What is the physical layer of the OSI model?

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.