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Echo is nagging problem in primary rate private network.

Steve McConnell has an analog headache that won't go away in the digital world of ISDN. McConnell is director of the Advanced Concepts Lab at Vandenberg Air Force Base in California. What's bugging him is the problem of echo in a private ISDN network at the testbed at Vandenberg.

It's a problem with temporary solutions, but he's looking for a permanent one. He first noticed the problem about a year ago, and says he has talked to all sorts of vendors and ISDN-savvy engineers in his quest to quiet the echo.

"When we first noticed this, we weren't sure whether it was an anomaly. We didn't know if there might be a problem with our equipment or whatever. Ever since then, I've been working with other people who are smarter about ISDN to see whether I was going insane or if there was a real problem."

Here is how McConnell explains the situation:

"For many years, T1 echo cancelers have solved the problem of echo being generated by two- to four-wire hybrid telephones.

"As ISDN primary rate is brought on-line and private networks migrate to the new features that PBXs using primary rate will offer, the older two-wire 2500-type phones will still be needed."

The result, he says, is a hybrid network that has some telephones with intelligence and some without. What happens is that in voice calls on the private network between a four-wire "smart" phone and a two-wire "dumb" phone, the dumb phone creates an irritating echo.

This doesn't happen in networks that use public network ISDN facilities, McConnell explains, because the central office switches have built-in echo cancellation.

"DS1 echo cancelers will work with the primary rate, but the problem occurs when you want to use the B channels for data traffic," McConnell elaborates.

"A primary rate echo canceler should interpret the D channel message and disable the echo cancellation whenever the PBX signals that a data call is being set up.

"The echo cancelers with the in-band 2100 Hz tone method of turning off cancellation will not work because of the lack of in-band signaling access."

McConnell goes on to say that DS1 echo cancelers can be set up to turn off the cancellation on certain channels, but this solution falls apart when the number of data devices that need the echo cancellation turned off exceeds the number of channels that have cancellation off.

He is in the market for some sort of primary rate echo canceler, but in the meantime says a solution can be found in the intelligence of a smart multiplexer, such as the IDNX from Network Equipment Technologies.

The mux can do B channel negotiation between the network side and the PBX. The PBX can send a setup message via the D channel, McConnell says, "requesting a certain B channel for a particular type of traffic, data in this case.

"I can program the IDNX to accept only data traffic on channels 20 through 24, with the echo cancellation turned off on the echo canceler on those same channels. If the PBX requests a voice call on a channel that the IDNX has assigned for data, the IDNX will negotiate with the PBX to use one of the voice channels."

For example, he says, if echo cancellation has been turned off on channels 20-24, and the PBX sends a D channel setup message requesting a data call on channel 12, the mux and PBX can negotiate. In this case, the mux may tell the PBX to use channel 22.

This is a problem that an increasing number of users could face as they implement ISDN on a private network basis. It throws up a big obstacle to the expansion of ISDN, because one of the advantages of ISDN is the ability to set up calls on a dynamic basis, using the D channel's intelligence.

When users are forced into allocating channels to certain uses, such as data-only with echo cancellation off, they surrender some of the flexibility that ISDN can offer.

Pioneering users like McConnell are doing the rest of the user community a service by bringing glitches like this to everyone's attention now. But a permanent solution to the glitch would be an even bigger service. Anyone out there with answers, please contact me (419-663-8811) and I'll put you in touch with McConnell.
COPYRIGHT 1992 Nelson Publishing
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:ISDN Forum
Author:Tanzillo, Kevin
Publication:Communications News
Article Type:Column
Date:Oct 1, 1992
Words:722
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