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Chapter 2
Basic Communication
Technology

Questions Answered in This Chapter

What is an analog signal?

What is a digital signal?

What is analog-to-digital signal conversion?

How are digital signals multiplexed?

What is bandwidth?

What is Narrowband ISDN?

What is Broadband ISDN?

What does all this have to do with ATM and ADSL?

The Basics

To understand why ATM and ADSL are necessary for the continued evolution of communication systems (and the human creature) and how ATM and ADSL fit into the current communication technology, an understanding of some fundamental communication concepts and a review of the developments in transmission and switching systems since the advent of the telephone is necessary. Toward that end, this chapter presents those fundamental concepts in as straightforward and simplified a manner as possible without reliance on a difficult mathematical treatment of any topic. The presentation is of an introductory nature beginning with very basic electronic theory building blocks and only covering those topics necessary to understand ATM and ADSL. If the reader is interested in a more in-depth treatment of any topic, the bibliography provides sources rich in material.

The material is presented in a logical sequence. Unless you are already familiar with a topic, the best course is to read each of the following sections in the order presented, as each lays the foundation for the next. Upon completion you will be familiar with the concepts, approaches, and terminology used in modern communications technology.

AC/DC

Generally, there are two basic types of electrical current, alternating current (AC) and direct current (DC). Typically, AC current produces an AC voltage while a DC current produces a DC voltage. AC voltage/ current is alternating between a maximum positive level and a maximum negative level over some period of time. That is, AC voltage/ current is not static, but always changing. DC voltage/current never changes, always remaining the same level within some specified tolerance. See Figure 2-1. In communications equipment and systems, DC voltage is used mainly to bias, or set, the proper operating conditions for semiconductor and integrated circuit (IC) devices. AC voltage is used primarily to transmit signals over the various transmission systems, such as coaxial or microwave links. AC voltages are also used in electronic equipment to generate the DC voltages used to bias the equipment.


Figure 2-1.  Basic electrical signals

Analog Signals

An analog signal is characterized as continuous and changing with respect to time. An analog signal possesses level and periodic characteristics. An AC signal is always an analog signal. Yet, an analog signal is not necessarily an AC signal. The signal represented in Figure 2-2 is referred to as a sinusoidal waveform, as the value of the waveform, at any point in time, varies in accordance with the sine of the function. It is both an analog signal and an AC signal.


Figure 2-2.  Graphical representation of an analog signal

AC and analog signal levels are typically measured in volts either from peak to peak (pk-pk volts), just peak (pk volts), average (avg volts), or RMS (rms volts). Peak to peak levels are measured from the maximum (peak) positive value to the maximum (peak) negative level. Peak level is measured from the zero reference level (the point where the waveform goes from concave up to concave down) to either the positive or negative peak. Average level is the average DC level (zero in this case since there is as much "negative voltage" as there is "positive"). The RMS level is .707 times the maximum or minimum peak level.

An analog signal's time period is the time the waveform takes to go from any value through a positive peak and a negative peak and return to that same value. Typically, the period is determined by taking the time for the waveform to go from a positive peak to a negative peak back to a positive peak. One complete cycle represents one period of the waveform. Frequency is the number of periods of a signal in one second measured in hertz (abbreviated as Hz). See Figure 2-3. So, 1.5 MHz represents a signal that varies through its complete cycle (positive peak-negative peak-positive peak) 1,500,000 times per second.


Figure 2-3.  Signal period

Only a changing signal such as an analog or digital signal can convey intelligent information. A signal that never changes (DC) cannot be used to convey much useful information. Two analog signals, one representing the varying information such as voice and called the modulating frequency, and a constant frequency analog signal called the carrier, can be combined in a process called modulation (transmit side) and separated in a reverse process called demodulation (receive side) to transmit the information long distances.


Figure2-4.  Graphical representation of binary bit stream


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