What units of measurement are used at the physical layer?

At the physical layer, the units of measurement relate to the signal and the transmission speed, not to data (packets, frames, and so on).

Put simply: here you measure how fast, and in what form, the signal is transmitted.

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#Main units of measurement at the physical layer

#1. Bits per second (bps)

This is the main unit of the physical layer.

It shows:

how many bits (0 and 1) are transmitted in one second

Examples:

• 10 Mbps
• 100 Mbps
• 1 Gbps
• 10 Gbps

Important: bps is not bytes, 1 byte = 8 bits.

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#2. Hertz (Hz)

Used to describe:

• signal frequency
• channel bandwidth

Examples:

• 100 MHz
• 2.4 GHz (Wi-Fi)
• 5 GHz (Wi-Fi)

Put simply:

Hertz shows how fast the signal oscillates.

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#3. Volt (V)

Used in wired networks.

It shows:

• the electrical voltage level
• the difference between logical 0 and 1

Example:

• 0 V is logical 0
• 5 V is logical 1

(the exact values depend on the standard)

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#4. Decibel (dB)

Used to assess:

• signal attenuation
• interference level
• line quality

Put simply:

dB shows how much the signal has weakened or been amplified

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#5. Meters (m)

Yes, this is a physical layer unit too.

Used for:

• the maximum cable length

Examples:

• twisted pair Ethernet: up to 100 m
• fiber optic: kilometers

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#What is not measured here

The physical layer does NOT use:

• frames
• packets
• IP addresses
• MAC addresses
• number of users

That is all handled by higher layers.

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#Short answer for the interview

The physical layer uses units of measurement related to the signal: bps (transmission speed), hertz (frequency), volts (voltage), decibels (attenuation), and meters (line length).