The Physical Layer is Layer 1 of the five-layer TCP/IP model. It defines how raw bits travel from one network node to another across a physical medium. Bits may travel as electrical signals over copper, light over fiber-optic cable, or radio waves over a wireless connection. section 4.3.1 section 4.4
Physical-layer specifications describe details such as cables, connectors, ports, signal encoding, radio frequencies, and modulation. Ethernet and Wi-Fi both include Physical Layer requirements: Ethernet covers UTP and fiber-optic transmission, while Wi-Fi defines wireless radio behavior. section 4.3.1 section 4.4
Keep these Layer 1 distinctions in mind:
- Layer 1 provides the medium and signals; it does not provide addressing or routing. section 4.3.1 section 22.1
- Layer 2 prepares messages for transmission to the next hop, while Layer 3 provides end-to-end delivery. section 4.3.1 section 4.4
- A physical port is commonly treated as the connector for a cable, while an interface is its software representation; networking usage sometimes treats the terms interchangeably. section 8.0
From application data to bits
When a browser requests a web page, higher layers prepare and address the message. Layer 2 adds a frame for the next hop, and Layer 1 transmits the resulting bits over a medium such as UTP cable. At the destination, the host receives the bits and removes the encapsulation before the application processes the data. section 4.3.2
