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Practical limit for ADSL is 15000 feet (SBC won't sell it out any farther), and that may max out at as low as 384k speed. The limiting factor is actually how low the signal to noise ratio can go before the modem drops sync. When I was doing contract onsite maintenance for AOL Broadband, if I wasn't getting 10db or better, they would turn down the speed. In reality, I've seen sync as low as 6db. It's quite modem dependent, there are good modems and there are crappy modems. It's also quite sensitive to your wiring quality all the way from the demarc to your modem, and even to other devices on the line, filters notwithstanding. When we had real problems, we would install what's called a "NID splitter" out at the demarc, with a dedicated pair direct to the DSL modem - and the rest of the house the way it was, connected to the (heavy duty) filtered tap of the NID splitter - and if we had to, replace the modem with a Westell, those are the best modems out there for low SNR lines. The other modems AOL used were Actiontec (garbage) and Creative Broadband Blasters (pretty decent). Westells usually work down to 6db, Blasters go down to 7 or 8, but if you get an Actiontec below 10, you are lucky. I also noticed that the SNR readings can vary between modems on the same line hooked up the same way - again, Westells being the strongest and Actiontecs being the weakest. Stay AWAY from USB modems - they are more hassles than they are worth.
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