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From: brian%cyberpunk@ucsd.edu (Brian Kantor)
Newsgroups: comp.dcom.telecom
Subject: Re: References/Fixes Needed For "Slippage" on Dialins
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Date: 28 Sep 90 17:24:23 GMT
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X-Telecom-Digest: Volume 10, Issue 690, Message 9 of 10


When you get "twinklies", consisting of characters having a lot of
bits on (especially high order bits) like }, you are probably seeing
your modem attempting to resynchronize.  (1200 bps 212 modems use
synchronous transmission between each other even though you are using
async to talk to them.)

My experience is that the A-#1 cause of this is a defective or
misconfigured interface card on one or both ends of one or more of the
circuits that connect your university's phone switch to the local
telco's digital switch.

What happens is that the a/d and d/a conversions at opposite ends of
the trunk occasionally drop a little data.  In other words, one or
more of the 8k/sec samples was damaged and was discarded at the
receiving end.  This has NO measureable effect on voice - completely
inaudible - but it makes the modems lose sync and they blow 1's bits
at each other until they resync, so you see lots of twinklies.
Sometimes they switches are misclocked so that they drop one sample
out of every N, so you see a periodic burst of twinklies every M
seconds.

This is always repairable, but it will probably take a transmission
specialist to bring his special test equipment and check for it as the
normal telco voice quality measurement stuff won't show the problem.

We had this problem big-time here at UCSD when the main campus was on
one machine and the student housing on another in the same telco
office - the two switches in the same building couldn't talk to each
other without sync slips.  The DMS-100 switch was famous for this - I
heard they had a production run of line cards that came from the
factory misconfigured slightly so that they worked ok for voice but
got lots of slips.  I understand they had to pull every single card
out of the switch to check the jumpers or some equally boring task.

Now that PacBell has fixed that problem with our local switch, we see
sync slip storms only once or so a year - typically when they've just
upgraded one of the central office switches in some other part of
town.  A quick call to their technical people handling the campus gets
it fixed right fast.  I get the impression we find out about it before
they do, sometimes.  (We've got over 200 dialup lines and about 8,000
students and faculty using them 24 hours a day, so we have a large
window of opportunity.)

My experience parallels others in this regard - once you get high
enough in the telco to find someone who can understand what you're
saying, they'll get it fixed.  If you're not in a position to bang on
them from an official campus position, try to talk to whoever runs the
switchroom in your campus phone facility and explain to them what's
going on.  They can get to the right people in the telco, eventually.


Brian