COMPUTER NETWORKS AND PROTOCOLS
UNIT I LP 1
Network architecture - Topology - Types (LAN, MAN, WAN &PAN)– Network Switching
Types: Circuit and packet switching 1.1 Data Communication: When we communicate, we
are sharing information. This sharing can be local or remote. Between individuals, local
communication usually occurs face to face, while remote communication takes place over
distance. 1.1.1 Components: A data communications system has five components. 1.
Message. The message is the information (data) to be communicated. Popular forms of
information include text, numbers, pictures, audio, and video. 2. Sender. The sender is the
device that sends the data message. It can be a computer, workstation, telephone handset,
video camera, and so on. 3. Receiver. The receiver is the device that receives the message.
It can be a computer, workstation, telephone handset, television, and so on. 4. 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 5. Protocol. A protocol is a set of rules that govern
data communications. It represents an agreement between the communicating devices.
Without a protocol, two devices may be connected but not communicating, just as a person
speaking French cannot be understood by a person who speaks only Japanese. 1.1.2 Data
Representation: Information today 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. Today, the prevalent 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. Appendix B discusses several different numbering
systems. 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. 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 andwhite 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 pixelby 10, and a white pixel by 11. There are
several methods to represent color images. One methodis 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: 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: 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. 1.1.3 Data Flow Communication
between two devices can be simplex, half-duplex, or full-duplex as shown in Figure Simplex:
In simplex mode, the communication is unidirectional, as on a one-way street. Only one of
the two devices on a link can transmit; the other can only receive (see Figure a). Keyboards
and traditional monitors are examples of simplex devices. The keyboard can only introduce
input; the monitor can only accept output. The simplex mode can use the entire capacity of
the channel to send data in one direction. Half-Duplex: In half-duplex mode, 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 The half-duplex mode is like a one-lane road with traffic
allowed in both directions. When cars are traveling in one direction, cars going the other way
must wait. In a half-duplex transmission, the 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 both half-duplex systems. The half-duplex mode is used in cases where
there is no need for communication in both directions at the same time; the entire capacity of
the channel can be utilized for each direction. Full-Duplex: In full-duplex both stations can
transmit and receive simultaneously (see Figure c). The fullduplex mode is like a two-way
street with traffic flowing in both directions at the same time. 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 traveling in both directions. One common example of
full-duplex communication is the telephone network. When two people are communicating by
a telephone line, both can talk and listen at the same time. The full-duplex mode is used
when communication in both directions is required all the time. The capacity of the channel,
however, must be divided between the two directions. 1.2 NETWORKS A network is a set of
devices (often referred to as nodes) connected by communication links. A node can be a
computer, printer, or any other device capable of sending and/or receiving data generated by
other nodes on the network. 1.2.1 Distributed Processing Most networks use distributed
processing, in which a task is divided among multiple computers. Instead of one single large
machine being responsible for all aspects of a process, separatecomputers (usually a
personal computer or workstation) handle a subset. 1.2.2 Network Criteria A network must
be able to meet a certain number of criteria. The most important of these are performance,
reliability, and security. Performance: Performance can be measured in many ways,
including transit time and response time. Transit time is the amount of time required for a
message to travel from one device to another. Response time is the elapsed time between
an inquiry and a response. The performance of a network depends on a number of factors,
including the number of users, the type of transmission medium, the capabilities of the
connected hardware, and the efficiency of the software. Performance is often evaluated by
two networking metrics: throughput and delay. We often need more throughput and less
delay. However, these two criteria are often contradictory. If we try to send more data to the
network, we may increase throughput but we increase the delay because of traffic
congestion in the network. Reliability: In addition to accuracy of delivery, network reliability is
measured by the frequency of failure, the time it takes a link to recover from a failure, and
the network'srobustness in a catastrophe. Security: Network security issues include
protecting data from unauthorized access, protecting data from damage and development,
UNIT I LP 1
Network architecture - Topology - Types (LAN, MAN, WAN &PAN)– Network Switching
Types: Circuit and packet switching 1.1 Data Communication: When we communicate, we
are sharing information. This sharing can be local or remote. Between individuals, local
communication usually occurs face to face, while remote communication takes place over
distance. 1.1.1 Components: A data communications system has five components. 1.
Message. The message is the information (data) to be communicated. Popular forms of
information include text, numbers, pictures, audio, and video. 2. Sender. The sender is the
device that sends the data message. It can be a computer, workstation, telephone handset,
video camera, and so on. 3. Receiver. The receiver is the device that receives the message.
It can be a computer, workstation, telephone handset, television, and so on. 4. 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 5. Protocol. A protocol is a set of rules that govern
data communications. It represents an agreement between the communicating devices.
Without a protocol, two devices may be connected but not communicating, just as a person
speaking French cannot be understood by a person who speaks only Japanese. 1.1.2 Data
Representation: Information today 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. Today, the prevalent 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. Appendix B discusses several different numbering
systems. 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. 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 andwhite 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 pixelby 10, and a white pixel by 11. There are
several methods to represent color images. One methodis 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: 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: 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. 1.1.3 Data Flow Communication
between two devices can be simplex, half-duplex, or full-duplex as shown in Figure Simplex:
In simplex mode, the communication is unidirectional, as on a one-way street. Only one of
the two devices on a link can transmit; the other can only receive (see Figure a). Keyboards
and traditional monitors are examples of simplex devices. The keyboard can only introduce
input; the monitor can only accept output. The simplex mode can use the entire capacity of
the channel to send data in one direction. Half-Duplex: In half-duplex mode, 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 The half-duplex mode is like a one-lane road with traffic
allowed in both directions. When cars are traveling in one direction, cars going the other way
must wait. In a half-duplex transmission, the 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 both half-duplex systems. The half-duplex mode is used in cases where
there is no need for communication in both directions at the same time; the entire capacity of
the channel can be utilized for each direction. Full-Duplex: In full-duplex both stations can
transmit and receive simultaneously (see Figure c). The fullduplex mode is like a two-way
street with traffic flowing in both directions at the same time. 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 traveling in both directions. One common example of
full-duplex communication is the telephone network. When two people are communicating by
a telephone line, both can talk and listen at the same time. The full-duplex mode is used
when communication in both directions is required all the time. The capacity of the channel,
however, must be divided between the two directions. 1.2 NETWORKS A network is a set of
devices (often referred to as nodes) connected by communication links. A node can be a
computer, printer, or any other device capable of sending and/or receiving data generated by
other nodes on the network. 1.2.1 Distributed Processing Most networks use distributed
processing, in which a task is divided among multiple computers. Instead of one single large
machine being responsible for all aspects of a process, separatecomputers (usually a
personal computer or workstation) handle a subset. 1.2.2 Network Criteria A network must
be able to meet a certain number of criteria. The most important of these are performance,
reliability, and security. Performance: Performance can be measured in many ways,
including transit time and response time. Transit time is the amount of time required for a
message to travel from one device to another. Response time is the elapsed time between
an inquiry and a response. The performance of a network depends on a number of factors,
including the number of users, the type of transmission medium, the capabilities of the
connected hardware, and the efficiency of the software. Performance is often evaluated by
two networking metrics: throughput and delay. We often need more throughput and less
delay. However, these two criteria are often contradictory. If we try to send more data to the
network, we may increase throughput but we increase the delay because of traffic
congestion in the network. Reliability: In addition to accuracy of delivery, network reliability is
measured by the frequency of failure, the time it takes a link to recover from a failure, and
the network'srobustness in a catastrophe. Security: Network security issues include
protecting data from unauthorized access, protecting data from damage and development,
