In the context of the OSI model, data encapsulation refers to the process in which each layer of the OSI model adds its own header information to the original data that is being passed down from the higher layers. This process occurs as the data moves from the upper layers to the lower layers of the OSI model, and each layer adds its own control information to the data, which is then encapsulated in a new protocol data unit (PDU) before being passed down to the next lower layer.
The encapsulation process starts at the application layer, and as the data moves down the layers, it gets encapsulated with additional header information from each layer. The final encapsulated data is then transmitted over the physical layer of the network. When the data is received by the receiving end, the process of decapsulation takes place in which each layer removes its own header information and passes the remaining data up to the next higher layer until it reaches the application layer.