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Assessing telecommunications options for minicomputers

Library Systems Newsletter [July 1982]

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Most of the discussion on the computer usage implications of the restructuring of the telecommunications industry have addressed the issues in the context of the mainframe environment. However, since the majority of automated library systems are mounted on minicomputers, the editors offer the following discussion of telecommunications options from that perspective.

The data communications needs of minicomputer-based systems are significantly different from those of mainframes. For example, mini-based systems are typically concerned with communication over relatively short distances, typically under 100 miles, in contrast with the hundreds or even thousands of miles common to mainframe communications. Additionally, mini users usually require a limited number of linkages rather than a complex multinode telecommunications network. Finally, of particular significance to telecommunications, the terminals on mini-based systems are usually asynchronous "dumb" terminals, whereas a mainframe tends to be linked with synchronous terminals with some built-in communications protocols.

Communications protocols, or standards, such as IBM's BISYNC, permit terminals to be "multi-dropped" or grouped on a single telephone line because each terminal can be selectively "polled" or addressed by the computer. Minis are typically configured with only one terminal per computer port so that the computer does not need to ascertain which terminal is transmitting, and therefore normally do not have this feature.

A "dumb" terminal uses no communications protocol. It is a TeletypeŽ or TeletypeŽ compatible terminal that displays or prints data just as it receives it. A "dumb" terminal may incorporate a microcomputer for data reformatting and other local activities, but it is still "dumb" if it is TeletypeŽ compatible. Without a protocol a terminal cannot be polled, so it cannot be clustered with other terminals on the same telephone line unless linked to additional equipment which contains a communications protocol.

The mainframe protocols exist because manufacturers have had enough users with numerous terminals distant from their mainframes to warrant the development of protocols by the vendors. Most minicomputer-based systems use "dumb" terminals which are less expensive and completely adequate for the tasks to be accomplished, because so many of these terminals are located within a few hundred feet of the computer site and can thus be hard-wired to the computer. Unfortunately, when there is a need to link remote terminals, the options are limited because there is no standard communications protocol to adopt. Terminals in the same building as a computer are usually hard-wired to it over a simple twisted pair of wires using line drivers-signal converters which condition the signal to ensure reliable transmissionon each terminal and the associated computer ports. Twisted pair wires may be installed by the telephone company, the computer system vendor, or the system user.

Remotely located terminals, those that cannot be hard-wired, are usually connected to the minicomputer by a telephone line with modems at each end. (The Bell System uses the term data set rather than modem.) A modem or data set is an electronic device which converts the signal from the digital form used by computers to the analog form used for telephone voice transmission and vice versa. The signal is modified to appear to the telephone system as a voice communication and to the mini computer as a digital communication. A pair of modems usually costs $1,000 or less.

There are three common methods of establishing the telephone linkage: dial access, dedication of a leased line to a single terminal, and the sharing of a leased line among a number of terminals using multiplexing techniques.

The dial access or dial-up approach enables regular telephone installations to be used for data transmission. The line can be used for voice communication when not being used for data transmission. Dial access is appropriate for situations where a specific terminal-computer linkage is of relatively short and infrequent duration-entailing only a few dozen transactions a day. In an environment where a number of terminals need to access the computer on such a basis, it is possible to have more than one terminal assigned to a computer port or input channel, thus reducing the cost of the computer installation.

The efficiency of dial access is reduced by the limitations which apply to regular telephone traffic-peak period loads which can result in difficulty in establishing the connection-and restricted transmission speeds. Until recently, a major advantage of this method of terminal-computer linkage was economic. Most such installations are within local areas where relatively short distances are involved and local calling rates apply. This situation is changing as telephone companies redesign their pricing structure and charge business and institutional users by The minute for local telephone calls. Costs are expected to rise further as rates are restructured to reflect the loss of long distance traffic which currently and in the past has helped to subsidize local service.

Certain technical aspects of dial access linkage limit the extent to which techniques to increase the speed of data communications-and thus lower the costs-can be applied.

Transmission speed is usually expressed in "baud" one baud being one signal element per second or bits per second (bps). With voice-grade telephone lines and regular modems the practical upper limit is 1800 baud or 1800 bps. Higher speed modems are available. They achieve faster data transmission by encoding more data bits in a baud-a modem operating at 1200 baud, but encoding two bits in a baud, effectively transmits data at 2400 bits per second (bps). Some modems will transmit data over voice-grade lines at 4800 and 9600 bps. Unfortunately, another characteristic of "dumb" terminals is that they can only use full- duplex modems in dial-up mode, and this effectively limits transmission to the basic 1200 bps.

