Introduction to Data Communications
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23c. Nyquist Theorem

In a digital Public phone system, the signal leaving our telephone at our house is an analog signal. It goes to the Central Office through the Local Loop. The Local Loop is the name for the wires that run from our house to the Central Office. The Central Office (also called a local exchange) is the building that all the neighbourhood phones with the same local connect. A local is the 1st 3 digits of your 7 digit phone number or LDN (Listed Directory Number).

At the Central Office, the analog signal is converted into a digital signal consisting of 1s and 0s.

The Nyquist Theorem states that to accurately reproduce an analog signal with a digital signal, the analog signal must be sampled a minimum of 2x the highest frequency of the analog signal.

This means that for the Voice Channel (0 to 4 kHz) to be digitized, we must sample the Voice Channel at 2x the highest frequency (4 kHz) which would be 8 kHz. This means that as soon as you digitize an analog signal, you must immediately double the bandwidth.

Introduction to Data Communications
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