What is OSI model and how it works?

The Open Systems Interconnection model(OSI model) is a conceptual model that characterizes and standardizes the communication functions ofa telecommunication or computing system without regard to its underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard protocols. The model partitions a communication system into abstraction layer.

 The original version of the model defined seven layers.

1. PHYSICAL LAYER  :-

       The physical layer of the OSI model refers to the actual hardware specifications. The Physical Layer defines characteristics such as timing and voltage. The physical layer defines the hardware specifications used by network adapters and by the network cables (assuming that the connection is not wireless). To put it simply, the physical layer defines what it means to transmit and to receive data.

*Transmission Media:- 
- Wired:- coaxial cable, twisted pair cable and fibre optic cable etc.
- Wireless :- bluetooth, wi-fi,wi-max,infra red etc.

- Transmission mode :- simplex, half duplex and full duplex .
- Central device :- Hub

2.The Data Link Layer:-
                           The data link layer can be sub divided into two other layers:-
-  the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.

The MAC layer basically establishes the computer’s identity on the network, via its MAC address. A MAC address is the address that is assigned to a network adapter at the hardware level. This is the address that is ultimately used when sending and receiving packets.

The LLC layer controls frame synchronization and provides a degree of error checked.

 - Functions of Data Link Layer

1. Framing
2. Physical Addressing
3. Flow Control
4. Error Control
5. Access Control 

- Central device :- Switch, Bridge etc.

*3. Network layer :-

In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers.

Function :-

The network layer provides the means of transferring variable-length network packetsfrom a source to a destination host via one or more networks. Within the service layering semantics of the OSI network architecture, the network layer responds to service requests from the transport layer and issues service requests to the data link layer.

4.Transport layer :-

The Transport layer is responsible for maintaining flow control. As you are no doubt aware, the Windows operating system allows users to run multiple applications simultaneously. It is therefore possible that multiple applications, and the operating system itself, may need to communicate over the network simultaneously. The Transport Layer takes the data from each application, and integrates it all into a single stream. This layer is also responsible for providing error checking and performing data recovery when necessary. In essence, the Transport Layer is responsible for ensuring that all of the data makes it from the sending host to the receiving host.

5. Session layer :-

In the Open Systems Interconnection (OSI) model, the session layer is the fifth layer, which controls the connections between multiple computers. The session layer tracks the dialogs between computers, which are also called sessions. This layer establishes, controls and ends the sessions between local and remote applications.

6.Presentation layer :-



The presentation layer does some rather complex things, but everything that the presentation layer does can be summed up in one sentence. The presentation layer takes the data that is provided by the application layer, and converts it into a standard format that the other layers can understand. Likewise, this layer converts the inbound data that is received from the session layer into something that the application layer can understand. The reason why this layer is necessary is because applications handle data differently from one another. In order for network communications to function properly, the data needs to be structured in a standard way.

7 . Application layer :-

 is the top most layer of OSI Model. Manipulation of data(information) in various ways is done in this layer which enables user or software to get access to the network. Some services provided by this layer includes: E-Mail, transferring files, distributing the results to user, directory services, network resources, etc.

The Application Layer contains a variety of protocols that are commonly needed by users. One widely-used application protocol is HTTP(HyperText Transfer Protocol), which is the basis for the World Wide Web. When a browser wants a web page, it sends the name of the page it wants to the server using HTTP. The server then sends the page back.

Other Application protocols that are used are: File Transfer Protocol(FTP), Trivial File Transfer Protocol(TFTP), Simple Mail Transfer Protocol(SMTP), TELNET, Domain Name System(DNS) etc.