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GROUND-START TRUNKS:
OPERATIONS
Ground-start facilities were specifically introduced to solve the problems that PBXs encounter on loop-start trunks. A ground-start facility provides an immediate signal when it is seized and it provides a positive signal when one party disconnects.
When a ground-start facility is idle the CO provides battery voltage on the Ring lead, but the CPE does not provide a ground on the Ring lead, and no current flows. Meanwhile, the CPE (LEGEND) monitors the Tip lead for ground at the CO, but the CO does not provide a ground on the Tip lead when the trunk is idle.
When the CO needs to seize a facility for an incoming call it selects an idle trunk, makes sure that the CPE has not applied ground to the Ring lead (no current is flowing), and the CO applies ground to the Tip lead. The ground connection on the Tip lead completes an electrical path, current begins to flow, and the CPE recognizes immediately that the facility has been seized for an incoming call. The CO also super imposes the ringing generators onto the facility, but ringing may not occur at once.
The significant operational difference here is that, unlike the signal from the ringing generators, the ground signal on the Tip lead is synchronized with the start of the incoming call. The CPE knows immediately that the facility is not available for outgoing traffic, even if it does not begin to ring for several seconds.
Ground-start facilities also provide a positive indication of a disconnection. When the distant party goes on-hook the CO removes battery from the Tip and Ring leads. Ground-start CPE (LEGEND) is designed to recognize this disconnect signal and remove its party from the facility. (This prevents the party from waiting for dial to to return, and prevent a toll restricted station from by-passing the restrictions.
Nearly all ground-start facilities are trunks, connecting PBXs with COs. There are only a few cases where round-start lines might be encountered. Some pay phones, especially older ones, use ground-start lines to obtain a positive disconnect signal and prevent a user from making multiple calls on a single coin. Also, some unattended data equipment uses ground-start lines so the modem can determine when to start dialing after going off-hook. Most modem equipment uses loop-start lines and alternate systems, such as a dial- tone detector in the modem, instead of ground-start lines.