Understanding ADSL DMT Carrier Waves: Questions & Answers

[bradland]

Hello,
I am hoping someone out there can me better understand how an ADSL DMT carrier wave works. Is the carrier wave made of photons or electrons?
I have tested the dry loop at my house and from what i can tell this is not an electrical circuit, but two open wires. This begs the question why are two wires needed?
If the signal is composed of electrons what direction(s) does the current flow through the wires? How does this relate to a DC or AC current.
How does the ADSL line driver generate the carrier signal?
What does the Power spectrum density measure in layman terms ?
How does the PSD relate to an AC or DC voltage?

I understand it may be difficult to explain in one post but any reference to books or websites would be greatly appreciated. thanks, brad

 

[jsgruszynski]

Hello,
I am hoping someone out there can me better understand how an ADSL DMT carrier wave works.
Is the carrier wave made of photons or electrons?

Electrons. It's an electronic system, not a photonic system. You may have an upstream connection beyond the ADSL modem on the phone company side (called a DSLAM) that is fiber optic but you can completely separate the ADSL portion from it both conceptually and in terms of testing.

I have tested the dry loop at my house and from what i can tell this is not an electrical circuit, but two open wires. This begs the question why are two wires needed?

It simply means the wires are connected to a high impedance. If you have an ADSL filter installed, this is what you'd expect for a DC measurement because the filter is isolating the voice channel of the POTS from the ADSL channel, which is at frequencies above the voice channel. Only the voice channel goes down to DC (it uses DC for on-hook/off-hook signaling and for power).

You still need 2 wires for a transmission line for AC, which ADSL is.

If the signal is composed of electrons what direction(s) does the current flow through the wires? How does this relate to a DC or AC current. How does the ADSL line driver generate the carrier signal?

ADSL is primarily a modem signal that is purely AC. All modulated signals are AC. ADSL just happens to be a digital modulation that is outside the standard voice channel spectrum of POTS. Analog modulation includes stuff like AM, FM and PM. Digital modulation includes FSK, PSK, QAM, etc. ADSL uses QAM (quadrature amplitude modulation) in several variants. CAP is the simplest and uses straight QAM. DMT uses several varieties of QAM at once.

What does the Power spectrum density measure in layman terms ? How does the PSD relate to an AC or DC voltage?

The actual ADSL signal is "stochastic" or "randomly changing in a predictable statistical way" because the actual data going over the wire depends on what's transmitted and really can't be known before hand. And to unnecessarily confuse the issue, there are reasons having to do with maximizing channel capacity why you want the resultant modulation to be "extra random" in a predictable way.

Power Spectrum Density is a metric for stochastic signals that can be used for error detection and correction. It's basically looking at a signal with a spectrum analyzer but with long-term statistics about the power at each frequency. Most spectrum analyzers can extract a PSD approximation for you. When you have a PSD that doesn't match the channel frequency characteristics it generally means you are getting errors and/or lower effective transfer bandwidth. See the PDF link below for what the frequency characteristic of ADSL is suppose to look like.

You can also use the PSD to perform certain types of corrections for channel distortion and signal impairment. An ADSL modem will use an approximation of PSD to do this automatically using digital signal processing (DSP).

DC only in the sense of "What frequency DC?" There is mathematically ideal DC and then there is "DC" that engineers (and electronics hobbyists) use. Often folks think "DC" is ideal DC but it's not.

In a strict mathematical sense, "DC" is a very fuzzy concept because at some point you had to turn on the "DC" and that act introduces a low frequency AC component to the "DC". Thus "DC" is actually ideal DC and AC mixed together.

So EEs who do DSP and microwave work will often talk about "What frequency DC do you really mean?" because what constitutes "DC" is frequency dependent quantity in practice. Many microwave engineers consider 30 MHz to be "DC", for instance.

A signal's PSD can have a DC component with all the AC components. However because ADSL operates in a band above voice audio and this ideal DC (or most intuitive senses of DC frequency), DC doesn't matter with ADSL's PSD. For ADSL, "DC" is any frequency below the top of the POTS voice channel.

See also:

http://en.wikipedia.org/wiki/Spectral_density

I understand it may be difficult to explain in one post but any reference to books or websites would be greatly appreciated. thanks, brad

These PDF document will probably help:

http://home.pacbell.net/bitrider/tb1000.pdf

http://www.sce.carleton.ca/courses/sysc-5403/f06/DSL_ST.pdf

The math on the 2nd one is both essential to really understanding ADSL in detail but also very hairy if you don't have an engineering degree.

Also you can go look at the various ITU standards for modems that operate in the POTS voice channel because the more recent ones (e.g. V.32bis) are pretty similar to ADSL modulation except that ADSL operates above the voice channel to much higher bandwidth (hence the higher speed possible compared to voice channel modems).

http://www.itu.int/itu-t/recommendations/index.aspx?ser=V

 

[berkeman]

Very nice post, jsgruszynski!

 

[bradland]

Thanks jsgruszynski. That is an awesome post.