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When DSL (ADSL) is provided over voice lines, all devices in the house except the DSL modem must be connected through filters. The filter protects the phone devices from high frequency noise. They are low-pass filters. The filter is also there to protect the DSL signal from being contaminated by high frequency noise added by analog phone devices, answering machines, etc.
During a Line Sharing install, one important question is how many devices require filtering, as this determines how many filters are supplied with the install kits.
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