• OSI means Open System Interconnect model.
• Developed by the International Organization for Standardization in 1984.
• It consists of seven layers.
• Each layer has a different but specific processing function.

OSI Model Layers




Application Layer

• 
   Application Layer is responsible for providing Networking Services to user. It also known  as Desktop Layer. Identification of Services is done using Port Numbers.
  
   Ports are nothing but Socket i.e. Entry and Exit Point to the Layer
  
    Total No. Ports 0 – 65535 
    Reserved Ports 0 – 1023
    Open Ports 1024 – 65535
  

Examples of Networking Services

Service                                              Port No.
HTTP                                                  80 
FTP                                                      21
SMTP                                                  25
TELNET                                             23
TFTP                                                   69

How data flows from Application Layer

Presentation Layer

 Presentation Layer is responsible for converting data into standard format. 

 Examples : ASCII, EBCDIC, JPEG, MPEG, BMP, MIDI, WAV, MP3

 Following tasks are perform at Presentation layer :

 Encoding – Decoding
 Encryption – Decryption
 Compression – Decompression 

How data flows from Presentation Layer

Session Layer

 Session Layer is responsible establishing, maintaining and terminating session. 
 Session ID also works at Session Layer.

 Examples :

 RPC  Remote Procedure Call 
 SQL  Structured Query language
 NFS  Network File System

How data flows from Session Layer

Transport Layer

Transport Layer is responsible for end-to-end connectivity. It is also known as heart of OSI Layers. Following task are performed at Transport Layer : - 

• Identifying Service
• Multiplexing & De-multiplexing
• Segmentation
• Sequencing & Reassembling
• Flow Control
• Error Correction

Identifying Service

Multiplexing & De-multiplexing

Segmentation

Sequencing & Reassembling

Sequencing & Reassembling

Error Correction

Flow Control - Windowing

How data flows from Transport Layer

Network Layer

Network Layer is responsible for providing best path to data to reach destination. Logical Addressing sits on this layer. Device working on Network Layer is Router.

It is divided into two parts 
Routed Protocols
e.g. IP, IPX, Apple Talk.
Routing Protocols
e.g. RIP, IGRP, OSPF, EIGRP

Routed Protocols

Routing Protocols

How data flows from Network Layer

Datalink Layer

Datalink Layer is divided into two Sub Layers :

LLC – Logical Link Control
It talks about Wan protocols e.g. PPP, HDLC, Frame-relay

MAC – Media Access Control
It talks about Physical Address. It is 48 bit Addressing i.e. 12 digit Hexadecimal No. It is also responsible for Error Detection
Device working on Data Link Layer is Switch, Bridge, NIC.

Error Detection – CRC Check

Error Detection – CRC Check

How data flows from Data Link Layer

Physical Layer

Physical Layer is responsible for electrical, mechanical or procedural checks. Data will be converted in Binary that is 0’s & 1’s. Data will be in the form of electrical pulses if it is Coaxial or Twisted Pair cable and in the form of Light if it is Fiber Optic Cable.

Devices working at Physical Layer are Hubs, Repeaters, Cables, Modems etc.

Physical Layer Example

How data flows from Physical Layer

Data Encapsulation & De-capsulation

Comparing OSI with TCP/IP Layers