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DMVs Allow Data and Voice to Share Same Centrex Lines.

For years businesses have been wiring their premises twice--first for the telephone system and later for the computer system. Currently, however, many businesses are using the in-house lines of their Centrex telephone systems for both voice and data communications.

These businesses have found that data/voice multiplexers (DVMs), such as our Line Miser DOV, allow terminals and telephones to share this readily available cabling resource. With DVMs, simultaneous voice and high-speed, full-duplex data transmissions up to 64 kb/s are possible on a single wire pair for distances of up to several miles. Both synchronous and asynchronous models are available.

In many applications, DVMs can eliminate or significantly reduce the expense and trouble of pulling cables between a computer room and every user location. DVMs greatly simplify terminal installation and relocation because every telephone station is a potential pre-wired terminal site. When used with a data-only PBX (data PBX), DVMs can even turn an existing Centrex telephone system into the basis of a comprehensive data communications network.

Data Channeled at Higher Frequencies

DVMs work because voice signals occupy only the lower frequencies of the available bandwidth on telephone lines. DVMs take advantage of this and transmit data signals at the higher frequencies above the voice spectrum. A system of frequency filters maintains the integrity of the separate voice and data channels and prevents cross-interference.

A DVM link requires a remote and a central unit. The remote unit is installed at a terminal site and the central unit is typically in or near the computer room. Both of the units then connect to the computer system via RS-232-C interfaces. Most remote models plug directly into the telephone system with standard RJ11C phone jacks. Central units connect to the telephone system at a main distribution frame (MDF) by way of an RJ71C interface. Each such interface will support up to 12 central DVM units.

When a digital data signal enters either DVM, that unit generates an analog carrier frequency that it modulates with the data pattern. This carrier frequency is transmitted to the other DVM, which demodulates it and returns a digital signal to the computer system. For full-duplex operation, remote and central units use different carrier frequencies. DVMs do not affect voice signals in any way.

There Are Some Restrictions

There are, however, a few restrictions on DVM usage. In general, DVMs are used on privately owned in-house lines. If the lines are owned by the local telephone company, it must grant permission to use DVMs on its lines. Furthermore, although a DVM may be certified by the FCC, the DOC in Canada, or by similar agencies elsewhere, most telephone companies will not let it transmit on the public-switched network. This is why data is redirected at the MDF and kept on in-house lines.

Although commonly used to connect terminals to mainframes, DVMs can link other devices as well. Examples include links between microcomputers and mainframes and links between word processors and printers. They can also provide connections to such data communications equipment as switching multiplexers, T1 multiplexers, X.25 network PADs (packet assembly/disassembly) and microwave links.

In all of these applications, DVMs can provide direct one-to-one links between data devices on a single telephone-wire pair. Some models can even function strictly as two-wire local modems without being connected to a telephone handset. Users can, however, gain considerable additional benefits if they use DVMs in conjunction with data PBXs.

Data PBXs basically do for computer systems what Centrex does for telephone systems--provide switching and contention among system devices. Data PBXs, however, are specialized for data. As a result, they provide a combination of data-handling features and capabilities typically unavailable in such other data-networking solutions as integrated voice/data PBXs and many local-area networks.

For example, data PBXs address the diverse electrical and operational interface requirements that characterize the world of data. They also work with a variety of media, including twisted metallic-wire pairs, fiber-optic and coaxial cables, satellite and microwave links, and broadband and baseband channels.

Moreover, data PBXs offer features such as multiple levels of security, gateways to other networks, and statistics on system usage. Features such as these, as well as many others, help simplify network usage, management and maintenance. And all these features are available at a cost considerably less than that for most other data-networking solutions.

The lower cost, however, does not detract from the performance of a data PBX. Current models use state-of-the-art microprocessor technology. They are the result of an evolutionary process that refined them for the complex data-handling requirements of today's computer systems. One of the newest data PBXs is even specifically designed to interconnect intelligent data devices in distributed computer systems. Components of this model can be located wherever the data devices are concentrated.

As Close as Telephone Wall Jack

When DVMs are used with a data PBX, all the features and capabilities of the switch are literally as close as the nearest telephone wall jack. All the resources in a computer system are equally close. System users can even communicate with other networks and a variety of outside resources. In fact, anything interfaced to the data PBX is potentially accessible over existing in-house telephone lines via DVM-equipped terminals (see system illustration above).

DVM/data PBX networks, therefore, can dramatically reduce a computer system's overall cabling requirements. In addition, unlike many other methods of putting voice and data on telephone lines, DVMs require only one wire pair. Many alternatives need two or three wire pairs, which means pulling more cabling in buildings not equipped with enough pairs.

Maximum Use of Existing Facilities

DVM/data PBX networks also maximize the use of existing facilities. Besides using installed wires, the networks can extend the lifespan of a Centrex system that provides good voice service. There is no need to buy an integrated switch because, with DVMs, simultaneous voice and data transmissions are now also possible in the Centrex system. This allows Centrex users to continue to use the service that they feel optimally meets their specific voice requirements.

In fact, having an optimal voice system as well as an optimal data system is an important reason for implementing a separate DVM/data PBX network. Separate systems can take advantage of the best technology in their respective areas and changes made in one system will not affect the other. It will, therefore, be easier to adopt new voice or data technology when it becomes available.

DVM/data PBX networks also integrate voice and data only where it makes economic and functional sense--at the transmission level. Integration at the switching level, which is done in integrated voice/data PBXs, is a questionable practice, because the switching requirements of voice and data systems are radically different. Moreover, integrated switching provides a single point of failure for all communications.

Major Benefits Summarized

The major benefits of DVMs and data PBXs, therefore, can be summarized as follows:

* Simplified installation and relocation of data terminals and other data-handling devices.

* Use of existing Centrex-system telephone wiring for data communications with no effect on voice communications.

* Continuing use of a Centrex telephone system with which a business is comfortable and satisfied.

* Separate voice and data systems optimally suited to specific communications needs.

* Data-handling features and capabilities unavailable in other networking solutions often costing much more.

* Better protection against a total communications shutdown.

These are the benefits many Centrex users are currently enjoying. Their systems, together with DVMs and data PBXs, use existing-wiring networks to provide the advantages of integrated voice/data systems. Their data systems, however, have more data-handling capabilities and cost considerably less. Moreover, these Centrex users have an important benefit of specialization--voice and data systems that more precisely address their communications needs.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:Gallagher, J.
Publication:Communications News
Date:Dec 1, 1985
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