Computer networks

2 A Informatica prof.sse ALLEMANI e CARUSO

Yesterday….

Central architecture

Before the coming of the PC, the common architecture was made up of a central computer - the only one with a processing capability – with some “stupid” terminals.

Today…

Several computers linked in a network, with indipendent processing capability, which share resources and provide services

networks of networks

What is a network?

A computer network is a group of computers linked together by means of cables or aerials which communicate data and share resources through a series of protocols and rules.

Why a network?

It is possible: To share expensive peripherals, such as printers,

scanner, plotter. To send data to users without any other support.

Moreover there are fewer restrictions concerning the size of the file which can be transferred through a network.

To centralize fundamental programs. Sometimes users must be allowed to access the same program, in order to work on it simultaneously.

To create automatic backup systems of files.

Network classification

Typology

( distance among computers )

Topology

(geometric shape used to create the network )

Network Typologies

LAN (Local Area Network ) (1m – 2km)

it is a network in a restricted area, such as an office room, a building or a

campus

MAN (Metropolitan Area Network ) ( 2km - 10Km )

it is a network made of groups of devices or LANs linked by a phone line or

other types of cables in a large geographical area, such as a town.

WAN (Wide Area Network )

it is a network made of the devices which let the connection of LANs and MANs

over a large geographical area, or even around the world ( an example is the

Internet ).

Network Topologies

Bus

Ring

Star

BUS Topology

Each node is connected one right after the other along

the same backbone. Information sent from a node travels

along the backbone until it reaches its destination node.

All the computers can communicate with each other

without having to go through the server.

Advantages: A computer or device that has gone down

does not affect the others.

Disadvantages: It cannot connect a large number of

computers.

RING Topology

It connects the computers and other devices one to the

other in a circle.

There is no central hub that holds all the data.

Communication flows in one direction around the ring.

Advantages: It requires less cabling and so it is less

expensive.

Disadvantages: If one node goes down, it takes down

the whole network.

STAR Topology

The computers are connected to a central device called hub.

The hub takes a signal that comes from any node and passes it

along to all the other nodes in the network.

Advantages:

-It gives close control of data

-Each PC sees all the data

-If a computer (not the hub) fails, no other computer is affected

Disadvantages: If the hub or its software goes down, the whole

network is down.

What is the Internet?

The Internet is a network of networks, which links computers

able to share information and resources with one another.

The Internet can be defined as a “mean of transport” for the

information and resources which are inside the computers.

The Internet has been compared to a motorway.

The Internet itself does not contain any information; it is

wrong to say that a document has been found “in the Internet”

or “on the Internet”. It would be better to say that it has been

found “through the Internet” or “using the Internet”.

Internet Services

The computers connected to the Internet can take

advantage of several services:

E- MAIL – It allows you to send and receive e-mails and to

access to a forum.

FTP (File Transfer Protocol) – It allows you to have a quick

access to a file in another computer, save on your computer and

vice versa.

WWW (World Wide Web) - You must not confuse the Internet

with WWW; this is only a part of the Internet.

What is the World Wide Web?

The World Wide Web is a global network of computers.

This means that all the computers which belong to this

global network ( the web ) can share data and

information.

The computers must use a common language, in order

to understand each other.

The common language used by all the computers in the

web is ruled by the standard protocol HTTP.

How does the World Wide Web

work?

The information are stored in documents called web

pages.

The web pages are files placed into computers called

web server.

The computers which read the pages stored into web

servers are called web client.

The web clients show the user the web pages, by

using a program called web browser.

How does the browser show the web pages?

The web pages stored into web servers are

written in a standard language: the HTML.

The pages written in html contain the

instructions for the browser in order to be

correctly shown.