2. CONTENTS
What is OSI?
Layer Architecture
The 7 OSI Layers
Interaction between the OSI layers
Conclusion
3. What is OSI?
The Open Systems Interconnection (OSI) is a model
that was developed by the ISO (International
Organization for Standards ) and it describes how
information from a software application in one
computer, moves through a network medium to a
software application in another computer.
4. Layer Architecture
Layer architecture simplifies the network design.
It is easy to debug network applications in a layered
architecture network.
The network management is easier due to the layered
architecture.
Network layers follow a set of rules, called protocol.
The protocol defines the format of the data being
exchanged, and the control and timing for the handshake
between layers.
6.
The Upper Layers: This deals with Application issues
and generally are implemented only in software. The
highest layer, application layer is closest to the end
user. This layer is where communication from one
end user to another begins through the interaction
between the application layer processes and the end
user.
The Lower layers: This handles Data transport
issues. The physical layer and Data link layers are
implemented in hardware and software.
9. Physical Layer
Provides physical interface for transmission of information.
Defines rules by which bits are passed from one system to
another on a physical communication medium.
Such characteristics as voltage levels, timing of voltage
changes, physical data rates, maximum transmission
distances, physical connectors, and other similar attributes are
defined by physical layer specifications.
OSI Model
10. Data Link Layer
Breaks the outgoing data into frames and reassemble the
received frames.
Handle errors by implementing an acknowledgement and
retransmission scheme.
Implement flow control.
half-duplex or full-duplex
Supports simplex,
communication.
OSI Model
11. Network Layer
Implements routing of packets through the network.
Defines the most optimum path the packet should take
from the source to the destination
Defines logical addressing so that any endpoint can be
identified.
Routers and L3 Switches works here.
IP address understandable layer
OSI Model
13. Transport Layer
Purpose of this layer is to provide a reliable mechanism for the
exchange of data between two processes in different
computers.
Ensures that the data units are delivered error free.
Ensures that data units are delivered in sequence.
Ensures that there is no loss or duplication of data units.
Provides connectionless or connection oriented service.
OSI Model
14. Session Layer
Session layer provides mechanism for controlling the dialogue
between the two end systems. It defines how to start, control and
end conversations (called sessions) between applications.
This layer requests for a logical connection to be established on an
end-user’s request.
Any necessary log-on or password validation is also handled by
this layer.
Session layer is also responsible for terminating the connection.
OSI Model
15. Presentation Layer
Presentation layer defines the format in which the data is to be
exchanged between the two communicating entities.
Also handles data compression and data encryption
(cryptography).
OSI Model
16. Application Layer
Application layer interacts with application programs and
is the highest level of OSI model.
Examples of application layer are applications such as file
transfer, electronic mail, remote login etc.
OSI Model
17. Interaction Between OSI Model
Layers
Information sent from a software application
in one system to a software application in
another, must pass through each of the OSI
layers. An example of this is shown in the
diagram below:
18.
19. OSI in Action
A message begins at the top
application layer and moves down
the OSI layers to the bottom
physical layer.
As the message descends, each
successive OSI model layer adds a
header to it.
A header is layer-specific
information that basically explains
what functions the layer carried
out.
Conversely, at the receiving end,
headers are striped from the
the
message as it travels up
corresponding layers.
OSI Model
20. 7 1 Application Application I 7
7-6 interface 7-6 interface
6 Presentation Presentation 6
6-5 interface 6-5 interface
5 Session Session 5
5-4 interface 5-4 interface
4 Transport Transport I 4
4-3 interface 4-3 interface
3 Network Network Network 3
3-2 interface 3-2 interface
2 1 Data link Data link Data link Data link 2
2- 1interface
Physical Physical Physical
2-1 interface
Physical I 1
F i g u r e 3 . 2 OSIlayers
Device
A Intermediate
node
Device
B
Link Link
Physical communication Physical communication
21. A given layer within the OSI model
communicates with three other layers:the
layer directly above it, the layer directly
below it and it’s peer layer in other networked
computer system. In the diagram below, the
data link layer in system A, communicates
with the network layer and physical layer of
system A. It also communicates with the data
link layer of system B.
23. Conclusion
Overall, this presentation has tried to
describe the OSI model,what it is all about
and how communication occurs through the
layers of the model from one user to
another .
