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Homebrew DSL Revisited

Members of the ISP-DSL list delve into the thorny topic of equipment options for homebrew DSL. While the wrong equipment won't work at all, the right modems can converse over barbed wire (for a short distance).

[February 28, 2002]

On the ISP-DSL list in February, SS inquired,

"I've read the archives regarding 'home-brewing' DSL using dry copper pairs from the phone company, but one thing still isn't clear. I have a pair of SDSL modems, one from 3Com and one from Efficient Networks, and I plugged them directly into each other to see if they would synch. They do not. Do I need to have a special kind of modem? Is it a problem that they're two different brands?"

A number of respondents explained that not all modems will do the trick:

[VB noted] "Neither DSL adapter you have is capable of working back to back, without a DSLAM. The 3Com SDSL modems do not have the smarts to work in CO mode, only in CPE mode. The intelligence of the DSL connection is in the CO side unit (usually a DSLAM port); the CPE mode unit is a relatively dumb device. To operate back to back, you must have one unit capable of operating in CO mode and one capable of operating in CPE mode."

[MS agreed] "You have to have two units that will work together. You can't play tennis with a badminton racket. They both hit things and look similar, but… well, you get the idea. Get rid of what you have and get the right tools."

Others noted that, despite the limitations, there are a number of options to choose from:

[BS suggested] "Look for some Net to Net SDSL boxes."

[JG added] "FlowPoint modems will do it as well, and so will Lucent DSL-HSTs."

[JJ observed] "Elastic modems work in a back to back configuration. In fact, as one of their demos, they actually run them over a short pair of barbed wire!"

[JB offered] "PairGains are a great product, too, but the most cost-effective I've found are from Xpeed."

SS asked a key follow-up question:

"How do I know if my installed line will support DSL, without first buying the equipment?"

Unfortunately, DH did not have the answer one might hope for:

"You can't know. Still, you can always re-eBay them if it doesn't work…"

—End