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The Last Mile: Alternative Ways To Implement ISDN over the Local Loop.

Will the concept of the Integrated Services Digital Network (ISDN) ever become a reality? At the moment, the answer seems to be "yes." Engineers around the world have been developing and refining the technology necessary for ISDN, and the International Telegraph and Telephone Consultative Committee (CCITT) is expected to set its first international ISDN standards by the end of this year. Several telephone companies even now believe that they will be able to introduce some ISDN-type services within the next year or two.

But in spite of these plans and activities, the full implementation of ISDAN will probably not occur until sometime in the next decade. Although more teleephone companies will soon join those that have already introduced digital carrier service for trunking applications, few telephone companies will be able to take ISDN service that "last mile" between the nearest central office and the subscriber's premises in the near future. Need Interim Alternative

Completing the local loop and providing digital service over the last mile will pose a formidable problem for ISDN--not so much because of standards and technology as because of economics. Customer demand will perhaps eventually justify the cost of converting local loops for ISDN service, but an alternative method of providing these services over existing facilities will be needed in the interim.

Some operating companies have already stated that they will not wait for full ISDN specification development to become available. Instead, these companies plan to utilize existing near-term alternatives to offer direct digital connections from the central office to the customers. Such an alternative is short-haul technology (Figure 1).

Few would argue that there is an increasing need for such services as the ISDN would offer. The information industry is becoming the largest and fastest-growing sector in the economies of many countries; in several countries, it has already achieved this distinction. To satisfy the industry's communications requirements, the exchange of information must be made simple and cost effective.

Information exchange, however, has typicall been neither simple nor cost effective, at least not for data. The lack of standardization, for example, often makes communications difficult between data services from different manufacturers. Communication distances, changing requirements and technology, and accessibility are also problems that can involve complicated and expensive solutions. More Data on Public Net

Currently, the public switched telephone network is a widely used medium for the exchange of information. For 100 years, it has done a creditable job of meeting the requirements of voice communications, and more recently, if has been handling some data communications as well. In light of the growing popularity of the personal computer and the trend toward the automated office and distributed networks, the use of the public switched telephone network for data communications will certainly become even more common in the immediate future.

The network as it exists, however, is largely an analog facility, which severely limits its effectiveness for data communications. Users who transmit data over the public switched telephone network must use long-haul modems to interface with the network. This limits users to a fairly low speed of operation, typically 1200 b/s.

Low-speed operation may be satisfactory for some applications, but many other applications, such as file transfer, video and rapid facsimile transmission, require much-higher data rates. Moreover, one can expect that these higher-speed applications will become even more widespread within the next few years.

It is also reasonable to expect that, with the public switched telephone network will become economically burdensome to many organizations, if it is not so already. This is particularly true in toll applications. Existing telephone lines will be tied up for long periods of time, and more lines will be needed. In addition, there will be considerable expenditures for more long-haul modems as network interfacing requirements increase.

Rising communications costs and the need for greater capabilities, therefore, will soon make the existing analog public switched telephone network an unsatisfactory choice for a total communications medium. On the other hand, the services that would be provided by ISDN will become increasing more attractive. Imagine the benefits of being able to place a high-speed data call anywhere in the world as easily as a voice call is placed today.

In fact, the appeal of ISDN-type services is shown by the growing popularity of such alternatives as bypass technology, the X.25 packet switched networks and T-1 distribution lines.

The public switched network is currently considered an analog facility because it has been optimized to transmit analog voice signals. As such, it has been designed to provide channels for the efficient transmission of analog signals between frequencies of 300 to 3,000 Hz. To provide ISDN services, the network must be converted from an analog to a digital transmission facility, that is, it must be optimized to transmit digital signals. Keep Existing Wiring

Conversion does not mean that existing telephone wires are analog and must be replaced with digital wiring; there is nothing inherently digital or analog about wiring. Furthermore, an analog facility does not necessarily limit the type of signals that can be transmitted over it to analog signals. An analog facility does, however, affect the speed at which other types of signals can be transmitted, their transmission distance and the points between which they will travel.

