Networks
Before
talking about networks we should know what a network is.
When
two or more computers are connected together they form a network;
Some
strategies are used to connect computers together such as:
Point
to point divides into:
point
to point on dedicated line
point
to point on commutated line
Point
to Point on dedicated line
In
this strategy we need a number of transmission lines equal to the number of
terminals we have to link.
The
PC is able to communicate simultaneously with all the others terminals even if
the most common consequence is a high interrupt frequency.
Point
to point on commutated line
Telephone
numbers are assigned to the lines linked to the elaborator and the number of
users that can dialogue with that elaborator is equal to the number of the
telephone lines assigned to it.
This
kind of linkage links more than one terminal on a single line, it means that
when a computer is sending data to another one the other PC can’t transmit
data until the previous trasmission hasn't finished.
It
turns out that it is necessary a protocol of communication that assigns a master
task to the central elaborator that will manage the information traffic while
the other terminals have the task of secondary stations. It means that
it will be the master to decide which slave can send data
and which slave can receive data.
The
evolution of these techniques drives to a Multiplexing linkage that consists in
dividing the time, a unique line uses with a strategy so similar to Time
sharing
Physically
the central PC will be linked through a high speed line to the multiplexer where
other short and low speed line are linked to the terminals.
Line
by line the MP will capture a byte number that will be sent to the Central
computer.
To
be sure of a correct data transmission we need some checks on the transmitted
data.
There
are two control techniques: Longitudinal parity and CRC Cyclic redundancy
Control
Cyclic
redundancy Control
When
data are transmitted a complex
mathematical algorithm of 16 or 32 bits is generated it is called CRC and is
transmitted after all the data have been transmitted.
The
receiver, to control the correct data transmission, will create another CRC and if
this CRC and the received CRC are equal, the transmission has been correct.
Before
talking about network topologies we have to give a definition of the following
terms:
Topology
and Architecture
Topology
: it is the
network from a physical point of view
Architecture:
it
is the way users use the lines to communicate data.
(Users network + Elaborator network)
We
have three network topologies: Star topology,
Ring topology and
Bus topology.
Bus
Topology (Multipoint type)
All
the nodes are linked to the same communication channel (shared line or bus).
This
topology is used for linking PCs far between them. The transmission is a broadcast
transmission so that information can
be sent in either direction, from any PC to any other PC.
It
means that the transmitted signal is picked up by all the nodes but only the
destination node can capture it.
The
advantage is that when a node breaks down the whole network isn’t disrupted.
Ring
Topology (Point to Point type)
Each
PC is linked to other two PCs to form a Ring.
From
a mono-directional Ring to a bidirectional Ring the transmission costs reduce
while the security level increases.
Star
topology (Point to Point type)
All
the nodes are linked to the central node called Hub via a central cable.
The
network has a communication cost that is directly proportional to the number of
nodes that can be linked. It is limited by the number of interfaces that are on the
central node.
This
topology presents a short communication time because the PC A to communicate to
the PC B just needs two steps.
In
this topology we can have a so long tail because all the PCs in the network
can’t send messages simultaneously at the same time, they can only transmit
one at time.
Since
we have different network topologies, if we want to link different networks
together we need OSI that is an open system interconnection.
It
was born in the end of 1970 like a standard communication system:
This
OSI protocol is formed by seven layers:
1.
physic layer
2.
data link layer
3.
network layer
4.
transport layer
5.
session layer
6.
presentation
layer
7.
application
layer