Ground start

In telephony, a ground start or GST is a method of signaling from a terminal or subscriber local loop to a telephone exchange, in which method a cable pair is temporarily grounded to request dial tone. Most middle 20th-century American payphones used "coin first" ground start lines, with the starting ground passing through the coin itself.

Local telephone companies typically provide two types of dial tone switched trunks – ground start and loop start. Private branch exchanges (PBX) work best on ground start trunks because those trunks can give them an on hook signal allowing for timely clearing. Many will work – albeit intermittently – on both types.

Normal single line phones and key systems typically work on loop start lines. The issue with loop start lines is that the PBX and central office can seize the line simultaneously; since neither gets the response it is expecting, the call is not initiated. The resulting condition is called glare or also known as call collision.

In an idle circuit, the central office supplies -48V (nominally) on the ring and open on the tip. A ground start PBX initiates an outgoing trunk seizure on an idle circuit by connecting of the ring lead to ground (maximum local resistance of 550 ohms). The central office senses this condition and grounds the tip lead. When the PBX senses this, it goes off-hook, then removes the ground on ring. The central office sends dial tone and the rest of the call proceeds normally.

In ground start signaling, the central office initiates a call by grounding tip and putting the ringing signal on the line. The PBX has 100ms to sense this condition. The PBX goes off hook; if it had been trying to seize the line by grounding ring, it releases ring from ground and the call proceeds normally.

At the end of either an incoming or outgoing call, the PBX initiates disconnect by going on hook, or the central office initiates disconnect by opening tip. When the other end detects the loss of loop current, it also goes on hook and the call clears normally.

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