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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 camera
le
ns 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
T
CP/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 interlaced
sc
anning 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 simultaneously.
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
a
nalog 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
p
oint 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 fo
rmats 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 VOP's is the basic unit of an
M
PEG-4 video stream. The GOV contains different types and numbers
of VOP's (I-VOP's, P-VOP's, etc.) as determined by the GOV length
and GOV structure. See also VOP.
GOV length - The GOV length determines the number of images
(VOP'
s) in the GOV structure. See also GOV and VOP.
GOV structure - The GOV structure describes the composition of an
M
PEG-4 video stream, as regards the type of images (I-VOP's or
P-VOP's) included in the stream, and their internal order. See also
GOV and VOP.
Half-duplex - See Full-duplex.
HTML (Hypertext Markup Language) - HTML is the set of “markup”
sym
bols or codes inserted in a file intended for display in web
browser. The markup tells the browser how to display the page's
words and images for the user.
HTTP (Hypertext Transfer Protocol) - HTTP is the set of rules for
e
xchanging files (text, graphic images, sound, video, and other
multimedia files) on the web. The HTTP protocol runs on top of the
TCP/IP suite of protocols.
Hub - A (network) hub is used to connect multiple devices to the
ne
twork. The hub transmits all data to all devices connected to it,
whereas a switch will only transmit the data to the device it is
specifically intended for.
IEEE 802.11 - A family of standards for wireless LANs. The 802.11
stan
dard supports 1 or 2 Mbit/s transmission on the 2.4 GHz band.
IEEE 802.11b specifies an 11 Mbit/s data rate on the 2.4 GHz band,
while 802.11a allows up to 54 Mbit/s on the 5 GHz band.
Image compression - Image compression minimizes the file size (in
b
ytes) of an image. Two of the most common compressed image
formats are JPEG and GIF.
Interlacing - Interlaced video is video captured at 50 pictures (known
as
fields) per second, of which every 2 consecutive fields (at half
height) are then combined into 1 frame. Interlacing was developed
many years ago for the analog TV world and is still used widely
today. It provides good results when viewing motion in standard TV
pictures, although there is always some degree of distortion in the
image.
To view interlaced video on e.g. a computer monitor, the video must
first b
e de-interlaced, to produce progressive video, which consists of
complete images, one after the other, at 25 frames per second. See
also Progressive scan.
IP (Internet Protocol) - The Internet Protocol is a method
transmitting
data over a network. Data to be sent is divided into
individual and completely independent “packets.” Each computer (or
host) on the Internet has at least one address that uniquely identifies
it from all others, and each data packet contains both the sender's
address and the receiver's address.
The Internet Protocol ensures that the data packets all arrive at the
intende
d address. As IP is a connectionless protocol, which means
that there is no established connection between the communication
end-points, packets can be sent via different routes and do not need
to arrive at the destination in the correct order.
Once the data packets have arrived at the correct destination, another
protocol
- Transmission Control Protocol (TCP) - puts them in the
right order. See also TCP.
IP Address - An IP address is simply an address on an IP network
used
by a computer/device connected to that network. IP addresses
allow all the connected computers/devices to find each other and to
pass data back and forth.
To avoid conflicts, each IP address on any given network must be
uniq
ue. An IP address can be assigned as fixed, so that it does not
change, or it can be assigned dynamically (and automatically) by
DHCP.
An IP address consists of four groups (or quads) of decimal digits
s
eparated by periods, e.g. 130.5.5.25. Different parts of the address
represent different things. Some part will represent the network
number or address, and some other part will represent the local
machine address.
See also IP (Internet Protoc
ol).
I-VOP - See VOP.
JPEG (Joint Photographi
c Experts Group) - Together with the GIF
file format, JPEG is an image file type commonly used on the web. A
JPEG image is a bitmap, and usually has the file suffix '.jpg' or
“.jpeg.” When creating a JPEG image, it is possible to configure the
level of compression to use. As the lowest compression (i.e. the
highest quality) results in the largest file, there is a trade-off between
image quality and file size.
kbit/s (kilobits per second) - A measure of the bit rate, i.e. the rate at
which
bits are passing a given point. See also Bit rate.
LAN (Local Area Network) - A LAN is a group of computers and
associate
d devices that typically share common resources within a
limited geographical area.
Linux - Linux is an open source operating system within the Unix
fam
ily. Because of its robustness and availability, Linux has won
popularity in the open source community and among commercial
application developers.
MAC address (Media Access Control address) - A MAC address is a
uniq
ue identifier associated with a piece of networking equipment, or
more specifically, its interface with the network. For example, the
network card in a computer has its own MAC address.
Manual iris - This is the opposite to an autoiris, i.e. the camera iris
mus
t be adjusted manually to regulate the amount of light allowed to
reach the image sensor.
Mbit/s (Megabits per second) - A measure of the bit rate, i.e. the rate
at
which bits are passing a given point. Commonly used to give the
“speed” of a network. A LAN might run at 10 or 100 Mbit/s. See also
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