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Cabinet

A cabinet is an enclosure for housing your equipment with pre-drilled holes (or squares) along a 4-post support system for mounting equipment. When you look at the specs for a cabinet, it may list the inches or centimeters, but in the data center industry, the important specification is called U, or Rack Units. A rack is something different (see below), but the unit of measure is the same between a cabinet and a rack. The most common size for a data center cabinet is 42 rack units, or 42U. Not all 42U cabinets will have the same exterior dimensions - what the 42U refers to is the inside height, and there are various options that may extend the width or depth of a cabinet by varying amounts. Most cabinets today have an outside footprint of 24" wide by 42" deep, but just 10 years ago it was nearly unheard of to have a cabinet with a depth greater than 36". As hardware evolves, so does the infrastructure.

CDU / Cabinet Distribution Unit

In other words, a Power Strip. Many people use the term CDU because (a) it's cool to use acronyms and (b) it helps to differentiate it from what most people use at home - something picked up from the mega market when it was on sale for $6.99. Today's data center CDUs are typically IP addressable and intelligent enough to provide near real-time (we say near real-time, because there is a standard for energy billing precision, and most CDUs don't meet that standard) statistics on power consumption. Some models also have ports that allow you to connect environmental sensors to them for monitoring things like temperature and humidity at the cabinet level, and some even have remote on/off switching per port.

Point of Demarcation / Point of Entry

Depending on your fault tolerance design, this will be one or more locations in your facility where your service providers (electrical utility, ISP) enter and then terminate into some device that allows you, the customer, to extend that service as needed. In other words, it's the point where your utility no longer worries about the physical infrastructure. If you have designed your facility to be concurrently maintainable, there would be a minimum of two separate points of entry to your facility for all utilities.

Power Distribution Unit / PDU

A good rule of thumb to remember the difference between a PDU and an RDU (Remote Distribution Unit) is that a PDU has a transformer. In medium to larger commercial settings (and nearly always in industrial settings) a higher voltage than your data center equipment can operate at is typically provided by your utility. It is then the customer's (your) responsibility to use step-down transformers to get the voltage to something usable. In the US, a typical input voltage would be 480VAC 3-phase, but power supplies on your server can only operate in the 100-240VAC (single phase) range, so we have to step it down to something usable. We'll use a PDU, such as the one pictured here, and step it down to 208VAC 3-phase. If you don't understand electricity enough to know what is meant by 3-phase vs. single-phase, that's ok for now, but you should look into some electrical primers. The main thing is to realize when you need single phase versus 3-phase power (nearly all IT equipment uses single phase, but the CDUs will often take input from a 3-phase source and break it down into single phases at the plug level).

Rack

A rack is actually referring to infrastructure that was very commonly found in telecom closets and central offices - it is a 2-post system designed to house devices that don't need that additional support. As most hardware has gotten heavier since the early 2000s, it is becoming less and less common to see these racks in a data center other than at the point of demarcation.