|| || +---------
+------->--| R
|+------>--| X
|| || | TE1 - ISDN telephone
|| +----<--| T
|| |+---<--| X
|| || +---------
|| ||
|| || +---------
+------->--| R
|+------>--| X
|| || | TE2 - i4b machine
|| +----<--| T
|| |+---<--| X
|| || +---------
^^ vv
|| ||
+------------+
| TX RX |
| |
| NT |
The NT's TX lines is connected to every TE's RX lines, so every TE can
"hear" what the NT "says": This way anybody on the S0 bus "hears" who is
calling.
The NT's RX lines are connected to every TE's TX lines so the NT is the
_only_ (!) device, which can "hear" all the TE's TX lines: this means TE2
cannot "hear" whom TE1 is calling and vice versa.
In case you want to "hear" everything on the S0 bus, you have to put a
second card into the i4b machine and connect that cards RX lines to the
NT's RX lines, that way you are able to "hear" what every other device
sends to the NT. This is supported in i4b in the isdntrace program by means
of the so called "analyze mode" which i use heavily in the development of
i4b: put 2 cheap card into a 386, solder a cable (documented in cable.txt)
install FreeBSD & i4b and you have an ISDN bus analyzer almost for free
(in case you recognize what commercial vendors will charge you for such a
device).
Now, there is a mechanism specified to prevent contention of the S0 bus which
involves some "echo" bits (explaining this here goes much too far, so in
case you want to understand that, study the respective standards text) with
which you might be able to read out the NT's RX lines on the TE side.
Wether a manufacturer of a given chipset allows a programmer to access those
echo bits depends on the chipset being used: the Siemens chipsets i4b
currently supports do not have a mechanism built in to allow access to the
echo bits.
Thats it.
hellmuth
--
Hellmuth Michaelis hm@kts.org Hamburg, Europe
We all live in a yellow subroutine, yellow subroutine, yellow subroutine ...
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