Data starts at the Application layer (L7) and travels down through each layer. At each layer, a header (and sometimes a trailer) is added to the data. This process is called encapsulation.
Receiver
L7Application
L6Presentation
L5Session
L4Transport
L3Network
L2Data Link
L1Physical
Layer 7Application
Layer 6Presentation
Layer 5Session
Layer 4Transport
Layer 3Network
Layer 2Data Link
Layer 1Physical
Scenario:
Layer 7 of 7 — Start at the Application layer and work down
PDU Encapsulation — live structure at current layer