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strength or an attenuator pad to reduce the signal.
signaling unit in the line.
n. Two-wire VF channel 20-IHz ringdown opera tion
The level adjust stage also ensures that the signal on
the Rx line is sent to the user at the same 0 dBm level
is similar to dc-dialing operation described above except
during a receive operation.
for E and M-lead conditions.
e. In the signaling unit, during idle (on-hook) con
o. In 20-Hz ringdown, during the on-hook or traffic
ditions at both terminals, both transmit signal (M) leads
conditions, the M-lead is at -48 volts, the E-lead  is
are grounded, a continuous tone is transmitted  from
grounded, and tone is off.
p. During signaling, the M-lead is grounded, the E-
both terminals, and both receive signal (E) leads are
open.
lead is open, and a tone is transmitted for the duration of
f. When a call is originated at one of the terminals,
the 20-Hz ringdown.
q. For ringdown operation, a two to four-wire Hy
dc-dial pulses are placed on the M-lead and the transmit
section of the signaling unit produces corresponding
brid termination device-must be used to interface the
tone-bursts on the send line for transmission  to the
four-wire dropside of the signaling unit and the two wire
distant terminal.
ringdown circuit.
g. The receive section of the signaling unit at the
r. In addition, a ringdown converter must be used
distant terminal converts the received tones into open or
to convert E and M signaling to 20 Hz (and vice versa)
ground E-lead conditions.
between the signaling unit and ringdown circuit.
h. When dialing is completed, the M-lead at the
s. The 12 two and/or four-wire VF channels are
calling terminal assumes a steady -48 volt level, which
equipped with dual 1600 Hz, 2600 Hz, and 20 Hz/48 vdc
halts transmission of the tone (from the calling terminal)
sources to provide automatic switch over from an online
and causes the E-lead at the distant terminal to assume
source to a standby source in the event of equipment
a steady-ground condition.
failure.  The redundancy helps to provide  continuous
i. When the distant terminal answers, the distant
operation under most conditions.
t. The outputs from the 12-line conditioning cir
M-lead assumes a steady -48 volt level.
j. Also, transmission of the tone ceases, and the
cuits can be patched through the echo suppressor to the
E-lead at the calling terminal assumes a steady ground
multiplexer via the VF patch panel.
u. The echo suppressor, which has 14 channels
condition.
k. This action allows the Tx and Rx lines to be
(two for spares), is used to reduce or eliminate feed back
cleared for normal voice channel communications.
signals in the VF channels.
l. When the call is completed and both terminals
v. The receive operation is the same except that
are in the on-hook condition, ground is applied to the M-
the signals flow in the reverse direction from the
leads, the transmit sections transmit tones, and the E-
multiplexer back to the field users.
leads are open.
m. In trunk applicators, supervisory signals are
2-5.
Digital Baseband Processing System
transmitted from one terminal to the other through
activation of the M-lead.  Multiply-frequency dial tone
a. Figure 2-4 is a simplified diagram of the Digital
pulses may also be used for signaling with the
Baseband
Processing
System
in
the
CSS.
2-7

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