Jim Hickstein <jim.hickstein@icdwest.teradyne.com> (06/06/91)
One of my users tried to dial in from home recently, only to discover
that the modem spit out garbage continually. He thought it looked
like a bad case of a noisy line, but voice calls sounds "crystal
clear." I noted that his use of "crystal" indicated that something
had gotten better than it was; he said that seemed to be the case.
Two such 1200-baud modems showed the same behavior (although one had a
history of such problems, and had had its 600-Ohm transformer replaced
a while back).
Even on calls from one of his residential lines to the other, the same
symptoms arise. So this lets out the modem on my end, and the network
between his serving office and anywhere else; the two directory
numbers have the same exchange, so I suppose they are in the same
switch.
Thinking fast, I explained that certain adaptive compression and other
fancy techniques can cause trouble for modem users. He mentioned that
a representative of Pac*Bell had said that they "guarantee service
only for voice data [sic]". And the modem worked last week; it just
suddenly stopped working. But I thought these techniques were only
economical in the long-haul network.
Is it possible that the little box on the corner suddenly has a
newfangled subscriber loop carrier terminal in it that does some
really sophisticated compression, or otherwise makes assumptions about
the sort of traffic that will be handled? There has been enormous
growth in his neighborhood (he said they had run low on directory
numbers, but this doesn't necessarily imply a shortage in the loop
plant, I suppose). Are they starting to use something other than
straight digitizing on SLCs? Or did both of his modems go south in
the same week?
Jim Hickstein, Teradyne/Attain, San Jose CA, (408) 434-0822 FAX -0252
jxh@attain.teradyne.com ...!{decwrl!teda,apple}!attain!jxhJohn Higdon <john@zygot.ati.com> (06/07/91)
Jim Hickstein <jim.hickstein@icdwest.teradyne.com> writes: > One of my users tried to dial in from home recently, only to discover > that the modem spit out garbage continually. I notice in your .signature a number that is in the Junction Ave. office. This place is packed with 5ESS, so what you describe could very well be a bad subscriber interface card in the switch. With digital, it can sound perfect and be totally unusable for data, for reasons I'll let others explain. > Thinking fast, I explained that certain adaptive compression and other > fancy techniques can cause trouble for modem users. He mentioned that > a representative of Pac*Bell had said that they "guarantee service > only for voice data [sic]". Call them back and tell them to check the handbook again. Pac*Bell considers the line to be meeting minimal spec IF it passes 2400 bps data. This they do guarantee. In other words, if you cannot get your 2400 bps modem to work through Pac*Bell facilities, then there is prima facie evidence that the line is not meeting minimal voice spec and someone should look into it. Oh, and do not be afraid to suggest things such as subscriber interface circuits (on the 5ESS). The days are long gone where those who knew ANYTHING about telco were immediately suspect. John Higdon | P. O. Box 7648 | +1 408 723 1395 john@zygot.ati.com | San Jose, CA 95150 | M o o !