Encapsulation and de-encapsulation

Encapsulation and de-encapsulation are the conversion processes at each end of a transmission. They’re what make horizontal (peer-to-peer) layer communication actually work.

The analogy that sticks: nested gift boxes. Each layer puts the data in a new box and writes its own info on the outside. The receiver opens boxes one at a time, reading each label, until it reaches the gift.

Encapsulation (sending, down the stack)

1. Application: takes user data, adds its header. The PDU is now data.
2. Transport: adds its header (which includes which application protocol sent this). Now it’s a segment.
3. Internet: adds its header (info for getting from source to final destination across one or more networks). Now a packet.
4. Link: adds a header and a trailer, making a frame, then converts the frame to a physical signal and puts it on the media. The trailer is a data-dependent sequence for error checking: the FCS (Frame Check Sequence).

Each added header grows the PDU. The added bytes are overhead, as opposed to the user data itself.

De-encapsulation (receiving, up the stack)

Each layer reads the header its counterpart added, does its job, strips the header, and passes the rest up. Like checking the address on a package, then opening it if it’s yours.

1. Link: reads the whole frame, checks header and trailer (FCS) for errors. An error means the frame is discarded (upper layers may request retransmission). Clean: read the frame header (which protocol is encapsulated), strip header and trailer, pass up.
2. Internet: examines the packet header and decides to process at this layer or pass up. Strips the packet header first.
3. Transport: the segment header says which application protocol gets the data. Strips it, hands the data over.
4. Application: strips the data header, uses it to process the data, delivers to the user application.

How deep do devices look?

Not every device processes all layers:

• Switch: frame header and trailer only (L2). Forward out a specific port, flood out all ports except the incoming one, or discard on error. The frame passes through unchanged.
• Router: strips the frame and reads the packet header (L3). The forwarding decision comes from the packet header; if filtering, it may look deeper into the segment header (L4).
• Host: all layers. Encapsulates when sending, de-encapsulates when receiving, and does both at once across multiple simultaneous communications.

Worth remembering

• The frame is the only PDU with a trailer.
• OSI and TCP/IP terms get used interchangeably in the field. Know both models to talk with other engineers.