| LAN |
(Local Area Network)
A communications network that serves users within a confined
geographical area. It is made up of servers, workstations, a network
operating system and a communications link.
Servers are high-speed machines that hold programs and data shared by
network users. The workstations (clients) are the users' personal
computers, which perform stand-alone processing and access the network
servers as required (see client/server).
Diskless and floppy-only workstations are sometimes used, which
retrieve all software and data from the server. Increasingly,
"thin client" network computers (NCs) and Windows terminals
are also used. A printer can be attached locally to a workstation or
to a server and be shared by network users.
Small LANs can allow certain workstations to function as a server,
allowing users access to data on another user's machine. These
peer-to-peer networks are often simpler to install and manage, but
dedicated servers provide better performance and can handle higher
transaction volume. Multiple servers are used in large networks.
The controlling software in a LAN is the network operating system
(NetWare, UNIX, Windows NT, etc.) that resides in the server. A
component part of the software resides in each client and allows the
application to read and write data from the server as if it were on
the local machine.
The message transfer is managed by a transport protocol such as TCP/IP
and IPX. The physical transmission of data is performed by the access
method (Ethernet, Token Ring, etc.) which is implemented in the
network adapters that are plugged into the machines. The actual
communications path is the cable (twisted pair, coax, optical fiber)
that interconnects each network adapter. See WAN.
Clients and Servers in a LAN This
illustration shows one server for each type of service on a LAN. In
practice, several functions can be combined in one machine and, for
large volumes, multiple machines can be used to balance the traffic
for the same service. For example, a large Internet Web site is often
comprised of several computer systems (servers).
The Software in a Network Client This
shows the various software components that reside in a user's client
workstation in a network.
The Software in a Network Server This
shows the network operating system and various system software
components in a network server.
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|
| laser printer |
A printer that uses a laser and the
electrophotographic method to print a full page at a time. The laser is used to
"paint" a charged drum with light, to which toner is applied and then
transferred onto paper (see electrophotographic
for more details). Desktop laser printers use cut sheets like a copy machine. Large
machines use cut sheets or paper rolls that are cut after printing.
In 1975, IBM introduced the first laser printer, the model 3800. Later, Siemens came out
with the ND 2 and Xerox with the 9700. These self-contained printing presses were online
to a mainframe or offline, accepting print image data on tape or disk.
In 1984, HP introduced the LaserJet, the first desktop laser printer, which rapidly became
a huge success and a major part of the company's business. Desktop lasers obsoleted the
clackety daisy wheel printers, but did not eliminate the dot matrix printers that are
still widely used for labels and multipart forms.
Low-end laser printers print in the 4 to 8 ppm range, while typical office workgroup units
print 17 to 32 ppm. HP is the major vendor, and many quality companies compete such as
Lexmark, IBM and Okidata. Midrange units print in the 40-60 ppm range, with a large jump
to high-end printers that print from 150 to more than 1,000 ppm.
Laser printer resolution is typically from 300 to 1200 dpi, but specialty printers can
reach imagesetter resolution of 2400 dpi. Some units offer paper handling functions found
on copy machines such as collation, stapling and 3-hole punching.
Color laser printers are slower than their monochrome counterparts, typically in the 4 to
10 ppm range. At the other end of the spectrum, high-end "digital printing
presses" can print 70 or more duplexed color pages per minute, producing finished
booklets and manuals (see digital printing).
There are several printer technologies that fall into the laser class category, but do not
actually use a laser. LED printers use an array of LEDs to beam the image onto the drum,
and electron beam imaging (ion deposition) creates the image with electricity rather than
light. Solid ink printers propel a waxlike ink onto the drum.
The Laser Mechanism The laser printer uses electrostatic charges to (1) create an
image on the drum, (2) adhere toner to the image, (3) transfer the toned image to the
paper, and (4) fuse the toner to the paper. The laser creates the image by
"painting" a negative of the page to be printed on the charged drum. Where light
falls, the charge is dissipated, leaving a positive image to be printed.
The LaserJet Noisier than today's models, but built like a tank, HP created a
revolution in desktop printing with its 1984 introduction of the LaserJet. The LaserJet's
reliability became legendary and caused HP to become the world leader in desktop laser
printers. (Image courtesy of Hewlett-Packard Company.)
Midrange Laser Printer This Dataproducts laser printer prints up to 60 ppm and uses
a unique drum system. The unit does not have to be stopped except once each 100,000 pages
to advance the drum sleeve. The drum lasts for a million pages, because it contains 10
sleeves within its cylinder. (Image courtesy of Hitachi Koki Imaging Solutions, Inc.)
Continuous Forms Laser Printing This Printronix laser printer uses continuous
forms. Continuous forms printers provide tighter registration over cut sheets and support
non-standard paper sizes. In addition, a huge amount of paper can be printed without
operator intervention. (Image courtesy of Printronix, Inc.)
The Versatile Network Printer This Xerox printer offers paper handling options,
including an envelope feeder (middle) and private collator (top) that keeps sensitive
documents locked in bins that must be opened with a password. (Image courtesy of Xerox
Corporation.)
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|
| latency |
(1) The time between initiating a
request for data and the beginning of the actual data transfer. On a disk, latency is the
time it takes for the selected sector to come around and be positioned under the
read/write head. Channel latency is the time it takes for a computer channel to become
unoccupied in order to transfer data. Network latency is the delay introduced when a
packet is momentarily stored, analyzed and then forwarded.
