TAC "noise"

Frank J. Wancho (WANCHO@SIMTEL20.ARPA)
Tue, 4 Aug 1987 14:28 MDT

     This one has got me stumped, and I'd appreciate it if this
message was redirected to a more appropriate forum if there is one.
     We have noticed this "noise" problem on and off for quite some
time and are now realizing that perhaps what we and our users are
seeing is not line noise but something else.
     In general, when a user calls to complain about seeing "noise"
from a TAC connection, we naturally assume the problem is caused by
line noise in the phone circuit between the user and the TAC port
modem. However, it has turned out that we have not been asking the
right question in determining the noise characteristic. The question
is: are you seeing "noise" when there is supposed to be no activity on
the line, or are you seeing it as garbage only when the remote host is
sending a long string of characters. If the answer is yes to the
former, the problem is probably a "bad" circuit. However, if the
answer is yes to the latter, then we have a whole new problem, or so
it seems.
     I've seen the latter problem only at 2400bps. Others have
reported the problem at 1200, and some even at 300. With the rumored
eventual upgrade to 9600bps dialup TAC access, I am concerned that
there may be an underlying basic TAC-to-modem interface problem that
needs to be found and fixed before higher speeds become commonly
available and useable.
     The characteristic of this form of "noise" is that the first
several lines of expected output appear OK, then garbage (noise).
This is consistently repeatable. We have empirically ruled out
several possible causes, including real line noise, and noise caused
by level settings in the modems themselves. It seems one possibility
remains: that there is a "slight" speed mismatch between what the TAC
outputs and what the modem expects and bits eventually get shifted...
     There is one further complication that makes things harder to
track: it appears that the TAC software *may* have been undergoing
"minor" changes during this time without the version number changing
in the TAC herald. Thus, it is somewhat difficult to pinpoint when
this problem started to occur relative to TAC version.
     I suggest that followups to this message be made privately amongst
interested parties unless there is some general interest in this
subject on this list. The only reason for bringing this up here is
that I have several high level users wanting answers *now* and I can't
evade the queries for much longer...


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