Rack

A rack is a frame or enclosure where people mount a system's electronic parts.  A rack has front panels 19 inches (more common) or 23 inches wide, and vertical spaces 1.75 inches, called a rack unit or U. 

Railroad workers originally mounted switching relays in racks, but today people use them for many purposes.  The most common are telephone and computer equipment.  They're ideal for server equipment because they store servers, battery backups, drive bays, and other devices without taking up a lot of floor space or needing a lot of tables or shelves.  Industrial engineers use racks to house industrial equipment such as control and power units.

Typical racks include space for 42 U, but manufacturers also sell half racks for some configurations and spaces.  Rack mounted equipment is only 1.719 inches high, leaving a small space between components so people can easily slide them in and out without scraping other components or binding on them.

Rack mount equipment fits into a rack.  Such equipment fits into 1 or more U (height) and 19 inches width, and comes with rails so engineers and technicians can slide the equipment in and out of the space.  Sometimes engineers bolt equipment into the rack for better security once they slide it into place.  They can easily remove the bolts so they can access the equipment for maintenance, or access it such as with rack mount monitors and keyboards.  Rails fully support equipment when it's pulled clear of the rack, which is very important for maintenance.  Mounting rail specifications vary from hardware manufacturer to hardware manufacturer – there is no standard.  As a result, people acquire rails from hardware manufacturers rather than from rack manufacturers.

Rack mount computer equipment includes many features that help accommodate easy access:

• Locking rails prevent movement when someone extends the rails, making maintenance easier.
• Rails fasten to equipment with locking pins rather than screws.  Individuals can add equipment to a rack without assistance.
• Folding cable trays keep cables neat.  The equipment extends fully without unplugging.
• Spaces include indicator lights.  These lights show faulty hardware, so engineers can more easily locate failed equipment.  This is critical in a rack environment with 10 or more devices.
• Handles on the rails so people can easily pull and push the equipment without touching cables or cords.

People use a KVM switch in a rack mounted server situation.  A KVM switch has inputs for one keyboard, mouse, monitor combination and outputs to multiple computers (or servers,) along with a switch.  A user switches control between many rack mounted computers using only one keyboard, mouse, monitor.

Rack mounted equipment accommodates upside down rack mounting.  In some cases, such as with DVD drives, this doesn't make sense because the device won't function.  This helps more equipment fit into the rack.

Racks come pre-assembled, or in pieces for self assembly in cases where the rack wouldn't fit through an entrance.  People accustomed to rack assembly complete assembly in an hour or less, even though it looks very complicated.

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