DACS device (Digital Access Carrier System)

If BT have too many people wanting a phone line and not enough copper line to supply they put on a DACS box  this splits one line into two.
If you modem will not connect faster than 28000K or suddenly drops you have been  DACS'D!)

To see what a dax box looks like see here  http://www.telephonesuk.co.uk/miscellaneous.htm#dacs

http://btsuck.org/?article=125
I recently found my connection speed using my BT line dropped from a very reliable 49K down to 28.8kbps.
 I managed to get an admission they had put a 'DAX unit' on the line (presumably to fix up a line for some other subscriber), and they couldn't guarantee a speed of more than 28 - 33 K now. To get a reasonable speed my only solution seems to be to get ADSL or some other such product !

http://www.btsuck.org/?article=102
About 3 years ago we had our first Internet line installed... We asked for an 'INTERNET LINE' not a voice line so BT went ahead and installed a DAX voice line that ran at an amazing 28.8K. It took 15 months of nagging before BT finally replaced it with a fully working line that ran at 48.2K though they messed that up and it took 3 days to repair!

A DACS allows two different lines to be connected back to the BT exchange over a single twisted pair copper cable. This device is used by BT when they run out of cables between the exchange and a customers home.
If you have had a DACS installed, you cannot get ADSL until the device is removed. BT will only be able to do this currently if there are no spare lines between your home and the exchange.
There are 2 types : see
If you have one of the internal white boxes you have definitely been DACSed. However, I think this indicates that at some point a 2nd phone line has been installed in the house so if you cancel one of these it can be removed and you should be OK for ADSL.

If you have one of the black rectangular boxes (look at the telegraph pole supplying your house) you MAY be DACSed - but not definitely so. Best way to tell is if you have a V.90 modem but never get above 33.6Kbps then you are most likely DACSed.

BT has no legal obligation to remove such devices from your line - they are only bound to provide services within the terms of your contract and their licence which I believe currently only covers speech & fax services. So you may be able to get them to remove a device if you kick up enough fuss and they have enough spare lines BUT they mat well replace it again at some future point to service other customers. I believe they have a rolling programme of laying new cables - but not sure how fast that is progressing or what areas they are concentrating on.

The only certain way to make sure that DACS is removed (or that your line is never DACSed in the first place) is to pay for the bandwidth you desire by upgrading to ISDN, Home / Business Highway or DSL.