Lesson 1/Learning Event 1
connections, the pulses of the teletypewriter signal would seriously
interfere with the telephone listener, and information would not be
transferred.
(3) Frequency compatibility. The signals transferred at the interface
must be sent over transmission systems that have similar impedance
characteristics.
These characteristics determine the power loss at
various frequencies passing over the circuits. Therefore the operator at
the interface must know whether the circuits so connected are capable of
transmitting all the normal frequencies required to reconstruct an
understandable reproduction of the transmitted signal to the receiving end
of the system.
In other words, narrow-band telephone signals are
successfully
transmitted
over
narrow-band
communication
facilities.
However, when the attempt is made to transmit a wideband signal over
narrow-band facilities, the serious loss of essential frequencies modifies
the characteristics of the received signals.
(4) Level compatibility. Circuits to be joined at the interface must
have level compatibility; that is, the output level of one circuit must be
accepted by the second circuit without changing the level. Level may be
changed within the second circuit, but this must be accomplished by
suitable
devices
having
matching
characteristics
(pads,
attenuators, amplifiers, etc). In technical control centers, at lease one
patching bay will be equal-level; all signals entering and leaving the bay
will do so at the same level.
(5) Polarity compatibility. The polarity of a VF signal is important,
but polarity of a DC signal is highly important.
VF telegraph circuits
therefore escape the polarity matching problems at line interfaces.
c. Equipment Compatibility.
Neither size, circuitry, or equipment
types are important in establishing equipment compatibility.
The most
important consideration is compatibility with the characteristics of the
signals received. This principle applies to both the electrical makeup of
the signals and to their channel matching.
Assume, for example, that
multiplexed signals coming through the interface are intended to terminate
on channels 1, 3, 5, and 7 of a multiplex terminal.
If the terminal
happens to be conditioned to receive channels 2, 4, 6, and 8,
communication obviously cannot exist.
d. Interconnecting Devices.
Many types of devices are used for
interconnecting the various items that make up an integrated communication
system.
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