Lesson 2/Learning Event 6
allowable channel noise is measured by a noise measuring set with C-message
weighting. The unit of measurement is dBrnc0, which is noise power in dBrnc
measured at a zero transmission level point.
EXAMPLE 1.
Parameter S2 allows a maximum variation in net loss of 6
dB, 3 dB above or below the average received level.
The
level is taken at the test frequency of 1.4 kHz.
If the
average received level is -1 dBm, the received level must
not rise above 2 dBm or fall below -4 dB at 1.04 kHz.
EXAMPLE 2.
Noise level is usually given in terms of dBrnc0.
This
phrase refers to C-message weighting of a circuit whose
noise level is compared with the transmission level point
(TLP).
Noise parameters are identical for all similar
mileage distances in table 2 for schedules S1 through D2.
All of these figures are positive, indicating that noise
levels are above 0 dBrn.
However, these figures are not
levels;
they
are
the
decibel
requirements
of
the
parameters.
If the requirement is 37 dBrn, the actual
noise level is 37 dB above -90 dBm. Numerically, it is the
difference between -90 and 37, or -53 dBm.
EXAMPLE 3.
Assume that a communication circuit is 980 miles long and
the measured noise level is -38 dBm. Does the circuit meet
requirements? No, because the difference between -38 and -
90 dBm is 52 dB, greater than the 41 dB allowed in the
parameters.
Learning Event 6: EXPANDED CIRCUIT PARAMETERS.
a. Circuit parameters are greatly expanded to provide AUTOSEVOCOM
service
schedules Z1, Z2, and Z3 in tables 3 and 4 establish the parameters for 10
to 50,000 Hz over metallic facilities without regenerators.
These
parameters are based on full-duplex operation using four-wire termination,
with impedance and source load at 135 ohms. The level of the baseband input
signal is 0 dBm, which is 1.04 volts peak to peak at 135 ohms impedance.
b. Circuit
parameters
are
also
greatly
expanded
for
the
50
kilobit/secencrypted voice transmission system (AUTOSEVOCOM) designed to
provide service within a bandwidth of 48 kHz over long-distance carrier
facilities.
With this method of use, four normal voice channels in the
carrier terminal are replaced by one 48-kHz special service band. Schedule
Z4 shown in table 4 shows the parameters for the carrier method of
operation.
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