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M120 Cooling System Description


The cooling system consists of the following components:

  • Two front fan trays

  • Two rear fan trays

  • Front air filter

The cooling system components work together to keep all router components within the acceptable temperature range (see Figure 1, Figure 2, and Figure 3). The router cooling system comprises two front and two rear fan trays (see Front View of a Fully Configured Router Chassis and Rear View of a Fully Configured AC-Powered Router Chassis). Both the front and rear fan trays install horizontally above and below the front and rear card cages. Each fan tray contains eight fans. The two front fan trays are interchangeable, the two rear fan trays are interchangeable, but the front and rear fan trays are not interchangeable. The fan trays are hot-insertable and hot-removable. The front fan trays cool the components installed in the front card cage (the FPCs, CFPCs, PICs, and craft interface). The rear fan trays cool the components installed in the rear card cage (the Routing Engines, CBs, power supplies, and FEBs). Figure 1 shows the airflow through the router.

An air filter in the front in the chassis located beneath the lower fan tray helps keep dust and other particles from entering the cooling system. To function properly, the entire cooling system requires an unobstructed airflow and proper clearance around the site, as described in M120 Clearance Requirements for Airflow and Hardware Maintenance.

Figure 1: Airflow Through the M120 Chassis
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 2: M120 Front Fan Tray
M120 Front Fan Tray
Figure 3: M120 Rear Fan Tray
M120 Rear Fan Tray