As HP has alluded to, it was a way (back in the day), of providing 2 dial tones over just 1 MPF (Pair of wires). They were introduced as a short-term solution to provide service, whilst the planning department worked on a way of providing more capacity to DP's (Distribution Points - IE: Telegraph Poles).
As is the case, it became the 'preferred method' to deliver Dial Tone to a new EU (End User), and they were quite prevalent within our networks.
With the advent of Broadband (DSL), the powers that be rolled out a DACS removal programme that saw dedicated teams working on this each and every day. The reason being that DSL will not work over DACS.
There are still quite a few out there, and funnily enough I had to remove one just recently to provide service at my Dad's new flat. How the process
should work (in days of yore), is that upon receipt of an EU's order, Openreach will check the routing and records of the feeding DP and if indeed a DACS is found to be in play, an additional task should then be 'built' for the planners to come up with a solution. More often than not, this will require underground intervention to 'push a pair' on from another DP with spare capacity, to the DP with the shortage, or to 'pull a pair back' from a DP further on from the one that has the shortage. These are called 'Pair Divert' tasks.
Once the UG (Underground) engineer reports back that the task can be, and has been completed, the order can then progress to the next stages. This will be things like, Frames Jumpering tasks, OMC tasks, Robotic Line Test tasks, Billing tasks ...... etc etc.
DACS removal has got quite easy these days, and planning very rarely get involved. The reason being is that once upon a time, a lot of folk would have two lines per household, as back then you could not get on the Internet (Via the 56k dial-up Modem), and use the landline at the same time. Mobile phones weren't as marketable then, as they are today.
So with the onset of DSL, which allows dual phone and Broadband usage at the same time, spare capacity to DP's increased along with the timeline.
I hope this gives some kind of insight, and feel free to ask if any more info is needed.
By way of a PS ...... we were told that the DACS technology had been developed by Iran (or Iraq, can never remember which), in order that the other country couldn't 'tap' into their PSTN lines and listen in to conversations. If they did, they would just hear 'White Noise'. I can't prove whether this is true or not I'm afraid. What I can say is true, is that BT at one point were going to introduce HAPS, which effectively provided 10 dial tones over the 1 MPF !!!!!!!!! Thank the good lord for DSL.
PPS .... DACS work at 140Vdc, when you're up a pole and it's raining, touching the 'live' is an experience.
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Topic: what is a DACS line and how do openreach DE-DACS the line (Read 8232 times)