Lesson 1/Learning Event 5
d. In-Band Supervision.
In-band supervision is accomplished by
sending a constant tone over the interswitchboard trunk in the on-hook
condition.
As soon as the switchboard operator or sole-user party
(dedicated circuit) creates the off-hook condition, the tone ceases. One
important advantage gained by this system (sometimes call pilot system) is
an automatic warning in case of trunk failure.
e. In-Band Signaling. A ringing signal of 20 Hz is detrimental to the
quality of adjacent channels of voice communication.
Further, the low-
frequency cutoff of channel filters in telephone carrier terminals
prevents the transmission of 20 Hz ringing, A converter is therefore
necessary to change the 20 Hz ringing signal to a tone for transmission,
and restoration of he tone to 20 Hz upon reception. The tone can travel
easily through the voice channels. An interface problem develops when two
systems using different frequencies for in-band signaling are joined.
A
compatible converter for each system must also be joined at the interface
to make possible in-band signaling.
Learning Event 5: E AND M SIGNALING
The bandwidth of each multiplexed voice channel in a telephone
carrier terminal is limited by the bandwidth of the channel loop filter.
All signals entering and leaving each channel therefore lie within the
normal voice range of approximately 300 to 3,400 Hz.
Since in-band
signaling is the only type of signaling permissible on telephone carrier
voice channels, a single-frequency (SF) unit must be interposed on the
trunk between the switchboard and each channel of a telephone carrier
terminal. SF units are available to operate at 1,600 or 2,600 Hz. Each
SF unit connects to the switchboard with a pair of wires separate from the
voice path.
One wire is designated the M lead and the other is the E
lead. Identification of these DC leads indicates the direction of in-band
ringing signal through SF units.
The M lead stands for "mount,"
indicating the sending SF unit originates the ringing tone.
The E lead
stands for "ear" indicating reception of the ringing tone.
In other
words, application of DC voltage to the M lead at the sending station
results in the appearance of a DC voltage on the E lead at the receiving
station.
Each SF unit has both E and M leads so that it can either
generate or receive a VF ringing signal.
Both E and M leads use ground
return.
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