I explained OSI in Simple Manner

1. OSI
Open System Interconnect

2. 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

3. Why OSI
1. Connecting two different
vendor device
2. Ease of troubleshooting
PDU – Protocol Data Units
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
L7
L6
L5
L4
L3
L2
L1
Data
Segment
Packet
Frame
Bits
Data
Data

4. Data Encapsulation Terminology
● Data encapsulates
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4

5. Physical Layer
All your physical components of the network connectivity comes under this
layer.
Ex. Ethernet Cable, HUB, RJ45, Fiber Cables.
PDU : Bits

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 Creating, Maintaining and Terminating a session. 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 datato 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, SMTP, Telnet, SNMP etc.,
Application Is not your Google Chrome
Internet Chrome
application
Application Layer

12. 1. Application layer: HTTP, FTP, SMTP, POP3, IMAP, DNS, Telnet, SSH,
SNMP, SIP, DHCP, etc.
2. Presentation layer: ASCII, EBCDIC, JPEG, MPEG, TIFF, SSL/TLS, etc.
3. Session layer: NetBIOS, SAP, SOCKS, PPTP, RPC, etc.
4. Transport layer: TCP, UDP, SCTP, DCCP, etc.
5. Network layer: IP, ICMP, ARP, RARP, IGMP, OSPF, BGP, IS-IS, etc.
6. Data link layer: Ethernet, Token Ring, PPP, HDLC, Frame Relay, ATM,
MPLS, etc.
7. Physical layer: 802.3 (Ethernet), 802.11 (Wi-Fi), RS-232, V.35, DSL, T1/E1,
etc.