FTTC broadband only line voltage

[TobyOneKenobi]

Hi,

hope I'm posting on the right board as this is FTTC but obviously copper line too, my question is this:

On a broadband only FTTC  connection (Digital voice phone) what would I expect to see voltage wise?

As I understand it, when you get switched over to DV the -50v is removed from the line so what does the ADSL use? 

Just curious.

Thanks

[Keith_Beddoe]

No voltage once the phone connection in the exchange is removed. FTTC comes from the cabinet. 

ADSL is a totally different service, with speed up to 20Mbs, and comes from the exchange, but even then, the 50V would not be needed if there was no phone service from the exchange.

[TobyOneKenobi]

Thanks,  perhaps I have my terminology wrong here. I have FTTC with VOIP so there is no dial tone, as I understand it the -48v supply has been turned off as it is not required. In order for the broadband connection to work over the copper lines to my house there must be a current of a specified voltage? I was wondering what that voltage is.

[Keith_Beddoe]

No voltage at all, its just a digital (binary) signal carried over high frequency slots, with zero DC content by design.

Any voltage you may measure would be possibly due to poor insulation between cable pairs.

[Keith_Beddoe]

See https://en.wikipedia.org/wiki/VDSL

[TobyOneKenobi]

OK, so it's a standard a digital signal based around 0 and 3.3/5 volts between 25-kHz to 1.1-MHz? That leads me to another question then, if I still had an analog phone on that line how does the digital signal work with the -48 volts present on the line?

[Keith_Beddoe]

The DC voltage does not affect the transmission of high frequencies. The digital component is separated at each end by filters.

Many older private circuits used by business, used a DC power feed of 100V to power the customer end, and the digital signal was carried along with it.

I would not be surprised if some of those are still working, although the limit of 2Mbs in both directions, may be too restrictive.

[TobyOneKenobi]

Of course! You wouldn't be able to measure anything by putting a multimeter on the line if there was nothing connected to it, once you have a router connected there will be a signal as the router transmits and receives data.