(DMT) Discrete Multitone Modulation and others

[danielshoes]

My university engineering group is making a presentation discussing the xDSL technologies. We are focusing the ISO/OSI layers 1 and 2 and my part covers the modulation/codification techniques. Most of the information I find in the Internet is too much superficial and the ITU-T documents are simply huge and cover too much math and specification details. I would appreciate if someone could help me with a non-superficial font of information. Specially from telecom/computing students...

 

[unclebabar]

What do you mean by superficial? Seems to me that unless you throw in some formulae or at least some really good charts any engineering discussion is going to seem superficial. Who's going to be attending this presentation?

If I recall correctly OSI layer 1 is the physical layer and OSI2 is the data link layer. So you're supposed to run on about how to pipe bits down a telephone link. How does it work? I'd like to know myself. What are some of the link's you've checked out? I thought that DSL used high frequenciy (well > 8000 hz) modulation to send data (hence the need for LPFs on your voice lines). It seems to me that DMT (someone come shoot me down if I'm wrong) is just the name for sending your bits over those bands with some standard digital modulation (which should be listed in the spec).

 

[danielshoes]

unclebabar, thank you for your answer. My professors and my colleagues will attend. By saying superficial, I mean that most of DSL information in the net applies to the end user, mentioning the types of DSL, data rates, user oriented overview, etc... The technical papers don't go much deep, just slightly briefieng something about the technology itself. And the ITU-T documents are too much specific, pointed to the manufacturers, with more than 250 pages each one. I would like to interchange technical information with Telecom/Computing students, who could help me by forwarding me something they had done in the past, university papers, hot links, etc... I have already searched tons of sites on the net, but I wondered that maybe some "anandtecher" would help me with a valuable contribution.

You are right about the layers 1 and 2! Actually, DMT is much more than a simple modulation technique. It takes the bandwidth between 25KHz and 1.1Mhz, divides it in hundreds of narrowband slices (aprox. 4KHz each one) with individual carriers (positioned at the center frequency of its narrowband) being modulated in paralel, by using QAM, at a slow bit rate (up to 16Bit/Hz/sec) by the information signal (wich is splitted and distributed between the individual carriers). The RX device takes this stuff and does the opposite to get the original signal. There is a lot of math involved, like Fourier Transforms, considerations in the frequency domain, echo cancellation, noise immunity, rate adaptation, and much more. And I would like to gather these informations and compare the techniques used. Still, I want to take the layer 2 and, for example, discuss about the ATM or IP based networks, etc...

Thanks in advance for any contribution.

 

[unclebabar]

sorry I doubt I could help you. I took a Comm class about 7 years ago. My only advice is: better bring coffee if you plan on talking at length about different QAM signal constellations.

But if you get any good stuff, please share!

 

[danielshoes]

Actually, constellation diagram is a basic modulation concept well known in my class (I am graduating), nothing to be afraid or to fry the brain. After a long time searching, I found something good discussing the ADSL physical implementation (though quite a bit brief). Check it here. Is something like this I am searching for. I still would like to receive suggestions/links/papers/ppt/doc/pdf for my Telecom/Computing e-friends. Thanks in advance for those who can give me a helping hand! I am waiting for you guys!!!