ADSL communications? - Page 1
Forum section: electronics / projects

author: jwhitmore
author_rank: Regular Contributor
timestamp: November 21, 2015, 12:03:44 am
content: I'm struggling to find out information on ADSL as when you search on that term you just get people trying to sell you routers or modems.My broadband, at home, is from an ADSL line, and I recently had a thought "how does that work?" I've been trying to find out but the books that I found on Amazon are expensive as. One was over 100 sterling on amazon.co.uk and at that price you really would want to be sure that it was what you were looking for. For most communications protocols you can get an IC which does all the heavy lifting and you could connect it to a processor, of what ever grunt is required. Say for example Ethernet you'd get an NIC chip. ADSL seems not to fit into that category.I'm curious as ADSL is a protocol which, as far as I know, uses a spectrum of frequencies for communications, leaving the low, below 4KHz for audio. How hard is it to connect up a Chip or would you have to design multiple circuits for all the various frequencies?There is a book by Walter J. Goralski and that might be the way to go. I don't want to know too much information, but if I could connect an ADSL chip to a processor and establish some sort of connection to an exchange I'd love to try.Maybe all I'm missing is a search term that bypasses routers.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 21, 2015, 12:52:53 am
content: http://www.itu.int/rec/T-REC-G.992.3-200904-IHave fun.

author: tautech
author_rank: Super Contributor
timestamp: November 21, 2015, 01:24:19 am
content: Of course you'll have much better luck searching for "Asymmetric digital subscriber line"

author: Phoenix
author_rank: Frequent Contributor
timestamp: November 21, 2015, 01:47:50 am
content: Quote from: tautech on November 21, 2015, 01:24:19 amOr just Digital Subscriber Line.The A refers to the type of implementation - Asymmetric meaning upstream bandwidth is different to downstream bandwidth. This occurs because DSL uses OFDM and the individual frequency bands can be assigned as upstream or downstream.

author: tautech
author_rank: Super Contributor
timestamp: November 21, 2015, 02:12:19 am
content: Quote from: Phoenix on November 21, 2015, 01:47:50 amYep, wish I had that, up to 80 MBps here in NZ if you're very close to your local telco cabinet or exchange.

author: nowlan
author_rank: Frequent Contributor
timestamp: November 21, 2015, 03:00:21 am
content: Do you goto university? I was able to access expensive books online via my school's library.You may want to try google books to glean some information, see if it has what you're interested in.Fundamentals of DSL TechnologyImplementation and Applications of DSL TechnologyI dont think the telecoms bodies would like you messing with their phone lines. You might find it easier to work with an existing router that has linux. Use the provided drivers to see what information the chipset can offer.

author: crazy horse
author_rank: Contributor
timestamp: November 21, 2015, 06:22:07 am
content: I have designed adsl exchange equipment a long time ago.Basically you have a series of discrete multi tone (DMT) spaced at 4.3125kHz.The upstream band in something like 30-100kHz (exact numbers I forget, because it was a long time ago) and the downstream (exchange to customer) from there to 1MHz ADSL1/2 or 2MHz for ADSL2+. The filter you use is to seperate out POTS frequency, ie the 4kHz anologue voice signsl you mention and the ADSL.There is multiple bits encoded on each 4.3125kHz tone using QAM. The data is split and transmitted across the range of tones, then received on the various tones and recombined.All this magic gets done in some ASIC's. These ASIC's were then combined with processors so less chips are needed.That's a quick basic rundown. Feel free to ask any questions.

author: rs20
author_rank: Super Contributor
timestamp: November 21, 2015, 07:16:14 am
content: And IIRC, the actual implementation is not as complicated as the previous description might suggest. In practice, to transmit, you distribute your bits for one symbol amongst a bunch of complex numbers (according to the QAM constellations). Then, a simple IFFT will translate that into a time domain signal, with each complex number become a tone in the time domain signal.On the receiving end, a FFT (the inverse of the IFFT) recovers the original complex numbers (obviously with error introduced). I believe Veterbi decoders are used for error correction.Due to the number of ADSL routers out there, ASICs are obviously the most cost-effective solution; but I imagine a half-decent ADC + FPGA + DAC/DDS could easily to the job.I have a question: For a soft-decision Viterbi decoder designed for QAM, what does the branch metric look like?

author: coppice
author_rank: Super Contributor
timestamp: November 21, 2015, 08:23:02 am
content: Quote from: rs20 on November 21, 2015, 07:16:14 amThe branch metric depends on the encoding you are trying to decode. You need to find a cost function appropriate to that coding scheme, and the rest is the Viterbi algorithm you'll find in many text books. For example, if QAM + a trellis code is used the cost function will be the Euclidean distance of the actual constellation point from the set of nearest possible constellation points. That is the cost function results in a vector of 4, 8 16, or whatever values, which are the costs for each of the potential branches.

author: coppice
author_rank: Super Contributor
timestamp: November 21, 2015, 08:44:09 am
content: Quote from: jwhitmore on November 21, 2015, 12:03:44 amThe ADSL specs can be downloaded fromwww.itu.int. If you are interested in the basics of the technology, rather that the details of the protocol, its based on OFDM (or more specifically COFDM), although most most material calls it DTM for historic reasons. There is lots of material about OFDM on the web.Quote from: jwhitmore on November 21, 2015, 12:03:44 amADSL falls exactly into that category. Almost all ADSL equipment consists of an ASIC doing the heavy grunt, and a processor dealing with the higher protocol layers, just like any Ethernet system.

author: dom0
author_rank: Super Contributor
timestamp: November 21, 2015, 09:11:20 am
content: You just don't get to buy the chips, because DSL is telco stuff and that means they try hard to close the market up.

author: jwhitmore
author_rank: Regular Contributor
timestamp: November 21, 2015, 10:10:30 am
content: Thanks for all that information, it's been very helpful, even the basic comments about searching for digital subscriber line. That makes a world of difference.OK I'm sure Telcos wouldn't want me playing on their property but that's a really interesting protocol. I'll just read as much as I can.It's great that it does fit into my NIC example but a pity telcos close the market. An expansion board for the RaspberryPi which turned the RPi into a ADSL Modem would be the bog's bits.Like I say it's very interesting, but maybe I'm a nerd. Just sounds like the most complicated coper based form of communications. I mean even Ethernet is just serial comms over twisted pairs. Actually I should check that. I've just assumed that and as a result it's no more complicated then USB. Well in my head.Anyhow thanks a million for all your help. I'll be sipping tea and reading for a while. I studied FFT back in Uni years ago but it never made any sense. Maybe this is my chance to finally understand the frequency domain.John