Specifications

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Appendix A Glossary
Raw/2:1 compressed capacities for a single tape are
2/4GB for DDS, 4/8GB for DDS-2, 12/24GB for
DDS-3, 20/40GB for DDS-4, and 36/72GB for DAT
72 (the latest standard).
data 1) Groups of facts processed into informa-
tion. A graphic or textural representation of facts,
concepts, numbers, letters, symbols, or instructions
used for communication or processing. 2) An
android from the twenty-fourth century with a pro-
cessing speed of 60 trillion operations per second
and a storage capacity of 800 quadrillion bits, and
who serves on the USS Enterprise NCC-1701-D with
the rank of lieutenant commander.
data bus The connection that transmits data
between the processor and the rest of the system.
The width of the data bus defines the number of
data bits that can be moved into or out of the
processor in one cycle.
data cable Generically, a cable that carries data.
Specific to HD connections, the narrower (20-pin)
of two cables that connects an ST-506/412 or ESDI
hard disk drive to a controller card.
data communications A type of communica-
tion in which computers and terminals can
exchange data over an electronic medium.
data compression A technique in which math-
ematical algorithms are applied to the data in a file
to eliminate redundancies and thus reduce the size
of the file. See also lossless compression and lossy
compression.
Data Link Layer In networking, the layer of
the OSI reference model that controls how the elec-
trical impulses enter or leave the network cable.
Ethernet and Token-Ring are the two most com-
mon examples of Data Link Layer protocols. See
also OSI.
data separator A device that separates data and
clock signals from a single encoded signal pattern.
Usually, the same device performs both data separa-
tion and combination and is sometimes called an
endec, or encoder/decoder.
data transfer rate The maximum rate at which
data can be transferred from one device to another.
daughterboard Add-on board to increase func-
tionality and/or memory. Attaches to the existing
board.
DB-9 A 9-pin D-shell connector, primarily used
for PC serial ports.
DB-25 A 25-pin D-shell connector, primarily used
for PC parallel ports.
DC Direct current, such as that provided by a
power supply or batteries.
DC-600 (Data Cartridge 600) A data-storage
medium invented by 3M in 1971 that uses a
1/4"-wide tape 600 feet in length.
DCE (data communications equipment)
The hardware that performs communication—usu-
ally a dialup modem that establishes and controls
the data link through the telephone network.
See also DTE.
DDE (dynamic data exchange) A form of
interprocess communications used by Microsoft
Windows to support the exchange of commands
and data between two applications running simul-
taneously. This capability has been enhanced fur-
ther with object linking and embedding (OLE).
DDoS (distributed denial of service) Refers
to a type of denial-of-service attack that uses multi-
ple computers that have been taken over by an
intruder to attack a targeted system. See also DoS.
DDR (double data rate) A type of SDRAM that
allows two accesses per clock cycle, doubling the
effective speed of the memory. The most common
types of DDR include PC2100 (also known as DDR
266MHz), PC2700 (also known as DDR 333MHz)
and PC3200 (also known as DDR 400MHz). See also
SDRAM.
DDR2 (double data rate 2) A type of SDRAM
that enables two accesses per clock cycle, doubling
the effective speed of the memory. DDR2 has more
robust signaling allowing for higher speeds than
conventional DDR. See also DDR and SDRAM.
DDR3 (double data rate 3) A type of SDRAM
that enables two accesses per clock cycle, doubling
the effective speed of the memory. DDR3 has more
robust signaling allowing for higher speeds than
DDR2. See also DDR, DDR2 and SDRAM.
de facto standard A software or hardware tech-
nology not officially made a standard by any recog-
nized standards organization but that is used as a
reference for consumers and vendors because of its
dominance in the marketplace.
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