The main feature of the fan unit is to ventilate the 19"/ 1U cabinet used for ONS 15305. The fan unit is a plug-in device consisting of a circuit board with 4 fans. The air is sucked in through 4 circular openings in the left sidewall, and emerges through holes in the right side cabinet wall. Four fans are used to improve reliability.The ONS15305 provides facilities to report a minimum of 4 auxiliary alarm inputs for associated equipment, e.g. power unit failure battery condition, cabinet door etc. and 2 dedicated alarm outputs. The alarm outputs are related to the unit alarm indicator and the traffic alarm indicator. The input alarms are reported to the management system and are activated by a closed or open loop condition between a pair of contacts.