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AXIS 231D+/232D+ - Glossary of Terms
Compression - See Image Compression.
Contrast - Defines the degree of difference between the
lightest and darkest parts
of an image or video stream.
DC-Iris - This special type of iris is electrically controlled
by
the camera, to automatically regulate the amount of light
allowed to enter.
DHCP (Dynamic Host Configuration Protocol) - DHCP is a
proto
col that lets network administrators automate and
centrally manage the assignment of Internet Protocol (IP)
addresses to network devices in a network.
DHCP uses the concept of a "lease" or amount of time that a
giv
en IP address will be valid for a computer. The lease time
can vary, depending on how long a user is likely to require
the network connection at a particular location.
DHCP also supports static addresses for e.g. computers
running
web servers, which need a permanent IP address.
DNS (Domain Name System) - DNS is used to locate and
transla
te Internet domain names into IP (Internet Protocol)
addresses. A domain name is a meaningful and
easy-to-remember name for an Internet address. For
example the domain name www.example.com is much
easier to remember than 192.0.34.166. The translation tables
for domain names are contained in Domain name servers.
Domain Server - Domains can also be used by
orga
nizations who wish to centralize the management of
their (Windows) computers. Each user within a domain has
an account that usually allows them to log in to and use any
computer in the domain, although restrictions may also
apply. The domain server is the server that authenticates the
users on the network.
Duplex - See Full-
duplex.
Ethernet - Ethernet is the most widely installed local area
ne
twork technology. An Ethernet LAN typically uses special
grades of twisted pair wires. The most commonly installed
Ethernet systems are 10BASE-T and 100BASE-T10, which
provide transmission speeds up to 10 Mbps and 100 Mbps
respectively.
ETRAX (Ethernet Token Ring AXIS) - Axis' own
mic
roprocessor.
Factory default settings - These are the settings that
origi
nally applied for a device when it was first delivered
from the factory. If it should become necessary to reset a
device to its factory default settings, this will, for many
devices, completely reset any settings that were changed by
the user.
Firewall - A firewall works as a barrier between networks,
e.
g. between a Local Area Network and the Internet. The
firewall ensures that only authorized users are allowed to
access the one network from the other. A firewall can be
software running on a computer, or it can be a standalone
hardware device.
Focal length - Measured in millimeters, the focal length of
a came
ra lens determines the width of the horizontal field of
view, which in turn is measured in degrees.
FTP (File Transfer Protocol) - An application protocol that
uses the
TCP/IP protocols. It is used to exchange files
between computers/devices on networks.
Frame - A frame is a complete video image. In the 2:1
interlace
d scanning format of the RS-170 and CCIR formats,
a frame is made up of two separate fields of 262.5 or 312.5
lines interlaced at 60 or 50 Hz to form a complete frame,
which appears at 30 or 25 Hz. In video cameras with a
progressive scan, each frame is scanned line-by-line and not
interlaced; most are also displayed at 30 and 25 Hz.
Frame rate - The frame rate used to describe the frequency
at
which a video stream is updated is measured in frames
per second (fps). A higher frame rate is advantageous when
there is movement in the video stream, as it maintains
image quality throughout.
Full-duplex - Transmission of data in two directions
s
imultaneously. In an audio system this would describe e.g.
a telephone systems. Half-duplex also provides
bi-directional communication, but only in one direction at a
time, as in a walkie-talkie system. See also Simplex.
Gain - Gain is the amplification factor and the extent to
which
an analog amplifier boosts the strength of a signal.
Amplification factors are usually expressed in terms of
power. The decibel (dB) is the most common way of
quantifying the gain of an amplifier.
Gateway - A gateway is a point in a network that acts as an
entry point to
another network. In a corporate network for
example, a computer server acting as a gateway often also
acts as a proxy server and a firewall server. A gateway is
often associated with both a router, which knows where to
direct a given packet of data that arrives at the gateway, and
a switch, which furnishes the actual path in and out of the
gateway for a given packet.
GIF (Graphics Interchange Format) - GIF is one of the
most
common file formats used for images in web pages.
There are two versions of the format, 87a and 89a. Version
89a supports animations, i.e. a short sequence of images
within a single GIF file. A GIF89a can also be specified for
interlaced presentation.
GOV (Group Of VOPs) - A group of VOPs is the basic unit
of an MPEG-4 video
stream. The GOV contains different
types and numbers of VOPs (I-VOPs, P-VIPs) as determined
by the GOV length and GOV structure. See also VOP.
GOV length - The GOV length determines the number of
image
s (VOPs) in the GOV structure. See also GOV and VOP.
GOV structure - The GOV structure describes the
compo
sition of an MPEG-4 video stream, as regards the type
of images (I-VOPs or P-VOPs) included in the stream, and
their internal order. See also GOV and VOP.
Half-duplex - See Fu
ll-duplex.