This discussion will assume you are referring to analog circuits.
The typical analog circuit that connects between the customer premises and the central office (phone company) has two copper wires. These wires are referred to as TIP and RING. There is typically a -48 VDC signal between the TIP and RING wires which is typically powered by the central office switch. When a standard analog phone is connected to the analog circuit and the handset is picked up (going off hook) a switch on the phone closes the connection between the TIP and RING wires and current is drawn from the central office switch. The switch determines that current is being drawn and provides dial tone so the person on the phone knows it is time to dial a number.
A PBX FXO circuit running loop start looks like a standard analog phone to the central office switch. Now, one problem that is often seen with loop start circuits is a condition called "glare." Glare occurs when a call comes in on the analog line from the central office at the same time the PBX wants to seize the same line to make an outgoing call.
The ground start protocol was designed to eliminate this problem with glare. From the PBX side, the RING lead is grounded first, then the central office circuit must ground the TIP lead before the PBX can close the loop between TIP and RING.
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