They allow for dense hardware configurations without occupying excessive floorspace or requiring shelving. Nineteen-inch racks in two-post or four-post form hold most equipment in modern data centers, ISP facilities, and professionally designed corporate server rooms.
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The 19-inch (482.6 mm) standard rack arrangement is widely used throughout the telecommunication, computing, audio, video, entertainment and other industries, though the Western Electric 23-inch standard, with holes on 1-inch (25.4 mm) centers, is still used in legacy ILEC/ CLEC facilities.Ī 19-inch rack used for switches at the DE-CIX in Frankfurt, Germany The 19-inch rack format has remained constant while the technology that is mounted within it has changed considerably and the set of fields to which racks are applied has greatly expanded. īy 1934, it was an established standard with holes tapped for 12-24 screws with alternating spacings of 1.25 inches (31.75 mm) and 0.5 inches (12.70 mm) The EIA standard was revised again in 1992 to comply with the 1988 public law 100-418, setting the standard U as 15.875 mm (0.625 in) + 15.875 mm (0.625 in) + 12.7 mm (0.500 in), making each "U" 44.45 millimetres (1.75 in). in all cases to be a whole multiple of 1 3⁄ 4 inches". The height of the different panels will vary, . But in light of the rapid growth of the toll network, the engineering department of AT&T undertook a systematic redesign, resulting in a family of modular factory-assembled panels all "designed to mount on vertical supports spaced 19 1⁄ 2 inches between centers.
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Once serial production started, they were built into custom-made racks, one per repeater. The earliest repeaters from 1914 were installed in ad hoc fashion on shelves, in wooden boxes and cabinets. The 19-inch rack format with rack-units of 1.75 inches (44.45 mm) was established as a standard by AT&T around 1922 in order to reduce the space required for repeater and termination equipment in a telephone company central office. There is little evidence that the dimensions of these early racks were standardized. īy 1911, the term was also being used in railroad signaling. The term relay rack appeared first in the world of telephony. The industry-standard rack cabinet is 42U tall however, 45U racks are also common. The height of the electronic modules is also standardized as multiples of 1.75 inches (44.45 mm) or one rack unit or U (less commonly RU). 2.5.4 Fiberglass reinforced plastic case racksĮquipment designed to be placed in a rack is typically described as rack-mount, rack-mount instrument, a rack-mounted system, a rack-mount chassis, subrack, rack cabinet, rack-mountable, or occasionally simply shelf.