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I was reading a textbook which says:

Baseband transmission requires that we have a low-pass channel, a channel with a bandwidth that starts from zero. This is the case if we have a dedicated medium(such as coaxial or fiber optic cable) with a bandwidth constituting only one channel. For example, the entire bandwidth of a cable connecting two computers is one single channel.

I'm a little bit confused, if dedicated medium means low-pass channel, why we still need Broadband Transmission (Using Modulation) to have bandpass channel—a channel with a bandwidth that does not start from zero and we need to convert the digital signal to an analog signal before transmission. Because for example, broadband transmission using modulation is the sending of computer data througha telephone subscriber line, the line connecting a resident to the central telephone office.

Isn't the subscriber line also a dedicated medium, connecting a user and telephone office?

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Baseband signaling requires a (relatively) good medium. (More or less) Voice-grade Cat-3 twisted pair is only good for 10 Mbit/s and only for 100 m reach. Your local loop usually has even lower-grade cabling.

So for a decent throughput and better reach you require a sophisticated, multi-carrier modulation scheme like one of the DSL standards. Only when utilizing the full spectral capabilities of a cable can you actually run something like 50 Mbit/s over a few 100 m of low-grade telephone cable.

That requires very elaborate line encoding including quadrature amplitude modulation (QAM), discrete multi-tone modulation (DMT), and active noise cancellation. The encoding is easily a hundred times more complex than Ethernet's simple baseband code at the same speed.

As a side effect, broadband signaling enables you to e.g. leave out the low frequencies used by POTS, so you can use a frequency splitter to run POTS and ADSL in parallel on the same line (effectively using frequency-division multiplex FDM).

The multiplex generates multiple channels out of a single original one - which perhaps answers your question.

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  • Thanks for your answer. and I don't have EE background, so it is a little hard for me to understand, I got another simple answer as "a subscriber line is a dedicated TELEPHONE line. So its baseband channel is connected to the telephone. That means to run broadband across THE SAME line, it must be modulated on a higher frequency channel.", is this correct?
    – amjad
    Jul 7, 2020 at 23:36
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    "a baseband channel" is the actual usage, "a telephone line" is what you've got. "to run broadband across THE SAME line, it must be modulated on a higher frequency channel" - essentially, yes.You can only multiplex signals on a single pair of wires when each has its own frequency domain.
    – Zac67 ♦
    Jul 8, 2020 at 6:21
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On a typical telephone subscriber line, there are actually two channels. The baseband channel, used for voice, and a broadband channel used for data.

On data-only subscriber lines, the same broadband technology is used. No point in using two different technologies.

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