At which OSI layer is data added to a PDU during the encapsulation process?

During the encapsulation process in networking, data is added to a Protocol Data Unit (PDU) at various OSI layers when information is passed down the layers. The correct answer indicates that this occurs at the Application layer.

At the Application layer, the data generated by applications is packaged for transmission. This is where user data, such as messages from web browsers or email clients, is created and formatted. When the data is handed off to the next layer, it becomes the PDU that includes not only the raw data but also necessary application-related headers or control information.

The Application layer is the first layer in the OSI model that interacts with the end user’s software applications. Therefore, during encapsulation at this layer, specific protocols define how the data is structured. This is why any data being sent over a network starts its journey encapsulated at the Application layer, which is crucial for ensuring that it can be correctly interpreted by the receiving application.

Other layers such as the Transport layer, Presentation layer, and Session layer play significant roles in managing data but are involved in later stages of encapsulation and focus on different functionalities, such as establishing connections and data formatting respectively.