ADSL filtering wiring guide

Wiring and filtering ADSL

Encouraged by common questions and some evident misunderstandings/misconceptions held by recent ADSL Newbies, we've figured out that the best way to explain how things come together with wiring and filtering when ADSL arrives on your phone line is by simply covering the likely scenarios you'll encounter or use, detailing what's happening in each, and with some dinky little pictures into the bargain.

As always, feel free to pop a mail off to us at adsl@clarity.it where a suitably wise mentor will attempt to bestow enlightenment while he has a coffee and a feet-up. Be mindful that if there's an obvious answer contained in the great texts, and it looks like you just haven't bothered looking, we reserve the right to insult and jibe you a little before helping you.

All the following relates to a line with ADSL service enabled at your exchange...

Scenario 1
No filtering on the line.

(1) No Filtering - in detail...
Here we have the typical telephone layout with no ADSL filtering whatsoever. You'll have trouble immediately you make an ADSL connection on this line. With no filtering of the signal, the full bandwidth of the exchange line is available at every socket, from the master linebox to the furthest extension point, which means everything that's connected to this line can be affected by everything else.

Because the whole signal is present everywhere, that means that ADSL can be accessed from any point on this setup.

Unfortunately this also means that any regular telephony devices (such as phones, faxes, SKY boxes, alarm system autodialers and so on) are blasted with the full frequency range used by ADSL, resulting in screechy hissy noise all over your phone calls. The ferocity of the interference varies by individual telephone devices from a moderate annoying hiss to completely deafening noise.

The door swings both ways too, and if your phone equipment's sounds overlap the ADSL signal enough, it can interfere with your Internet connection, slow it down , or cause it to drop.

So unless you plan to never ever use any normal telephony equipment on this line, we need to add some filtering circuitry (also known as splitters) to keep ADSL and telephony apart from each other and let them get on with their own business without interference.

Interesting note: Sometimes, although rarely, a person may have a phone line installed specifically for ADSL use only, and will connect NO telephone equipment to the line. This is most common in business of course. In this case, they should in fact actually remain with Scenario 1 - no filtering is needed if the line will be used for ADSL and nothing else, ever.

Scenario 2
Microfilters.

(2) Microfilters - in detail...
Using microfilters is the fastest and easiest way to take care of the filtering problem. It's a no-brainer. One microfilter is used at EACH AND EVERY telephone point where a telephone or regular telephony-based equipment is to be used. Remember this includes SKY boxes, faxes, dial-up modems and the like.

The issues to be aware of are;

Cost. Good microfilters are in the ballpark of a tenner each. If you have four or five sockets in your home, the £ soon add up
Quality. There's lots of cheapos out there, and a few not-so-cheapos which have had plenty of berative comments from users. Of course, the one we do is one of, if not THE best performers available
Quantity. Manufacturers advise the use of no more than four microfilters on any one line. Never actually tried it, so comments/reports invited from anyone enjoying success with more than 4 microfilters.
Neatness. (lack of) - Sadly the dangleweed nature of most microfilters makes for a silly thingie hanging out of your wall at each location. While sometimes causing a feeling that it's not the best possible job of filtering among male householders, reports indicate that this can be of prime aesthetic concern to The Wife.
Accessibility. I wouldn't have thought this would be a problem, but from real live customer emails I've read, apparently Junior and/or Rover (and flatmates apparently) can be quite partial to messing around with any dangleweed-type devices hanging out of walls. Can't say I've ever had that problem myself, but don't say I didn't warn you.

Scenario 3
ADSL Adapter on the Master linebox - All phone extensions are now filtered

(3) ADSL Adapter faceplate on Master box - in detail...
This is the standard method used by your BT engineer when he sets you up if you buy a Fully Installed ADSL package from your ISP. You can buy these faceplates legitimately from Clarity in the UK, otherwise only BT provides them - but they're not sold seperately by BT - only provided as part of a full installation service.

The new master faceplate provides an unfiltered socket for you to connect your ADSL modem, and filters all other extensions right there and then, meaning there's no need for individual filters at each and every telephone point. A lot neater and cost-effective for most cases.

Remember that because the extension wiring is now filtered, that means it no longer has the ADSL signals present. So in this scenario, if you want an ADSL outlet in a location OTHER than at the master socket, you must install an ADSL extension. Again, this is what your BT engineer would do upon request as part of a full installation package from your ISP. The BT ADSL extension kits used are also available legitimately from Clarity to the UK public. Otherwise, as with the master faceplate adapter, they're not sold seperately by BT, only provided as part of a full install.

This page is a work in progress. We plan to show more simple overviews of various permutations of installations for ADSL, including dual-purpose ADSL/Voice wall boxes, ADSL extensions, and so forth. Any special requests might just be considered - seeing we've got the images for all our little boxes now anything's just a few cut 'n' pastes away.

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