The Internet is of course bursting with details and tech behind DSL data services, but good news! You're here! Where I'll now explain it in mere seconds without bending your mind round things you don't need to know, and with any luck help you to spend your lovely cash at this, the very site where you can buy the best ADSL filtering and installation parts as specified by BT itself! Brilliant. Everybody wins.

POTS

I'll use the term POTS here. Plain Ordinary Telephone Service. Which means "Everything on the phone line which isn't the DSL router (or modem)"

ADSL

This means all the variants of DSL service. ADSL, ADSL2+, VDSL - these are all variants of DSL technology and in the context of installation and filtering, should be considered the same thing.

Ironically, the only variant not relevant here is actual true "DSL" which is a dedicated line service and not used with any POTS service on the same line.

Where POTS and ADSL live on the telephone line

Below you'll see a couple of horizontal bars, representing your telephone line. On the left is low frequency, on the right is high frequency. The whole bar represents the entire "bandwidth" or you might also say "range" of frequency that the copper wires of your telephone line can carry as determined by Physics and the electrical conduction properties of Copper atoms and such stuff.

People like real-world analogies of stuff they can't see, so you might consider it like this; a busy road with a pavement alongside it; Loads and loads of traffic zooming along the big wide road bit in both directions, but there's also that little footpath at the side with some happy little people scampering along it.

Of course, the people mustn't stray onto the road, or the traffic flow will be thrown to heck as cars veer and screech to avoid them.

And neither can any cars mount the pavement. Obviously.

But drivers fall asleep and people wander around like twits, so to make sure they each stay where they're allowed to go, we stick a barrier up along the side of the road. And that's your DSL Microfilter.

I believe that's a well painted picture right there.

The bit of your line used by POTS:

Notice that the telephone signal (POTS), represented by the red section, only lives in a tiny part at the low end.

That leaves quite a lot of free "bandwidth" capacity on the line for something else to use.  And it's used of course for ADSL. The term "broadband" simply refers to the fact that the frequency range used by ADSL is very very wide compared to that of a telephone, or even old-shool dialup modems.

Phones, faxes, dial-up modems, SKY boxes, alarms with autodialers - anything which can make and/or receive telephone calls - live in this bit of the available "bandwidth".

Notice that it's fuzzy though. different telephones and equipment go slightly higher up the frequency range than others. And absolutely pointless too, because POTS only carries up to 3.3KHz anyway.

The bit of your line used by ADSL:

This is the bit ADSL works on. This isn't to scale - in reality the size of ADSL's chunk is bloody HUGE compared to the telephony part, but this is just an Idiot's Guide, so give me a break.

A line with BOTH telephones and ADSL running on it:

If you run both things on one line, there's an overlap, illustrated in this simplistic fashion by the use of the purple bit there. The enormous chunk of bandwidth on the line which is ADSL spills oer into the telephone bit slightly. In the Real World, it causes noise that you can hear on phones, and which interferes with low-speed data transmissions - like faxes, SKY data bursts, dial-up modems, that kind of thing. In the same way, when those things cause sound freqencies on the line which sneak up into the chunk where ADSL is working, your ADSL modem can get pretty miffed and slow down, stall, or even lose the connection with your ISP.

Thus we need to separate the two types of signal that co-exist on this single telephone line so that one doesn't interfere with the other.
We need to keep them in their own respective chunks of bandwidth so they don't trouble each other. And THAT, ladies and gentlemen, is what an ADSL filter essentially does. It gets rid of the horrible overlap and keeps the two types of service apart from each other.

To understand the logic of this "ADSL filtering" business, it's absolutely essential that you understand these following summary points;

• ADSL wiring is UNFILTERED. It's the full, pure, untampered signal from the ADSL equipment at your local exchange.
  
  
• To filter the signal for any given socket or sockets means to chop off all the unwanted ADSL signal, so that the ADSL signal can't be heard on, or interfere with, telephony equipment. By definition, that means you're removing the ADSL service from that socket.
  
  
• A filtered socket has had the ADSL chopped off. Once an extension has been filtered, there's no going back, it's dead as far as ADSL is concerend, it can ONLY be used for telephone-based equipment.
  
  
• Any wiring or socket which is NOT filtered is carrying BOTH telepone and ADSL service.  You can therefore access both services from these wires or sockets - but if you connect ANY TELEPHONE EQUIPMENT to these wires then they absolutely MUST be on the other side of an ADSL filter, so keep that in mind at all times.  If you ONLY use these wires or sockets for ADSL and connect NO telephone equipment, then no filters are needed.