UNIT- I
Introduction
An interconnected collection of autonomous computers is called a computer network. Two
computers are said to be interconnected if they are able to exchange the information. If one
computer can forcibly start, stop and control another one, the computers are not autonomous. A
system with one control unit and many slaves is not a network, nor is a large computer with
remote printers and terminals.
In a Distributed system, the existence of multiple autonomous computers is transparent(i.e., not
visible) to the user. He can type a command to run a program and it runs. It is up to the operating
system to select the best processor, find and transport all the files to that processor, and put the
results in the appropriate place.
The user of a distributed system is not aware of that there are multiple processors; it looks like a
virtual uniprocessor. Allocation of jobs to processors and files to disks, movement of files between
where they are stored and where they are needed, and all system function are automatic.
With a network, users must explicitly log onto one machine, explicitly submit jobs remotely,
explicitly move files around and generally handle all the network management personally. The
distinction between Network and distributed system lies with software (OS) rather than software.
In network user invokes, in distributed system the system invokes.
A network is a set of devices connected by media links. Anode can be a computer, printer or any
other device capable of sending and receiving data generated by other nodes on the network.
The links connecting the devices are often called communication channels.
Networks use Distributed processing, in which a task is divided among multiple computers.
Advantages of Distributed processing are
Security/ Encapsulation
Distributed data bases
Faster problem solving
Security through Redundancy
Collaborative processing
Network Criteria
Computer Networks S.SANDHYA
RANI
, Network Criteria
Performance Reliability Security
Performance:
The performance can be measured in many ways and depends on number of factors.
Number of users
Type of transmission medium
Hardware
Software
Reliability
This is measured by the following factors
Frequency of failure
Recovery time of a network after a failure.
Catastrophe.
Security
Network security issues include protecting data from the following
Unauthorized access
Viruses
Applications
Accessing Remote databases
Accessing Remote programs
Value added communication facility
Marketing and sales
Financial services
Manufacturing
Electronic message
Directory services
Information services
Teleconferencing
Cellular telephone
Cable television
Computer Networks S.SANDHYA
RANI
,Network Structure
Hosts
IMP
Subnet
The end systems are called the HOSTS. The hosts are connected through a
communication subnet or simply Subnet as shown in fig.
The subnet consists of two parts: a) Transmission lines b) Switching elements.
The Transmission lines transmit the raw bits. The Switching elements are specialized
computers, which switches packets. This is called Interface Message Processor (IMP) or
Router or data switching exchanges or packet switching nodes.
The data can be transmitted through the subnet in two ways. They are
a) Point to point or store and forward
b) Broad casting
Network Architecture
To reduce the design complexity, most networks are organized as a series of layers or
levels, each built upon on the one below it. The number of layers, the name of each layer
,the contents of each layer ,and the function of each layer differ from network to network
However, in all networks the purpose of each layer is to offer certain services to the higher
layers ,shielding those layers from the details of how the offered services are actually
implemented.
Computer Networks S.SANDHYA
RANI
, Layer n on one machine carries on a conversation with layer n on another machine. The
rules and conventions used in this conversation are collectively known as the layer n
Protocol.
The entities comprising the corresponding layers on different machines are called Peers.
Layer 5 protocol
Layer 5 Layer 5
Layer 4/5 interface
Layer 4 protocol
Layer 4 Layer 4
Layer 3/4 interface
Layer 3 protocol
Layer 3 Layer 3
Layer 2/3 interface
Layer 2 Layer 2 protocol Layer 2
Layer 1/2 interface
Layer 1 protocol
Layer 1 Layer 1
Physical Medium
Layers, protocols and interfaces.
The interface defines which primitive operation and services the lower layer offers to the
upper one.
A set of layers and protocol is called network architecture.
Data transfer methods:
a. Simplex communication:
Data will be transferred in one direction only.
Simplex transmission
A B
Ex: Keyboards, Monitors
b. Half -- duplex communication:
Data will be transferred in both the directions, but not simultaneously.
