From: bill.garfield@yob.sccsi.com (Bill Garfield)
Newsgroups: comp.dcom.telecom
Subject: InterOffice Trunk Help Needed
Date: 16 Nov 92 02:13:00 GMT
Reply-To: bill.garfield@yob.sccsi.com (Bill Garfield)
Organization: Ye Olde Bailey BBS - Houston, TX - 713-520-1569
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 851, Message 5 of 6

Background:

I have two MITEL SX2000SG 3-cabinet PBX's side-by-side in my office.
Southwestern Bell service arrives via 44.736 Mbps (DS3) fiber
terminating in Rockwell equipment.  The PBX's are digital, The trunks
are digital. There is no channel bank equipment at my end.  Network
synchronization is derived from SWB.  Span status logs indicate *NO*
frame slips, *NO* bit errors and no CRC errors. Everything is in sync,
fat, dumb and happy.

Two CO's across town (near a business associate) I have another MITEL
three-cabinet SG.  Just coincidentally, this machine is served by the
same CO as my business associate (about more later).

Only the PSTN service from SWB and my IXC feeds (USS) are non ISDN.
All the MITELS derive network sync from one of the machines here at
the corporate office, which in turn derives its sync from SWB.  We've
tried different sync sources, even let 'er go freerun without any
change in the problem. Zero suppression is AMI -- throughout the
network ... I've checked.

The problem: For a _very_ long time now I have been having fits trying
to RELIABLY receive 9600 bps FAX transmissions from a very large
business associate across town. (Two central offices away). The brand
of fax machine in use at either end has no bearing, as we've both
tried several brands, multiple machines on multiple extensions.

In desperation, I installed a copper POTS line (from SWB) direct to
ONE of my 170 fax machines (yes, 170!).  This works SWELL, but doesn't
address the central issue of the business associate being unable to
reliably send long, multiple-page fax messages from any of his 75 fax
machines to any of the other 169 machines at my corporate office.  In
answer to the obvious question, neither we nor the business associate
are experiencing any fax problems with anyone else that we know of.

Testing and research: A few nights ago, the business associate & I
attempted sending nearly 100 fax images from his location to mine via
the SWB PSTN. As it was off-peak, all my PBX's were basically idle, so
I was able to busy out selected DID T-1's, thereby effectively
"steering" his inbound fax calls from SWB onto specific individual
T-1's, and recording the span and channel number being seized.  I
thought I was going to find a bunch of bad trunks. I didn't. What I
found was that whenever a specific trunk WOULD pass his fax image to
me flawlessly, the next time the SAME trunk got selected the fax would
fail. We tried these same tests on nine different SWB DID T-1's,
busying out eight spans and leaving one span of 24 trunks open.  

The failure rate was nearly identical. Five of these spans are served
by a 5AESS and the other four by a 1A, but that too seems not to
matter.  When things failed, as they did at an alarming rate of 50% of
the time, the fax machines would experience difficult link
negotiations and then sync up at 4800 or 2400. The received image
captured from a marginal link contains SERIOUSLY misaligned pixels --
it looks like a noisey image, but at close scrutiny, it isn't noise,
as no "extra" gibberish is present.  The desired image was there, but
with literally thousands of "holes" in it from "missing" pixels. The
missing pixels are all there, but are scattered about the page,
appearing as noise dots.  The received image is so mutilated as to be
virtually unintelligible.

Brainstorm!  I took an unused DID number on my downtown PBX (near the
associate) and "routed" it via my internal ISDN trunks to send the
call to the same fax machine on the same extension, cable and pair,
but using my internal trunks instead of Bell's.  This WORKS! - So WHY
can't ma bell give me usable fax circuits over her PSTN?  What's
different?  I strongly suspect inter-machine trunk trouble between the
National and Clay central offices, but how do I prove this and how do
I get through to the teledroid at repair service that we're
experiencing this problem on ALL 216 DID trunks?  Profound thoughts
and ideas are welcomed.
     

Ye Olde Bailey BBS    Houston,Texas
Node 1: 1 713 520 1569  yob.sccsi.com