Unit -1
Introduction
''Computer Network'' is a collection of autonomous computers interconnected by a single
technology. Two computers are said to be interconnected if they are able to exchange
information.
The word data refers to information presented by the parties. Data communications are the
exchange of data between two devices via some form of transmission medium such as a
wire cable. For data communications to occur, the communicating devices must be part of a
communication system made up of a combination of hardware (physical equipment) and
software (programs).
The effectiveness of a data communications system depends on four fundamental
characteristics: delivery, accuracy, timeliness, and jitter.
I. Delivery. The system must deliver data to the correct destination. Data must be received
by the intended device or user and only by that device or user.
2. Accuracy. The system must deliver the data accurately. Data that have been altered in
transmission and left uncorrected are unusable.
3. Timeliness. The system must deliver data in a timely manner. Data delivered late are
useless. In the case of video and audio, timely delivery means delivering data as they are
produced, in the same order that they are produced, and without significant delay.
4. Jitter. Jitter refers to the variation in the packet arrival time. It is the uneven delay in the
delivery of audio or video packets. For example, let us assume that video packets are sent
every 3D ms. If some of the packets arrive with 3D-ms delay and others with 4D-ms delay,
results in poor video quality.
COMPONENTS OF DATA COMMUNICATION
A data communications system has five main components (Figure 1.1)
Message. The message is the information (data) to be communicated. Popular forms of
information include text, numbers, pictures, audio and video.
1
,Sender. The sender is the device that sends the data message. It can be a computer,
workstation, telephone handset, video camera and so on.
Receiver. The receiver is the device that receives the message. It can be a computer,
workstation, telephone handset, television and so on.
Transmission medium. The transmission medium is the physical path by which a message
travels from sender to receiver. Some examples of transmission media include twisted-pair
wire, coaxial cable, fiber-optic cable and radio waves.
Protocol. A protocol is a set of rules that govern the data communications. It represents an
agreement between the communicating devices. Without a protocol, two devices may be
connected but they can’t communicate.
Data Representation
Information or data comes in different forms such as text, numbers, images, audio, and video.
Text
In data communications, text is represented as a bit pattern, a sequence of bits (Os or Is). Different
sets of bit patterns have been designed to represent text symbols. Each set is called a code, and the
process of representing symbols is called coding. Current coding system is called Unicode, which
uses 32 bits to represent a symbol or character used in any language in the world. The American
Standard Code for Information Interchange (ASCII), developed some decades ago in the United
States, now constitutes the first 127 characters in Unicode and is also referred to as Basic Latin.
Numbers
Numbers are also represented by bit patterns. However, a code such as ASCII is not used to
represent numbers; the number is directly converted to a binary number to simplify mathematical
operations.
Images
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. For better
representation of the image (better resolution), 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 (made of a
combination of three primary colors: red, green, and blue), another method is called YCM (primary
colors: yellow, cyan, and magenta).
Audio
2
, 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.
Video
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.
DATA FLOW
Simplex: The communication is unidirectional, like one-way street. Only one of the two
devices on a link can transmit and the other can only receive. Eg., Communication between
Keyboard and monitors. The simplex mode can use the entire capacity of the channel to
send data in one direction.
Half-Duplex: Here, each station can both transmit and receive, but not at the same time.
When one device is sending, the other can only receive, and vice versa.
Entire capacity of a channel is taken over by whichever of the two devices is transmitting at
the time. Walkie-talkies and CB (citizens band) radios are the examples.
Full-Duplex: Here, both the stations can transmit and receive simultaneously. In full-duplex
mode, signals going in one direction share the capacity of the link with signals going in the
other direction. This sharing can occur in two ways: Either the link must contain two
physically separate transmission paths, one for sending and the other for receiving; or the
capacity of the channel is divided between signals travelling in both directions. Telephone
network is the example.
NETWORKS:
Physical structures
Before discussing networks, we need to define some network attributes.
Type of Connection
A network is two or more devices connected through links. A link is a communications pathway that
transfers data from one device to another. For visualization purposes, it is simplest to imagine any
link as a line drawn between two points. For communication to occur, two devices must be
connected in some way to the same link at the same time.
1. point-to-point
A point-to-point connection provides a dedicated link between two devices. The entire capacity of
the link is reserved for transmission between those two devices. Most point-to-point connections
use an actual length of wire or cable to connect the two ends, but other options, such as microwave
or satellite links, are also possible. When you change television channels by infrared remote control,
you are establishing a point-to-point connection between the remote control and the television's
control system.
