Loop-Start Signaling
Loop-start signaling allows a user or the telephone company
to seize a line or trunk when a subscriber is initiating a call. It is primarily
used on local loops connecting to residences rather than on trunks
interconnecting telephone switches.
A telephone connection exists in one of the following states,
as illustrated in Figure 2-10:
A summary of the loop-start signaling process is as
follows:
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1. |
When the line is in the idle state, or on hook, the telephone
or PBX opens the two-wire loop. The CO or FXS has battery on ring and ground on tip.
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|
2. |
If a user lifts the handset off the cradle to place a call,
the switch hook goes off hook and closes the loop (line seizure). The current
can now flow through the telephone circuit. The CO or FXS module detects the
current and returns a dial tone.
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|
3. |
When the CO or FXS module detects an incoming call, it
applies AC ring voltage superimposed over the 48 VDC battery, causing the ring
generator to notify the recipient of a telephone call. When the telephone or PBX
answers the call, thus closing the loop, the CO or FXS module removes the ring
voltage.
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Loop-start signaling is a poor solution
for high-volume trunks because it leads to glare,
which is the simultaneous seizure of the trunk from both ends. Glare occurs, for
example, when you pick up your home telephone and find that someone is already
at the other end.
Glare is not a significant problem at home. It is, however, a
major problem when it occurs between switches at high-volume switching centers,
such as long-distance carriers or large PBX systems.
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