Introduction To the OSI Model

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23 Οκτ 2013 (πριν από 4 χρόνια και 16 μέρες)

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Introduction To the OSI Model


The OSI


Open Systems Interconnect model is a standard that was introduced by the
ISO International Organization for Standards which is part of the United Nations.
The standard was published in 1983 and specifies a layered

approach to network
communication.


The purpose of the
OSI reference model

is to make networks more manageable and
to aid the problem of moving data between computers. The main objectives of the
ISO OSI reference model were

a.

allow manufactures of differen
t systems to interconnect their equipment
through standard interfaces.

b.

Allow software and hardware to integration well and be portable on differing
systems.

c.

Create a model which all the countries of the world use.



The model divides the problem of moving
data between computers into seven
smaller, more manageable tasks, which equate to the
seven layers

of the OSI
reference model.


The seven layers are:


1.

The Physical Layer

2.

The Data Link Layer

3.

The Network Layer

4.

The Transport Layer

5.

The Session Layer

6.

The Presen
tation Layer

7.

The Application Layer


The Application Layer is the top layer and is also referred to as layer 7. Data passes
from the top layer of the sender through the other layers to the Physical Layer


layer
1, then across the network media


e.g. netw
ork cabling, and up from the Physical
Layer of the receiver to the Application Layer. There is virtual data flow between
corresponding layers on the sender and the receiver.


It must be noted that the OSI model is an abstract model that provides a framewo
rk
that defines the functions of each layer.


In effect the OSI standard defines the interfaces between the layers. The packages of
data passed between layers are called
PDUs

(
Protocol Data Units
). These PDUs
have specific names when referring the four l
ower layers:


Layer

PDU Name

Transport

Segments

Network

Packets

Data Link

Frames

Physical

Bits


As the data is passed from layer to layer, each layer adds its own information. This is
called
encapsulation
.



The layers of the OSI model can be grouped

into two layers


the
Media layers
,
consisting of the physical layer and the datalink layer, and the
Host layers
, consisting
of the application, presentation, session and transport layers. The Media layers so
called because it controls delivery of data o
ver the network, and the Host layers so
called because they provide for accurate delivery of data between computers, or hosts,
on the network.