Telephone Switching > General PABX Talk

Why Ground Start trunks on PABX's? Please Explain

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Here in Norway, I have never heard about Ground start in the regular network. ???
Quit many office telephones had this button when connected to a PABX, but this was for transferring to another extension, not to get the dial tone.  Some German telephones had an extra contact set on the dial, grounding at the same interval the receiver was shorted out.

Could you tell me why it has been used, and whats the positive, and negative arguments?   :)


Stephen Furley:
I don't know what the reason for ground start lines was; I did read it somewhere in the past but I've forgotten.  I think ground start lines were used mainly for analogue trunk lines to pabx systems; these days most trunks are digital, so I would guess that ground start lines are not as common as they once were.

I think Donna may be able to advise on the differences between loop start and ground start.  I have long since forgotten this from back in my implementation days.   I agree with the posts here so far that ground start usually applied to trunk lines associated with PBX systems, but that's as far as my knowledge level goes anymore.


Its my understanding that ground start was much faster at activating the CO equipment than loop start, and on a large business system, that avoided collisions between an incoming call being intercepted by someone dialing a 9 to get an outside line.  I guess it still happened, but it was much rarer to happen than with loop start.

Prepay pay phones, where you had to deposit the coin to get a dial tone were always ground start.  This was set up so that the ground could not only be the reference for starting the call but for coin collect and return voltages.

Ground start still exists on PBX lines, but pay phones work entirely different these days.

this probably explains a more succinctly than i would ;) actually, is pretty much what bill said.

coincidentally, i had occasion to deal with ground start loop today. customer ordered 14 lines as ground start. actually needed 2 to be loop start for the fax machines.

at&t does not provide coin service at all any more. the coin vendor gets a POTS line & the coin service is thru their equipment.

i do take issue with the term trunk line. it is just a line or pair. trunk line is between central offices.


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