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Networking Fundamentals · Foundational

Physical Layer (L1)

OSI Layer 1, which transmits raw bits as electrical, optical or radio signals over a medium.

Definition

The Physical layer defines cables, connectors, pinouts, voltages, light wavelengths, signaling, encoding and data rates. Its PDU is the bit. Hubs, repeaters, media converters, cables and transceivers are Layer 1 devices because they move signals without reading addresses.

Example

A bad RJ45 crimp or a dirty fiber connector is a Layer 1 problem.

Exam tip

In bottom-up troubleshooting you check Layer 1 first: link lights, cables, and interface status.

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