Ground Start Signaling
The following diagrams depict Ground Start signaling states:
IDLE STATE - Telephone or PBX On-Hook. PBX/Telephone monitors for ground on Tip lead. BAT from CO/FXS appears on RING lead.
CENTRAL OFFICE SIEZURE
GROUND START FAR-END DISCONNECT
Both Loop Start & Ground Start signalling will return the line to the "idle state" upon local release. But only Ground Start provides FAR-END DISCONNECT supervision.
Advantages of Ground Start:
Minimizes the possibility of “Glare”.
Provides FAR-END Disconnect Supervision (e.g. the remote user can disconnect and the local PBX can be made aware of this and also disconnect).
Disadvantages of Ground Start:
TIP/RING leads cannot be reversed.
C.O./FXS and PBX/Telephone grounds must be at the same potential (Earth ground).
There is limited support for the Ground Start interface in most PBXs. If it is available, Ground Start is usually only incorporated on Trunk Interface Cards (e.g. no Line/Station Cards). Also, it is likely that only FX Trunk applications are supported; that is to say, that the PBX acts like a station.