jhf@cblpe.att.com (11/08/88)
I remember reading about a known problem with DMS-100s. It had to
do with a synchronization problem with one of the cards in
the switch. The manifestation of the problem is an "occasional"
single bit error that makes it barely tolerable to use a 1200 baud
modem over the line and nearly impossible to use a 2400 baud.
Unfortunately, the error rate is so low that it's undetectable using
the "standard" tests that the craftperson performs in response to
noisy line complaints.
Apparently it's quite easy for the craftperson to figure out that
card in question is flakey *IF* you can tell them the particular
card to check. Then, it's just a simple matter of a board swap
and you're back in business at 2400 baud.
A friend recently moved to a new exchange and found that his modem
won't hold carrier for more than 5 minutes at 2400 baud and gets lots
of curly brackets (another symptom of the DMS problem) at 1200 baud.
Repeated calls to Ohio Bell about noisy lines have gotten him nowhere
("But sir, when our technician listened to the line, he didn't HEAR
any noise.") except that he was told that the switch in question was
a DMS-100.
Any info would be appreciated.
Thanks,
Jeff
---
Jeff Frontz Work: 614/860-2797
AT&T-Bell Labs (CB 1C-356) Cornet: 353-2797
att!cblpe!jhf jeff.frontz@attbl.att.com Home: 614/794-3986goldstein%delni.DEC@decwrl.dec.com (Fred R. Goldstein dtn226-7388) (11/09/88)
In issue 174, someone at AT&T asked about a synchronization problem
with the DMS-100. To the best of my knowledge, this is not a DMS-100
problem per se, as similar problems occur in other exchanges where
T1 carrier is in use between switches (many or most). The problem
is that every line is supposed to be synchronized to a master clock
in a heirarchy. The DMS is supposed to be slaved to one line (with
another as backup), and all of the incoming T1 lines are also
supposed to be slaved, eventually, from the master. (There's a
multi-strata set of rules for this.)
Since you can't _hear_ slips, telco people get sloppy and sometimes
don't set up things right. It's possible, then, that one of the T1
lines is not synchronized to the switch. This "plesiochronous"
operation ($10 word for "just out of sync") usually results in a
very regular (every n seconds) error on modems. You have to pester
the telco to get it fixed; it matter what exchange the call is to/from,
since it's usually on the interexchange trunks.
fred