Computer Networks S.SANDHYA
RANI
Introduction
An interconnected collection of autonomous computers is called a computer network. Two
computers are said to be interconnected if they are able to exchange the information. If one
computer can forcibly start, stop and control another one, the computers are not autonomous. A
system with one control unit and many slaves is not a network, nor is a large computer with
remote printers and terminals.
In a Distributed system, the existence of multiple autonomous computers is transparent(i.e., not
visible) to the user. He can type a command to run a program and it runs. It is up to the operating
system to select the best processor, find and transport all the files to that processor, and put the
results in the appropriate place.
The user of a distributed system is not aware of that there are multiple processors; it looks like a
virtual uniprocessor. Allocation of jobs to processors and files to disks, movement of files between
where they are stored and where they are needed, and all system function are automatic.
With a network, users must explicitly log onto one machine, explicitly submit jobs remotely,
explicitly move files around and generally handle all the network management personally. The
distinction between Network and distributed system lies with software (OS) rather than software.
In network user invokes, in distributed system the system invokes.
A network is a set of devices connected by media links. Anode can be a computer, printer or any
other device capable of sending and receiving data generated by other nodes on the network.
The links connecting the devices are often called communication channels.
Networks use Distributed processing, in which a task is divided among multiple computers.
Advantages of Distributed processing are
Security/ Encapsulation
Distributed data bases
Faster problem solving
Security through Redundancy
Collaborative processing
Network Criteria
Computer Networks S.SANDHYA
RANI
, Network Criteria
Performance Reliability Security
Performance:
The performance can be measured in many ways and depends on number of factors.
Number of users
Type of transmission medium
Hardware
Software
Reliability
This is measured by the following factors
Frequency of failure
Recovery time of a network after a failure.
Catastrophe.
Security
Network security issues include protecting data from the following
Unauthorized access
Viruses
Applications
Accessing Remote databases
Accessing Remote programs
Value added communication facility
Marketing and sales
Financial services
Manufacturing
Electronic message
Directory services
Information services
Teleconferencing
Cellular telephone
Cable television
Computer Networks S.SANDHYA
RANI
,Network Structure
Hosts
IMP
Subnet
The end systems are called the HOSTS. The hosts are connected through a
communication subnet or simply Subnet as shown in fig.
The subnet consists of two parts: a) Transmission lines b) Switching elements.
The Transmission lines transmit the raw bits. The Switching elements are specialized
computers, which switches packets. This is called Interface Message Processor (IMP) or
Router or data switching exchanges or packet switching nodes.
The data can be transmitted through the subnet in two ways. They are
a) Point to point or store and forward
b) Broad casting
Network Architecture
To reduce the design complexity, most networks are organized as a series of layers or
levels, each built upon on the one below it. The number of layers, the name of each layer
,the contents of each layer ,and the function of each layer differ from network to network
However, in all networks the purpose of each layer is to offer certain services to the higher
layers ,shielding those layers from the details of how the offered services are actually
implemented.
Computer Networks S.SANDHYA
RANI
, Layer n on one machine carries on a conversation with layer n on another machine. The
rules and conventions used in this conversation are collectively known as the layer n
Protocol.
The entities comprising the corresponding layers on different machines are called Peers.
Layer 5 protocol
Layer 5 Layer 5
Layer 4/5 interface
Layer 4 protocol
Layer 4 Layer 4
Layer 3/4 interface
Layer 3 protocol
Layer 3 Layer 3
Layer 2/3 interface
Layer 2 Layer 2 protocol Layer 2
Layer 1/2 interface
Layer 1 protocol
Layer 1 Layer 1
Physical Medium
Layers, protocols and interfaces.
The interface defines which primitive operation and services the lower layer offers to the
upper one.
A set of layers and protocol is called network architecture.
Data transfer methods:
a. Simplex communication:
Data will be transferred in one direction only.
Simplex transmission
A B
Ex: Keyboards, Monitors
b. Half -- duplex communication:
Data will be transferred in both the directions, but not simultaneously.
Computer Networks S.SANDHYA
RANI
