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
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.
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.