How ADSL filters work and where they go

[Saturday]

Along with your nice new router you got a couple of little filters.

What are they for?

Well, ADSL, the technology O2 uses for its Broadband service runs on the same line you use for normal telephone calls. It is able to do this because voice and ADSL signals use different frequencies. Problems can arise however when stray frequencies generated by telephone equipment cross over into the part used for ADSL and this will corrupt the data being transmitted. That’s why you need filters on an ADSL line.

An ADSL filter is a small electronic gadget which restricts the frequency of the signal passing through it. To get a good ADSL connection your router needs as much of the available frequency bandwidth as possible. Voice calls only need a comparatively small share of the bandwidth so the filters supplied by O2 in effect share this out between the router and any telephony equipment you have.

You will see that the filter has one connection in and two out. This is important. The one marked with the phone symbol is filtered and only lets through the frequencies needed for voice/fax etc. The one marked as modem is completely unfiltered so when the router is connected to this it gets all the bandwidth including any voice signals - which the router simply ignores.

Where do you fit them?

Your router needs a very large part of the available bandwidth to function properly; your telephony equipment needs a tiny part. As you’ll have read above, this means that the phone socket of the micro filter only lets a small part of the frequency spectrum through; the part needed for voice/fax. You must never connect your router to the phone outlet of a filter either directly or indirectly via an extension. If you do your router will at best have a terrible connection, more likely none at all.

How many do you need?

Every device in your home that is connected to the phone line must have a filter. That obviously includes all phones and fax machines but don’t forget other devices such as a Sky TV box if you have one. At a pinch you can plug a splitter into the filter and so connect two telephones etc. to one filter but you really should buy additional filters as needed if you want to avoid problems.

Are all filters the same?

Like most electronic devices you get what you pay for. The supplied O2 filters are OK but there are better ones on the market.

Even better, consider buying and fitting a replacement filtered faceplate to your master socket. This is a simple 5 minute DIY job and will cost about £15. With a filtered faceplate, you are likely to eliminate all ADSL degrading noise generated by any extensions in your house. As a side benefit you will also be able to remove all or most of your micro filters depending on configuration.