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White paper on IT Racks
A rack unit is a unit of measure used to describe the height of a server, network switch or other similar device mounted in a 19-inch rack or a 23-inch rack (refers to width of rack). One rack unit is 44.45 mm (1.75 in) high.
One rack unit is commonly designated as "1U"; similarly, 2 rack units are "2U" and so on. The size of a piece of rack mounted equipment is usually described as a number in "U". One rack unit is also sometimes referred to as "1RU"; most server racks have 42 U of height.
Professional audio and video gear often comes with rack mount options and use the same measurement specifications.
Half-rack units typically describe units that fit in a certain number of rack units, but occupy only half the width of the rack (9.5 inches or 241 mm). For example, a "4U half-rack" DVCAM deck would occupy 4U (4 1.75 inches) height 9.5 inches width. In theory, two half-rack decks could occupy the 4U space.
A front panel or filler panel in a rack is not an exact multiple of 1.75-inches (44.45 mm). To allow space between adjacent rack mounted components, a panel is 1/32 inch (0.031 inch or 0.79 mm) less in height than the full number of rack units would imply. Thus, a 1U front panel would be 1.719 inches (43.66 mm) high. If n is number of rack units, the formula for panel height is h = (1.750n - 0.031) inch = (44.45n - 0.79) mm.
Coincidentally, a rack unit is equal to a vershok, which is an obsolete Russian length unit.
The rack unit size is based on a standard rack specification as defined in EIA-310.
A rack server, also called a rack-mounted server, is a computer dedicated to use as a server and designed to be installed in a framework called a rack. The rack contains multiple mounting slots called bays, each designed to hold a hardware unit secured in place with screws. A rack server has a low-profile enclosure, in contrast to a tower server, which is built into an upright, standalone cabinet.
A single rack can contain multiple servers stacked one above the other, consolidating network resources and minimizing the required floor space. The rack server configuration also simplifies cabling among network components. In an equipment rack filled with servers, a special cooling system is necessary to prevent excessive heat buildup that would otherwise occur when many power-dissipating components are confined in a small space.
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