telecom@ucbvax.ARPA (12/25/84)
From: Jon Solomon (the Moderator) <Telecom-Request@BBNCCA>
TELECOM Digest Mon, 24 Dec 84 17:45:26 EST Volume 4 : Issue 138
Today's Topics:
Getting {'s to death?
Maintaining the network and invading the house
----------------------------------------------------------------------
Date: Friday, 21 Dec 1984 05:57:25-PST
From: goldstein%donjon.DEC@decwrl.ARPA (Fred R. Goldstein)
To: telecom@bbncca.ARPA
Subject: Getting {'s to death?
The V.22 (212) type modems use a "scrambling" scheme to keep the bits
density up to par, and the chip set decodes the bit stream as it comes
in. If for some reason (like noise on the line) it can't decode a
character correctly, then it puts out a { character. It's not really
being received; instead, it's your modem's way of telling you it missed
something (or missed a "nothing").
If your CO is digital, one possible explanation of why it has a regular
error every 80 seconds is if the T-carrier span lines connecting it to
the CO where the computer is, or the toll switch, aren't in sync. Then
you may see a "slip", as the 1.544000 Mbps clocks drop a bit here and
there. This will manifest itself as an inaudible phase shift in the
received signal. V.22 modems are phase modulated (4psk), and generally
work by timing the peaks of the waveforms. So if you lose 1/8000 of a
second, the 2400 Hz modem tone will be phase shifted, changing the
meaning of the bit and munging the descrambler.
Although all digital transmission systems in the public network try
to maintain sync to the master clock in Kansas City, most long-haul
toll circuits are still analog, so you have "digital islands". With
neither end of the circuit officially in sync, somebody at telco may
have slipped up and let both ends be free-running (instead of master-
slave). That's my guess.
At 300 bps you shouldn't have any problems with this, not that it's much
consolation.
------------------------------
Date: Sun, 23 Dec 84 23:35 EST
From: Frankston.SoftArts@MIT-MULTICS.ARPA
Subject: Maintaining the network and invading the house
To: telecom@MIT-MC.ARPA
The planned converted of Newton Mass to a DMS-100 switch
apparently did occur as scheduled last weekend. One side
effect was to misprogram my residental hunt group so that all
incoming calls came into the same number. I reported it to
service. The next day I got a call at the office that the
problem was resolved. Fine.
But than a while later a get a message from home that there is
a telco person on my premises trying to fix a problem. My
first reaction was to tell him to get out. Hist story is that
he was sent since they were checking out my repair order and
couldn't clear it until they dealt with an "80% short" they
found on my line.
There are a number of issues here:
1. People first reaction is to allow telco people in the house
instead of requiring positive confirmation from me that it is
at my request. This is a long standing problem for those of us
with foreign equipment such as computers on the network. I've
trained my wife but people still don't understand that I must
be present before people muck with the circuits in my house.
2. I presume the "80% short" is found by diagnostic support in
the new exchange? What is an 80% short?
3. The real issue here is that if there truly is this problem,
then they should phone me, or better yet, send me a letter,
saying that they have found this problem and advise that I
resolve it. If it doesn't threaten the network, then it is my
problem. If it threatens the network, then they should install
an interface to isolate my house from the network.
I am not objecting to their willingness to check out my house
to resolve the problem, even if they do charge me. What I am
concerned about is that telco (Nynex in this case) doesn't seem
to be prepared for the new world in which I am responsible for
maintaining my premise system. They should simply inform me of
the problem. If I elect, they can come and fix it.
Since the problem, I have not heard anything from them about
the problem.
------------------------------
End of TELECOM Digest
******************************