Network Rack
19 Inch Racks
A 19-inch rack is a standardized (EIA 310-D,
IEC 60297 and DIN 41494 SC48D) system for mounting various
electronic modules in a "stack", or rack, 19 inches (482.6 mm)
wide. Equipment designed to be placed in a rack is typically
described as rack-mount, a rack mounted system, a rack mount
chassis, subrack, or occasionally, simply shelf. The slang
expression for a subrack (generally 1U height) is "pizza box"
due to the similarity in size and shape.
Because of their origin as mounting systems for telephone switching
equipment, they are still sometimes called relay racks, but the
19-inch rack format has remained a constant while the technology
that is mounted within it has changed to completely different fields.
This standard rack arrangement is widely used throughout the
telecommunication, computing, and entertainment industries, as
well as others.
The rack's mounting fixture consists of two parallel metal strips
(also referred to as "rails") standing vertically. The strips are
each 0.625 inches (15.875 mm) wide, and are separated by a gap of
17.75 inches (450.85 mm), giving an overall rack width of 19 inches
(482.6 mm). The strips have holes in them at regular intervals,
with both strips matching, so that each hole is part of a horizontal
pair with a center-to-center distance of 18.3 inches (464.82 mm).
The holes in the strips are arranged vertically in repeating sets
of three, with center-to-center separations of 0.5 inch (12.7 mm),
0.625 inch (15.875 mm), 0.625 inch (15.875 mm). The hole pattern thus
repeats every 1.75 inches (44.45 mm). Racks are divided into regions,
1.75 inches in height, within which there are three complete hole pairs
in a vertically symmetric pattern, the holes being centered 0.25 inch
(6.35 mm), 0.875 inch (22.225 mm), and 1.5 inch (38.1 mm) from the top
or bottom of the region. Such a region is commonly known as a "U", for
"unit", and heights within racks are measured by this unit.
Rack-mountable equipment is usually designed to occupy some integral
number of U. For example, an oscilloscope might be 4U high, and
rack-mountable computers are most often 2U or 1U high.
Excerpt from "19-inch rack." Wikipedia, The Free Encyclopedia.
20 Oct 2006, 13:36 UTC. Wikimedia Foundation, Inc. 30 Oct 2006
http://en.wikipedia.org/w/index.php?title=19-inch_rack&oldid=82620559 Local Area Network (LAN)
A local area network (LAN) is a computer
network covering a local area, like a home, office, or group
of buildings. Current LANs are most likely to be based on
switched IEEE 802.3 Ethernet running at 10, 100 or 1,000
Mbit/s or on Wi-Fi technology .
The defining characteristics of LANs in contrast to WANs
(wide area networks) are: their much higher data rates;
smaller geographic range; and that they do not require
leased telecommunication lines.
...
Technical aspects of LAN
Although switched Ethernet is now most common at the physical
and data link layers, and TCP/IP as a protocol, historically
many different options have been used (see below), and some
continue to be popular in niche areas. Larger LANs may have
redundant links, and routers or switches capable of using
spanning tree protocol and similar techniques to recover from
failed links. LANs may have connections to other LANs via
routers and leased lines to create a WAN (Wide Area Network).
Most will also have connections to the large public network
known as the Internet, and links to other LANs can be
'tunnelled' across this using VPN technologies.
Excerpt from "Local area network." Wikipedia, The Free Encyclopedia.
27 Oct 2006, 02:00 UTC. Wikimedia Foundation, Inc. 30 Oct 2006
http://en.wikipedia.org/w/index.php?title=Local_area_network&oldid=83970048
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