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.
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