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.