Lesson 1/Learning Event 2
(3) Only
signal-compatible
terminals
can
be
interfaced
by
interconnecting the line sides.
However, altogether different types of
terminals can be interfaced by interconnecting the VF loops. For example,
an installation may have an intermix of frequency-division-multiplex (FDM)
and time-division-multiplex (TDM) terminals.
The line sides cannot be
interconnected because the composite signals from these types of terminals
are entirely different. So the interface must take place at the patching
panel where the individual channel voice signals appear.
Learning Event 2: TWO-WIRE AND FOUR-WIRE PRINCIPLES
Most telephones are two-wire instruments; that is, a pair of wires
connects them together or to other telephone facilities.
When two
telephones are connected by a pair of wires, the communication circuit is
satisfactory as long as the signal loss in the wire is small.
When the
signal loss is great, the listener will have difficulty hearing the
message. An amplifier can raise the level of signal in one direction of
transmission, but will do nothing for the signal traveling in the opposite
direction.
It is therefore necessary to use two pairs of wire, one for
each direction of transmission, with an amplifier in each pair.
Such a
circuit is called four-wire (4W), while a circuit using one pair is called
two-wire (2W).
The principle of a 2W-4W voice communication circuit is
illustrated in figure 1-1.
The line facility may include long-line
circuits, radio sets, or telephone carrier terminals, or combinations of
them.
Many times the components of the system (other than lines and
telephones) are included within the carrier terminals or radio sets.
5
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