DSL Network Guide Chapter 6: Filtering your phone lines
On your phone lines the telephone service operates on a 0 Khz - 4 Khz, the DSL Upload operates on 20 Khz - 160 Khz and the DSL download operates on 240 Khz - 1100 Khz. Click here for a graph of this. Both the telephones and the DSL Modems cause noise on the other devices frequency. To prevent interference a filter needs to be places on every single device that uses the phone lines. This would include anything at all that uses the phone lines including fax machine and alarm systems. Each filter adds resistance. No more than five filters should be used on one phone line. If you need more than five filters you might want to consider a NID splitter.
The DSL modem should never have a filter on it. Some filters, called splitters, have an unfiltered port on them allowing both a phone device and the DSL modem to share one outlet.
If a phone device is missing a filter, a filter has gone bad, or the splitter isn't operating properly you can experience problems with your DSL or phone service. If you have static or computer-like noises on your phone lines turn off the DSL modem and unplug it from the telephone jack. If the noise continues it is not DSL related. If that fixes the noise then you probably have a bad splitter or filter.
If your modem needs to be rebooted frequently or you lose synch (i.e. your DSL light goes out) this could be caused by a bad splitter or filter. You can verify this by unplugging all the phone devices in your house and ensuring your modem plugs directly in to the wall jack with a chord no longer than about 5 or 6 feet. If this cures the problem then it's a filtering issue. If it doesn't then you need to contact your ISP. You can further verify it's not an issue inside your house by plugging the modem into your NID. If it's good at the NID then it's definitely something the ISP needs to be contacted about. If this cures the issue then there's a problem with your internal wiring.
If you find the filters annoying, have an alarm system, or have other reasons why the single line filters don't cut it you probably need a NID splitter. This is all covered in the next chapter.
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