Ground Start Trunk
Local telephone companies typically provide two types of dial tone switched trunks -- ground start and loop start. PBXs 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 side 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 is clears normally.
A PBX user must be careful to order the correct type of trunk line from the local phone company and correctly install the telephone system at the PBX end -- so that they match. Line equipment in most 20th-century central office switches had to be specially rewired to create a ground start DDCO line. Crossbar switch did it with a paper sleeve on the Vertical Off Normal contact, 5ESS switch by translation, and DMS-100 by a slide switch on the line card, all according to what the customer ordered.
Read more about this topic: Ground Start
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