OSI Model

This page provides introduction to OSI Model.

Overview

OSI stands for Open System Interconnection Model. It is a conceptual model that categorizes and standardizes the different functions in a network. Functions are divided into $7$ layers. Application developer works with the top $3$ layers of OSI model and Network Engineer works with last $4$ layers of OSI model.

Below are $7$ layers of OSI model:

Application

• Application layer is closest to end user and intereacts with software applications.
• HTTP & HTTPS are layer $7$ protocols.
• For example identifying communication partners and synchronizing communication.

Presentation

• Presentation layer translate between application and network formats.
• For example encryption of data as it is sent, and decrypting the data as it is received.

Session

• Session layer establishes, manages, and terminates connections between local application and remote applications.

Transport

• Transport layer segments and reassembles the data for communications between end hosts.
• It adds header to data and each data plus header combination is called segment.

Network

• Network layer provides connectivity between end hosts on different networks.(i.e. outside the LAN).
• It provides logical addressing(IP addresses) and path selection between source and destination.
• It addes layer $3$ header, which includes information like source and destionation IP address to the segment.
• Combination of Data, layer $4$ header, and layer $3$ header is called packet.
• Routers operates at layer $3$.

Data Link

• Data Link layer provides node to node connectivity and data transfer (for example, PC to Switch, Switch to Router, Router to Router).
• It defines how data is formatted for transmission over a physical medium(for example, copper UTP cables).
• It also detects and possibly corrects physical layer errors.
• Like layer $3$ layer $2$ also uses addressing.
• Layer $2$ adds layer $2$ header and layer $2$ trailer to packet and it is called frame.
• Switches operates at layer $2$.

Physical

• Defines physical characteristics of the medium used to transfer data between devices. For example voltage level, maximum transmission distance, physical connectors, cable specifications etc.
• Digital bits are converted into electrical signals(for wired connections) or radio signals(for wireless connections).
• Cables and Pins operates at layer $1$.

TCP/IP Suite

TCP/IP is conceptual model and set of communications protocols used in the internet and other networks. There are only $4$ layers in TCP/IP protocol. The diagram below illustrates the comparison between the OSI model and the TCP/IP model: