Baseband signal is a digital signal transmitted directly without modulation, which is different from frequency band signal. The band signal is transmitted by loading the baseband signal on the carrier, and the band of the baseband signal is very wide (theoretically infinite). However, due to the band-pass, there is almost no transmission medium with infinite bandwidth, so the baseband signal can't be transmitted over a long distance on ordinary media, otherwise the signal can't be recovered by intersymbol interference and attenuation. Therefore, modulating the baseband signal with carrier can reduce the bandwidth and make the signal reliable.

The so-called bandwidth refers to the spectrum width of the signal, that is, the sum of the frequencies of all sine waves that it can be decomposed into. The carrier frequency is relatively single, so the bandwidth of the modulated signal is small.

Broadband signal is a relative concept, which means that its transmission medium has wide band-pass capability. This advantage is that it can multiplex a large number of signals on a transmission medium and save the cost of line laying. A signal transmitted over a broadband medium is called a broadband signal. At present, the widest bandwidth medium is single-mode fiber.

Narrowband: It is a traditional telephone line access method, including the familiar 33.6KMODEM, 56K MODEM (collectively referred to as public telephone network) and the use of wireless phones (that is, surfing the Internet through mobile phones). ISDN, that is, integrated services digital network, still belongs to the category of narrowband network.