What is the PDU of the physical layer?

• A. bytes
• B. frames
• C. bits
• D. packets

Answer: (C)

The important idea is that each layer defines its own unit of data to pass to the layer below. At the physical layer, the data being transmitted is the raw bitstream. The hardware and signals on the medium carry individual bits (0s and 1s) as electrical, optical, or radio signals, and the physical layer’s job is to turn the higher-layer data into that continuous stream of bits and back again. Because of that, the PDU for the physical layer is bits.

Frames belong to the data link layer, which adds headers and trailers to create a complete frame for transmission. Packets belong to the network layer, where data is wrapped with network addressing and routing information. Bytes are a storage or representation unit, not the PDU at the physical layer, since the physical layer deals with continuing bit sequences rather than structured packets or frames.