Although this document takes the format of a FAQ list it is probably best read from beginning to end.
Comments, complaints, etc. to ali@gwc.org.uk.
Digital Access Carrier System
It is a technology which allows two ordinary phone lines to be squeezed down a single copper pair. Normally each phone line requires its own copper pair all the way to the exchange.
Because someone (perhaps you) requested an additional line be installed (or someone's existing phone line went faulty) and BT couldn't find a spare pair going back to the exchange.
Although DACS units are expensive they are not nearly as costly as running additional underground / overhead cables, which could involve digging up roads and could take months.
Three possible reasons -
1. When the GPO / BT were building the local telephone infrastructure, the internet was not even thought of. Nobody had more than one phone line in their house. These days it is not uncommon for people to have two or more phone lines in their house. Also business use of telecommunications has increased hugely over the last 20 years. So sometimes when someone requests an additional line there are simply not enough pairs in the ground.
2. Copper pairs are subject to corrosion and can occasionally fail. In this case BT are obliged to repair the fault quickly and for the reasons above may be unable to find an unused pair.
3. Sometimes, even when spare pairs exist all the way back to the exchange, there may be a lot of work involved in connecting up the pair. The engineer may have to visit several cable junctions, some of which may be at the top of poles, some of which may be in green BT cabinets, some may be underground and some may involve stopping traffic etc. to lift manholes. In such a case it is often easier for a lazy engineer to DACS an existing pair.
The analogue signals in each phone line are digitised in the DACS unit and multiplexed across the single pair to the exchange. At the exchange another unit de-multiplexes the signal and converts it back to two analogue lines for connection to a standard line card on the switching equipment.
In normal (voice or fax) use of a phone line there is no problem. But these days modems can connect at up to 56kbps to most ISPs. Unfortunately the technology used to achieve this speed will not work on a DACS line.
On a normal phone line, when dialling in to an ISP, the analogue signal from your modem travels down the copper phone line to the exchange, where it is converted to a digital signal and sent over the public telephone network to your ISP digitally. At the ISP, the digital signal terminates in a modem rack which processes the signal in its digital form. So there is only a single analogue to digital conversion (at your local phone exchange).
The V.90 modem standard relies on there being only a single analogue to digital conversion to work. With DACS, there is an additional A to D conversion going on, so the V.90 protocol breaks down.
A DACS line will usually support a connection speed of 33.6kbps, though never more than that.
56kbps modems can be very bad at coping with DACS lines. They often fail to successfully negotiate down to 33.6kbps. Better results can often be achieved by configuring the modem to V.34 mode thereby forcing it to negotiate a 33.6 kbps connection.
Occasionally, having a line DACS may *improve* the speed of connection. If a line previously worked only at 28.8 or 32.0 kbps, it is likely it will work at 33.6 following conversion to DACS.
Also, without DACS, if you ordered a second line you might have to wait months for BT to pull extra cables if there was not a spare pair available. A "slow" phone line is (usually for you, and always for BT) better than none at all.
First of all, if your phone line is from your local cable company then you are all right. Only BT are known to use DACS.
DACS in its current form is only used for standard analogue phone lines - not for ISDN, Highway, ADSL etc.
If your modem connection speed suddenly drops to 33.6 kbps, and nothing has changed in your setup, it is likely your line has been DACSed. If this happens immediately after you notice BT engineers working on a phone pole serving your property then it is even more likely.
UPDATE - you can now use the BT Broadband Availability checker to see if you have DACS. Either follow the links from the BT Broadband site or try this link to go directly to the checker. If after entering your phone number you get a note mentioning a "line sharing device" then your line is probably DACSed.
There are at least 3 different types of DACS from two different manufacturers (Telspec and ECI).
Of course there are two parts to a DACS installation - one part in the telephone exchange and the part which goes on a phone pole or underground cavity or whatever near to your property.
The exchange part is usually rackmounted.
The local part is often a plastic box about 8 inches high by 4 inches wide by 1 1/2 inches deep. It has a single cable coming out of it which will be connected to a nearby D.P. (distribution point - junction box). There will usually be the BT logo and the word DACS on the unit though it may be hard to read at the top of a pole. Outdoor units are always black in colour and indoor units are usually off-white.
A photo is available at http://frank.gwc.org.uk/~ali/dacs/dacs.jpeg. Both units in the photo are manufactured by Telspec. The unit on the left is the indoor type and the unit on the right the outdoor type.
Another, possibly older style of DACS unit takes the form of a black cylinder, about 8 inches long. These are usually pole mounted.
You can view a picture of the exchange end of DACS at this page about the inside of a telephone exchange.
First of all, be aware that BT is unlikely to be under any legal obligation to remove DACS from your line. Although you may feel aggrieved that your Internet connection speed has suddenly dropped, BT's only *obligation* is to provide a line within the terms of your contract with them and within the terms of their licence. That means for speech and fax only. 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.
Also remember that BT did not put the DACS there for the fun if it - it was put there for a reason and unless they can find either a spare pair or someone else's line to DACS then it will not be physically possible to remove it without running more cables.
That said, BT may well remove the DACS if you kick up enough fuss. However even then there is no guarantee that your line will not be DACSed again in the future.
In fact if you succeed in getting your line de-DACSed then there is a very good chance that it will be DACSed again next time someone nearby requests a new phone line. If data speeds are important to you then you might be well advised to upgrade to Highway or ADSL sooner rather than later in order to reserve your line.
Well the obvious answer is for BT to lay more cables. However that would involve huge investment and BT isn't exactly flush with cash at the moment.
The other answer is to do DACS the way it should have been done in the first place. Rather than converting from digital to analogue and then back again at the exchange, why not just eliminate the double conversion and feed the digital signal straight in to the switching equipment. That way your line would be better than ever since the only analogue part of the connection would be the short run of cable from the pole into your premises.
V.90 (56kbps) modems would have no problems coping with such a system - they would see the line as if you lived right next door to the phone exchange.
In fact BT are deploying such DACS systems now, although the older DACS systems are likely to remain in use for some time to come.
Of course, some people might say that analogue phone lines are obsolete anyway, and the phone system should be fully digital, including the local loop.....
Thanks to Phil McKerracher for comments and suggestions.
Content and Design © Copyright Alistair Riddell 2001-2004. modified 2004-01-06 ali@gwc.org.uk
