ISO – OSI (International Standard Organization - Open Systems Interconnection model) what is it?

ISO – OSI is a standard for communication between network devices that is impose by ISO (International Standard Organization) it consist of 7  layers

• Application
• Presentation
• Session
• Transport
• Network
• Data link
• Physical

All the network devices have one or more of these layers to perform a network task to communicate data with one end to another it needs to implement this at the both ends.

Devices such as Computers has implemented all the seven layers while   switch has ether Network, Data Link , Physical or Data Link Physical layers depending of the type of the switch which you may come across as L2 Switch and L3 Switch stands for Layer 2 switch and Layer 3 Switch which is the number of layers from the bottom of the list. L3 switch is mostly know as manageable switch since you can use them to program the flow of the data in and out of it. And the routers the things that we use to route our packets through network works on Network layer meaning Network, Data Link and Physical are implement on them.

So Let’s look at the layers from top to bottom

Application

This refer to the actual application that you use to communicate with the network google chrome, Mozzila Fire Fox, Putty, File Zella are some example of applications that you may already use to communicate with the network.

Presentation

This is the data representation system conversion since different system have different way of representing things so it converts to a format that every one recognize and wise versa

Session

Session is a layer that contains session details like session Id. which is reference to one of the connection of the same application since there can be more than one for example google chrome can have many tabs opened and it says which tab the data needed to go

Transport

This layer controls the end to end delivery it use TCP (Transmission Control Protocol and UDP (User Datagram Protocol) to achieve that. TCP is a connection oriented one so end to end delivery is guaranteed and UDP is not connection oriented so it is not that reliable for sending data. In this phrase that TCP Header is added to the data which is telling that destination that it is a TCP packet or UDP packet also it have some flags that can be set one if syn another is ack and also fin which is use by the three way hand shake of the TCP to successfully establish a connection.

Network

The data with the TCP header is send to this layer which adds another header to it which is the Network Header it contains the source IP address, Destination IP address, source port and destination port. In this the packet is fragmented that is divided to small parts if it is too big to send as one. This header is use for transferring the packets end to end.

Data Link

 In data link layer it adds another header which is data link header it consist of two mac (Media access control) address of host and a destination this use to navigate through the network the destination and source mac are not always the address of the beginning and end nodes it is hop to hop that mean if you send data from your computer and if it goes through router A,B,C and Destination then fist source mac is of your computer and destination is of router A and when it get to the router A it check it for the destination IP by opening the IP layer and it finds that the packet need to send to router B to get to the destination so it does not change IP Header but when it come to data link it change it now the source mac is the mac of router A and destination mac is the mac of router B and this process goes on until it reach the destination. The data is also segmented at the if it is big and if is known as a frame error control and flow control is added by this stage. This layer is further divided to LLC and Mac

Physical Layer

Till now the data is of bits and it is not physical thing and we need to convert is to a physical thing so we can send it across a physical media Physical Layer dose this for wire Cables it converts to electrical Current for Wireless it convert to wireless signals for fiber cables it convert to laser light etc.

At the other end the data is reached by the physical layer which convert it to bits and pass to data link which check for errors and then pass to network which check for destination and Transport for connection  session to find out which instance presentation convert the data back to local application type and finally application which you will see the result at your PC