Given the changes in dial access pricing structures and an appropriate number of transactions per remote terminal per day-300 or more- most libraries are in a situation that justifies the use of a dedicated leased line. The cost of such service now varies from $4 to $12 per month per mile, but it can be expected to rise dramatically in The next few years.

Although in many parts of the U.S. the phone companies offer Dataphone Digital Service (DDS) as an alternative to voice-grade lines, most leased lines are voice-grade. With present tariffs, there are virtually no cost savings to be realized through the use of DDS even though no modems are required on equipment attached to such lines. There is, however, greater reliability and better performance.

Terminals located in groups at remote sites may share a leased line by being "multiplexed" to a minicomputer using data terminal equipment designed to combine the transmissions of multiple terminals into one composite signal. At the other end of the line on which this signal is transmitted, an identical multiplexor reconstitutes the original input from each terminal and routes them to the appropriate ports of the computer or to yet another multiplexor. Neither terminal equipment nor the computer hardware or software need be changed when multiplexing is undertaken. Dial-up lines may also be shared this way.

There are several multiplexing techniques, the most cost effective of which is usually statistical multiplexing. A statistical multiplexor uses a small microprocessor and a buffer memory so that data can be stored temporarily during periods of peak activity. This permits more terminals to share a line because the "stat mux," as it is often called, smoothes out the traffic flow. It allocates the shared line in such a way that up to eight terminals, each operating at 1200 bps, can share a single 1200 baud voice-grade telephone line transmitting at 2400 bps. Another term now more commonly used by manufacturers of this equipment is "data concentrator." Stat muxes or data concentrators have a price range of $1,500 to $10,000 each depending upon the number of terminals they handle. In addition, the special modems required for the data concentrators may cost as much as $6,000 a pair.

To add to the complexity of all this, it is possible to network stat minces or data concentrators. For example, several terminals at a branch library may share a multiplexor (unit A) that is connected to another multiplexor (unit B) through a pair of modems. In turn the second multiplexor (unit B) connects with yet another multiplexor (unit C). Several other terminals and terminal clusters may also come into unit C so that all share a single line to the central site. The multiplexor (unit D) at the central site splits all the transmissions among the appropriate ports of the computer.

In addition to the equipment already mentioned, there are concentrating terminals, multidrop concentrators, port concentrators, and intelligent port selectors which further stretch the number of telecommunications options. These will be pursued in a future issue of LSN. Given the range of the options available to minicomputer users and the expected increase in the number of suppliers of telecommunications hardware and transmission lines as the telephone industry is deregulated, librarians managing automated systems will need to periodically reexamine their telecommunications approaches to ensure that they continue to take advantage of the most cost effective options available.

The editors offer the following rules of thumb as valid for the time being:

  1. If the terminals are within 2000 feet of the computer, hard-wired or direct connections using line drivers are generally most cost effective.
  2. When terminals are remote from the computer, but widely scattered, direct connection through telephone lines with modems may be cost effective. Normally dedicated lines are better than dial-up. If a library' s telecommunications costs are more than $1,000 a month or have risen more than 20% in the past year, a review should be undertaken.
  3. If the remote terminals are concentrated at a small number of sites; there are more than 25 remote terminals; or telecommunications costs are more than $1,500 per month, it is quite likely that the use of statistical multiplexing will be cost effective.
  4. If there are more than 50 remote terminals, it may be possible to network statistical multiplexors to realize even greater cost savings.

Some libraries may be able to reduce ongoing telecommunications costs by up to 90% by investing in telecommunications hardware. The "payback" period--the time required, to recover the capital outlay for telecommunications hardware-may be as short as two to three years.

While a library may wish to have the vendor of its computer system review and modify its telecommunications, it is not mandatory that it do so. Virtually all telecommunications hardware requires no changes in software. A library may choose to retain an expert in telecommunications because its vendor does not have appropriate expertise or because the vendor's prices are felt to be too high.

The vendor should be notified of planned changes as should the telephone company (ies). Despite frequent protestations by telephone companies, they may not deny the connection of telecommunications equipment to their lines. Until 1968 only telephone companies could connect telecommunications or other devices to their own facilities. As a result of an FCC ruling known as the Carterfone Decision, independent data and voice communication vendors may now attach a wide variety of devices on behalf of their customers.

View Citation
Publication Year:1982
Type of Material:Article
Language English
Published in: Library Systems Newsletter
Publication Info:Volume 2 Number 07
Issue:July 1982
Publisher:American Library Association
Place of Publication:Chicago, IL
Notes:Howard S. White, Editor-in-Chief; Richard W. Boss, Contributing Editor
Subject: Telecommunications
Record Number:3828
Last Update:2022-07-27 16:28:53
Date Created:0000-00-00 00:00:00