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
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 different systems to interconnect their equipment
through standard interfaces.
b. Allow software and hardware to integration well and be portable on differing
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
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 Presentation 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. network 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 framework
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 lower layers:
Layer PDU Name
Data Link Frames
As the data is passed from layer to layer, each layer adds its own information. This is
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 over the network, and the Host layers so
called because they provide for accurate delivery of data between computers, or hosts,
on the network.