On this week’s show, Jason Zerbe learns about the technology behind DSL.

The topics for R&D are:

  1. Do DSL soft modems exist and how do they work?
  2. Is DSL soft modem the source code available?
  3. Can an end-user create their own DSL links?

According to the Wikipedia article on DSL soft modems, the current lack of DSL soft modems is because of SOHO (small office/home office) networking. Hence the embedded DSL gateways take care of the modulation and networking. I’ve found out that speed of the DSL service is directly proportional to the modulation type. In order from least efficient modulation to most efficient: T1413, GDMT, GLITE, ADSL2, ADSL2Plus. Then there is also MMODE modulation (multi-mode) that automatically detects what type of modulation is being used. In my area (central Woodbury, Minnesota) QWest is using GDMT modulation that is auto detected by the MMODE setting on their DSL gateways. For a list of DSL modems supported by customer service, you can check out this page.

There are plenty of papers and articles about how DSL works (a 2001 Microsoft paper) and twice as much documentation and source code about analog soft modems. I could not find any source code that deals with building a DSL soft modem. I did find a 1999 precursor to “full DSL” done by Andreas Gustafsson. Too, bad guess we’re going to have to stick with those little embedded devices and wall-wart combos.

Yes, you can create your own DSL modem to DSL modem links, with the cooperation of the reigning TelCo (telephone company). To do so you must purchase a “dry copper” line, typically used for alarm systems, and have the TelCo route the dry line to another dry line. “Dry copper” is called such, because it carries no power or dial-tone, as opposed to “wet copper” or a line carrying dial-tone. At the termination of this other dry line, you have your buddy place a DSL modem on his end, and you place a DSL modem on your end. Presto, long-range LAN/WAN. For more information and ideas about this: check out the pbs.org article from Robert X. Cringely.

12/22/2007 – Add: I was poking around and I found a retailer of DSL modems and Ethernet extenders. I have been experiencing some trouble with the 100Mbps Ethernet link & Telephone combo that runs on the Cat5 that I installed last summer. It seems as though I might have exceeded the Ethernet spec’s distance.