2.1.The OSI Model

The OSI model consists of seven layers:

• Layer 1: The physical layer

• Layer 2: The data link layer

• Layer 3: The network layer

• Layer 4: The transport layer

• Layer 5: The session layer

• Layer 6: The presentation layer

• Layer 7: The application layer

top-down : All People Seem To Need Data Processing

bottom-up : Please Do Not Throw Sausage Pizza Away

Protocol Data Unit (PDU)

• The layer 1 (Physical layer) PDU is the bit.

• The layer 2 (Data link layer) PDU is the frame.

• The layer 3 (Network layer) PDU is the packet.

• The layer 4 (Transport layer) PDU is the segment for TCP or the datagram for UDP.