What happens during the encapsulation process?

Encapsulation is a top-down process where each layer adds its own header (and the data link layer also a trailer) to the data from the layer above, until it leaves as a bit stream.

* Each layer adds its header (and L2 a trailer) going down the stack. *

Each layer wraps the data from the layer above with its own control information so the receiving device knows how to process each layer.

Process (sending):

1. Application creates Data
2. Transport layer adds header → Segment (Transport header + Data)
3. Network layer adds header → Packet (Network header + Segment)
4. Data Link layer adds header AND trailer → Frame (Frame header + Packet + Frame trailer)
5. Physical layer converts to Bits for transmission

Application: DATA
Transport:   [TCP Header] + DATA = SEGMENT
Network:     [IP Header] + SEGMENT = PACKET
Data Link:   [Frame Header] + PACKET + [Frame Trailer] = FRAME
Physical:    BITS

Each layer processes and passes data down to the next level until it is sent as a bit stream.

Analogy: Encapsulation is like putting a letter in an envelope, then in a box, then wrapping the box — each layer adds another wrapper.

Go deeper:

• Encapsulation (networking) — the formal definition of headers/trailers added per layer.
• The OSI Model — Practical Networking — animated view of data being encapsulated down the stack.