gardner@uxc.cso.uiuc.edu.UUCP (04/29/87)
We have recently installed a Northern Telecom DMS100 switch on campus.
We have been experiencing a number fo problems with 1200 and 2400 modems and
spurious line noise. Most characteristic are the two characters { and del,
together received on lines comming from the switch. The local telephone
company has corrected the problem with most local phones, only by repairing/
replacing local trunk line equipment not associated with the switch, but
through which calls may come in to the dms100 and university mainframes.
We still have unacceptable problems on long distance lines, which they are
now tracing out through long distance trunks.....maybe they have to "fix"
the rest of the country to work with us. Has anyone out there had
similiar problems with DMS100's or similiar digital switches? Please respond
by mail, I'll summarize.
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Assistant Director - Computer Services Office - University of Illinois
Michael G. Gardner 217-244-0914
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US Mail: Univ of Illinois, CSO, 1304 W Springfield Ave, Urbana, IL 61801W8SDZ@SIMTEL20.ARPA (Keith Petersen) (05/06/87)
The 1200/2400 bps modem "noise" problems with the Northern Telecom
DMS100 digital switch are well known by your vender. If they are not
telling you about them, ASK!
There are 25-30 software revisions for the digital switch software
which must be installed to help eliminate this problem. There are
also some hardware modifications to the DMS100 which involve
synchronizing all the oscillators in each frame and locking them
together to a master oscillator - which itself must be locked to a
master that services other exchanges in your area.
The problem is caused by the fact that the "switch" your call goes
through is a multiplex switch. That means it's "chopping" the signal
(at an 8 KiloHertz rate). This causes little holes in your modem
tones. Normally these will be smoothed out by the characteristics of
the subscriber local loop. The problem comes when your call goes
through two switches which are not synchonized. The result is that
the holes produced by each switch have random coincidence with each
other. The result - pulse width modulation. This really messes up
the 1200 and 2400 bps modems!
--Keith Petersen
Arpa: W8SDZ@SIMTEL20.ARPA
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GEnie Mail: W8SDZ
RCP/M Royal Oak: 313-759-6569 (300, 1200, 2400 bps)