Hardware should be installed as close to the front of the rack as possible to allow for proper airflow in the back. Minimizing cable clutter can also help prevent airflow blockages. In a typical rack system, airflow through the chassis is typically designed to move from front to back. Rack-monitoring systems should be installed to sound an alarm if the rack' s overall temperature rises too much, if air quality (moisture or smoke) falls below acceptable levels, or if airflow is restricted. Because rack security is also a concern, these monitoring systems can alert technicians when a door is left open, or use motion sensors to detect the presence of an intruder. In data rooms containing multiple rows of racks, a hot aisle/cold aisle layout, pulls cool air from vents in the floor or from nearby, low-lying wall vents into the rows of racks. The hot air aisles are used to direct the heated air away from the racks into exhaust vents for cooling.