The OSI Reference Model

one of the first topic that faces any one beginning network career is osi model , i will try to make it very simple , so i made a video to approach osi model concepts and layers.

after seeing the video lets go more deep in osi model 

The Open Systems Interconnection (OSI) reference model provides a means of describing how data is transmitted over a network. The model addresses hardware, software, and data transmission. This topic describes the purpose of the OSI model

The OSI reference model provides a number of benefits in understanding how networks function, by doing the following:

- Reducing complexity: The OSI model breaks network communications into smaller, simpler parts.

- Standardizing interfaces: The OSI model standardizes network components to allow multiple-vendor development and support.

- Facilitating modular engineering: The OSI model allows different types of network hardware and software to communicate with one another.

- Ensuring interoperable technology: The OSI model prevents changes in one layer from affecting the other layers, allowing for quicker development.

- Accelerating evolution: The OSI model provides for effective updates and improvements to individual components without affecting other components or having to rewrite the entire protocol.

- Simplifying teaching and learning: The OSI model breaks network communications into smaller components to make learning easier

The OSI Model Layers and Their Functions

The OSI isn’t a physical model. it's a set of guidelines that application developers can use to create and implement applications that run on a network.

The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how Data is transmitted end to end.

The OSI reference model has seven layers:

-        Application layer (layer 7)

-        Presentation layer (layer 6)

-        Session layer (layer 5)

-        Transport layer (layer 4)

-        Network layer (layer 3)

-        Data Link layer (layer 2)

-        Physical layer (layer 1)

The Application Layer

The application layer is the OSI layer that is closest to the user, acting as an interface between the actual application program—which isn’t  part of the layered structure—and the next layer down.

This layer provides network services to the applications of the user, such as e-mail, file transfer, and terminal emulation.

The Application layer is also responsible for identifying and establishing the availability of the communication partner and determining whether sufficient resources for the intended communication exist.

The Presentation Layer

The Presentation is responsible for data translation and code formatting. Tasks like data compression, decompression, encryption, and decryption are associated with this layer.

The Session Layer

The Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities (keeps different applications’ data separate from other applications’ data.).

For example, web servers have many users, so there are many communication processes open at a given time. It is important, then, to keep track of which user communicates on which path.

The Transport Layer

The transport layer segments data from the system of the sending host and reassembles the data into a data stream on the system of the receiving host.

The transport layer establishes, maintains, and properly terminates virtual circuits. Transport error detection and recovery and information flow control ensure reliable service.

The Network Layer

The network layer provides connectivity and path selection between two host systems that may be located on geographically separated networks.

The Data Link Layer

The data link layer defines how data is formatted for transmission and how access to the physical media is controlled. This layer also typically includes error detection and correction to ensure reliable delivery of the data.

The Physical Layer

the Physical layer does two things: It sends bits and receives bits.

This layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

Characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.