At which OSI layer is data added to a PDU during the encapsulation process?
transport layer
network layer
application layer
data link layer
The correct answer and explanation is :
The correct answer is Data Link Layer.
Explanation:
In the OSI (Open Systems Interconnection) model, encapsulation refers to the process of adding headers (and sometimes trailers) to data as it moves down the layers of the OSI model. This process ensures that the data can be properly routed, delivered, and understood by the receiving system.
To understand where data is added to the Protocol Data Unit (PDU), we need to examine the function of each layer and how they contribute to the overall transmission process:
- Application Layer (Layer 7): This is where user-level data is generated. At this point, the data doesn’t yet have any protocol-related information; it’s just raw data from the application, such as text or files.
- Transport Layer (Layer 4): The transport layer adds a header containing information for end-to-end communication, such as source and destination ports. The PDU at this layer is referred to as a segment.
- Network Layer (Layer 3): The network layer adds a header that contains routing information, such as source and destination IP addresses. This PDU is called a packet.
- Data Link Layer (Layer 2): This is where the actual framing of data occurs. The data link layer adds both a header and a trailer, including information for physical addressing (MAC address) and error checking (such as CRC). The PDU at this layer is known as a frame.
Thus, it is at the data link layer where the actual PDU is framed with a header and trailer, which enables the physical transfer of data across the network.
The encapsulation process starts at the application layer and proceeds down to the physical layer. However, it’s the data link layer that directly adds the final layer of protocol-specific information (such as physical addressing and error detection).
Here is the diagram illustrating the OSI model encapsulation process. It shows how data is encapsulated into a PDU (Protocol Data Unit) at each layer, starting from raw data at the Application layer, moving through the Transport, Network, and Data Link layers, and finally reaching the Physical layer as bits.