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((refimprove|date=January 2008))
In [[telephony]], '''ground start''' is a method of signaling from a [[terminal (telecommunication)|terminal]] of a subscriber [[local loop]] to a [[telephone exchange]], where one side of a cable pair is temporarily grounded to request [[dial tone]].  Most middle 20th-century American [[payphone]]s used coin-first ground start lines, with the starting ground connection provided by the  coin itself, bridging a set of contacts as it passed through the coin chute.

==Ground start trunk==
Telephone companies typically provide two types of dial-tone switched circuits – ground start and [[loop start]]. [[Private branch exchange]]s (PBX) work best on ground start trunks because those trunks can give them an on-hook signal allowing for timely clearing.

Normal single-line telephones and [[Business telephone system|key systems]] typically work on loop-start lines. On loop-start lines the PBX and [[telephone exchange|central office]] may inadvertently seize the line simultaneously, a condition called ''glare'', resulting in [[call collision]] since neither gets the expected response and no call can be initiated.

In an idle circuit, the central office supplies –48V (nominally) on the ring conductor with respect to the tip side.  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. To avoid glare, before the PBX originates an outgoing call, it must first verify that the CO has not already applied ground to tip. The PBX has 100ms to sense this condition.  

At the end of either an incoming or outgoing call, the PBX initiates [[Disconnect supervision|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 [[Clearing (telecommunications)|clears normally]].  

A PBX user must be careful to order the correct type of [[Trunking|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 line.  [[Crossbar switch]]es did it with a paper sleeve on the ''vertical off-normal'' contact, [[5ESS switch]]es by translation, and [[DMS-100]]s by a slide switch on the line card, all according to what the customer ordered.

==See also==
*[[Foreign exchange office]]
*[[Foreign exchange station]]
*[[BORSHT]]

==References==
*[http://www.its.bldrdoc.gov/fs-1037/dir-017/_2501.htm Glossary of telecommunications terms]


[[Category:Telephony signals]]

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