From the front of the chassis, there is a single air intake on the left side of the services gateway. Air is
pushed from the fan tray through the air filter and then to the card cage. The air is exhausted out the right
of the system. Figure 31 on page 52 illustrates the air flow from the top of the chassis and the front of
the chassis.
Figure 31: Airflow Through the Chassis
At startup, the Routing Engine runs the fans at full speed until it can obtain reliable temperature data from
its temperature sensors. After it finishes booting and has reliable temperature data, the Routing Engine
monitors the temperature of certain critical services gateway components and adjusts the speed of the
fans accordingly.
The fans have three speeds: low, intermediate, and high. Table 11 on page 52 shows the correlations
between fan speed and critical component temperature. Note that there is hysteresis built into the
correlation, so that the fans are not subject to spurious speed changes. Also, if one of the fans in the fan
tray fails, the speeds of the remaining fans are automatically adjusted to keep the temperature within the
acceptable range.
Table 11: Critical Component Temperatures and Fan Speed Transitions
Fan Speed ResponseCritical Component Temperature
Increase from Low to MediumIncrease to 60°C
Increase from Medium to HighIncrease to 75°C
Decrease from High to MediumDecrease to 65°C
Decrease from Medium to LowDecrease to 55°C
52
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