3
Introduction
''Computer Network'' is a collection of autonomous computers interconnected by a single
technology. Two computers are said to be interconnected if they are able to exchange
information.
The word data refers to information presented by the parties. Data communications are the
exchange of data between two devices via some form of transmission medium such as a
wire cable. For data communications to occur, the communicating devices must be part of a
communication system made up of a combination of hardware (physical equipment) and
software (programs).
The effectiveness of a data communications system depends on four fundamental
characteristics: delivery, accuracy, timeliness, and jitter.
I. Delivery. The system must deliver data to the correct destination. Data must be received
by the intended device or user and only by that device or user.
2. Accuracy. The system must deliver the data accurately. Data that have been altered in
transmission and left uncorrected are unusable.
3. Timeliness. The system must deliver data in a timely manner. Data delivered late are
useless. In the case of video and audio, timely delivery means delivering data as they are
produced, in the same order that they are produced, and without significant delay.
4. Jitter. Jitter refers to the variation in the packet arrival time. It is the uneven delay in the
delivery of audio or video packets. For example, let us assume that video packets are sent
every 3D ms. If some of the packets arrive with 3D-ms delay and others with 4D-ms delay,
results in poor video quality.
COMPONENTS OF DATA COMMUNICATION
A data communications system has five main components (Figure 1.1)
Message. The message is the information (data) to be communicated. Popular forms of
information include text, numbers, pictures, audio and video.
1
,Sender. The sender is the device that sends the data message. It can be a computer,
workstation, telephone handset, video camera and so on.
Receiver. The receiver is the device that receives the message. It can be a computer,
workstation, telephone handset, television and so on.
Transmission medium. The transmission medium is the physical path by which a message
travels from sender to receiver. Some examples of transmission media include twisted-pair
wire, coaxial cable, fiber-optic cable and radio waves.
Protocol. A protocol is a set of rules that govern the data communications. It represents an
agreement between the communicating devices. Without a protocol, two devices may be
connected but they can’t communicate.
Data Representation
Information or data comes in different forms such as text, numbers, images, audio, and video.
Text
In data communications, text is represented as a bit pattern, a sequence of bits (Os or Is). Different
sets of bit patterns have been designed to represent text symbols. Each set is called a code, and the
process of representing symbols is called coding. Current coding system is called Unicode, which
uses 32 bits to represent a symbol or character used in any language in the world. The American
Standard Code for Information Interchange (ASCII), developed some decades ago in the United
States, now constitutes the first 127 characters in Unicode and is also referred to as Basic Latin.
Numbers
Numbers are also represented by bit patterns. However, a code such as ASCII is not used to
represent numbers; the number is directly converted to a binary number to simplify mathematical
operations.
Images
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. For better
representation of the image (better resolution), 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 (made of a
combination of three primary colors: red, green, and blue), another method is called YCM (primary
colors: yellow, cyan, and magenta).
Audio
2
, 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.
Video
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.
DATA FLOW
Simplex: The communication is unidirectional, like one-way street. Only one of the two
devices on a link can transmit and the other can only receive. Eg., Communication between
Keyboard and monitors. The simplex mode can use the entire capacity of the channel to
send data in one direction.
Half-Duplex: Here, each station can both transmit and receive, but not at the same time.
When one device is sending, the other can only receive, and vice versa.
Entire capacity of a channel is taken over by whichever of the two devices is transmitting at
the time. Walkie-talkies and CB (citizens band) radios are the examples.
Full-Duplex: Here, both the stations can transmit and receive simultaneously. In full-duplex
mode, signals going in one direction share the capacity of the link with signals going in the
other direction. This sharing can occur in two ways: Either the link must contain two
physically separate transmission paths, one for sending and the other for receiving; or the
capacity of the channel is divided between signals travelling in both directions. Telephone
network is the example.
NETWORKS:
Physical structures
Before discussing networks, we need to define some network attributes.
Type of Connection
A network is two or more devices connected through links. A link is a communications pathway that
transfers data from one device to another. For visualization purposes, it is simplest to imagine any
link as a line drawn between two points. For communication to occur, two devices must be
connected in some way to the same link at the same time.
1. point-to-point
A point-to-point connection provides a dedicated link between two devices. The entire capacity of
the link is reserved for transmission between those two devices. Most point-to-point connections
use an actual length of wire or cable to connect the two ends, but other options, such as microwave
or satellite links, are also possible. When you change television channels by infrared remote control,
you are establishing a point-to-point connection between the remote control and the television's
control system.
3
