What data is transmitted at the physical layer?

At the physical layer, what gets transmitted is not data in the usual sense, but raw bits, 0s and 1s, converted into a physical signal.

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#Short answer for the interview

The physical layer transmits bits (0 and 1) in the form of electrical, optical, or radio signals.

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#In detail, in plain language

#What exactly is transmitted

The physical layer works with:

• bits: 1010100110
• signals that represent these bits:
  • voltage
  • light pulses
  • radio waves

It does not know what these bits mean: text, video, or an IP packet.

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#What this data looks like at the physical layer

Depending on the medium:

#Copper cable (Ethernet)

• 1 is a high voltage level
• 0 is a low voltage level

#Fiber optic

• 1 means there is a light pulse
• 0 means there is no light pulse

#Wi-Fi

• 1 and 0 are different radio signal parameters (frequency, phase, amplitude)

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#What is not in physical layer data

A very important point.

At the physical layer, the following are NOT transmitted:

• frames
• packets
• IP addresses
• MAC addresses
• ports
• protocol headers

All of this appears at higher layers.

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#A visual analogy

Imagine Morse code:

• The physical layer is:
  • short and long signals
  • sound, light, or a radio wave
• The meaning of the message is already not the physical layer's concern.

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#Summary

The physical layer transmits:

• a stream of bits (0 and 1)
• in the form of physical signals
• without understanding the structure or meaning of the data