In terms of the last mile, the local loop is typically designed for separation distances of up to about three miles. This is the maximum distance over which attentuation permits voice frequencies to travel effectively without amplification at some point along the line. Local loops longer than three miles require loading coils. But because local loops are limited to three miles whenever possible, the overwhelming majority of them are essentially comprised of unloaded wire strung between two locations.

Both loaded and unloaded local loops, however, pose problems for digital transmissions. When the local loop is unloaded, digital signal can only be sent over it at extremely low speeds and/or for relatively short distances. In those instances when loading coils are used, the coils would act as bandpass filters and eliminate most digital signals from the line as they passed through.

These and other characteristics of the last mile necessitate several changes in the local loop before it can be optimized for digital transmissions. For example, all loading coils would have to be removed, and regenerative repeaters would have to be installed at various points along the lines. Once the lines are thus conditioned for digital signals, however, they will no longer be a satisfactory medium on which to transmit analog voice signals. Problem Not Insurmountable

Theis means that either separate lines must be used for digital and analog signals, or that voice signals must be digitized before entering the local loop. The first solution is less than optimal because, once trunking applications are all digitized, the voice signals would have to be digitized at some point anyway before they enter a trunk. On the other hand, with the high-speed digital transmission envisioned for ISDN, local loops may still require multiple lines to handle the problems with cross-talk that would occur.

The necessary changes in the local loop, and even those in the trunking applications, should not, however, present an insurmountable technological problem to optimizing the public switched telephone network for ISDN service. There will, though, be a problem in terms of the time and effort required in implementing the changes. And, of course, the changes will involve considerable expense.

Although converting the public switched telephone network to a digital facility can be accomplished, the determining factor in the implementation of ISDN will be one of economics. Telephone companies worldwide have an enormous investment in existing cable plant facilities of varying vintages. Similarly, the subscribers have a substantial investment in their existing communications equipment for both voice and data. Need Assured Payback

For this reason, neither the telephone companies nor their subscribers are going to rush headfirst into something before being reasonable assured of an economic payback. The implementation of ISDN, therefore, will probably proceed in a stepwise fashion, beginning with full digitization in trunking applications. Each future step will be taken as it becomes economically justifiable.

The key to the total provision of ISDN services, however, ultimately lies in the network's ability to bring digital service to the subscriber's premises. Estimates claim that approximately 40 percent of the telephone companies' total plant investment is in the local loop. This represents about $80 billion in the United States alone. Upgrading the local loop to a digital facility will require an additional huge investment.

To justify this additional investment, the telephone companies will have to be very certain that they will eventually get a sufficient return. They will also have to digitize local loops with existing technology as it becomes economically viable. The problems with this is that, unless the digital services are provided quickly and cost effectively, potential subscribers will go elsewhere.

Already some subscribers are turning to X.25 packet switched networks and bypass technology for their communications needs. These alternatives have a twofold benefit--they are currently available and allow users to make the most out of their existing equipment. If subscribers continue to turn to these alternatives, the implementation of ISDN could be seriously delayed or perhaps even scuttled.

For ISDN to become an eventual reality, therefore, an interim method of providing digital service over the last mile is needed until the local loop is converted into a digital facility and all communications are digitized. This will help to forestall the defection of subscribers to alternate transmission facilities and bring in new subscribers. The revenues generated from these subscribers, in turn, will help to pay for future changes in the local loop.

The most likely candidate for bringing digital service to the local loop is existing short-haul technology. Short-haul technology is a branch of telecommunications specifically aimed at providing high-speed data communications over relatively short distances. Currently, short-haul technology is used in such data communications equipment as local data sets (line drivers), local multiplexers and data-over-voice (DOV) multiplexing devices (Figure 2.

