OSI LAYERS INTERACTIONS

OSI Layer Interactions

Various protocols are implemented at each of the seven layers defined in the OSI reference model. The protocols function based on the guidelines defined for each layer.
In the OSI model, a layer at one end of the communication can interact with a peer layer at the other end. The message travels down the layer of the sending end through the transmission medium to the peer layer of the receiving end.
The following figure shows the communication between the Transport layers of two peers:

Communication Between Transport Layers Of Two Peers

In case two peers need to communicate with each other, the Application layer initiates the message for the recipient end. However, the two Application layers cannot communicate directly. As a result, the message is passed to the Presentation layer at the sender end, which adds its own header to the message and passes it to the Session layer. The Session layer, in turn, adds its own header and passes it to the next lower layer, the Transport layer. In this manner, each layer at the sender end receives the message, adds its own header, and passes the message to the next lower layer. This is because the message is converted into bits and placed on the transmission medium at the Physical layer. The Physical layer does not append any header.
When the recipient's Physical layer receives the message, it passes the message to the Data Link layer. The header added by the peer Data Link layer is stripped and passed to the Network layer, which, in turn, strips its own header. In this manner, each time the message is passed to the upper layer, the corresponding header is stripped off. When the message reaches the Application layer, it is read and interpreted.
The following figure shows the communication between the layers of two communicating ends: