What is the PDU at Layer 1 (Physical)?

The Layer 1 (Physical) PDU is bits — the frame converted to a raw bit stream encoded as electrical, light, or radio signals; there is no addressing at this layer.

* The Layer 1 bit stream encoded as electrical, light, or radio signals. *

The Physical layer converts frames into bits (binary 1s and 0s) for transmission.

What happens at Layer 1:

1. Frame is converted to bit stream
2. Bits encoded as signals:
   • Electrical impulses (copper)
   • Light pulses (fiber)
   • Radio waves (wireless)
3. Transmitted over physical medium

No addressing at Layer 1 - just raw signal transmission.

Memory aid: Bits are the building blocks — everything becomes 1s and 0s at the physical layer.

Go deeper:

• Physical layer — bit-by-bit transmission, signaling, encoding, and media.