Path: utzoo!attcan!uunet!pilchuck!ssc!tad
From: tad@ssc.UUCP (Tad Cook)
Newsgroups: comp.dcom.modems
Subject: Re: Help: My telco has mid and high frequency loss
Summary: frequency response, twisted pair
Message-ID: <764@ssc.UUCP>
Date: 11 Jan 91 20:28:49 GMT
References: <88953@lll-winken.LLNL.GOV>
Organization: very little
Lines: 78

In article <88953@lll-winken.LLNL.GOV>, casey@gauss.llnl.gov (Casey Leedom) writes:

(Various stuff about modem problems deleted)
> 
>   I asked PacBell to come out and test the frequency response of the line
> outside and inside the house to determine whether it was ``their
> problem'' or my house wiring, but they don't seem to be able to do that.

Doing a 1 KHz test is pretty standard.  Doing a sweep is a little more
exotic.  Pac Bell in your area MAY be installing some Proctor 51200
Dialup Test Sets in the CO that will allow for frequency response testing,
but I don't think they have done it yet.

>   The service technician who came out also mentioned that my house is
> just about as far from the central office as I could get without being
> assigned to a different CO.

This could be a problem.  I live about 5 blocks from my CO, a modern
5ESS, and the lines are great.  If I lived several miles away, I could
still have what the telco judges as adequate voice-grade service,
but I might have problems with fast modems like yours.
> 
>     1.	I welcome any comments about frequency response testing and
> 	getting PacBell to fix their wiring if it's the problem.
> 	And just what are the nominal levels of service that they
> 	do promise to provide?

They are obligated to provide VOICE GRADE service.  They will probably
tell you that you need to pay Big Buck$ for a data quality line, which
is really just a voice line with all the problems engineered out of it.
> 
>     2.	I welcome any comments about the potential problems that very
> 	old, untwisted aluminum wiring might generate and in particular,
> 	does anyone think it could be responsible for my frequency
> 	response loss?

I would replace the wiring from the demarc (Pac Bell's protector) to
your modem with it's own 22 guage twisted pair.

Although if the old wiring is in good shape it is probably not
causing any problem, it would be best to do this so that you
know that your wiring is good.

After that, it might be interesting to do your OWN frequency
response tests between 300-3000 Hz.  Get a freq counter, a
signal generator that will match a 600-900 ohm load, and use
a quality DVM in the AC volts mode (preferably with true RMS
reading) to measure the end-to-end loss at various fixed
freqs across the spectrum (say every half-octave...300, 424,
600, 850, 1200, 1700, 2400 and 3400 Hz).  Do this from
one of your lines to the line that you are using for the
modem (I think you said you had more than one phone line
to your house?)  Then run a comparison at another location
where there are two convenient lines for a loop-around test
like this, where you think the service is "good."

You will need to dial from one line to the other, then terminate
each side with a load that will hold the call, but not put noise
on the line.  An easy alternative would be to use old fashioned
phones and unscrew the carbon mic in the handset.


Another test...take your DVM, in the milliamps scale, and
with the line idle, measure the current from tip to ring
(across the line) that the meter draws from the line.
If it is anything under about 25 milliamps, you are on
a LONG loop from the C.O.



Tad Cook
Seattle, WA
Packet: KT7H @ N7HFZ.WA.USA.NA
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