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Sleuth solves ISDN puzzler.


Mack Dobkins is a beleiver in testing.

Dobkins, with US West in Phoenix, tells how a company there ran into trouble recently with its phone lines and found fault in all the wrong places.

The company has two Primary Rate Interface (PRI) ISDN trunks off the US West DMS-100 switch to a Northern Telecom SL-PBX.

This customer uses one D channel and 47 B channels to connect remote sites to headquarters and to facilitate outbound collection calls.

The customer was plagued with fast busy signals and suspected a network problem.

"They were getting nothing reported on their Level 1 physical layer analysis," Dobkins remembers. "But I wondered if it was really coming from our network.

"We pulled it up in the central office and found channels 1 through 5 were call processing and so were channels 32 through 46. I called right in and it was blocked with a fast busy. At that point," he says, "there was only one thing to do: take the Chameleon 32 [a Tekelec protocol analyzer] to the customer."

Not Our PBX!

At the site, Dobkins asked if the fast busies couldn't be from the customer's carrier or in-house switch.

He was told, "No, our PBX doesn't give fast busies."

"That's the first one I've ever known that doesn't," answered a skeptical Dobkins.

"From looking at the traffic and the protocol traces," he says now, "I'd pretty well figured out what was happening.

"He said there was a fast busy coming from our DMS. But there was a call setup coming in from the DMS and a release complete from the SL-1. It was refusing to answer calls.

"They weren't sending setups out the link toward the DMS. Their system thought it had fewer channels than it had," Dobkins says. "The software had gone wild and lost track of its channels. I suggested they reboot the whole system."

That seven- or eight-minute procedure would get all the processors back into the same state, he figured.

"They did reboot, and all of a sudden the primary rate just opened up with calls. There were tons of calls going through. They found and fixed a problem in the SL-1 software and haven't had any trouble since," he adds.

Do it Yourself

Before Dobkins left, he urged the user to get his own testing equipment.

"Their lifeblood is making collection calls and getting calls from regional offices. They are getting economies from fast call setup and primary rate, but they need a datascope. It's a data world."

Dobkins, manager of ISDN product development, is sold on testing in his own operation, where US West employees are users as well as providers of the Integrated Services Digital Network.

In the 25-story CPAC (Central Phoenix Administration Consolidation) building in the financial center of Phoenix, 1700 US West employees use ISDN. Dobkins does a lot of testing there with equipment from Idacom, Hewlett-Packard, and Tekelec.

"It was a very aggressive move-in schedule," says Dobkins. The 1700 employees moved in within seven months.

"we were under a tight crunch to come up with multipoint wiring standards off a 5ESS (AT & T switch). We had to do a lot of different things, such a length tests and stress tests, using protocol analyzers."

Installers Goofed

Dobkins caught one problem early, when only about one fourth of the CPAC users were on-line.

"We were having a problem with certain sets. Those farther out were having a problem with calls not completing. They would dial a number and it wouldn't go through," he says.

"We took a datascope out there and it told us the symptom: all the digits weren't leaving the sets.

"We thought it might have been a CPE problem. We had to put a TDR [Time Domain Reflectometer, which sends a pulse down a transmission line and gauges the reflection from the other en] on the line and found a very slight discontinuity."

It turned out that installers had used ISDN wiring only so far then mistakenly switched back to traditional USOC (Universal Service Order Code) wiring the rest of the way. The crosstalk that resulted caused distant phone sets to shut down--but only some of the time.

"The only commonality was that these were long (farther from the switch) users, so we knew it had to be a physical problem," says Dobkins.

"We knew what the problem was on the datascope; the TDR looked at the physical level. That's the thing the datascopes don't do well, look at the physical level."

US West was fortunate the problem was caught early.

"It could have been real devastating. At that time not a lot of people were using data, so most of the D channel was being used for call control," he says.

Dodged A Bullet

"If we'd gotten to where we are now, using a lot of packet data, it would have been a catastrophic problem. Lots of people would be getting corrupted data and not communicating," he says.

Dobkins, who travels all over US West territory doing testing, says more people need to better understand sophisticated diagnostics, not just in ISDN but other areas of voice and data communications. "I can't expect everyone in the phone company to become a protocol expert," he says, "but if we're really going to provide integrated services, we need pools of key people available to remotely access and diagnose problems quickly and efficiently."
COPYRIGHT 1990 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

Article Details
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Title Annotation:data/voice test equipment
Author:Tanzillo, Kevin
Publication:Communications News
Date:May 1, 1990
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