Lesson 1/Learning Event 5
b. Standard.
The operating standard for E&M signaling in the DCS is
as follows:
(1) On-hook.
Ground is applied to the M lead, resulting in
transmission of tone to the distant SF unit over the send line. Reception
of this tone by the distant SF unit causes the E lead to open, so ground
is removed from the distant switchboard.
This condition remains so long
as the on-hook condition prevails.
(2) Off-hook. Ground is removed from the M lead and battery is applied
to it, resulting in removal of the tone from the line.
Absence of the
tone causes the distant SF unit E lead to apply ground to the switchboard
circuit.
(3) 4W operation.
Since most telephone trunks operate 4-wire, either
pair can carry identical on-hook tone without interference.
This
arrangement gives the advantage of in-band signaling as well as in-band
supervision.
The conditions for on- and off-hook ringing are tabulated
below.
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