5.
System Reliability.
There are many factors that affect system reliability.
System
reliability is the ratio of usable circuit time to total circuit
time.
The circuit is usable when traffic can be passed without
errors.
For voice telephone traffic, the required level of
reliability is 98%.
For digital data traffic, the required
reliability rate increases to 99.9%.
Two major factors affect system reliability; the propagation support
received, and the equipment reliability. You achieve a high state of
equipment reliability through proper maintenance and by establishing
sufficient backup equipment to prevent system down time due to
equipment failure.
There are several techniques at your disposal to help you increase
the reliability of your system.
a. Length/Number of Hops. One way to obtain greater reliability
is to decrease the distance between radio relay points. Conversely,
the fewer hops required also increases system reliability. You must
weigh the benefit and loss of each factor in determining the number
and distance of system links.
b. Threshold Levels. For a signal from a receiver to be usable,
it must be stronger than the receiver noise level.
You find this
threshold level in the equipment TMs. To increase reliability, plan
hops so that the signal level is higher than the threshold level of
the receiver you use.
The antenna gain and transmission line loss
affect your signal. Ensure your signal is sufficient to overcome the
effect of signal fade.
NOTE: Generally, the relationship between reliability and fade margin
is in the applicable TM.
Experience shows that a 40 decibel
(dB) fade margin yields a reliability of 99.9%.
For example,
if the signal strength at the receiver is -10 dB and the
receiver threshold level is -50 dB, then the fade margin is 40
dB.
c. Equipment Location.
Locate your antenna as close to the
equipment as possible. Radio equipment used in multichannel systems
operates best when you use only one section of coaxial cable between
the antenna and the radio. Adding sections increases the line loss.
Using two or more sections reduces the output signal by more than
50%.
d. Antenna Orientation.
You should rotate your transmitter and
receiver antennas to reduce effects of reflected signals caused by
rough terrain. Rotate the antennas alternately until you achieve the
best orientation between them.
2-17
SS0016
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