OSI Layer Model

OSI (Open Systems Interconnection) has been developed by International Organization for Standardization (ISO).

OSI Layer Model is the system’s conceptual model; it is abstract and not physical as the motherboard or RAM, but the working of the computer system is based on the layering of the OSI model.

OSI model relates the computers’ functions together, characterizing the functions, as well as interconnecting and assembling the tasks to be performed. It is layered, therefore one look at the ‘layers’ of the OSI model will give you an idea about the order of fulfillment of computer tasks.

The model has seven layers enumerated as follows:

7.         Application layer
6.         Presentation layer
5          Session layer
4.         Transport layer
3.         Network layer
2.         Data link layer
1          Physical layer

Functionality of the 7 layers

Upper layers:

The upper layers perform application-specific functions such as connection management, appropriate designing & formatting, structure composition, encoding messages and encryption. The application layer identifies communication partners, limiting or permitting the data exposure and dealing with privacy related issues. File transfer, emailing, network and software services are a part of this layer. Upper layers require the architecture to be open-ended since the application protocols are continuously evolving.
Let’s have a look at the Upper layer segments of the OSI Model in a detail:

7-Application Layer:

Application layer fundamentally works by using some building blocks which act as significant components for other Application Layer Protocols. Application Layer works for building up the Application association, which is a mutual agreement between the two application entities. Multi-peer associations are built up in Application layer and it is bound to continue evolving in its usage and applications with the growing changes and needs of the time.

6-Presentation Layer:

Presentation Layer as the very name signifies deals with the ‘representation’ of the information between two application entities. In order to get the pieces of information agreed upon by the two systems, the conformity in the ‘representation’ of the sent information is highly significant. The accuracy of the representation between the communicating system helps in ruling out the possibility of translation needs between the two. Presentation protocol thus determines suitable transfer syntax for every abstract syntax sent over by the application entities. The combination of an abstract syntax and transfer syntax in presentation layer is known as presentation context.

5-Session Layer:

The upper layer of OSI Model starts with the Session Layer. It mainly supports the synchronization needs of application entities and looks over the controlled use of transfer facilities by application entities. The fundamental working of synchronization encapsulates some of the basic features which include, transmission of synchronization points, originating and assigning serial numbers of synchronization points, sifting the data sent before and after synchronization, notifying when known activity of an application entity has started, interrupted, resumed or ended etc.

Lower layers:

The lower layers are concerned with more network-specific functions such as routing, flow control and addressing. Lower layers entail the four remaining layers as mentioned in the beginning namely Physical layer, Data link layer, Network layer and Transport layer. All these vertically hiking up layers support the other in the best transference and comprehension of the useful data and communicate the command to the Upper layers. Each layer protocol utilizes the service provided by the next lower layer and provides an increased service to the next hierarchical upper layer.

4-Transport Layer:

Transport layer acts as a final bridge between the lower and the upper layers. It transfers the information between the lower layer and the upper layers, or among different systems or hosts. This layer is responsible for getting the entire message across ensuring the accuracy, keeping track of fragmentation and out-of-order packets. Transference of transparent data and controlling the IP traffic specifically are some of the unavoidable characteristics of the Transport Layer which is certainly placing it in the “hot” seat for prompt network services.

3-Network Layer:

The network layer provides the system with basic switching and routing technologies. It works in the manner to develop end-to-end communication as per the communication provided by the Data Link Layer. Network layer acts as a boundary between the host and the subnet. It deals with routing issues, deadlock and congestion issues caused by increased number of packet data transfer and decreasing the performance etc. The two basic methods for sending packets used by network layer interface are virtual circuit and datagram methods which are also named as connectionless network or connection-oriented network.

2-Data Link Layer:

Data layer encodes and decodes the data packets for a smoother and reliable communication between the machines or systems connected by a single communication channel. It groups the stream of raw bits being received from the physical layer into the ‘frames’ The detailed mechanism taking place guarantees the reliable delivery, acknowledgement of the sent information, error controlling tendency, control of the flow of information etc.

1-Physical Layer:

Physical layer conveys the impulses and signals through the network. It controls the transmission of the actual data. It defines and paves way for the electrical signals and line states. It encodes the data and the connector types for instance determining the number of pins and function of each pin hence taking over the mechanical functions. It physically transfers the raw bits over a transmission medium and is responsible for the establishment or breaking of the connection.

Reference: http://www.certiology.com