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**Physical Layer** is the **lowest, first layer of the OSI model**, responsible for the **actual transmission of bits (0 and 1)** over a physical communication medium.

Put **very simply**:
the physical layer is **how exactly data gets turned into a signal and sent over wires or through the air**.

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## What the physical layer actually does

The physical layer **does not think about data as files, packets, or messages**.
It works only with **raw bits**.

It defines:

### 1. **Signal type**

- electrical (copper cable)
- optical (fiber optic)
- radio signal (Wi-Fi)

### 2. **What 0 and 1 look like**

For example:

- 1 is a voltage of 5 V
- 0 is a voltage of 0 V
(or different light levels in fiber optic cable)

### 3. **Transmission speed**

- 100 Mbps
- 1 Gbps
- 10 Gbps

### 4. **Physical medium**

- twisted pair (Ethernet cable)
- fiber optic
- coaxial cable
- radio channel

### 5. **Connectors and pins**

- RJ-45
- SFP
- USB
- the shape and purpose of the pins

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## What the physical layer **does NOT do**

Important for interviews.

The physical layer:

- **does not know** who the data is being sent to
- **does not check for errors**
- **does not split data into packets**
- **does not use IP addresses or MAC addresses**

It simply passes along a stream of bits:

> "Here's a signal, send it on"

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## A real-life example

Imagine a phone call:

- The **physical layer** is:
  - the wire
  - the radio waves
  - the sound converted into an electrical signal
- **What you say and to whom** is already a higher layer's job.

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