Ground start
Encyclopedia
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.
. 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.
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
, a ground start or GST is a method of signaling from a terminal
Terminal (telecommunication)
In the context of telecommunications, a terminal is a device which is capable of communicating over a line. Examples of terminals are telephones, fax machines, and network devices - printers and workstations....
or subscriber local loop
Local loop
In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...
to a telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
, in which method a cable pair is temporarily grounded to request dial tone
Dial tone
A dial tone is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed...
. Most middle 20th century American payphones used "coin first" ground start lines, with the starting ground passing through the coin itself.
Ground start trunk
Local telephone companies typically provide two types of dial tone switched trunks -- ground start and loop startLoop start
In telecommunications, a loop start is a supervisory signal given by a telephone or PBX in response to the completion of the loop circuit, commonly referred to as 'off-hook'. When idle, or 'on-hook', the loop is at 48V DC...
. PBXs work best on ground start trunks because those trunks can give them an on hook signal allowing for timely clearing
Clearing (telecommunications)
Clearing, in telecommunications means:* A sequence of events used to disconnect a call and return to the ready state. It is sometimes, particularly in the context of common channel signaling, called teardown....
. Many will work -- albeit intermittently -- on both types.
Normal single line phones and key systems
Key telephone system
A business telephone system is any of a range of a multiline telephone systems typically used in business environments, encompassing systems ranging from small key systems to large scale private branch....
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
Call collision
In telecommunications, a call collision is one of two things:#The contention that occurs when a terminal and data circuit-terminating equipment specify the same channel at the same time to transfer a call request and handle an incoming call...
.
In an idle circuit, the central office supplies -48V (nominally) on the Ring side
Tip and ring
"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...
and open on the Tip
Tip and ring
"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...
. A ground start PBX initiates an outgoing trunk seizure on an idle circuit by connecting of the Ring
Tip and ring
"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...
lead to ground (maximum local resistance of 550 ohms). The central office senses this condition and grounds the Tip
Tip and ring
"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...
lead. When the PBX senses this, it goes off hook, then removes the ground on Ring
Tip and ring
"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...
. The central office sends dial tone
Dial tone
A dial tone is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed...
and the rest of the call proceeds normally.
In ground start signaling, the central office initiates a call by grounding Tip
Tip and ring
"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...
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
Tip and ring
"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...
, it releases Ring
Tip and ring
"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...
from ground and the call proceeds normally.
At the end of either an incoming or outgoing call, the PBX initiates disconnect
Disconnect supervision
Disconnect supervision is a term in telephony describing signaling between the telephone exchange and a connected party used to indicate that the connected call is being disconnected....
by going on hook, or the central office initiates disconnect by opening Tip
Tip and ring
"Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...
. When the other end detects the loss of loop current, it also goes on hook and the call is clears normally
Clearing (telecommunications)
Clearing, in telecommunications means:* A sequence of events used to disconnect a call and return to the ready state. It is sometimes, particularly in the context of common channel signaling, called teardown....
.
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
Crossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
did it with a paper sleeve on the Vertical Off Normal contact, 5ESS switch
5ESS Switch
The 5ESS Switch is a Class 5 telephone electronic switching system sold by Alcatel-Lucent. This digital central office telephone circuit switching system is used by many telecommunications service providers.-History:...
by translation, and DMS-100
DMS-100
The DMS-100 Switch is a line of Digital Multiplex System telephone exchange switches manufactured by Nortel Networks.The purpose of the DMS-100 Switch is to provide local service and connections to the PSTN public telephone network. It is designed to deliver services over subscribers' telephone...
by a slide switch on the line card, all according to what the customer ordered.