OSI Model (7 Layers of OSI Model, and How it works).

The most basic thing to understand network is to understand about OSI model. It is one of the most important tools which help us to know how networking devices like router, switch, hub, computer, etc... works.

To understand OSI model easily, let’s take a real life example. Many of us have posted a Greeting card or any Letter to anyone.
 

Generally to do it, we have to follow at least below steps:-
1 - Write your letter or Purchase a Greeting card.
2 - Pack it into an envelope.
3 - Write sender and receiver address on that envelope
4 - Stamp it.
5 - Go to the post box and drop it into it.

The above example, signify that we have to go through some steps in a specific order to send any message with each other. The networking too use a set of steps to send and receive any data which is called OSI model. The ISO model has 7 steps. Each step is called as a Layer. Each layer is arranged from top to bottom (numbering from 7 to 1). When a device send information to another the data goes from top to bottom layer (7 to 1). But if a device receives any data it will go from bottom to top (1 to 7). When data goes from top to bottom layer, a header is added to it at every layer. This is called encapsulation.

Lets Understand each layer how they work.

# Layer 7 – Application layer
 This is the closest layer to the user. It provides the interface between the applications we use and the next layers. But don't thing programs you are using (like a web browser – IE, Firefox, Opera, etc) are not a part of Application layer. Telnet, FTP, SMTP, HTTP, HTTPs, POP3 are examples of Application layer.

# Layer 6 – Presentation layer
Presentation layer ensures that the data passing is in a appropriate form for receiving device. It can be also called as translator of the network. For example, if we want to send an song the Presentation layer will format our data into MP3 format. Or we want to send photos to any of our friend, the Presentation layer will format our data into GIF, JPG, PNG etc... format.

# Layer 5 – Session layer
The Session Layer establishes, maintains and ends communication with the receiving device.

# Layer 4 – Transport layer
This layer maintains data flow and provides for error checking and recovery between the devices. The best example of Transport layer is Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

# Layer 3 – Network layer
This layer provides logical addresses which routers will use to determine the path to the destination (the last destination). In most cases, the logic addresses here means the IP addresses (including source & destination IP addresses).
 
# Layer 2 – Data Link Layer
The Data Link layer formats the message into a data frame, and adds a header containing the hardware destination and source address to it. This header is responsible for finding the next destination device on a local network.

NOTE:- Network layer (layer 3) is responsible for finding the path to the last destination (network) but it doesn’t care about who will be the next receiver. It is the Layer 2 that helps data to reach the next destination.

Network layer is divide into 2 sub-layers:
>> Logical link control (LLC)
>> Media access control (MAC).

The LLC functions include:
+ Managing frames to upper and lower layers
+ Error Control
+ Flow control

The MAC sublayer carries the physical address of each device on the network. This address is more commonly called a device’s MAC address. MAC address is a 48 bits address which is burned into the NIC card on the device by its manufacturer.

# Layer 1 – Physical layer
The Physical Layer defines the physical characteristics of the network such as connections, voltage levels and timing.

I summarize all the important functions of each layer in the table:-

Layer	Description	Popular Protocols	Protocol Data Unit	Devices operate in this layer
Application	# User interface	HTTP, FTP, TFTP, Telnet, SNMP, DNS…	Data
Presentation	# Data representation, encryption & decryption	# Video (WMV, AVI…)	Data
		# Bitmap (JPG, BMP, PNG…)
		# Audio (WAV, MP3, WMA…)
Session	# Setup, monitor & terminate the connection session	# SQL, RPC, NETBIOS names…	Data
Transport	# Flow control (Buffering, Windowing, Congestion Avoidance) helps prevent the loss of segments on the network and the need for retransmission	# TCP (Connection-Oriented, reliable)	Segment
		# UDP (Connectionless, unreliable)
Network	# Path determination	# IP	Packet/Datagram	Router
	# Source & Destination logical addresses	# IPX
		# AppleTalk
Data Link	# Physical addresses	# LAN	Frame	Switch, Bridge
		# WAN (HDLC, PPP, Frame Relay…)
	Includes 2 layers:
	# Upper layer: Logical Link Control (LLC)
	# Lower layer: Media Access Control (MAC)
Physical	Encodes and transmits data bits	# FDDI, Ethernet	Bit (0, 1)	Hub, Repeater…
	# Electric signals
	# Radio signals