Analog and Digital Signaling

Everything you hear, including human speech, is in analog form. Until several decades ago, the telephony network was based on an analog infrastructure as well.

Although analog communication is ideal for human interaction, it is neither robust nor efficient at recovering from line noise. (Line noise is normally caused by the introduction of static into a voice network.) In the early telephony network, analog transmission was passed through amplifiers to boost the signal. But, this practice amplified not just the voice, but the line noise as well. This line noise resulted in an often unusable connection.

Analog communication is a mix of time and amplitude. Figure 1-4, which takes a high-level view of an analog waveform, shows what your voice looks like through an oscilloscope.

Figure 1-4. Analog Waveform

Figure 1-4. Analog Waveform

If you were far away from the end office switch (which provides the physical cable to your home), an amplifier might be required to boost the analog transmission (your voice). Analog signals that receive line noise can distort the analog waveform and cause garbled reception. This is more obvious to the listener if many amplifiers are located between your home and the end office switch. Figure 1-5 shows that an amplifier does not clean the signal as it amplifies, but simply amplifies the distorted signal. This process of going through several amplifiers with one voice signal is called accumulated noise .

Figure 1-5. Analog Line Distortion

Figure 1-5. Analog Line Distortion

In digital networks, line noise is less of an issue because repeaters not only amplify the signal, but clean it to its original condition. This is possible with digital communication because such communication is based on 1s and 0s. So, as shown in Figure 1-6, the repeater (a digital amplifier) only has to decide whether to regenerate a 1 or a 0.

Figure 1-6. Digital Line Distortion

Figure 1-6. Digital Line Distortion

Therefore, when signals are repeated, a clean sound is maintained. When the benefits of this digital representation became evident, the telephony network migrated to pulse code modulation (PCM).

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