Computer Networks – Computer Science Notes – For W.B.C.S. Examination.

কম্পিউটার নেটওয়ার্ক – কম্পিউটার সায়েন্স নোট – WBCS পরীক্ষা।

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.Continue Reading  Computer Networks – Computer Science Notes – For W.B.C.S. Examination.
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

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

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.

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