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Amplifiers (Analog)

The boosting devices in an analog network are known as amplifiers. Amplifiers boost, or amplify the weak incoming signal, much like an amplifier in a radio or TV. As it traverses the network the signal accumulates noise. Through every step of the transmission and through each amplifier, the noise is amplified along with the signal, creating the potential for significant accumulated noise at the receiving end of the transmission. The resulting signal-to-noise ratio can be unacceptable. Amplifiers typically are spaced every 18,000 feet or so in an analog network.

If you take a bale of hay and tie it to the tail of a mule and then strike a match and set the bale of hay on fire, and if you then compare the energy expended shortly thereafter by the mule with the energy expended by yourself in the striking of the match, you will understand the concept of amplification.

William Shockley, co-inventor of the transistor [2-4].

The impact of amplification on voice communications generally is tolerable, as humans are relatively intelligent receivers who can filter out the noise or, at least adjust to it. In the event of a truly garbled transmission, the human-to-human error detection and correction process simply involves a request for re-transmission. Should the quality of the connection be totally unacceptable, the connection can be terminated and re-established. Computer systems, however, are not so forgiving, and garbled data is of decidedly negative value.

Repeaters (Digital)

In a digital system, periodic amplifiers are replaced by regenerative repeaters, which regenerate the signal, rather than simply amplifying it. The repeater guesses the binary value (1 or 0) of the weak incoming signal based on its relative voltage level and regenerates a strong signal of the same value, without the noise. This process immensely enhances the signal quality. Repeaters are spaced at approximately the same intervals as amplifiers, although spacing is sensitive to the carrier frequency, which affects both transmission speed, or bandwidth provided, and the level of attenuation experienced.

The performance advantage of digital networks can be illustrated by comparing the error rate of amplifiers and regenerative repeaters. For example, a twisted-pair, analog network can be expected to yield an error rate on the order of 10-5. In other words, digital data sent across an analog network will suffer 1 errored bit for every 100,000 bits transmitted (Figure 2.7). The very same twisted-pair network, if digitized and equipped with repeaters, will yield an expected error rate of 10-7, or 1 errored bit in every 10,000,000. This is an improvement of two orders of magnitude. Digital fiber optic systems, currently considered to be the ultimate, yield error rates in the range of 10-11 to 10-14, or an error rate as low as 1 bit for every 100,000,000,000,000 transmitted—virtually perfect [2-3]!


Figure 2.7  Comparative error performance of analog vs. digital transmission over twisted pair.

The Conversion Process: Digital to Analog (D to A) and Analog to Digital (A to D)

Regardless of the relative merits of analog and digital transmission, both technologies are available. Local telephone loops, which connect the user premise to the central office exchange generally are analog, as are US cellular radio networks. High-capacity, backbone carrier transmission generally is digital. Therefore, analog-to-digital and digital-to-analog conversions routinely take place in the carrier-provided networks.

Digital to Analog: Modems

As local loops generally are analog, computer communications across such circuits is not possible without the assistance of a device to accomplish the digital-to-analog conversion. Of course, one might gain access to a more expensive digital circuit, by so specifying, if it is available.

The device which accomplishes D-to-A conversion is known as a modem. Modems MOdulate and DEModulate the analog sine wave represent digital bit streams across the analog local loop, reconstructing the digital signal on the receiving end through a process of A-to-D conversion (Figure 2.8). A variety of techniques are used to accomplish this process and are discussed in Chapter 7.


Figure 2.8  Modem: Digital-to-Analog Conversion.

Analog to Digital: Codecs

The reverse conversion process is necessary to send analog information across a digital circuit. This is often the case in the carrier networks, where huge volumes of analog voice are digitized and sent across high capacity, digital circuits. This requirement also exists where high capacity digital circuits connect premise-based, PBX voice systems to central office exchanges or to other PBXs, assuming that the PBXs or COs have not already performed the conversion. As video also is analog in its native form, a similar process must be employed to send video across a digital circuit.

The device that accomplishes the A-to-D conversion is known as a codec. Codecs COde an analog input into a digital (data) format on the transmit side of the connection, reversing the process, or DECoding the information, on the receive side, in order to reconstitute the analog signal (Figure 2.9).


Figure 2.9  Codec: Analog-to-Digital Conversion.

Encoding is the process of converting an analog information stream (e.g., voice or video) into a digital data stream. The voice or video signal is sampled at frequent intervals with each sample of amplitude then being expressed in terms of a binary (computer) value, which is usually a 4-bit or 8-bit byte. The reverse process of decoding takes place on the receiving end, resulting in recomposition of the information in its original form, or at least a reasonable approximation thereof.


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