Signal bandwidth: A signal can be decomposed into a series of weighted sums of different frequency sine and cosine functions. Bandwidth is the frequency width of the trigonometric function corresponding to the weighted non-zero part. The width of the signal spectrum is the difference between the highest frequency component and the lowest frequency component of the signal. For example: a square wave signal superimposed by several sine waves, its lowest frequency component is its fundamental frequency, assuming f = 2kHz, and its highest frequency component is its 7th harmonic frequency, that is, 7f =7×2=14kHz , So the signal bandwidth is 7f-f =14-2=12kHz. In computer networks, bandwidth is used to indicate the ability of a channel in the network to transmit data, so network bandwidth indicates the "highest data rate" that a channel in the network can pass through in unit time.
Channel bandwidth: It defines the lower limit frequency and upper limit frequency of the signal allowed to pass through the channel, that is, it limits a frequency passband. For example, the allowable passband of a channel is 1.5kHz to 15kHz, and its bandwidth is 13.5kHz.
How to judge that the signal can pass through the channel: The lowest frequency component and highest frequency component of any composite signal can pass through the channel within the frequency range of the channel. For example, the allowable passband of a channel is 1.5kHz to 15kHz, and its bandwidth is 13.5kHz. Various single-frequency waves with frequencies of 1.5kHz, 4kHz, 6kHz, 9kHz, 12kHz, 15kHz and any within this frequency band can also pass through this channel. However, if a square wave with a fundamental frequency of 1kHz passes through the channel, the distortion will be serious; if the fundamental frequency of a square wave signal is 2kHz, but the highest harmonic frequency is 18kHz, the bandwidth exceeds the channel bandwidth, and its 9th harmonic will Filtered out by the channel, the square wave received through this channel is not of good quality; then, if the fundamental frequency of the square wave signal is 500Hz, the highest frequency component is 5.5kHz for the 11th harmonic, and its bandwidth only needs 5kHz, If it is much smaller than the channel bandwidth, can it pass the channel well? In fact, when the signal is transmitted on the channel, the fundamental frequency is filtered out, and only the harmonics can pass, and the signal waveform must be unsightly.