What IS OSI Model and Working of each Layer

OSI stands for Open System Interconnections, developed by International Organization for Standardization (ISO) in 1980. This OSI layer is developed, to make a standard process to understand data communication over the internet. This is a standard communication process model.

OSI is a reference model, it developed to understand the data communication in the network from end to end devices.


What OSI do–  It defines the communication process using 7 layers from one endpoint to another endpoint devices. Each layer performs own functionality and forwards to the next layer. The Same process happens at another endpoint.

Advantage of OSI- It divide a big communication process into smaller components.

OSI is the combination of 7 layers, each layer has own functions on data.


PDU– PDU stands for Protocol Data unit, that describes the data for each layer. It means how information/data share from one layer to another.

OSI Layers-

Layer 7- Application Layer

Layer 6- Presentation Layer

Layer 5- Session Layer

Layer 4- Transport Layer

Layer 3- Network Layer

Layer 2- Data Link Layer

Layer 1- Physical Layer


Top 3 layers called software layer because it performs the functions in software or in protocol and bottom 4 layers called Hardware layer.


Application Layer-

This layer responsible to provide interface for the users to interface with networking services. Identify the service using Port Number.

Example- Web Browser, Outlook etc.

Protocols-Http, Telnet, SSH, SMTP, IMAP, DNS, SNMP, Telnet, DHCP etc.

PDU-  Data


Presentation Layer- It responsible that how data would be represented. For example, Encoding-decoding, Encryption-decryption, compression-decompression etc.


PDU- Data


Session Layer-  This layer is responsible for establishing, maintain or terminate the session. Session layer creates a Session between sender and receiver, once the data got transferred it terminate the session.  Session ID used to identify the session. From client to server, a data communication process organized by 3 modes: Simplex, Half-Duplex, and Full-Duplex.

Protocols- NFS(Network File System), Apple Talk, SQL, RPC etc.


Transport Layer- This layer is responsible for end to end the transportation of data between the application. This layer is Reliable, means It responsible for 3 things: Sequencing, Error Free, and Flow control. Sequencing- data should go in the same sequence and data would be error free and should go with the same flow.

This layer communicates do by 2 types:  Connection Oriented and Connection Less. In connection less, it’s not giving the error free data guarantee while in Connection oriented it provide error free and sequencing data.

Connection Less using UDP and Connection Oriented using TCP/IP protocols.

This layer is also responsible for break a data into small segments.

Functions of Transport Layer-

  • Create a session using 3-way handshaking between sender and receiver before sending the data.
  • Multiplexing and De-multiplexing
  • Segmentation
  • Error Correction
  • Sequencing
  • Flow Control
  • Acknowledge

Protocols-            TCP/IP- Https, Http, FTP, SSH, Telnet,

UDP- DNS, DHCP, IP Telephone, SNMP

PDU– Segments


Network Layer-  This layer is responsible to provide a path to data and responsible for logical addressing. This layer used to Route the packet among the network. In this layer, data converts into a packet.

Network layer has two responsibility, Logical Addressing and Path selection (Routing).

Devices- Router

Protocols- Routed Protocol (IP, IPX etc) and Routing Protocols (Eigrp, RIP, OSPF etc)

PDU- Packets.


Data Link Layer- This layer is responsible data transport between LAN. Data link layer is responsible for error free data transmission, error detection and flow control over the physical layer. In this layer data packed into frames and this process called data framing.

This layer has 2 sub-layer:

LLC: Logical Link Control, provide flow control, error control data transmission (Check Error)

MAC: Media Access Control, provide physical addressing as well as logical topology.

Devices- Switches, Bridge.

Protocols- ARP, RARP

PDU- Frame


Physical Layer- Physical layer is responsible for a physical medium for data transmission and transfers the data in Bit format ( 0 and 1 Binary) and This layer sends bits and receives bits.

Data Rate



Devices- Network Interface Card (NIC), Hubs etc.

Protocols- Ethernet, Token Ring


How OSI Layer Works:





8 thoughts on “What IS OSI Model and Working of each Layer

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s