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From: rab@well.UUCP (Bob Bickford)
Newsgroups: comp.dcom.modems
Subject: Re: What number am I calling from?
Message-ID: <2263@well.UUCP>
Date: Sat, 27-Dec-86 06:01:22 EST
Article-I.D.: well.2263
Posted: Sat Dec 27 06:01:22 1986
Date-Received: Sat, 27-Dec-86 08:37:26 EST
References: <750@sdcc12.ucsd.EDU> <1706@sunybcs.UUCP> <722@cooper.UUCP> <1498@kitty.UUCP>
Distribution: na
Organization: Whole Earth Lectronic Link, Sausalito, CA
Lines: 77
Summary: 900 ohms , ANAC

In a previous article Larry Lippman writes:
>   	Dialing a three-digit access code which immediately provides an
> 	announcement over the telephone circuit.  This unrestricted
> 	implementation has generally been considered a "security" problem
> 	(use your imagination) by telephone company management; I am
> 	somewhat surprised to see people posting articles reporting the
> 	implementation of unrestricted ANAC.

   Why?  I'm afraid my imagination does not present me with any
compelling reason to belive this a 'security risk'.  Certainly it
is of little or no use to a telco cracker.

 [context deleted]
> 	You are referring to milliwatt test lines which provide a 1,000 Hz
> signal at 0 dBm (1 mw) into a 900 ohm termination.  Some milliwatt test
> lines provide a continuous tone; others have 9 seconds of tone followed by
> 1 second of silence (with or without answer supervision).

   Ours are 8 seconds and 2 seconds; the tone by the way is 1004 Hz
and NOT 1000 Hz.

> 	A word of caution concerning the use of milliwatt test lines: if
> you don't know what you are doing and don't have the proper test equipment,
> you will be fooling yourself with incorrect measurements.

   Quite true.  We wasted several weeks working on the audio portion of
our video conferencing system when I was at Vitalink Communications.
Finally we purchased a Halcyon tester (don't recall model #) and
stopped listening to the linemen who kept insisting that the line was
a 600 ohm line.

 [some text deleted]
> 	Note that I said the termination impedance is 900 ohms and NOT
> 600 ohms.  End office termination impedances (i.e., like your telephone)
> are ALWAYS 900 ohms.  Toll offices and intertoll trunks are 600 ohms.
> Most dedicated data lines and 4-wire private lines are also 600 ohms.
> There is a slight but definite error if 600 ohms is used for subscriber
> loop transmission measurement.

   Only about a dB (or less), but enough to give you the screaming fits if
you know what the measurement "is supposed to be".

> >    There are also numbers that give you a terminated line (good
> > for balancing bridges) and a sweep tone.
> 
> 	Almost all CO's provide three "quiet lines" for noise measurement
> purposes and repeater test purposes:  (1) a balanced termination; (2) an
> open-circuit termination; and (3) a short-circuit termination.  The latter
> two lines are primarily used to test negative impedance repeaters for
> noise and "singing".
> 	It is useless to attempt any noise measurement unless (1) you are
> certain what type of termination you have dialed; (2) you have a proper
> noise measurement test set (which reads in dBRN, among other things); and
> (3) you know what you are doing.

   That's strange, we seem to only have one variety of quiet line around
here, and that's the terminated one.  (Compare measurements on it with
measurements made on a call to a known quiet set, such as another of
our conferencing systems in 'mute' mode, and it's almost identical,
certainly not 6dB off.)

> 	The test line that gives a "sweep tone" is called a "loop checker"
> test line.  The amplitude of the tone is NOT constant over the swept
> frequency range.

    I did not say it was, it is merely interesting.

> Larry Lippman @ Recognition Research Corp., Clarence, New York


-- 
Robert Bickford    {hplabs, ucbvax, lll-lcc, ptsfa, msudoc}!well!rab
terrorist cryptography DES drugs cipher secret decode NSA CIA NRO IRS
coke crack pot LSD russian missile atom nuclear assassinate libyan RSA
 The above is food for the NSA line eater.  Add it to your .signature and
 you too can help overflow the NSA's ability to scan all traffic going in or
 out of the USA looking for "significant" words.  (This is not a joke, sadly.)