Information Technology Reference
In-Depth Information
signal to the other that it is fixing to go off-hook by “grounding” the line. This helps
in preventing both sides from trying to seize access at the same time (called glare).
E&M (PBX to PBX) is another analog switch-to-switch (PSTN-to-PBX or PBX-to-
PBX) signaling type that provides additional signaling capability and can be a two-
wire or four-wire implementation.
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CAS T1/E1 circuits get their name from the fact that switch to switch signaling (di-
aled digits, caller ID, and so on) occurs in band along on each individual channel with
the voice traffic.
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CCS ISDN PRI circuits set aside one channel specifically for carrying signaling infor-
mation for all the other channels.
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Q Signaling (Q.SIG).
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SS7 interswitch PSTN signaling.
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Loop-Start Signaling
Loop-start signaling is an analog signaling technique used to indicate on-hook and off-
hook conditions in the network. It is commonly used between the telephone set and the
CO, PBX, or FXS module. As shown in Figure 14-3, with loop start, the local loop is open
when the phone is on-hook. When the phone is taken off-hook, a -48 direct current (DC)
voltage loops from the CO through the phone and back. Loop-start signaling is used for
residential lines.
Te l e p h o n e
CO or FXS Module
Tip
Ring
Gen
On-Hook
(Open Loop)
Ring
-48 DC Battery
Te l e p h o n e
CO or FXS Module
Tip
Off-Hook
(Closed Loop)
Ring
Gen
Ring
-48 DC Battery
Figure 14-3
Loop-Start Signaling
Ground-Start Signaling
Ground-start signaling is an analog signaling technique used to indicate on-hook and off-
