Data Encapsulation & Decapsulation in the OSI Model :

Remember :

Encapsulation starts from the Uppermost Layer  à Lowest Layer

Decapsulation starts from Lowest Layer  à Uppermost Layer.

Data in Different Layer :

1.     Transport Layer: Segment is the PDU

2.     Network Layer: Packet is the PDU

3.     Data Link Layer: Frame is the PDU

4.     Physical Layer: Bit is the PDU

** PDU: Protocol Data Unit

Data Encapsulation:

• Application layer is the GUI interface for user. So, data starts from Application layer.
• Data Application - > Data Presentation Layer - >Data Session Layer.
• As we are sending larger amount of data so in Transport Layer Data need to be segmented with proper sequence.
• Transport layer we have 2 protocol UDP / TCP. Here we will discuss about TCP.
• The data in the transport layer is called as segment. 
• Each sequenced segment can be assembled together on the receiver side. Then each segment are handled in network layer for network addressing or logical addressing and routing through internet. The data is named here as a packet. 
• The network layer adds its IP header to send it to the data link layer. Here, the data is named as frame. 
• The data link layer receives packet from network layer and place them into network medium. Data link layer encapsulates each packet in a frame consisted of hardware (MAC) address of the source and destination computer and the LLC information to identify the previous layer protocol, and FCS (Frame Check Sequence for Error detection ) it is needed for the packet to pass it when it will arrive the destination. 
• A frame is a logical group of 1’s and 0’s, the physical layer is responsible to digitalize the signal which is used by devices on the same local network.

Data Decapsulation:

• The receiving computer first will synchronize with the digital signal by reading a few extra 1’s and 0’s. Then it will receive the whole frame to pass it to the above data link layer. 
• The Datalink layer will do a Cyclic Redundancy Check (CRC) on the frame. This is a computation which the computer does and if the result it gets matches the value in the FCS field, then it assumes that the frame has been received without any errors. Once that's out of the way, the Datalink layer will strip off any information or header which was put on by the remote system's Datalink layer and pass the rest (now we are moving from the Datalink layer to the Network layer, 
• Network layer will check and match the IP address, if it matches then it will remove the IP header from the packet and rest is sent to above layer i.e. transport layer. 
• The data, named as segment in transport layer, is processed to rebuild the data tream and acknowledges to the transmitting computer that it has received the data. 
• Then it hands over the data to application layer.

For Better Visualisation :