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OSI Reference Model: 7 Layers Defined and Functions Explained
Neeraj Yadav

OSI Reference Model: 7 Layers Defined and Functions Explained

16-Dec-2015 | | Total View : 1205 |

OSI (Open Systems Interconnection) is reference model for how applications can communicate over a network. The Open Systems Interconnect (OSI) model has seven layers. Networking protocols need to be established for low level computer communication up to how application programs communicate. Each step in this protocol is called a layer.


The main benefits of the OSI model include the following:

• Helps users understand the big picture of networking

• Helps users understand how hardware and software elements function together

• Makes troubleshooting easier by separating networks into manageable pieces

• Defines terms that networking professionals can use to compare basic functional relationships on different networks

• Helps users understand new technologies as they are developed

• Aids in interpreting vendor explanations of product functionality


Layer 1 – The Physical Layer:

The physical layer, the lowest layer of the OSI model, Deals with all aspects of physically moving data from one computer to the next over a physical medium.

The layer transmits bits (ones and zeros) and defines how the data is transmitted over the network.”

“The physical layer of the OSI model defines connector and interface specifications, as well as the medium (cable) requirements. Electrical, mechanical, functional, and procedural specifications are provided for sending a bit stream on a computer network.”


Layer 2 – The Data Link Layer:

Data Link Layer Provides low-level error detection and correction.”

This layer sets up links across the physical network, putting packets into network frames.

So that the data link layer provides error-free transfer of data frames from one node to another over the physical layer.

For example -  if a packet is corrupted this layer is responsible for retransmitting the packet.

NOTE: Data Link Layer has 2 sub layer

  1. Logical Link Control (LLC)
  2. Media Access Control (MAC)


Layer 3 – The Network Layer:

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. Or you can say this layer sets up links across the physical network, putting packets into network frames.

In short ”The Network Layer responsible for routing packets of data across the network”

It requires logical address such as IP address. Ex – Router.


Layer 4 – The Transport Layer:

“The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications.”

The transport layer of the OSI model, offers end-to-end communication between end devices through a network. Depending on the application, the transport layer either offers reliable, connection-oriented or connectionless, best-effort communications.

If I divide the work of Transport Layer then it works like –

  • Takes data from higher levels of OSI Model and breaks it into segments that can be sent to lower-level layers for data transmission
  • Conversely, reassembles data segments into data that higher-level protocols and applications can use
  • Also puts segments in correct order (called sequencing ) so they can be reassembled in correct order at destination.


Layer 5 – The Session Layer:

“The session layer allows session establishment between processes running on different stations”

The session layer, provides various services, including tracking the number of bytes that each end of the session has acknowledged receiving from the other end of the session. This session layer allows applications functioning on devices to establish, manage, and terminate a dialog through a network.

Session Layer Provides duplex, half-duplex, or simplex communications between devices.


Layer 6 – The Presentation Layer:

“This layer is usually part of an operating system (OS) and converts incoming and outgoing data from one presentation format to another”

Ex - Convert clear text to encrypted text at one end and back to clear text at the other.

It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.


Layer 7 – The Application Layer:

“The application layer serves as the window for users and application processes to access network services.”

You can understand it by example –

  • Web browser uses HTTP (Hyper-Text Transport Protocol)
  • E-mail program may use POP3 (Post Office Protocol version 3) to read e-mails and SMTP (Simple Mail Transport Protocol) to send e-mails


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