What is the OSI Model ?

The OSI model is an architectural model that represents networking communications. It was introduced in 1978 by the International Organization for Standardization (ISO) to standardize the levels of services and types of interactions for computers communicating over a network.

What does the OSI model do ?

The OSI model defines the generic tasks that are performed for network communication. You can think of each layer of the OSI model as a piece of software that performs specific tasks for that layer. Each layer communicates with the layer below and the layer above. Data that is transmitted over the network must pass through all layers.

How to use the OSI model ?

The OSI model is used as a common reference point when comparing the function of different protocols and types of network hardware. Understanding the OSI model is important for comparing different products.

The architecture of the OSI model :

The OSI model divides network communication into seven layers. Each layer has a defined networking function, as described in below

1. Application =  The application layer provides the interface between the networking protocol stack and the software running on the computer. For example, this layer provides the interface for e-mail, file transfers, Telnet and File Transfer Protocol (FTP) applications. Applications use the services provided by applicationlayer protocols, which in turn use the services provided by the other layers beneath them.

2. Presentation layer =  The presentation layer (sometimes referred to as the syntax layer) is responsible for translating each computer’s native syntax into a common transfer syntax readable by the other computers on the network. In some cases, the transfer syntax can provide functions such as data compression and encryption.

3. Session Layer = The session layer provides many functions involved in the regulation of the dialog between two computers communicating over the network. For example, the session layer sets up, regulates, and terminates exchanges between the applications at each end of the communication.

4. Transport Layer = The transport layer provides functions that complement those of the network layer, including guaranteed delivery (which uses packet acknowledgments to ensure data is received), flow control (which regulates transmission speed to avoid dropped packets), and end-to-end error detection (which enables the receiving system to detect damaged packets).

5. Network Layer = The network layer defines the functions that provide end-to-end communications between computers on different networks. Chief among these functions is routing, which enables computers to relay traffic through intermediate networks to a destination on a remote network any distance away. Other functions include packet sequencing, end-to-end error detection from the sender to the recipient, congestion control, and addressing. While the data-link layer is responsible for local traffic on a single network, the network layer is responsible for directing traffic to its ultimate destination.

6. Data-Link Layer = The data-link layer defines the interface between the network medium and the software running on the computer. Among the data-link layer functions are packet addressing (which allows computers to direct traffic to specific destinations on the local network); media access control (which allows multiple computers to share a single network medium without conflicting); and formatting the frame used to encapsulate data for transmission over the network. The data-link layer divides into two sublayers. The logical link control (LLC) sublayer controls elements such as error checking from node to node on the same LAN, frame synchronization, and flow control. The media access control (MAC) sublayer controls the movement of data packets to and from one network interface card (NIC) to another across a shared channel.

7. Physical Layer = The physical layer defines the nature of the network medium—the actual fabric of the network that joins all the computers together—and the nature of the signals transmitted over the medium. In most cases, the network medium is a form of copper cable that uses electric currents for signaling, but fiber-optic and wireless media are becoming increasingly prevalent.