Specifications

Glossary
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available memory Memory currently not in
use by the operating system, drivers, or applica-
tions, which can be used to load additional
software.
average access time The average time it takes a
disk drive to begin reading any data placed any-
where on the drive. This includes the average seek
time, which is when the heads are moved, as well
as the latency, which is the average amount of time
required for any given data sector to pass under-
neath the heads. Together, these factors make up
the average access time. See also average seek time
and latency.
average latency The average time required for
any byte of data stored on a disk to rotate under
the disk drive’s read/write head. Equal to one-half
the time required for a single rotation of a platter.
average seek time The average amount of time
it takes to move the heads from one random cylin-
der location to another, usually including any head
settling time. In many cases, the average seek time
is defined as the seek time across one-third of the
total number of cylinders.
AVI (audio video interleave) A storage tech-
nique developed by Microsoft for its Video for
Windows product that combines audio and video
into a single frame or track, saving valuable disk
space and keeping audio in synchronization with
the corresponding video. AVI files are widely sup-
ported by media players and video production
programs.
AWG American Wire Gauge, a U.S. standard for
measuring the thickness of copper and aluminum
wire for electrical and data-transmission use.
Thinner wire is used to save space and for short
distances, but thicker wire has less resistance and
is better for long wire runs.
B Channel The bearer channel in an ISDN net-
work. It’s used to carry data at a rate of 64KBps.
See also BRI.
backbone The portion of the Internet or wide
area network (WAN) transmission wiring that con-
nects the main Internet/WAN servers and routers
and is responsible for carrying the bulk of the
Internet/WAN data.
backplane A rarely used motherboard design in
which the components typically found on a
motherboard are instead located on an expansion
adapter card plugged into a slot. In these systems,
the board with the slots is the backplane. The PCI
Industrial Computer Manufacturers Group (PICMG)
single-board computer designs for rackmount sys-
tems are the primary users of backplane designs
today.
backup The process of duplicating a file or
library onto a separate piece of media. It’s good
insurance against the loss of an original. Depend-
ing on how the backup was made, the data might
need to be restored with a special program before
reuse.
backup disk Contains information copied from
another disk. Used to ensure that original informa-
tion is not destroyed or altered.
backward compatibility The design of soft-
ware and hardware to work with previous versions
of the same software or hardware.
bad sector A disk sector that can’t hold data
reliably because of a media flaw or damaged format
markings.
bad track table A label affixed to the casing of
an ST412/506 or ESDI hard disk drive that tells
which tracks are flawed and incapable of holding
data. The listing is entered into the low-level for-
matting program. Modern ATA (IDE) and SCSI
drives are low-level formatted during manufacture
and don’t have (or need) a bad track table.
balanced signal Refers to signals consisting of
equal currents moving in opposite directions.
When balanced or nearly balanced signals pass
through twisted-pair lines, the electromagnetic
interference effects—such as crosstalk caused by the
two opposite currents—largely cancel each other
out. Differential signaling (used by some types of
SCSI interfaces) is a method that uses balanced
signals.
balun Short for balanced/unbalanced. A type of
transformer that enables balanced cables to be
joined with unbalanced cables. Twisted-pair
(balanced) cables, for example, can be joined
with coaxial (unbalanced) cables if the proper
balun transformer is used.
Appendix A
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