Clarity UK ADSL filtering explanation


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Why you need filters with ADSL and when (UK)

It's a common thing for people new to ADSL to have no idea how ADSL and "telephone" services manage to exist on a single phone line at the same time. Fair enough, it's not immediately obvious by any means. What, in fact, is "broadband" actually referring to?

Here's a quick explanation, made up on the spot and grossly understated, but it conveys the nature of the beast without delving into the frequency spectrum stuff. Boffins will observe that it's not really technically correct, but this isn't for boffins IS IT?

Roughly what's going on
Bellow you'll see a couple of images of bars. This horizontal bar represents your telephone line. On the left is low frequency, on the right is high frequency. The entire bar represents the entire "bandwidth" (the full range of frequencies), that the copper wires of your telephone line can carry.

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

That leaves quite a lot of usable "bandwidth" on the line for something else to use. 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 indeed a 56k modem. (boffins: yeh yeh, I know, but it'll do for illustration of the concept!)

The bit of your line used by telephony:

Phones, faxes, 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. It's not particularly related to quality or anything, it's just One Of Those Things.

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.