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Subject: Re: Microsoft "Get ISDN"?
To: aledm@routers.co.uk (Aled Morris)
Date: Thu, 14 Mar 96 15:07:18 MET
From: Greg Lehey <lehey.pad@sni.de>
Cc: hackers@freebsd.org (Hackers; FreeBSD), isdn@muc.ditec.de
In-Reply-To: <199603141251.MAA09029@deputy.pavilion.co.uk>; from "Aled Morris" at Mar 14, 96 11:43 am
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> At 09:06 14/03/96 MET, Greg Lehey wrote:
>> (S0 is a German term for BRI)
>
> Being pedantic, S0 is the name of the "reference point" where you
> interface to the ISDN.

I *love* being pedantic :-)  In this particular case, all the more so
because I searched the literature for this abbreviation a while back.

To quote from Helgert, "Integrated Systems Digital Networks", first
(only?) edition, 1991, p. 25:

     The S reference point defines the demarcation between the TE1 or
     TA on the one side and the NT2 on the other.  If we consider the
     functions of NT2 to be primarily  ntwok related, S provides a
     separation of the logical and possibly physical functions of the
     end-user and the network and thus corresponds to a boundary
     between the two.

No mention here or in any other literature I have seen of S0 or S2m
(which Deutsche Telekom uses to refer to Primary Rate Interface).

> ISDN is delivered on a copper pair, ("U" interface) which is adapted
> to a two pair "S" (or "S0") interface by an "NT1".  In the US, the customer
> is responsible for the NT1 (hence many ISDN devices boast "integrated
> NT1") whereas in Europe, the telco provides the NT1.

BRI is usually delivered on UTP within a larger (minimum 2UTP) cable.
Helgert reprints a diagram from the 1988 Blue Book on page 23, showing
the interfaces U (transmission), T and S.  U and T are separated by an
NT1, and T and S are separated by an NT2.  On the S bus you can hang
either a TE1 (such as an ISDN BRI board or "modem"), or a TA, which
creates an R interface to a TE2 (such as an analogue phone).

> Same with DSU/CSUs for regular leased lines I believe.

I wouldn't know about that.  I would guess that that depends on
whether they're analogue or digital.

Greg