1. COMPUTER NETWORKS Ms. Mrinmoyee Mukherjee Assistant Professor
St. Francis Institute of Technology,
Mount Poinsur, S.V.P Road, Borivli (west),
Mumbai

2. OBJECTIVES OF THE DAY Today we will focus on- Example
Understand OSI model

3. EXAMPLE
let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.

4. NEEDS OF STANDARDS Communication Established standards
Standards are known as protocols
Implementation
A framework is helpful in the design of hardware and software for communication
ISO-OSI Model serves this purpose
ISO-OSI supersedes the TCP/IP model
ISO
International Standards Organization
OSI
Open Systems Interconnect

5. 1. Introduced in 1978 and revised in 1984
OSI MODEL BACKGROUND
1. Introduced in 1978 and revised in 1984
2. Formulates the communication process into structured layers
3. There are seven layers in the model, hence the name the 7-Layer model
4. The model acts as a frame of reference in the design of communications and networking products

6. ALL PEOPLE SEEM TO NEED DATA PROCESSING
OSI MODEL -7 LAYERS
AL-Everything else
2. PL-Encodes/decodes messages
3. Session-manage connections
4. Transport-How to send
packets to the application
5. Network-How to route packets to the node
6. Data- How to transmit frames
7. Physical-How to transmit bits
ALL PEOPLE SEEM TO NEED DATA PROCESSING

7. OSI MODEL -7 LAYERS Application Layer-Data Presentation Layer-Data
Session Layer-Data
Transport-Segments
Network-Packets
Data Link-Frames
Physical-Bits
Some video time

8. APPLICATION LAYER It is the highest level of OSI model
Provides the interface (interacts) with the application programs.
The application layer uses SMTP commands to tell the server how to handle the data
Application layer contains management functions to support distributed applications
Examples of application layer are applications such as file transfer, electronic mail, remote login etc (FTP,HTTP,SMTP)

9. 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).
3. Converts from system specific format to network format
4. It converts the text to ASCII format and specifies the format of image

10. SESSION LAYER
1. 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.
2. This layer requests for a logical connection to be established on an end-user’s request.
3. Any necessary log-on or password validation is also handled by this layer.
4. Session layer is also responsible for terminating the connection.
5. This layer provides services like dialogue discipline which can be full duplex or half duplex.
6. Session layer can also provide check-pointing mechanism such that if a failure of some sort occurs between checkpoints, all data can be retransmitted from the last checkpoint.
7. Adds a header to handle sessions

11. 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 and in sequence.
Ensures that there is no loss or duplication of data units.
Provides connectionless or connection oriented service.
Multiplex multiple connection over a single channel.
Basically it tries to provide- Reliability, Flow control, Fragmentation, and port numbers are used to differentiate between different applications running on same node

12. NETWORK LAYER
1. Implements routing of frames (packets) through the network.
2. Defines the most optimum path the packet should take from the source to the destination
3. Defines logical addressing so that any endpoint can be identified and handles congestion in the network.
5.Facilitates interconnection between heterogeneous networks (Internetworking).
6. The network layer also defines how to fragment a packet into smaller packets to accommodate different media.

13. DATA LINK LAYER
1. Data link layer attempts to provide reliable communication over the physical layer interface.
2. Breaks the outgoing data into frames and reassemble the received frames.
3. Create and detect frame boundaries.
4. Handle errors by implementing an acknowledgement and retransmission scheme.
5. Implement flow control.
6. Supports points-to-point as well as broadcast communication .
7. Supports simplex, half-duplex or full-duplex communication.

14. 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. Covers all - mechanical, electrical, functional and procedural - aspects for physical communication. The physical layer is responsible for movements of individual bits from one hop (node) to the next.

15. SUMMARY
VIDEO TIME!!!!!!

16. Layer Operations
At each layer, additional information is added to the data packet
An example would be information related to the IP protocol that is added at Layer 3

17. Formatting of Data Through the Layers
Application Header
Presentation Header
Session Header
Network Header
Transport Header
Physical Frame Preamble
Data Link Header and Trailer

18. Packet : General Format
Header
Trailer
Data
A general concept of packets serves as a prerequisite to
the understanding of the ISO-OSI model.

19. Some Header Information Added at Various Layers
Packet arrival information
Receiver’s address
Sender’s address
Synchronization character

20. Data Actual data May contain error correction code Size may vary
Performed on individual characters of the data
Example: Parity
Size may vary
Depending on the protocol
Example
802.3 specifies range of data packet length