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Multi-channel diagnostic modems trim networks.


With the introduction of Accunet Spectrum of Digital Services (ASDS) in June, AT&T reaffirmed its decision to phase out voice-grade private-line offerings in favor of digital services.

AT&T anticipates this transition will be completed by end of 1991.

Though this is only a billing transition--analog services have already been "swung" onto digital facilities--the effect will be to increase the cost of local voice-grade private lines in relation to long-haul digital lines.

Telephone companies will continue to offer voice-grade private lines well into the future.

Banks, retailers, brokerage houses, and other transaction-oriented businesses had better look for ways to streamline local voice-grade private-line networks to save on line changes.

At the same time, such businesses must not disrupt service to customers.

That would jeopardize their competitive position in local markets.

One way to accomplish both objectives is through the use of intelligent diagnostic modems capable of supporting several multipoint applications.

In Banking

In the case of a bank, whose host runs ATM and teller applications over separate lines, a multiport/multipoint modem may be used to combine the applications over a single multidrop line for cost savings of 25-60% on voice-grade private lines.

Say a bank has an ATM network of six drops per line and a teller terminal (TT) network of four drops per line.

Combining the two applications onto a single multidrop line of six drops results in a $1188 monthly savings on line charges.

Now let's say a bank has a TT network consisting of three lines, each connecting six branch offices.

It also has a two-line ATM network, each line connecting nine branch offices.

Thirty-six drops would be needed to support both applications.

Using an intelligent diagnostic modem to combine both applications over three lines of only 18 drops produces a monthly line cost savings of about 50%, or $1372.

The use of such modems can even improve host-to-terminal response time, particularly if the bank runs multiple applications over single lines polled by a front-end processor. Separating applications to operate from dedicated front-end ports and dedicated diagnostic-modem ports enables you to shorten polling tables and polling cycles per application.

This is likely to translate into a higher response per application, which in turn imporves customer satisfaction with bank service.

Because customers tend to choose and efficiency at which speed and efficiency at which they deliver services, these modems provide banks with a key competitive advantage.


Historically, two technologies have been used to derive multiple channels over a single transmission facility:

* frequency-division multiplexing (FDM),

* and time-division multiplexing (TDM).

FDM creates multiple channels by segmenting the 300-Hz frequency spectrum (bandwith) of the circuit and assigning each channel its own segment (frequency range). But FDM is risky for multipoint, multiport applications because of its susceptibility to adjacent-channel interference when line conditions are less than ideal.

Adjacent-channel interference results when varying individual channel strengths from multiple drops interfere with each other, inhibiting the ability of the modem to distinguish noise from data.

The more drops on a circuit, the greater the potential for noise and varying signal levels.

This kind of interference decreases error prevention, lowers throughput, and degrades service to users.

There are other potential problems with FDM technology in multipoint, multiport applications.

The modem may exhibit unreliable operation over the switched network when dial backup becomes necessary.

There is also the potential need for line conditioning at data rates in excess of 9.6kb/s, which inflates operating cost.

TDM is more reliable for deriving multiple channels over the same facility.

It combines several channels over a single, wider band to eliminate the potential for adjacent-channel interference and its consequent error-performance and throughput problems.

To improve data integrity, some manufacturers use "Trellis coding" with their FDM-based modems.

But this has a negative impact on throughput, especially in a polling network at lower data rates, because more processing time is required for modem training in the switched-carrier mode. As TDM solves the problem of adjacent-channel interference, Trellis coding is not needed. High throughput is thus sustained. Aside from cost savings on line charges, intelligent diagnostic modems provide numerous features that can enhance the overall efficiency of multipoint, multidrop networks.

THe "intelligence" is derived from the modem's software, which allows the unit to be controlled from a central location.

In this way, all modem options, including channel data rates and network addresses, are configured from the management console at the central site.

Once selected, the configuration opttions are downline-loaded to the remote locations.

Diagnostic testing and system restoral can also be initiated through the network-management system, eliminating the need to send technicians to remote sites.

THe integral diagnostic design overlays an "out of band" or "secondary" diagnostic channel onto the unit's communications link. This low-speed diagnostic channel provides an independent communications path for the network-management system's controller, which permits continuous non-interferring monitoring of voice-frequency (VF) line parameters and digital EIA leads, as well as the modem's own internal operation.

VF line parameters for which measurements are made outbound include:

* overall signal quality;

* receive level;

* impulse, phase, and gain hits;

* signal-to-noise ratio;

* phase jitter;

* nonlinear distortion;

* retrains;

* dropouts;

* and loss of data-carrier detect (DCD).

Alarm Conditions

When a parameter exceeds a user-defined threshold, an alarm condition is reported to the central site. Alarm conditions may be reported, automatically or on demand by operator selection, upon the loss of any of a number of EIA-connector and modem-related states.

Fault isolation on the modem network is facilitated by comprehensive loopback and testing capabilities.

System faults can be isolated to the particular terminal, modem, or line.

Some intelligent diagnostic modems support an integral dial-backup unit, which allows private-line communications to be temporarily rerouted through the public switched network, minimizing the impact of circuit failures. Since many users do not want dial backup, some vendors offer it separately as an add-on option.

Other vendors provided integral dial backup whether or not their customers want it, thus inflating the hardware investment.

Because the features of intelligent diagnostic modems are defined in software, new levels of functionality are possible, such as "day/night operation," which permits multiple lower-speed interactive daytime applications to be replaced at night with a single high-speed channel for file transfers.
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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Author:DiSario, Dom
Publication:Communications News
Date:Feb 1, 1990
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