DATA COMMUNICATION AND COMPUTER NETWORK
There are 10 digits i.e. 1, 2, 3, 4, 5, 6, 7, 8, 9 available in decimal number system. It is known as Base
10 system. The value of a digit in a number depends upon its position in the number e.g. the number
546 in this system is represented as (546)10
546 = (4*102) + (8*101) + (6*100)
Images are also represented by bit patterns. In its simplest form, an image is composed of a matrix
of pixels (picture elements), where each pixel is a small dot. The size of the pixel depends on the
resolution. For example, an image can be divided into 1000 pixels or 10,000 pixels. In the second
case, there is a better representation of the image (better resolution), but more memory is needed
to store the image. After an image is divided into pixels, each pixel is assigned a bit pattern. The size
and the value of the pattern depend on the image. For an image made of only black-and-white dots
(e.g., a chessboard), a I-bit pattern is enough to represent a pixel. If an image is not made of pure
white and pure black pixels, you can increase the size of the bit pattern to include gray scale. For
example, to show four levels of gray scale, you can use 2-bit patterns. A black pixel can be
represented by 00, a dark gray pixel by 01, a light gray pixel by 10, and a white pixel by 11. There are
several methods to represent color images. One method is called RGB, so called because each color
is made of a combination of three primary colors: red, green, and blue. The intensity of each color is
measured, and a bit pattern is assigned to it. Another method is called YCM, in which a color is made
of a combination of three other primary colors: yellow, cyan, and magenta.
Audio refers to the recording or broadcasting of sound or music. Audio is by nature different from
text, numbers, or images. It is continuous, not discrete. Even when we use a microphone to change
voice or music to an electric signal, we create a continuous signal. In Chapters 4 and 5, we learn how
to change sound or music to a digital or an analog signal.
Video refers to the recording or broadcasting of a picture or movie. Video can either be produced as
a continuous entity (e.g., by a TV camera), or it can be a combination of images, each a discrete
entity, arranged to convey the idea of motion. Again we can change video to a digital or an analog
signal, as we will see in Chapters 4 and 5.
What does Data Communications (DC) mean?
Data communication refers to the exchange of data between a source and a receiver. Data
communication is said to be local if communicating devices are in the same building or a similarly
restricted geographical area. Some devices/technologies used in data communications are known as
data communication equipment (DCE) and data terminal equipment (DTE). DCE is used at the
sending node, and DTE is used at the receiving node.
The meanings of source and receiver are very simple. The device that transmits the data is known as
source and the device that receives the transmitted data is known as receiver. Data communication
aims at the transfer of data and maintenance of the data during the process but not the actual
generation of the information at the source and receiver.
© Er. PRAKASH POUDEL JIGYASU