2. Why OSI
1. Connecting two different vendor device
2. Ease of troubleshooting
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
3. Overview
It is a 7 layer model developed by ISO at 1984.
Each layer defines specific function.
Note that every communication no need to be go through all the layers. It can
skip some layers if needed - ** Will be discussed later
Each layer is a package of protocol
5. Physical Layer
All your physical components of the network connectivity comes under this layer.
Ex. Ethernet Cable, HUB, RJ45, Fiber Cables.
6. Data Link Layer
Switching technology lies here in DLL Layer
MAC addresses are used for communication
MAC address header will be attached in this layer.
PDU : Frame
Ex. Switch
7. Network Layer
IP header will be encapsulated to the data in this layer.
Routers are of Network Layer devices.
Functions :
Logical Addressing : It helps in assigning an IP address to a device
Routing : Helps in providing reachability to a destination device via routers
Path determination : Helps in selecting the best path when we have multiple
route to reach same destination.
Example : Router
PDU : Packet
8. Transport Layer
This layer decides how to transfer a data.
It checks the reliability of the data sent.
Functions :
Segmentation : Data received by session is segmented and each segment will use sequence number to re-
arrange the segments by the receiver.
Flow Control : To slow down or faster the data transmission between two devices
Error Control : The segment uses Checksum to check the error in the data received.
TCP : Transmission Control Protocol - Its a connection oriented protocol each and every packet sent, the
source should receive an acknowledgement if not received then source will retransmit the packet.
UDP : User Datagram Protocol - Its a connectionless protocol, no acknowledgement will be there for the packet
sent.
PDU : Segment
9. Session Layer
Helps in setting up and maintaining a connections. It uses API’s
Functions :
Authentication : Used by the server to check the user’s username and
password
Authorization : Used by the server to check if the user has the access to the
data he is trying to access or not.
PDU : Data
10. Presentation Layer
How a data is going to be presented to the user. It can be a image, video or
audio file.
Functions :
Translation : converts or translates date to machine language which comes
from Application layer
Compression : It does Data Compression so that data transmission can be
faster and to reduce size of data
Encryption/Decryption : It does encryption so that the data sent is secured.
PDU : Data
11. Application Layer
If you have any software in the computer that dont want to communicate to the
internet then it wont use this layer ex. VLC player, but the web browsers need to
communicate to the internet so behind all the web browsers we have a layer called
application layer so that it can talk with the internet .
Application Layer Protocol example : IP, FTP, SMPT, Telnet, SNMP etc.,
PDU : Data
Internet Chrome
application
Application Layer
