Lesson 1/Learning Event 4
Learning Event 4: SIGNALING
Signaling is just as important as voice communication.
Without
signaling, one party cannot call another in a telephone network. The type
of signaling depends mostly on the telephone system in use. Most tactical
operations use local-battery telephones, while higher level systems use
common-battery telephones.
The definition of the type of signaling is
related more to a switchboard than a telephone, because the switchboard
makes telephone networks possible.
a. Ringing. Each telephone has a bell for ringing; this is the signal
which alerts a party that someone is calling. The normal ringing signal
is 20 Hz at about 90 volts. This type of signal is satisfactory between
telephones and between a telephone and switchboard.
It is not
satisfactory for signaling over long-distance trunks between switchboards
because the relatively high ringing voltage interferes with signals on
other trunks.
b. Ringdown Supervision.
A switchboard operator is required to
supervise every call going through his board. The work of supervising a
tactical switchboard is simplified by the ringdown feature. As each party
initiates his call with a ring (local-battery operation), a drop leaf
falls or a light illuminates to indicate the fact on the switchboard.
After the operator makes the connection and rings the called party, he
restores the drop leaf.
When each party completes his call, he is
required to ring again, thus causing the drop leaf to fall. The operator
then disconnects the parties.
The ringdown feature is essential for
local-battery operation; but is not used in common battery operation or on
long-distance trunks.
c. Plug-Supervision. In a common-battery telephone system, a calling
party merely raises his telephone handset off the cradle closing the
switchboard loop and cause a light to show on the switchboard panel. The
current which actuates the relay through the closed telephone contacts is
furnished by the switchboard.
The signal is therefore the same current
which is used to carry the voice over the wire.
When the operator
completes the call, both the calling and the called parties' lights
extinguish.
When the parties hang up after completing the calls, both
lights illuminate and the operator pulls down the cords.
An important
point to notice is that when the telephones are "on hook" (idle), there is
no current through the wire, even though battery voltage remains applied
to it.
Plug supervision is used in both common-battery operation and
supervision of interswitchboard trunks. This method of operation is also
known as "off-hook" signaling since line current flows only during the
off-hook condition.
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