Data Encapsulation and Decapsulation

Understanding data encapsulation

Data encapsulation inserts additional information onto the Protocol Data Unit (PDU) as it passes from the upper layers to the lower layers of the protocol suite.
Data encapsulation inserts source and destination addressing information, which is called headers to help the message to arrive to its destination.
Each layer inserts addressing information onto the PDU until it is placed on the actual network.

At the Application Layer, the application layer protocol creates the Protocol Data Unit (PDU) which is known as Data (raw message).
Next, the raw message is passed down to the Transport Layer which inserts a Layer 4 header that contains the source and destination service port numbers. The PDU is now known as a Segment.
The Segment is passed down to the Network Layer which inserts a Layer 3 header that contains the source and destination IP addresses. The PDU is now known as a Packet.
The Packet is passed down to the Data Link Layer which inserts a Layer 2 header which contains the source and destination MAC addresses. A Trailer is inserted at the end which contains the Frame Check Sequence (FCS). A Preamble is also inserted to indicate the start of the message and the sequence number. The PDU is now known as a Frame.
Lastly, the Frame is then translated into Bits which are then placed onto the network at the Physical Layer.

This is the process of removing each header as the message is moved from the lower layers to the upper layer of the protocol suite.
As the message moves upwards, each layer ensures the message is delivered to the correct device and to the appropriate Application Layer protocol.
Each layer process the message before sending it to the next upper layer.

The MTU is the maximum size of a message which can be transmitted across a network.
The MTU is 1518 bytes, if a message is greater than the MTU of the network, the message will be discarded.

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