In the OSI (Open Systems Interconnection) model, the Protocol Data Unit (PDU) is a term used to describe a unit of data at each layer of the network protocol stack. Each layer of the OSI model has its own specific PDU, which includes both the data and the protocol-specific control information required for communication.

The OSI model consists of seven layers, each with a distinct function:

1. Physical Layer

2. Data Link Layer

3. Network Layer

4. Transport Layer

5. Session Layer

6. Presentation Layer

7. Application Layer

At the Physical Layer (Layer 1), the PDU is referred to as a bit. This layer is responsible for the actual transmission of raw data bits over a physical medium, such as a wire, fiber optic cable, or wireless channel. The Physical Layer converts the data into electrical, optical, or radio signals that can be transmitted over the chosen medium¹².

The process of data transmission involves the following steps: