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I have heard DSL prevents some items that use phone lines from functioning properly, such as home alarms.

First of all, is it possible to use a fax machine on the same line as a DSL modem and connection? And if so, should the fax machine sit behind a DSL filter or not?

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A fax machine uses the same frequencies that an analog telephone uses, so it should be connected in exactly the same way: behind the splitter.

The splitter filters out the higher ADSL frequencies for those devices, which otherwise might see interference and fail to function or even interfere with your ADSL signal.

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  • A fax machine uses the same frequencies that an analog telephone uses - Ah, perfect! That is the key idea I was looking for. I assumed since more data is transmitted in a fax, it would use those higher frequencies as well. – IQAndreas Aug 7 '15 at 5:02
  • If you think that answers your question, please mark the answer as accepted. :-) – Sebastian R. Aug 7 '15 at 15:45
  • And some security systems will choke on DSL filters because they operate on the same frequency as a DSL signal. To a human ear the screeches might sound the same, but technically they are not. – Giacomo1968 Dec 19 '15 at 22:55
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Should the fax machine sit behind a DSL filter or not?

Yes it should.


ADSL Filters & Splitters

Generally speaking the terms "micro-filter", "splitter" and "ADSL filter" mean the same thing.

When your telephone line is ADSL enabled it is able to carry information on different frequency ranges: ADSL uses the higher frequencies to transmit data, whilst voice uses the lower frequency band.

Although you will still be able to use your telephone as normal once your line is activated, the transmission of adsl data on the line will cause interference.

In order that the adsl data does not interfere with the normal voice data (and vice-versa), you need to ensure you install a filter on each device that uses the lower frequencies. These devices are telephones, fax machines, answer phones, sky boxes.

Source ADSL Filters & Splitters


Why do I need ADSL filters (microfilters) and where do they go?

A microfilter is a device that allows broadband to work at the same time as your telephone service. Without microfilters you may get problems with your BT broadband or experience noise on your telephone line.

Normally, you'll need a microfilter for every telephone socket in your home where you've got some kind of telephony or broadband equipment plugged in. This equipment includes:

  • Your BT Home Hub, or another type of broadband router or modem
  • Telephones
  • Answering machines
  • Digital TV boxes (for example, Sky boxes)
  • Fax machines
  • Alarm systems

Source Why do I need ADSL filters (microfilters) and where do they go?

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