why don't we use Broadband technology to transmit digital signals ,and use Baseband technology to transmit analog signals?
&quot;Broadband technology&quot; is not inherently coupled with &quot;analog signal transmission&quot;, as &quot;baseband technology&quot; is not coupled with &quot;digital signal transmission&quot;. Both wire cables and optical fiber cables transmit signals in the form of electromagnetic waves, in RF frequency range for a wire medium and in IR/light frequency range for an optical fiber medium.
For digital transmission, the source data are transformed to changes in amplitude/frequency/phase parameters of a generated output signal and then the pulse shaping and filtering operations are performed on this signal. Resulting voltage/current or amplitude/phase waveforms have no sharp rise/fall jumps like those you see on the logic analyzer display. In this way, the resulting waveforms look similar to an analog signal.
In a baseband transmission the signal is sent &quot;as is&quot;; in a broadband transmission a modulator device creates a passband signal from a baseband signal and a carrier. The passband signal frequency is shifted by modulation process and occupies the frequency band around the carrier frequency with the width of the baseband signal spectrum. If the entire channel is used for a baseband transmission, we can use a medium's full bandwidth for the baseband signal. On the other hand, passband techniques allow frequency-division multiplexing, and we can share the channel for multiple concurrent transmissions. The number of subchannels is defined by a medium's full bandwidth divided by a baseband signal spectrum width.
Cable television systems use &quot;broadband technology&quot; for distribution of multiple analog video channels. Digital networks also can use frequency-division multiplexing (the &quot;broadband technology&quot;, as your reference calls it). In 1980s, 10BROAD36 Ethernet standard was developed in an attempt to use analog TV cables for Ethernet networking. The 10BROAD36 communication line was even able to share a cable with TV channels. The attempt failed for a number of reasons. What's more, the frequency multiplexing is not a single (and so unavoidable) solution for sharing communication lines in packet-switching networks of Internet. The sharing of a communication line to multiple data streams is performed with packet switching. Still, frequency-division multiplexing is used in wired (Discrete Multitone Transmission) and wireless (WiFi, OFDM) digital transmissions.
As for analog transmissions in rare survived analog telecom systems, I do not know of analog signals with spectrum that wide as to occupy the entire bandwith of transmission lines. So these systems can safely use frequency multiplexing to share cables for multiple transmissions (&quot;broadband technology&quot;, as your reference calls it).