Figure 7: Airflow Through The M120 Chassis; Figure 8: M120 Front Fan Tray; Figure 9: M120 Rear Fan Tray - Juniper M120 Hardware Manual

M120 Multiservice Edge Router Hardware Guide
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Figure 7: Airflow Through the M120 Chassis

The host subsystem monitors the temperature of the router components. When the
router is operating normally, the fans function at lower than full speed. If a fan fails or
the ambient temperature rises above a threshold, the speed of the remaining fans is
automatically adjusted to keep the temperature within the acceptable range. If the
ambient maximum temperature specification is exceeded and the system cannot be
adequately cooled, the Routing Engine shuts down the system by disabling output power
from each PEM.
Both the front and rear card cages pull air from a single intake in the front of the router.
Air is pushed up through both card cages where it combines in a common exhaust plenum
and is exhausted out the upper sides and rear of the system. Some air is circulated back
down through the power supplies and is exhausted out the lower rear area of the chassis
below the power supplies.

Figure 8: M120 Front Fan Tray

Figure 9: M120 Rear Fan Tray

Troubleshooting the M120 Cooling System on page 286
Maintaining the M120 Air Filter on page 263
Replacing an M120 Air Filter on page 187
Maintaining the M120 Fan Trays on page 264
Replacing an M120 Fan Tray on page 190
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