telephony
Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...
, ground start is a method of signaling from a
terminal
Terminal may refer to:
Computing Hardware
* Terminal (electronics), a device for joining electrical circuits together
* Terminal (telecommunication), a device communicating over a line
* Computer terminal, a set of primary input and output dev ...
of a subscriber
local loop
In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the commo ...
to a telephone exchange, where one side of 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 connection provided by the coin itself, bridging a set of contacts as it passes through the coin chute.
Ground start trunk
Telephone companies typically provide two types of dial tone switched circuits – ground start and
loop start
Loop start is a telecommunications supervisory protocol between a central office or private branch exchange (PBX) and a subscriber telephone or other terminal for the purpose of starting and terminating a telephone call. It is the simplest of the ...
. Private branch exchanges (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 key systems typically work on loop start lines. On loop start lines the PBX and 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 In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...
, 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
A ringtone, ring tone or ring is the sound made by a telephone to indicate an incoming call. Originally referring to and made by the electromechanical striking of bells, the term now refers to any sound on any device alerting of a new incoming ...
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 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.
A PBX user must be careful to order the correct type of
trunk line
In telecommunications, trunking is a technology for providing network access to multiple clients simultaneously by sharing a set of circuits, carriers, channels, or frequencies, instead of providing individual circuits or channels for each clie ...
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 did it with a paper sleeve on the Vertical Off Normal contact, 5ESS switch by translation, and
DMS-100
The DMS-100 is a member of the Digital Multiplex System (DMS) product line of telephone exchange switches manufactured by Northern Telecom. Designed during the 1970s and released in 1979, it can control 100,000 telephone lines.
The purpose of ...
by a slide switch on the line card, all according to what the customer ordered.
See also
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Foreign exchange office
Foreign exchange service (FX) is an access service in a telecommunications network in which a telephone in a given exchange area is connected, via a private line, as opposed to a switched line, to a telephone exchange or central office in another e ...
*
Foreign exchange station
Foreign exchange service (FX) is an access service in a telecommunications network in which a telephone in a given exchange area is connected, via a private line, as opposed to a switched line, to a telephone exchange or central office in another e ...