Data Centre Terminology

Commonly Used Industry Terms

A

Amp, a unit of electrical current.

AC
Alternating Current, the designation given to power that is delivered in the form of a sinusoidal wave form. AC won out over DC as the preferred method of delivering and using power in the industrial age due to the ease of voltage transformation using static devices (transformers).

ACAE
Air Conditioning Airflow Efficiency, the amount of heat removed per standard cubic foot of airflow per minute.

AHU
Air Handling Unit.

Cold Aisle
An aisle where rack fronts face into the aisle. Chilled airflow is directed into this aisle so that it can then enter the fronts of the racks in a highly efficient manner.

Containment
Using either long curtains or rigid plastic to maintain a physical barrier between a hot and cold aisle. Keeping warm exhaust air away from the intake of the server racks is a crucial part of making any data centre more efficient.

CRAC
Computer room air conditioner (pronounced crack) which uses refrigerant and a compressor. Cooling of the air in the data centre is accomplished by airflow over the evaporation coils where the refrigerant is being “directly expanded”.

DC
Direct Current, a non-time varying method of delivering power. While slightly more efficient then AC if utilized between the DC portion of the UPS and the power supplies in IT equipment, it has not won wide acceptance in modern data centres.

DCiE
Data Centre infrastructure Efficiency, a metric developed by the Green Grid, data centre infrastructure efficiency is an efficiency measure that is calculated by dividing the IT equipment power consumption by the power consumption of the entire data centre. This measure is the inverse of PUE.

Dewpoint
The temperature at which air reaches water vapour saturation. Dewpoint is constant for a specific amount of water in a specific amount of air while relative humidity varies with temperature. The latest ASHRAE spec for data centre environmental conditions includes an upper limit for humidity based on dewpoint.

Hot Aisle
An aisle where rack backs face into the aisle. Heated exhaust air from the equipment in the racks enters this aisle and is then directed to the CRAC return vents.

HVAC
Heating, Ventilation and Air Conditioning system, the set of components used to condition interior air including heating and cooling equipment as well as ducting and related airflow devices.

In-Row Cooling
Cooling technology installed between racks in a row that draws warm air from the hot aisle and delivers cool air to the cold aisle, minimising the path of the air.

kW
Kilowatts, one thousand watts.

kWh
Kilowatt-Hour, one thousand watt hours. kWh is a common unit of electrical energy.

kVA
Kilovolt Amperes = voltage x current (amperage).

KVM
Keyboard, Video, Mouse, an interface technology that enables users to access multiple servers remotely from one or more KVM sites. More obscurely, can also mean Kernel-base Virtual Machine: a version of the Java Virtual Machine for small devices with limited memory.

MW
Mega Watt, a measure of power equal to one million watts. Often used to describe the size of data centres in terms of power capacity.

N+1
Need plus one, a redundancy concept where capacity is configured to include used capacity plus one additional device to enable continued operations with the failure of one system in the configuration.

PDU
Power Distribution Unit, this typically refers to one of two pieces of equipment in the power delivery chain. One is the combination transformer/breaker panel that is often used between a UPS supplying voltage higher than that used by the IT equipment and the cabinets. The other is the smaller “power strip” like device that is used inside the rack to distribute power to the IT equipment.

PH
Phase, a term that describes the relationship between multiple time-varying waveforms which have a constant frequency but differ in their position relative to time. It is also used to refer to the number of sinusoidal voltages that make up the power delivery to a device. Most common are three-phase and single-phase. Single-phase consists of 2 conductors between which a sinusoidal voltage is present. Three-phase is a set of 3 or 4 conductors. In the case of 3 conductors, a sinusoidal voltage of a constant magnitude and frequency but differing relationship with respect to time exists between any 2 conductors. In a 4 wire system, the same voltage as in the 3 wire case exists between any of the three “hot” conductors and in addition, between any of the three “hot” conductors and fourth neutral conductor there exists a voltage that is smaller by a factor of the square root of three than the voltage between any of the “hot” conductors. An example of this is a 208/120 three-phase system. 208 volts exists between any of the three “hot” conductors and 120 volts exists between any of the “hot” conductors and the neutral conductor.

PUE
Power Usage Effectiveness, a metric defined by the Green Grid, which is a measure of data centre efficiency calculated by dividing the total data centre energy consumption by the energy consumption of the IT computing equipment. This measure is the inverse of DCiE.

U
A unit of space in a rack, equal to 1.75″. The vertical dimension of racks and IT equipment is often specified in “Us” such as 42U.

UPS
Uninterruptible Power Supply, a device placed in series with the supply of power from the utility which contains energy storage such that the supply of power from the UPS is continuous even when the utility supply is removed. While battery-based energy storage is the most common, flywheel-based energy storage is gaining in popularity due to the reduced maintenance cost.

UPS
Uninterruptible Power Supply, a device placed in series with the supply of power from the utility which contains energy storage such that the supply of power from the UPS is continuous even when the utility supply is removed. While battery-based energy storage is the most common, flywheel-based energy storage is gaining in popularity due to the reduced maintenance cost.

V
Volt, a unit of electrical potential.

W
Watt, a unit of power, commonly used in electrical discussion, watts are the product of potential and current. If the current and voltage are AC, the relationship between watts, volts and amps includes power factor, watts = volts x amps’ x PF.