OSI Model in Networking.

The OSI 7 layers model has clear characteristics at each layer.
Basically, layers 7 through 4 deal with end-to-end communications between data source and destinations, while layers 3 to 1 deal with communications between network devices.

On the other hand, the seven layers of the OSI model can be divided into two groups: upper layers (layers 7, 6 & 5) and lower layers (layers 4, 3, 2, 1).

The upper layers of the OSI model deal with application issues and generally are implemented only in software.

The highest layer, the application layer, is closest to the end user. The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software.
The lowest layer, the physical layer, is closest to the physical network medium (the wires, for example) and is responsible for placing data on the medium.

Layer 7: Application Layer: Provides standardized services such as virtual terminal, file and job transfer and operations.

Layer 6: Presentation Layer: Specifies architecture-independent data transfer format. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Layer 5: Session Layer: Controls establishment and termination of logic links between users. It provides for full-duplex, half-duplex, or simplex operation, and establishes check pointing, adjournment, termination, and restart procedures.

Layer 4: Transport Layer: Provides reliable and sequential packet delivery through error recovery and flow control mechanisms. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control.

Layer 3: Network Layer: Routes packets according to unique network device addresses. The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors.

Layer 2: Data Link Layer: Frames packets. It is concern with physical addressing, physical link management, network topology, error notification and the flow of control .

Layer 1: Physical Layer: Interfaces between network medium and devices. The Physical Layer defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network adapters, Host Bus Adapters (HBAs used in Storage Area Networks) and more.