In digital communication: there are baseband transmission and frequency band transmission. Where are they used? Seek expert interpretation, urgent!

Simply put, the baseband signal of digital signal is the low-frequency signal electrical signal that is not modulated by digital code words, and baseband transmission is the transmission of baseband signal; The frequency band transmission must be a frequency modulated signal. In order to transmit data better, the signal is frequency-modulated to a higher transmission band. For example, the signals in the computer network cable are definitely baseband signals, while the IF, HF and UHF signals received by TV broadcasting are band signals.

The signal transmitted on the optical fiber is encoded by the photoelectric conversion module of the optical transceiver, and the HDB3 code is a telecom number, so it must be encoded and converted before it can be transmitted on the optical fiber. The code type of SDH transmission is scrambling code NRZ, while there are many PDH codes, and NRZ scrambling code and 5b6b code can be used.

Finally, I personally feel that SDH and PDH systems do not need to modulate signals when transmitting signals on optical fibers, but only need to multiplex electrical signals.