this describes about OSi model, its seven layers and their functionalities in a simpler way to understand

1. THE ISO_OSI MODEL
BY,
G. VISHNU VARDHAN,
ASSISTANT PROFESSOR,
DEPT OF ECE.

2. INTRODUCTION
• The International Standards Organization (ISO) is a
multinational body dedicated to worldwide agreement on
standards.
• ISO proposed a reference model as a first step toward
international standardization of protocols used at different
layers.
• This model is called Open System Interconnection model (OSI).
An open system is model that allow any two different systems
to communicate regardless of their underlying architecture.

3. 7LAYERS OF OSI MODEL

4. ORGANIZATION OF LAYERS

5. The principles that were applied to arrive at the seven layers can
be briefly summarized as follows:
1. A layer should be created where a different abstraction is
needed.
2. Each layer should perform a well-defined function.
3. The function of each layer should be chosen with an eye
toward defining internationally standardized protocols.
4. The layer boundaries should be chosen to minimize the
information flow across the interfaces.
5. The number of layers should be large enough that distinct
functions need not be thrown together in the same layer out of
necessity and small enough that the architecture does not
become unwieldy.

6. PHYSICAL LAYER
The physical layer is responsible for movements of individual bits from
over a physical medium

7. • The physical layer coordinates the functions required to carry a bit stream over
a physical medium.
• It deals with the mechanical and electrical specifications of the interface and
transmission medium.
It also concerns about
 Physical characteristics of interfaces and medium.
 Representation of bits.
 Data rate.
 Synchronization of bits.
 Line configuration.
 Physical topology.
 Transmission mode.

8. DATA LINK LAYER
The data link layer is responsible for reliable transmission of frames from
one node to the next.

9. The data link layer transforms the physical layer, a raw transmission facility, to
a reliable link and is responsible for node to node delivery.
The other functions of data link layer include
 Framing : The data link layer divides the stream of bits received from the
network layer into manageable data units called frames.
 Physical addressing : the data link layer adds a header to the frame to
define the sender and/or receiver of the frame.
 Flow control : If the rate at which the data are absorbed by the receiver is
less than the rate at which data are produced in the sender, the data link
layer imposes a flow control mechanism.
 Error control : It provides the mechanism of error control in which it detects and
retransmits damaged or lost frames. Error control is normally achieved through
a trailer added to the end of the frame.
 Access control : When two or more devices are connected to the same link,
datalink layer protocols are necessary to determine which device has control
over the link at any given time.

10. NETWORK LAYER
The network layer is responsible for the delivery of individual
packets from the source host to the destination host.

11. It is responsible for the source to destination delivery of a packets
across multiple networks.
Other functions include
 Routing : Provide mechanism to transmit data over independent
networks that are linked together.
 Logical addressing : If a packet passes the network boundary,
we need another addressing system to help distinguish the
source and destination systems. Network layer adds Logical
addresses of sender and Receiver of the another network.

12. TRANSPORT LAYER
It is responsible for source process to destination process
delivery of entire message.

13. Transport layer provides two types of services:
 Connection Oriented Transmission: In this type of transmission the
receiving device sends an acknowledgment back to the source after a
packet or group of packet is received.
 Connectionless Transmission: In this type of transmission the
receiver does not acknowledge receipt of a packet.

14. Other functions of Transport layer include :
 Segmentation and Reassembly: Divide the message received
from Session layer into Segments and number them to make a
sequence for reassembly at the receiving side.
 Service point addressing: Transport layer makes sure that the
message is delivered to the correct process on destination
machine.
 Error Control: Make sure that the entire message arrives
without errors else retransmit.
 Flow Control: Transport layer makes sure that the sender and
the receiver communicate at a rate they both can handle

16. SESSION LAYER
It is responsible for beginning, maintaining & ending
the communication between two devices, which is called
session.

17. Functions of session layer include
 Dialog Control: The session layer allows two systems to enter
into a dialog. It allows the communication between two
processes to take place in either half-duplex (one way at a
time) or full-duplex (two ways at a time) mode.
 Synchronization: Allows a process to add checkpoints to a
stream of data.

18. PRESENTATION LAYER
This layer is concerned with the syntax and semantics of the
information exchanged between two systems.

19. The responsibilities of presentation layer include:
 Data Translation: Encoding and Decoding
Sender to Common format on Sending side
Common to Receiving format on Receiver side
 Data Encryption: For security and privacy purpose.
 Data Compression: Data compression reduces the number of
bits contained in the information.

20. APPLICATION LAYER
The application layer is responsible for providing services to
the user.
It Provides User interfaces and support for Services, like e-
mail, file transfer.

21. Functions of application layer include:
 Network Virtual terminal: It allows a user to log on to a remote
host.
 File Transfer Access, and Management: This application allows
a user to access files in a remote host.
 Mail Services: This application provides various e-mail
services.
 Directory Services: This application provides the distributed
database sources and access for global information about

22. SUMMARY