Introduction to OSI layer model – The inter networking.
Why we need a model for inter networking . If a model has a flaw why cant we just change the model according to our needs and then use it . This is what WE do while coding but this is not in case of networking.
Imagine you being an English reader and I am writing this article in French. Will you be able to understand it . Probably not .This is the same case with networks they need to work on the same architecture through.
In the earlier days of internet there was no defined model for networking which was a pain in the neck (I wanted to use something else in place of neck but 
) So OSI(Open System Interconnect came up with A layered approach .Which was consistent for all and it divided the network architecture into 7 layers .Why we needed a division . Hmmm consider a post office . The post office is divided into several departments one receives the parcel and add some info to it . The next department parses the parcel based on the info from previous department and send it to the higher department. If the parcel reaches directly to second department it cannot process as it needs some information which can only be present if it comes visa department1. All the work can be done by a single department also but it will lead to a disastrous situation. It is just like a man who is working as a peon , clerk, manager accountant etc a one man army example
) So OSI(Open System Interconnect came up with A layered approach .Which was consistent for all and it divided the network architecture into 7 layers .Why we needed a division . Hmmm consider a post office . The post office is divided into several departments one receives the parcel and add some info to it . The next department parses the parcel based on the info from previous department and send it to the higher department. If the parcel reaches directly to second department it cannot process as it needs some information which can only be present if it comes visa department1. All the work can be done by a single department also but it will lead to a disastrous situation. It is just like a man who is working as a peon , clerk, manager accountant etc a one man army example
Similarly this division was necessary for networking . The 7 layers were Application, Presentation, Session, Transport, Network, Data Link, and Physical . CHEAT SHEET( Please Do Not Take Sausage Pizza Away ) .The first letter of each word is the starting character of a layers name but in opposite order .Each OSI Layerprovide services to the layer above it and receives service from the layer below it.Same as I mentioned in the above example.
Application layer:-The top most layer of OSI .Generally people say that http,ftp etc are example of application layer but actually they are application programs they act as an interface with the application layer .The primary role of the application layer is that it checks resource usability and synchronization with the remote partner.There is no layer above it so it doesn’t have to server anyone.
Presentation Layer:- The function of this layer is very critical as it provides encryption services .for eg consider a connection between a MAC and PC , MAC might be using some other data format like(EBCDIC) and PC using ASCII , so this layer converts the data into standard form so that it can be understood irrespective of the Host machine. Other service apart from encryption are decryption,data compression and decompression
Session layer:- This layer establishes the connection with the remote machines.The mode of communication like simplex,duplex or half duplex is decided at this layer.It also separates the data of different applications from each other .
Transport Layer:- The transport layer is responsible for S&R also called segmentation and reassembly .The data from upper layers is combined together and sent as a single data stream .In broad terms end to end data transport .It used 2 protocols TCP and UDP. To make the connection reliable it is combined with techniques like Flow Control,Windowing,Acknowledgments, re transmission and Not ready stop(Discussed later)
Network Layer:-This layer basically is used for routing .It tracks location of devices (they belong to which network).Data travels into form of packets data packets and route update packets.
Data Link Layer:-This layer is divided into 2 parts the upper part being the LLC(Logical Link Control)And the lower being MAC(Medium Access control).The DLL deals with movement of data on Local area networks .The data movement is in forms of frames and forwarded on the basis of Hardware address called the MAC address
Physical Layer:- This layer defines the specifications of data transmission , the data travels in form of digital signal eg 01110001000. The real transmission takes place here i.e. traveling through a medium whether Cable,fiber or Air.
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