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From: rpw3@rigden.wpd.sgi.com (Rob Warnock)
Newsgroups: comp.dcom.modems
Subject: Re: Measuring Line Loss
Message-ID: <64297@sgi.sgi.com>
Date: 17 Jul 90 05:16:53 GMT
References: <1532@mitisft.Convergent.COM>
Sender: rpw3@rigden.wpd.sgi.com
Reply-To: rpw3@sgi.com (Rob Warnock)
Distribution: usa
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 29

In article <1532@mitisft.Convergent.COM> dold@mitisft.Convergent.COM
(Clarence Dold) writes:
+---------------
| The following chart gives DBM equivalences, as measured on a 600 ohm line...
| 	dBm	RMS mVolts	mWatts
| 	0	775		1000
+---------------
                                1.000   [missing decimal point in original]

But notice that most phone lines are closer to *900* ohms, not 600. The 600
ohm reference is for use inside the CO plant, and most lines have (or at
least, used to have) 900:600 transformers on them. Most "DAA" (Part 68)
modules also convert from the 900 ohm line impedance to 600 ohms internally
to the modem, so if you measure the A.C. voltage on the inside of that module
you'll get the numbers indicated. If you just stick a meter across the line,
you'll see more like:

 	dBm	RMS mVolts	mWatts
 	0	949		1.000
	etc.


-Rob

-----
Rob Warnock, MS-9U/510		rpw3@sgi.com		rpw3@pei.com
Silicon Graphics, Inc.		(415)335-1673		Protocol Engines, Inc.
2011 N. Shoreline Blvd.
Mountain View, CA  94039-7311