OK
some basic bits I've picked up:
1) filters are generically rubbish
By which I mean the filters you tend to get when you buy your modem - the ones with a maile phone socket on one end, an inch of cable and then a box which you plug your modem cable and phone line into.
2) all modems are not the same.
Generically, a 'proper' modem will be connected to your PC with wireless or ethernet. Connecting to the 'net with a USB modem is not going to give you such good results.
3) all ISPs are far from the same.
Some throttle (cap) your speeds, some have very restrictive 'fair use policies' (and massively cut your speeds if you are deemed to download too much - although they won't tell you what 'too much' is)
4) THE biggest difference is if your local exchange has been 'LLU'd (Local Loop Unbundled)
5) Line noise is bad.
6) Line length is king - but don't expect too much...
7) internal wiring...
Before I go into more detail on those, some basic definitions:
SNR - Signal Noise Ratio. Basically, the higher the number, the slower the connection. BUT (and it's a huge BUT!), on (a lot of) 'proper' modems, you can adjust the SNR downwards - and as you do, your speed goes up
Attenuation - pretty much relates to the length of your line - again high numbers are bad - but there is no
much you can do about this one.
Mast Socket. The first box on the wall where your phone line enters your house. This may then have extensions wired off it, leading to other phone points around your home. It will (if fitted in the last good few years) have a removeable front cover (2 screws) behind which is a normal looking phone socket. This is the test socket, and it has the most 'pure' phone signal that your house can get.
Removing it is completly fine and allowed. Touch the wires
behind it though, and your touching BT owned bits -
not a good idea...
I'll go through each point in more detail below - please bear with me...