The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology.

1. OSI REFERENCE MODEL

2. INTRODUCTION TO OSI
 OSI stands for“ Open Systems Interconnection".
 OSI model was first introduced in 1984 by the International Organization for
Standardization (ISO).
 Outlines WHAT needs to be done to send data from one computer to another.
 Protocols stacks handle how data is prepared for transmittal (to be
transmitted)
 In the OSI model, The specification needed are contained in 7 different layers
that interact with each other.

3. OSI LAYERS:

4. THE INTERACTION BEWEEN LAYERS IN OSI MODEL

5. 1. APPLICATION LAYER
 First layer from (Top to Bottom)
 Last layer from (bottom to top)
 Closest to the end-user
 Enables the user to communicate its data to the receiver by providing certain
services.
 Functions:
1.General network access
2.Flow control
3.Error recovery

6.  Provides protocols that are commonly needed.
Examples
1.File Transfer Protocol (FTP)
2.HyperText Transfer Protocol (HTTP)
3.Simple Mail Transfer Protocol (SMTP)
4.Simple Network Management Protocol (SNMP)
5.Network File System (NFS) Telnet’

7. PRESENTATION LAYER

8. Presentation Layer
 Sixth level of the seven layer OSI model.
 Responds to service requests and issues service requests.
 Concerned with syntax and semantics.
 data translator for network.
 The Presentation Layer is composed of two sub-layers. They are:
Common Application Service Element (CASE)
Specific Application Service Element (SASE)

9. Presentation Format
 Agreeing to a message format, called a presentation format.
 Represent (encode) the data in various ways (e.g., data compression, or
encryption)
 Receiving peer will convert the encoding back into its original meaning
(decode).

10. Responsibilities:
 Specific responsibilities of presentation layer:
 Data Representation
peers agree to the format of the data being exchanged
 Data Compression(audio, video, image)
Reduces the number of bits contained in the information
 Encryption
plaintext to ciphertext(encrypted form)

11. MAJOR PROTOCOLS IN
PRESENTATION LAYER :
 Remote Desktop Protocol
 ASCII
 Network Data Representation
 External Data Representation

12. SESSION LAYER

13. Session Layer
 Lies between Presentation and transport layer
 Responsible for dialog control and synchronization
 Responsible for beginning, maintaining & ending the
communication between two devices.
 Maintains sessions between remote hosts

14. PURPOSE OF SESSION LAYER
Purpose of session layer is to assist in support of the interaction between
co-operating presentation entities which are classified in two categories.
1. Session Administration Service and
2. Session Dialogue Service.
Functions:
 Breaks the data supplied into segments
 Ensures the sequence of data received
 Provides end-to-end delivery
 Detect and handle disconnectivity

15. PROTOCOLS IN SESSION LAYER:
 AppleTalk Data Stream Protocol (ADSP)
 AppleTalk Session Protocol (ASP)
 Call Control Protocol for Multimedia Communication
 Internet Storage Name Service
 Network Basic Input Output (NetBIOS)
 Password Authentication Protocol (PAP)
 Point-to-Point Tunnelling Protocol (PPTP)
 Remote Procedure Call Protocol (RPC)
 Short Message Peer-to-Peer (SMPP)
 Secure Shell (SSH)

16. TRANSPORT LAYER
 The transport layer is responsible for the process to process delivery of the
message.
 A process is an application program running on the host.
 The transport layer is also responsible for the delivery of the message
 From one process to another

17. RESPONSIBILITIES OF TRANSPORT
LAYERS
 Service point addressing
 Segmentation and reassembly
 Connection control
 Flow control
 Error control

18. NETWORK LAYER
 Delivery of individual packets from source host to the destination host
 Ensures that each packet gets from the point of origin to its destination
 If two or systems are attached to different network (links) with the connecting
devices in the network links there is often a need of network for the source to
destination delivery
Other responsibilities of network layer are
 Logical addressing
 Routing

19. DATA LINK LAYER
 Second layer from (Bottom to Top)
 Sixth layer from (Top to Bottom)
 The Data Link layer adds reliability to the physical layer by providing error
detection and correction mechanisms.
 It makes the physical layer appear error-free to the upper layer & lower layer
 The data link layer is responsible for moving frames from one hop (node) to
the next.

20.  On the sender side, the Data Link layer receives the data from Network Layer and
divides in FRAMES.
 On the receiver side, the data link layer receives the stream of bits from the physical
layer and regroups them into frames and sends them to the Network layer.
 This process is called Framing.

21. FUNCTIONS OF DATA LINK LAYER
 FRAMING
 Physical Addressing
 Flow control
 Error control
 Access Control
 Main Responsibility

22. PHYSICAL LAYER
 First layer from (Bottom to top)
 Last layer from (top to bottom)
 Provides a standardized interface to physical transmission medium.
 Its responsible for movements of individual bits from one hop (Node) to
next.
 Both data and the signals can be either analog or digital.

23.  On the sender side, Receives the data from Data Link Layer and encodes it into
signals to be transmitted onto the medium.
 On the receiver side, the physical layer receives the signals from the
transmission medium decodes it back into data .

24. FUNCTIONS OF PHYSICAL
LAYER
 Interface
 Representation of bits
 Data rate
 Synchronization of bits
 Line configuration
 Physical Topology
 Transmission mode
 Bit-to-Bit Transmission