Air conditioning systems consist of an indoor unit that can cool, this is connected via two refrigerant pipes to an outdoor unit. In cooling mode this refrigerant circuit to the indoor unit cools the indoor heat exchanger. Air is drawn across the cooled heat exchanger to produce an output of cold air. Sensors built into the indoor unit can adjust the amount of refrigerant in the circuit and hence the cooling output.
All modem systems are in fact reverse cycle heat pumps and can offer heating as well as air conditioning cooling, although in communication room types of environment the heating option is rarely used.
Applications where it is vital that communications are not lost will often have separate independent cooling systems. Typically two systems off the same type will be operated with one system in operation and one in stand bye, each will share the duty cycle with one running for 12 hours and the other then taking over the cooling duty for the next 12 hours.
Safety circuits can be built in so that if the air conditioner that is running should fail the other system will come on line to ensure that the server room temperature is maintained at the correct level.