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ISDN Architecture



ISDN Implementation Standards



Growth and Adaptation of ISDN









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Understanding Data Communications, Sixth Edition

From: Understanding Data Communications, Sixth Edition
Author: Gilbert Held
Publisher: New Riders Trade
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13. ISDN

ISDN, which is the acronym for Integrated Services Digital Network, augurs the potential for the development of a universal digital network that provides integrated voice and data on common telephone company facilities. In this chapter, you'll examine the idea behind ISDN, its architecture, and some of the benefits that can be expected from its use.


The need to transmit human speech resulted in the development of a telephone system that was originally designed for the transmission of analog data. Although the telephone system satisfied the basic need to transmit human speech, its design required the conversion of digital signals produced by computers and terminals into an analog format for the transmission of digital data. This conversion was awkward and expensive because modems were required at both ends of a telephone channel to do the digital-to-analog and analog-to-digital conversions.

A rapid decrease in the cost of semiconductors and the evolution in digital signal processing resulted in the application of digital technology to the telephone network. In the 1960s, telephone companies began to replace the electromechanical switches located in their central offices with digital switches. By the early 1970s, several communications carriers were offering end-to-end transmission services. In these services, unipolar digital data from terminals and computers was converted first into a modified bipolar digital format. Then, through a series of digital repeaters in the network, the data was transmitted to its destination. At its destination, data was converted back to its original unipolar digital format. The unipolar-to-bipolar signal conversion enabled the telephone company to space repeaters farther apart, reducing the construction cost of the digital network. Figure 13.1 illustrates the use of amplifiers and repeaters on analog and digital circuits.

The amplifier used on analog circuits amplifies the entire signal, including any signal impairments such as noise. The digital repeater, also known as a data regenerator, regenerates a new digital pulse, eliminating any distortion to the pulse that occurs as it travels on a digital circuit.

Because analog amplifiers increase the size of an analog signal including any previous distortion, whereas digital repeaters regenerate a pure digital pulse and eliminate any previous distortion to the digital signal, the error rate on a digital network is significantly lower than that on an analog facility. In addition, the devices required to perform the unipolar-to-bipolar and bipolar-to-unipolar signal conversion are much less expensive than the modems required for signal conversion on analog facilities.

By the mid-1980s, most telephone companies had incorporated a large amount of digital technology into their plant facilities so that a significant portion of the lines connecting telephone company central offices transported speech in digital form, although speech continued to be carried in analog form from the subscriber to the central office. At the central office, speech is digitized for transmission over the backbone network of the telephone system. Similarly, at the central office closest to the destination of the telephone conversation, the digitized speech is reconverted into its analog format and then transmitted to the receiver telephone.

The progression of telephone systems in the use of digital technology forms the basis for ISDN. Thus, ISDN can be viewed as an evolutionary progression in the conversion of the analog telephone system into an eventual all-digital network, enabling both voice and data to be transported end-to-end in a digital format.

Besides integrating voice and data, ISDN provides a level of communications capability above that obtainable with conventional analog technology. When voice and data are integrated, subscribers are able to talk on the telephone and use a computer or terminal at the same time over a regular telephone line. For business, this capability improves the productivity of office workers and reduces the cost of wiring buildings and offices because there should be no need to install separate wires to each desk for voice and data. Thus, ISDN offers subscribers a level of efficiency beyond that obtainable with conventional facilities.

Figure 13.1. Amplifiers versus repeaters.

For individual subscribers, ISDN can result in the offering of a series of new functions accessible to their homes over existing telephone wire. Electronic meter reading, slow scan video, Internet access for surfing the World Wide Web and transmitting and receiving electronic mail, and other applications are either offered or can be expected to be offered to individual subscribers and businesses.


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