OSI stands for Open Systems Interconnection and refers to the way data is transmitted from one point to another. OSI was created in 1984by International Standards Organization (ISO), which included American National Standards Institute (ANSI) and British Standards Institute (BSI). It was created mainly for describing the behavior of electronics. It divides the system of communications into small parts known as layers. Each layer provides services to the instances of other upper layers and requests for services to the lower layer. Its main purpose was to describe how telecommunication should be made.

How OSI works

OSI works in a managed manner, it is finely distributed into layers and each layer has its own function. Each layer provides services to the upper layer and requests for services to the lower layer. There are seven layers in OSI namely Physical, Data Link, Network, Transport, Session, Presentation and Application. Here we will discuss the function of these layers.

1. Physical

It is responsible for defining a connection between a device and physical medium like wires and hubs etc. It is responsible for various encoding schemes. It also provides electrical as well as mechanical interfaces for a network and specifies the way signals are transmitted on a network. It communicates through various devices such as

  • Switches
  • Connectors
  • Transmitters
  • Multiplexers
  • Repeaters
  • Hubs

It is connected to the second layer known as Data Link layer and messages are sent between these two layers for performing various functions.

2. Data Link

The main responsibility of this layer is linking data from one device to another. It identifies a specific computer on the network. This layer is also responsible for detecting errors as CRC is added to it. Data Link layer consists of two sub-layers, Logical link Control (LLC) and Medium Access Control (MAC).

Logic Link Control sub-layer is responsible for providing interface between media access methods and network layer protocols. This sub-layer is also responsible for frame error correction.

Medium Access Control sub-layer provides services for making connection with physical media. At this sub-layer, the physical address of the device known as MAC address is added to the data packets, which contain necessary information for traveling from one source to another). Some devices using this layer are

  • Brouters
  • Bridges
  • Switches

Some protocols which use this layer are

  • HDLC (High Level Data Link Control)
  • SLIP
  • PPP

It is connected to the physical layer and network layer.

3. Network

This layer routes data between hosts and networks (IP and IPX). It also reports errors regarding packet delivery (ICMP) and reports host group membership (IGMP). Some devices through, which it communicates are

  • Routers
  • Brouters

It is used by a number of protocols like

  • IP
  • IPX
  • DecNET
  • DLC
  • ICMP
  • RIP
  • ARP
  • OSPF

It is further linked to the transport layer.

4. Transport

It is responsible for taking action to correct faulty transmissions, controlling flow of data, packet handling and acknowledging successful receipt of data. It is used by gateways. Some of the corresponding protocols are

  • TCP
  • UDP
  • NetBEUI
  • SPX

It is linked to the next layer known as session layer.

5. Session

This layer provides services for opening, using and closing a session. It provides synchronization nodes and manages upper layer errors. Protocols which use this layer are:

  • Network Files system (NFS)
  • SQL
  • RPC

This layer is connected to the next layer which is Presentation.

6. Presentation

It translates the data into formats used by computers and  encodes and decodes the data. It is also responsible for determining the structure of data and communicating via gateways and applications. Formats using this layer are:

  • NCP
  • JPEG
  • MPEG
  • All other formats of video, audio and pictures

It is connected to the next layer- Application.

7. Application

This layer is responsible for handling network connections of all types. It provides interface between user and computer. API is also incorporated in this layer. Corresponding protocols are

  • SNMP
  • FTP
  • SMTP
  • NFS
  • NCP
  • TCP
  • SMB
  • HTTP
  • WWW
  • TelNET
  • TFTP

This OSI system is not necessary for applications to work but it is made for providing a basic structure for communicating software so that they would be compatible with other softwares.