Short-haul technology, however, is not a new development. Gandalf introduced the first local data set over 13 years ago as an alternative to long-haul modems for local applications. This local data set, which used an encoding rather than a modulating technique, allowed for high-speed data communications over privately owned and leased telephone lines for distances of up to 13 miles. Its purchase price was equivalent to the cost of leasing a long-haul modem for four months, and ait provided a data rate of 9600 b/s.

Short-haul technology, therefore, was first commercially developed in response to the need for cost-effective, high-speed localized data communications; and it is continuing to evolve as this need grows. Current estimates say that approximately 80 percent of data communications is over distances of less than 200 miles, and most of this is said to be less than 50 miles. Data communications within such arange typically does not require the complexity and expense of long-haul modem technology.

Long-haul technology is more complex, and consequently more expensive, than short-haul technology largely because long-distance data communications differs significally from local data communications in several important ways. For example, there are few, if any, equalization stages required for most local data communications. In contrast, long-distance data communications requires a considerable number of equalization filters.

Data echo can also be a problem in data communications over long distances. Data echo is harder to suppress in such communications because of the inherent longer dealy times. Group delay, which is the differences in the delay times for different frequency bands, is another problem. Group delay increases with distance, and it must be compensated for in long-distance data communications. On the other hand, since the distances involved in local data communications are relatively short, data echo and group delay are not problems with which short-haul technology must cope.

Because short-haul technology does not have to address the various problems associated with long-distance data communications, products designed for local data communications provide considerably more features for lower cost. These products also tend to be faster and smaller (Figure 3). For example, there now are synchronous and asynchronous, high-speed local data sets that are smaller than a pack of cigarettes and plug directly into the RS-232-C interface of data terminal equipment.

There are also models of local data sets that can operate at speeds in excess of 25 kb/s. Some local data sets provide such features as full and half-duplex operation, diagnostics and self-tests, synchronous and asynchronous transmissions, and controlled or continuous carrier operation. There are now even inexpensive four and eight-channel multiplexers that incorporate short-haul technology.

These cost-effective devices provide all the economic benefits of multiplexing to data transmissions over local distances. Included among the benefits are reduced cabling and modem requirements, as well as reduced leased-line costs.

The advantages of short-haul technology, therefore, are both operational and economic. They also have the advantage of being optimzed to work on precisely the type of analog facilities that currently make up the last mile. For these reasons, a number of telephone operating companies have been looking into short-haul technology or are aready using it to provide ISDN-type services to their customers.

Local-area data transports (LADTs), for example, incorporate the same fundamental short-haul technology that is used in the data-over-voice (DOV) multiplexing devices that manufacturers of data communications equipment have been marketing for several years. Similarly, digital data service (DDS) employs data devices that are based on technology that is not significantly different from the short-haul technology used in the first local data sets. Benefits Both Users and Telcos

By using such devices, the telephone operating companies are able to meet their subscribers' current demands for ISDN-type services; and they can do so without the expense and trouble of converting local loops to digital facilities. The subscribers, on the other hand, have the services they want and need without the expense and trouble of replacing existing telephone and data equipment.

As the demand for ISDN-type services continues to grow, telephone operating companies will feel increasing pressure to provide such services to avoid risking the permanent loss of important revenue sources. Because of the capabilities and benefits of devices based on short-haul technology, such devices are perhaps the only practical way to implement ISDN-type services in the immediate future. As a result, the use of these devices will continue to grow.

This growth, in turn, will spur even greated developments in short-haul technology and more innovations i the devices that use it. Consequently, short-haul technology has the potential to become more than merely an interim method of completing the local loop; it might very well become the long-term solution to the problem of the last mile. Thus, while ISDN will probably have an adverse effect on the long-haul modem market, the future looks much brighter for the short-haul industry.
COPYRIGHT 1984 Nelson Publishing
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Copyright 1984 Gale, Cengage Learning. All rights reserved.

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Author:Skene, J.
Publication:Communications News
Date:Dec 1, 1984
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