telephony
Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunications services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is ...
, ground start is a method of signaling from a terminal 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 co ...
to a
telephone exchange
A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...
, where one side of a cable pair is temporarily grounded to request
dial tone
A dial tone (dialling tone in the UK) is a telephony signal sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is ...
. Most middle 20th-century American
payphone
A payphone (alternative spelling: pay phone or pay telephone or public phone) is typically a coin-operated public telephone, often located in a telephone booth or in high-traffic public areas. Prepayment is required by inserting coins or tel ...
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
A business telephone system is a telephone system typically used in business environments, encompassing the range of technology from the key telephone system (KTS) to the private branch exchange (PBX).
A business telephone system differs from ...
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 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 ...
, then removes the ground on ring. The central office sends
dial tone
A dial tone (dialling tone in the UK) is a telephony signal sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is ...
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 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
In electronics and telecommunications, a crossbar switch (cross-point switch, matrix switch) is a collection of switches arranged in a Matrix (mathematics), matrix configuration. A crossbar switch has multiple input and output lines that form a ...
es did it with a paper sleeve on the ''vertical off-normal'' contact, 5ESS switches by translation, and DMS-100s by a slide switch on the line card, all according to what the customer ordered.