(2) With regard to malicious software, latency is the period between infection and
the first obvious damage to the host system. Many viruses or logic bombs written by
amateurs have a short latency and are therefore relatively easy to detect; however, more
vicious malware can lie dormant or replicate to many other hosts and then wreak havoc
unexpectedly. |
|
| LBA |
| (Logical Block Addressing)
A method used to support IDE hard disks larger than 504MB (528,482,304 bytes) on PCs. LBA
provides the necessary address conversion in the BIOS to support drives up to 8GB. BIOSs
after mid-1994, which are sometimes called "Enhanced BIOSs," generally provide
LBA conversion. LBA support is required for compatibility with the FAT32 directory. |
|
| LCD |
(Liquid Crystal Display)
A display technology that uses rod-shaped molecules (liquid crystals) that flow like
liquid and bend light. Unenergized, the crystals direct light through two polarizing
filters, allowing a natural background color to show. When energized, they redirect the
light to be absorbed in one of the polarizers, causing the dark appearance of crossed
polarizers to show. The more the molecules are twisted, the better the contrast and
viewing angle.
Because it takes less power to move molecules than to energize a light-emitting device,
LCDs replaced LEDs in digital watches years ago. The LCD was developed in 1963 at RCA's
Sarnoff Research Center in Princeton, NJ.
TYPES OF LCDs
Passive Display (TN, STN)
Called "passive matrix" when used for computer screens and
"passive display" when used for small readouts, all the active electronics
(transistors) are outside of the display screen. Passive displays have improved immensely,
but do not provide a wide viewing angle, and submarining is generally noticeable. Passive
display types are:
TN - Twisted Nematic - 90º twist
Low-cost displays for consumer products and
instruments. Black on gray/silver
background.
STN - Supertwisted Nematic- 180-270º twist
Used extensively on laptops for mono and
color displays. DSTN and FSTN provide
improvements over straight STN.
180º - green/blue on yellow background
270º - blue on white/blue background
Dual Scan STN
Improves STN display by dividing the screen
into two halves and scanning each half
simultaneously, doubling the number of lines
refreshed. Not as sharp as active matrix.
Active Display (TFT)
Typically used for laptop computer screens and flat panel desktop
monitors. Known as "active matrix" displays, transistors are built into each
pixel within the screen. For example, a 640x480 color VGA screen requires 921,600
transistors; one for each red, green and blue dot. Provides a sharp, clear image with good
contrast and eliminates submarining, but fabrication costs are high. Uses a 90º (TN)
twist. Also called "TFT LCD" (thin film transistor LCD).
Reflective Vs. Backlit
Reflective screens used in many consumer appliances and some lightweight
laptops require external light and only work well in a bright room or with a desk lamp.
Backlit and sidelit screens have their own light source and work well in dim lighting. |
|
| LCD monitor |
| A flat panel display that uses liquid
crystals. Although almost exclusively used in laptops, the LCD technology is also the most
popular thus far for flat panel desktop monitors. See flat panel
display and LCD. |
|
| LCD panel |
| Also called a "projection panel," it
is a data projector that accepts computer output and displays it on a see-through liquid
crystal screen that is placed on top of an overhead projector. |
|
| line matrix printer |
An impact printer that prints a line at a
time. Printronix pioneered this technology in 1974. Line matrix and band printers are the
surviving line printer technologies, but line matrix can print graphics, whereas band
printers cannot. Line matrix resolution is in the 70 to 140 dpi range and speeds range
from 400 to 1,400 lpm.
Line matrix printers offer medium resolution, monochrome printing with a very low ribbon
cost. They also provide high speed; for example, printing a three-part form at 1,200 lpm
is equivalent to a 65 ppm page printer. Line matrix printers can exist in harsh conditions
and are often found in warehouses and other industrial environments.
The print mechanism is a row of dot hammers that is almost as wide as the page. The
hammers are mounted on a shuttle that oscillates back and forth approximately two inches
in a track. The hammers are magnetically released at the appropriate time and bang into a
ribbon and onto the paper.
Printronix Line Matrix Printers Printronix's ProLine line matrix printers range
from 475 to 1,200 lpm. Today's line matrix printers are as quiet, if not more so, than
laser printers and can function in the harshest industrial environments. (Image
courtesy of Printronix, Inc.)
Printronix Printhead This printhead contains seven sets of print hammers that
oscillate back and forth to cover 16" wide paper. The hammers are held back by
magnets. The hammer is demagnetized and springs forward onto the ribbon. As it recoils, it
is remagnetized back in place. (Original drawing courtesy of Printronix, Inc.)
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|
| LZW |
(Lempel-Ziv-Welch)
A data compression method that stems from techniques introduced by
Jacob Ziv and Abraham Lempel. LZW has been used in many hardware and
software products, including V.42bis modems, GIF, TIF and PDF files
and PostScript Level 2. The LZ77 method creates pointers back to
repeating data, while LZ78 creates a dictionary of repeating phrases
with pointers to those phrases.
The Welch in Lempel-Ziv-Welch is for Terry Welch, the Unisys
researcher who created an enhanced version of these methods that was
patented by Unisys. Unisys collected royalties from thousands of
software publishers whose applications used this algorithm until the
patents expired in 2004. This was the most widely licensed data
compression patent in history. |
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