OSI Layers in Action

In the OSI Model lesson, you followed your data down the seven layers, from the application to the wire.
Now the bits arrive at their destination, and the whole journey runs in reverse.

When the receiving device receives the data, it performs decapsulation, which is the reverse of encapsulation.

The data moves up through the OSI layers, with each layer removing the information that its counterpart added on the sender’s side.

Layer 1 – Physical

The Physical Layer converts incoming electrical, optical, or radio signals back into raw bits. These bits are then passed to the Data Link Layer.

Figure 1 – Start of the De-encapsulation Process

Layer 2 – Data Link

The Data Link Layer groups the bits into a frame, verifies it using the FCS, and removes the Layer 2 header (MAC addresses). It then passes the packet to the Network Layer.

Figure 2 – OSI Layer 2: Data Link Layer in De-encapsulation

Layer 3 – Network

The Network Layer processes the packet, removes the IP header, and checks the source and destination addresses. It forwards the segment to the Transport Layer.

Figure 3 – OSI Layer 3: Network Layer in De-encapsulation

Layer 4 – Transport

The Transport Layer reassembles the segments into the original data stream. It removes the Layer 4 header (port numbers, reliability info) and identifies the correct service.

Figure 4 – OSI Layer 4: Transport Layer in De-encapsulation

Layers 5 to 7 – Upper Layers

Once the lower layers finish processing, the data moves into the upper layers of the OSI model: Session, Presentation, and Application. These layers make sure the data is correctly managed, formatted, and delivered to the right application.

Layer 5 – Session

The Session Layer establishes, maintains, and ends the communication session between devices. It keeps track of the session until the transfer is complete.

Figure 5 – De-encapsulation in the Upper OSI Layers