Telephone companies typically provide two types of dial-tone switched circuits – ground start and
loop start.
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, 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 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 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 switches 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. ==See also==