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Analog is best explained by examining the transmission of a natural form of information, such as sound or human speech, over an electrified copper wire. In its native form, human speech is an oscillatory disturbance in the air which varies in terms of its volume or power (amplitude) and its pitch or tone (frequency). As sound compression waves fall onto a transmitter, analogous (approximate) variations in electrical waveforms are created over an electrical circuit. Those waveforms maintain their various shapes across the wire until they fall on the receiver or speaker, which converts them back into their original form of variations in air pressure.
A similar, but more complicated, conversion process is employed to transmit video over networks. In its native form, video is a series of still images captured and transmitted in rapid succession in order to create the illusion of fluidity of motion; image information is reflected light waves. Analogous variations in electrical or radio waves are created in order to transmit the analog video image information signal over a network from a transmitter (TV station or CATV source) to a receiver (TV set), where an approximation (analog) of the original information is presented.
Information which is analog in its native form (voice and image) can vary continuously in terms of intensity (volume or brightness) and frequency (tone or color). Those variations in the native information stream are translated in an analog electrical network into variations in the amplitude and frequency of the carrier signal. In other words, the carrier signal is modulated (varied) in order to create an analog of the original information stream.
The electromagnetic sinusoidal wave form, or sine wave, can be varied in amplitude at a fixed frequency, using Amplitude Modulation (AM). Alternatively, the frequency of the sine wave can be varied at a constant amplitude, using Frequency Modulation (FM). Additionally, both frequency and amplitude can be modulated simultaneously in order to create an analog of the native signal, which generally varies along both parameters simultaneously. Finally, the position of the sine wave can appear to be manipulated, adding the third technique of Phase Modulation (also known as Phase Shift Keying or PSK). This provides additional benefits. These benefits will be discussed in Chapter 7.
Bandwidth, in the analog world, is measured in hertz (Hz). The available bandwidth for a particular signal is the difference between the highest and lowest frequencies supported. For example, a 3.3 kHz voice channel can be provided through a band-limiting filter supporting transmission at frequencies between 200 Hz and 3,500 Hz. Similarly, a 3.3 kHz channel is provided at frequencies between 7,000 Hz and 10,300 Hz. Passband refers to the upper and lower cutoff frequencies at which the filters operate [2-2].
Voice
A voice grade channel is approximately 4,000 Hz, or 4 kHz. Approximately 3.3 kHz (200 Hz to 3,500 Hz) is used for the voice signal itself. The remaining bandwidth is used for purposes of network signaling and control, and in order to maintain separation between information channels. While human speech transmission and reception encompasses a much wider range of frequencies, 3.3 kHz is considered to be quite satisfactory and is cost-effective. Band-limiting filters are used in carrier networks to constrain the amount of bandwidth provided for a voice application [2-2] and [2-3]. Figure 2.5 illustrates an analog local loop supporting voice communications.
Figure 2.5 Analog voice transmission over a two-wire local loop.
Video
A CATV video channel is approximately 6,000,000 Hz, or 6 MHz. Approximately 4.5 MHz is used for information transmission, while the balance is used for guard bands to separate the various adjacent channels riding the common, analog coaxial cable system.
While the natural world is analog in nature, computers (which are decidedly unnatural beings) are digital in nature. Computers process, store, and communicate information in binary form. That is to say that a unique combination of 1s and 0s has a specific meaning in a computer language. A bit (binary digit) is an individual 1 or 0. Multiple bits travel across a network in a digital bit stream.
Digital communications dates to telegraphy, in that the varying length of making and breaking an electrical circuit resulted in a series of dots and dashes which, in a particular combination, communicated a character or series of characters. Early mechanical computers used a similar concept for input and output; contemporary computer systems communicate in binary mode through variations in electrical voltage.
Digital signaling, in an electrical network, involves a signal which varies in voltage to represent one of two discrete and well-defined states: such as either a positive (+) voltage and a null, or zero (0), voltage (unipolar); or a positive (+) or a negative (-) voltage (bipolar). The receiver monitors the signal, at a specific frequency and for a specific duration (bit time) to determine the state of the signal. Various data transmission protocols employ different physical states of the signal, such as voltage level or voltage transition. Because of the discrete nature of each bit transmitted, the bit form is often referred to as a square wave. Digital devices (Figure 2.6) prefer digital transmission facilities.
Figure 2.6 Digital communications between terminal and mainframe.
Digital signaling in an optical network can involve either the pulsing on and off of a light stream, or a variation in the intensity of the light signal. Digital transmission over radio systems (e.g., microwave, cellular or satellite) can be accomplished by varying the amplitude of the signal.
In the digital world, bandwidth is measured in bits per second (bps). The amount of bandwidth required depends on the amount of raw data to be sent, the desired speed of transmission of that set of data, and issues of transmission cost. Additionally, data is routinely compressed by various means in order to enhance the efficiency of transmission and to reduce transmission costs. Additionally, analog voice commonly is converted to a digital bit stream, requiring a maximum of 64 Kbps for full fidelity or quality.
Although analog voice and video can be converted to digital, and digital data can be converted to analog, each format has its own advantages.
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