The OSI reference model is divided into seven layers, in descending order

The OSI reference model is divided into seven layers, in descending order: the physical layer, the data link layer, the network layer, the transport layer, the session layer, the representation layer, and the application layer.

The OSI reference model also employs a layered architecture technique that divides a network system into layers, each of which implements a different function, and each of which is formally described in the form of a protocol, which defines a set of rules and conventions used by a layer to communicate with a distant peer. Each layer provides a defined set of services to neighboring upper layers and uses the services provided by the lower layers adjacent to it.

Conceptually, each layer communicates with a distant peer, but in practice the protocol message units generated by that layer are transmitted with the help of services provided by neighboring lower layers. Thus, communication between peer layers is called virtual communication.

Expanded Information:

How it works

Data is generated by the uppermost layer of the transport (usually the application program), and is transmitted from the upper layer to the lower layer. At each layer, some information specific to that layer is added to the front end, called a header, before it is passed on to the next layer; think of the addition of a header as a sleeve of envelopes.

So by the time it reaches the bottom layer, the original data has been slipped into seven layers of envelopes, which are then transmitted to the receiving end through media such as network cables, telephone lines, and optical fibers.

The receiving end receives the data and transmits it from the bottom to the top, removing one layer of envelopes (i.e., removing the headers recognized by that layer) for each layer until the top layer, where the data is restored to its original form as it was when it was first generated from the top layer of the transmitting end.

In network terms, this addition of headers to the original data at each layer is called data encapsulation, and the original data before encapsulation is called the data bearer. On the transport side, the upper layer passes the data to the lower layer, which treats the data from the upper layer as a data bearer, and then encapsulates the data bearer into new data, which continues to be passed to the lower layer for encapsulation until the bottom layer.