Answer to Question 1
The guardband of frequency is an unused portion of the circuit. The guardband is necessary so that individual channels remain distinct within the single circuit. This means that the total capacity of the circuit cannot be used because guard-bands are required to ensure that individual channels maintain their integrity. Guardbands are a form of overhead. Overhead is an additional element that must be added to a technology in order for that technology to work. As a rule, lower overhead is more desirable because it is more efficient.No data are carried in the guardbands. The purpose of the guardband is to prevent the separate channels from interfering with each other, which would result in crosstalk.
Answer to Question 2
Modems are devices that modulate a digital signal into an analog signal on the sending end, and then, at the receiving end, demodulate the signal from analog back to digital. A digital device is at each end of the communication. The term modem stands for modulator/demodulator. As you read earlier, analog signals can be modulated using amplitude, frequency, or phase modulation. Modems also use simplex, half-duplex, or full-duplex connections, depending on the type of modem.
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